Public switched Telephone networks – Switching system principles–PABX switching– ISDN, Cellular mobile communication systems – GSM, GPRS, DECT, UMTS, IMT2000, Limited range Cordless Phones and Facsimile, Wifi and Bluetooth.

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TELEOMMUNICATION SYSTEMS
UNIT-IV
Public switched Telephone networks – Switching system principles–
PABX switching– ISDN, Cellular mobile communication systems –
GSM, GPRS, DECT, UMTS, IMT2000, Limited range Cordless
Phones and Facsimile, Wifi and Bluetooth.
Public switched Telephone networks
Telephone Services:
 Plain old telephone services (POTs) is the voice grade telephone service that remains the
basic form of residential and small business service connection to the telephone network in
many parts of the world.
 POTs has been available almost since the introduction of the public telephone system in
the 19th century in a form mostly unchanged to the normal user despite the introduction of
Touch-tone dialing, electronic telephone exchanges and fiber-optic communication into the
Public Switched Telephone Network (PSTN)
The system was originally known as the Post Office Telephone service. The term was dropped as
telephone services were removed from the control of national post offices.
POTs include:
 Bi-directional or full duplex, voice band path with limited frequency range of 300 to 3400 Hz.
In other words, a signal to carry the sound of the human voice both ways at once.
 Call - progress tones , such as dial tones and ringing signal
 Subscriber dialing
 Operator services such as directory assistance, long distance calling and conference calling
assistance.
Many calling features become available to POTs subscribers after computerization of telephone
exchanges during the 1980's 1970's. The services include:
 Voice mail
 Caller ID
 Call waiting
 Speed dialing
 Conference call
The communication circuits of PSTN continue to be modernized by advances in digital
communication; however than other improving sound quality these changes have been mainly
transparent to the POTs customers.

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Telephone Networks
 Public Switched Telephone Network is the network of the world's Public circuit switched
Telephone network. It consists of telephone cables, fiber optic cables, microwave
transmission, cellular networks, communication satellites, under sea telephone cables, thus
allowing any telephone in the world to connect to any user.
 Originally a network of fixed-line analog telephone systems, the PSTN is now almost
entirely local in its core and includes mobiles as well as telephones. The technical operation
of the PSTN utilizes the standards created by the ITU-T.
 These standards are for different networks in different countries to interconnect seamlessly.
There is also a single global address space for telephone numbers based on E.163 and
E.164 standards.
 The combination of the interconnected networks and the single numbering plan make it
possible for any phone in the world to dial to any other phone.
 The switching centers are organized into: regional offices (class 1), sectional offices (class
2), primary offices (class 3), toll offices (class 4) and end offices (class 5). The hierarchical
relationship between these offices is illustrated in the following Fig.1.
 Subscriber telephones are connected through loops, to end offices (or central offices). A
small network may have only one end office, whereas a large city must have several end
offices.
 Many end offices are connected to one toll office. Several toll offices are connected to one
primary office. Several primary offices are connected to a sectional office, which normally
serves more than one state. And finally several sectional offices are connected to one
regional office. All the regional offices are connected using mesh topology.
 Accessing the switching station at the end is accomplished through dialing. In the
telephones featured rotary or pulse dialing is a digital signal was sent to the end user for
each number dialed.
 This type of dialing is prone to errors due to the inconsistency of connections during the
dialing process.
 Today, dialing is accomplished through touch-tone technique. In this method, the user has
two small bursts of analog signals called dual signals. The frequency of signals depends on
rows and columns.

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Fig.1. Hierarchical relationship in telephone networks
SWITCHING SYSTEMS
 Telephony was invented in the 1870s, all the early exchanges used human operators to
establish and supervise calls. As networks grew it became impractical to continue to use
people to set up telephone calls.
 It has indeed been calculated that to carry today's telephone traffic using 19th century
practices would need more than half the total population of all major cities to be employed
as telephone operators.
 Modern telephones are a far cries from Bell's original model and even from the ones
available a couple of decades ago. They vary in shape and features.
 They have push buttons instead of dials and memories to store telephone numbers. Some
are cordless; the handset is not attached to the base unit, but is linked to it by short range
radio. However, the principles of telephony have remained the same from the beginning.
SWITCHING SYSTEM PRINCIPLES
 A switching system of some sort is needed to enable any terminal (e.g. a telephone, a
teleprinter, a facsimile unit) to pass information to any other terminal, as selected by the
calling customer.
 If the network is small, direct links, Fig.2. Can be provided between each possible pair of
terminals and a simple selecting switch installed at each terminal. If there are five terminals,
each must be able to access four links, so if there are N terminals there must be a total of
½N (N–1) links.

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Fig.2. Full interconnection
 A slightly different approach would be to have one link permanently connected to each terminal,
always used for calls to that particular terminal Fig.3.
 Again, each terminal would need a selection switch to choose the distant end wanted for a
particular conversation, but the number of links is reduced from 10 to 5 for a 5-terminal
system, and to N links for N terminals.

Fig.3. One link per terminal method
 As number of terminals and distances increase, this type of arrangement becomes
impossibly expensive with today's technologies. A variant of this is however in wide use in
radio telephone networks: all terminals use a single common channel to give the instructions for
setting up each call Fig.4.

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Fig.4. Use of a calling channel
 The terminals concerned then both switch to the allocated link or channel for their
conversation. The number of links may be reduced substantially by this method; enough
need to be provided only to carry the traffic generated by the system.
 But each terminal still needs access to several channels and must have its own selection
switch.
 So far as telephone networks are concerned it is at present more economical to perform all
switching functions at central point’s i.e. not to make each terminal do its own switching.
 This means the provision of only one circuit from the nearest switching point or exchange to
each subscriber's terminal Fig.5. As networks grow and expand into other areas more
switching points are provided, with special circuits between such points to carry traffic
between the areas concerned.
Fig.5. Use of a switching centre; the telephone exchange

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 The whole network has to be designed to strict transmission parameters so as to ensure that
any subscriber on any exchange can converse satisfactorily with all other subscribers,
anywhere in the world.
 Call accounting equipment also has to be provided so that appropriate charges can be levied
for all calls made.
 The first telephone exchanges were manual and all calls were established by operators.
Automatic exchanges using electromechanical relays and switches were however
developed very rapidly.
 Computer-controlled exchanges, with no moving parts at all, are now beginning to become
common. Most countries in the world now have automatic telephone systems fully
interconnected with the rest of the world's systems.
PBX SWITCHING
 The acronym PBX (or PABX in Europe) stands for Private Automatic Branch Exchange, a
device for switching telephone calls within a building such as an office block or factory.
 The fundamental principles of PBX switching are the same as those found in a much larger
central office switch, which is used in the public network to switch telephone traffic from the
caller to the person who is being called.
 The PBX is essentially a connecting device. When a call comes into the PBX from an
outside trunk line, it is either routed directly to telephone extension or to a console with a
human attendant.
 The console rings and is answered by an attendant, who then connects the incoming call to
the required extension by dialing the extension number required and, when this is
answered, hanging up so that both parties can be connected.
 Before PBXs were automated and computerized, the attendant manually connected both
the caller and the person being called by physically inserting a cord which contained the
incoming call, into a board full of sockets each of which was a designated telephone
extension.
 A modern digital PBX attendant's console looks much different. Indicator lights on the
console display which of the incoming lines is active, which callers are on hold, a display
showing which extension the incoming call is being connected to and so on.
 Since the early 1970s, PBX technology has moved through a number of distinct
technological generations. Early PBXs were entirely electromechanical, using either
Strowger or crossbar switching.
 With the introduction of computers, SPC exchanges replaced electromechanical switching.
These PBXs, however, still operated analog switching and transmission, and far from all the
PBX functions were computerized.
 The next step was to computerize digital switches fully, with all the moving parts replaced
by micro circuitry.
 Future PBX generations will continue the same digital theme, moving toward broadband
switching and convergence with local area networks.
The fundamental elements of a digital PBX are shown in Fig.6.
All incoming lines enter some sort of switching matrix

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 Depending on the number of extensions connected to the PBX, there can be hundreds or
even thousands of different switching paths to which the incoming call may be connected.
The paths within the switching matrix are controlled by a central processor computer
 Each extension line must be constantly scanned for activity and the processor alerted, when
activity occurs. The central processor must be informed when a user wishes to place a call
as the telephone is picked up. When the number has been dialed, the processor must be
able to read the required number and make the connection internally or if it is a call outside
PBX, the processor must allocate it an outside line. Similarly, when a caller replaces the
telephone handset after the call is finished, the processor needs to be informed that this
extension is now free to accept further calls. There are also various signals which must be
sent down the line, such as busy signals, signals to make the phone ring, on hook/off hook,
dialing and so on. The signaling equipment is controlled as a subset of the central processor.
 Finally, information about each call, its duration, and the number called, and so on, needs
to be gathered for accounting and analysis
 A digital PBX can handle both voice and data traffic and be programmed to support many
different functions and applications. The capacity of a PBX to process calls and undertake
the other functions that may be required of it is a function of the capability of the central
processing unit. Microprocessor technology expands ever upward, and eventually it will be
possible to put almost, all the functions of a PBX on to a single microchip. A digital PBX
exhibits all the attributes of a computer. It is programmable and, therefore software
controlled has few moving parts, and is highly reliable, it is expandable, can perform
extensive test and diagnostic routines, and is intelligent.
Fig.6. Basic elements of a computerized switching system
FUNCTIONS OF A SWITCHING OFFICE
The basic functions performed by a switching office are the same, whether it is manual,
electromechanical, or electronic. The basic stages that a call must go through are as follows:

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 When the subscriber picks up her telephone, the office must detect that service is needed.
In an automatic office, the dialing tone is switched to that line, and the mechanism waits for
the subscriber to dial.
 The dialed telephone number must now be used to set up an interconnection path. The
number is received as a train of pulses from a rotary dial or a train of frequency pairs from a
push-button telephone. These signals cause the equipment to set up a path through the
exchange to the appropriate outgoing line.
 A central office with 10,000 lines would need approximately 50 million interconnections if
there were to be a unique path between every input and every possible output. The
switches therefore concentrate the calls into a limited number of paths through the
exchange. Also, the office will be connected to a limited number of trunks, and again the
incoming calls are concentrated onto these outgoing lines. The calls, having been
concentrated onto the available interconnections, find their way to the requisite part of the
exchange and are switched to the appropriate output lines.
 The required output line might be busy. It is, therefore, necessary to detect a busy condition
and to notify the caller of it. Similarly, as there is only a limited number of paths through the
exchange, the exchange itself may not be capable of making the connection and again the
caller must be notified. If the exchange is unable to make a connection, it will switch a busy
signal onto the caller's line.
 It is necessary to make the telephone of the called person to ring. The terminating
automatic exchange sends a ringing signal down the line when the connection is made.
 The telephone of the called party has now rung. The ringing signal must be removed from
the line when that person answers. The exchange may, after a respectable wait, disconnect
the call.
 When the call is successfully established and completed, the parties put their telephones
down and the circuit must then be disconnected. The exchange circuitry detects that the
telephones are back on their rests. The exchange disconnects the circuit freeing the
interconnection paths.
 Last, the caller usually has to be charged. In an exchange there must be some mechanical
way of recording the number of calls each subscriber makes and the duration and distance
of trunk calls. Some exchanges have counters for each subscriber, which must be read
periodically. Computerized exchanges produce magnetic tape for processing on another
computer.
MESSAGE SWITCHING
 As one telegraph system after another was installed in the countries around the world,
nation wise communications networks evolved. A message could be sent from one point to
another even if the two points were not serviced by a common telegraph line.
 In this case, telegraph operators at intermediate points would receive a message on one line
and retransmit it on another when a telegraph office had several lines emanating from it, the
process of transferring a message from one line to another was, in essence, a switching
function.
 One of the world's largest message switches was completely automated in 1963 when
Collins Radio Company installed a computer-based message switch for airline companies
of North America.
 The processor in each node maintains message queues for each outgoing link, Fig.7.

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Fig.7. Message switching network
 This system and the more recent successors store incoming messages directly into a
computer memory (disk file) and forward them automatically to the appropriate output line
when available. Hence this mode of operation is often referred to as store-and-forward
message switching.
 Included with each message is a header containing an address and possibly routing
information so the message processor at each node can determine to which output line to
switch the message.
 These queues are normally serviced on a first come, first served basis. However, priority
information can sometimes be included in each header to establish different classes, or
grades of service, thereby allowing time-critical messages to be placed at the head of a
queue.
MESSAGE SWITCHING AND CIRCUIT SWITCHING
 A message switching network is fundamentally different from a circuit switching network in
that the source and destination do not react in real time.
 In fact, most message switching networks could deliver a message on a delayed basis if a
destination node is busy or otherwise unable to accept traffic.
 In a message switching network there is no need to determine the status of the destination node
before sending a message, as there is in circuit switching.
 Message switching networks are fundamentally different from circuit switching networks in
their response to traffic overloads.
 A circuit switching network blocks or rejects excess traffic while a message switching network
normally accepts all traffic but provides longer delivery times as a result of increased queue
lengths.
 Yet another important distinction of a message switching network is that the transmission
links are never idle while traffic is waiting to use them.
 In a circuit switching network, a circuit may be assigned to a particular connection but not
actually carrying traffic. Thus, some of the transmission capacity may be idle while some
users are denied service.

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 In contrast utilization of the transmission links of a messages witching network is directly related
to the actual flow of information. Arbitrarily high utilization efficiencies are possible if
increased store-and-forward queuing delays are acceptable.
PACKET SWITCHING
 Fig.8 depicts both the conceptual structure and the conceptual operation of a packet
switched network. A single message at the source is broken up into packets for transmission
through the network.
 Included in each packet is a header containing address and other control information? Each
packet is relayed through the network in a store-and-forward fashion similar to a message
switching network. At the destination node, the packets are reassembled into the original
contiguous message and delivered.
 The main feature of a packet switching operation is the manner, in which the transmission
links are shared on an as-needed basis. Each packet is transmitted as soon as the
appropriate link is available, but no transmission facilities are held by a source when it has
nothing to send.
 In this manner, a large number of relatively inactive sources can share the transmission
links. In essence, link utilization is improved at the expense of storage and control
complexity in the nodes.
 Considering the declining cost of digital memory and processing, the increased control
complexity becomes less and less significant as digital technology advances.
 One particular variation of packet switching Asynchronous Transfer Mode (ATM) is designed
to specifically support hardware implementations of control intensive functions, thereby
supporting very high traffic volumes with low delay.
Fig.8. Packet-switching network
 As the traffic load in a packet switched network increases, the average transmission delay increases
correspondingly. In contrast, a circuit switched network either grants service or rejects it.
Conversely when only a few circuits are in use in a circuit switched network, much network
transmission capacity is idle. When there is a light load on a packet switched network, the active
users benefit by shorter than usual delay times.
 Using automatic repeat request (ARQ) error control, packet switching networks traditionally
provided essentially error-free transmission for each node-to-node transfer.

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 When errors are detected a retransmission is requested (message NAK). Hence transmitting
nodes must hold all transmitted packets in memory until a positive response (message ACK)
is returned by the receiving terminal. Furthermore, an entire packet is usually received and
checked for errors before forwarding it to another node.
 Customers typically access packet networks by way of leased lines or dial-up connections.
Dial-up connections are used by infrequent users, while leased lines are preferred by heavy
users to achieve constant availability, higher data rates, and possibly low error rates.
PACKET SWITCHING AND MESSAGE SWITCHING
Despite the similarity to message switching operations, a packet switching network is different in
two important respects:
 The store-and-forward delay through a packet switched network is relatively short. Thus
interactive communications can occur in much the same manner as if a dedicated end-to-end
circuit is established.
 A packet switched network does not provide storage of messages, except in an incidental
manner while relaying packets from one node to another. The network is designed to
provide switched communication between two nodes, both of which are actively involved in
the communication process. A packet switching network does not normally store a
message for later delivery to an inactive or busy terminal.
PACKET FORMAT
 As indicated in Fig.9, a packet contains three major fields: the header, the message, and
the redundancy check bits.
 Generally speaking, the control information associated with a particular message or link is
included in the header of a message packet. Some packets may not contain a message field
if they are being used strictly for control purposes.
 Although a variety of techniques for generating redundancy checks are possible, the most
popular technique uses cyclic redundancy checks (CRCs).
 Basically, a CRC is nothing more than a set of parity bits that cover overlapping fields of
message bits. The fields overlap in such a way that small numbers of errors are always
detected and the probability of not detecting the occurrence of 2 large numbers of errors is
1 in 2M, where M is the number of bits in the check code.
A header typically contains numerous subfields in addition to the necessary address field.
Additional fields sometimes included are:
 An operation code to designate whether the packet is a message (text) packet or a control
packet. In a sense this field is a part of the destination address with the address specifying
the control element of a switching node.
 A source address for recovery purposes or identification of packets at a destination node that
is capable of simultaneously accepting more than one message.

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Fig.9.Typical packet format
 A sequence number to reassemble messages at the destination code, detect faults, and
facilitate recovery procedures.
 A length code to indicate the length of a packet when less than a standard size pack is
transmitted.
INTEGRATED SERVICES DIGITAL NETWORK (ISDN)
 Integrated services digital network is a single network able to carry and switch a wide variety of
telecommunication services, voice, video, data or packets over the public switched telephone
network (PSTN).
 It is expected to evolve from an IDN, an integrated digital network, which is a telephony
network in which digital transmission systems have been fully integrated with switching
systems.
 ISDN is a service offering that extends access to digital transport facilities and to the
signaling network. Access to the digital transport facilities occurs on 64 kbps (B) channels
while access to signaling network occurs on 16 or 64 kbps signaling (D) channels.
 Two levels of digital access to the ISDN network have been standardized: basic rate access
and primary rate access. As shown, the worldwide basic rate interface (BRI) standard is also
referred to as 2B+D interface. It consists of two bearer channels for customer voice or data
at 64 kbps. In addition, it has one 16 kbps signaling channel. It runs over a single pair of
twisted wires between the customer and the telephone company.
 The primary interface (PRI) is sometimes referred to 23B+D interface. PRI ISDN is a trunk
connection. It is installed on the trunk side of a PBX or into a multiplexer. BRI ISDN is a line-
side connection. It connects to the same ports in PBXs as do telephone sets.
 Many corporations use PRI ISDN for their direct inward dialing (DID) traffic. The local
telephone company sends the caller's names and phone number over the signaling
channel. The telephone system captures the information and sends it to the display equipped
ISDN telephone.
The purpose of ISDN is to provide fully integrated digital services to users. These services fall into
three categories: bearer services, tele-services and supplementary services which are explained
using Fig.10.

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Bearer Services:
 Bearer services provide the means to transfer information (voice, data and video) between
the users without the network manipulating the content of that information. The network
does not need to process the information and therefore does not change the content.
 Bearer services belong to the first three layers of the OSI model and are all well defined in
the ISDN Standards. They can be provided using circuit-switched, packet switched, frame
switched or cell switched networks.
Teleservices:
 In tele-servicing, network may change or process the contents of data. This service
corresponds to the layers 4-7 of the OSI model.
 Teleservices rely on the facilities of the bearer services and are designed to accommodate
complex user needs without the user having to be aware of the details of the process. Tele
services include telephony, teletex, telefax, video text, telex and teleconferencing. Although
the ISDN defines these services by name, they have not yet become standards.
Supplementary services:
 Supplementary services are those services that provide additional functionality to the
bearer services and teleservices. Examples of these services are reverse charging, call
waiting and message handling.
Fig.10. ISDN services
B channel:
 A bearer channel (B channel) is defined at a rate of 64 kbps. It is the basic user channel
and carry any type of digital information in full duplex mode as long as the required
information rate does not exceed 64 kbps.

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 Multiplexed transmission of this sort however must be destined for a single recipient. A B
channel carries transmission end to end. It is not designed to demultiplex a stream midway
in order to separate and divert transmissions to more than one recipient.
D Channels:
 A data channel (D channel) can be either 16 or 64 kbps, depending on the need of the user.
Although the name says data, the primary function of the D channel is to carry control
signaling for the B channel.
 Control information (such as call establishment, ringing, call interrupt or synchronization) is
carried by the same channel that carries the message data. The ISDN separates the control
signals onto a channel of their own, the D channel. D channel carries the control signaling
for all the channels in a given path, using a method called common channel (out-of-band)
signaling.
 In this mechanism, the subscriber uses the D channel to connect to the network and secure
a B channel connection. The subscriber then uses the B channel to send actual data to
another user.
 All the devices attached to a given subscriber loop use the same D channel for signaling
but each user sends data over a B channel dedicated to a single exchange for a duration of
the exchange.
 Using a D channel is similar to having a telephone operator to place a call, pick up the
phone and tell the operator what type of call you wish to place and the number you wish to
contact.
 The operator finds an open line appropriate for your needs rings your party and connects.
The D channel acts like an operator between the user and the network layer.
H Channels:
 H channels (Hybrid channels) are available with the data rates of 384 kbps (H0), 1536 kbps
(H11) or 1920 kbps (H12). These rates suits H channels for high data rate applications such
as video, teleconferencing and so on.
CELLULAR MOBILE COMMUNICATION SYSTEMS
 When an existing system undergoes a unique improvement, it is wise to give that system a
new name. This is what happened with the mobile two-way telephone communications system.
 Under the original plan, a single medium-power transmitter was placed at the centre of
population in an urban community having a service area of about 50 miles in diameter. The
central transceiver had a capacity of between 100 and 500 channels however, because
mobile telephones are full-duplex, the system was limited to about 250 conversations at a
time.
 Although the costs were high and subscriber lists were small, the system served the
community very well. As the community and the subscriber lists grew, users experienced
long waiting periods before they could get into the system.
 Most of the systems were owned and operated by the local telephone companies, which
were capable of supplying mobile-to-mobile as well as mobile-to-home or mobile-to-
business telephone interconnections.
 The geographical shape of the service area was controlled by the radiation pattern of the
transmitting antenna, but problems resulted when the community grew or changed shape.

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 The changes to this system had to be planned and implemented over a period of time. The
first step was to use new system-ready mobile units in new installations, which offered
capabilities far beyond those of the old system.
 Once the mobile units were in place, the second step was to convert the large service area
into several smaller service areas, called cells. The cells took on the hexagonal shape of a
honeycomb.
 The one high-power transmitter was replaced by six separate 5 W transceivers tied together
by land line interconnections through the branch exchanges of the telephone companies.
A mobile unit in cell C, A would transmit to cell transceiver, then through the telephone system to
cell where the message would be retransmitted to a mobile unit in cell C's area. These two steps
introduced several advantages. First, the cell transmitter and mobile transmitter each operate at a
lower power. Second, a mobile unit in cell A, one in cell C, and a third in cell F could use the same
frequencies without channel interference. The reuse of frequencies will increase the system's
capacity by as many cells as can use the same frequencies. The only restriction is that adjacent
cells may not use the same frequencies. Third, the system is easily expanded because adding a
new cell will not affect any of the established cells. Fourth, cells provide better management of the
total service area
 These are the most obvious differences. Each cell's transceiver is connected through the
telephone system to a central mobile switching office MSO for better overall system control.
 The cells are not independent of each other, or of the total system. Suppose a mobile unit
in cell A is using channel 24, and another unit in cell C is also using channel 24.
 So far, no problem since the same frequencies may be used in widely separated cells.
Suppose unit -A, being mobile, now moves into cell B as in Fig.11.
 The MSO keeps track of which frequencies are in use in each cell, knowing that adjacent cells
should not use the same frequency.
 As soon as cell A gives up control of unit A to cell B, the MSO will change the
transmit/receive frequencies of either mobile unit depending on the number of channels in
use of each cell.
 The communications link will thus continue on without co-channel interference. The switchover
takes about 250ms and will generally go unnoticed by either mobile unit operator.
Fig.11. Cellular mobile telephone system.

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 A cellular mobile telephone system divides the area to be served into hexagonal zones or
cells. Each cell uses a different set of frequencies to its immediate neighbors. The cells are
grouped into blocks in this illustration, blocks of seven cells.
 The pattern of frequencies is repeated in each block. Thus the tone-filled cells in this
illustration use identical frequencies. There could be more cells in a block than illustrated
here.
A step-by-step trip through a complete transaction will clarify other differences from the original
mobile telephone systems. To begin, each cell transceiver sends out an identification signal of
equal strength for all cells. When a mobile unit operator picks up the handset, a scanning system in
the unit measures the signal strength of all cells’ identification code signals.
The mobile unit then sets up contact with the cell having the strongest signal. The data channel in
each cell transceiver, called a setup channel operates at a 300 band and uses the ASCII 7-bit
code. The mobile unit also sends the cell its identification code, which is then passed on to the
MSO by telephone line for recognition, frequency assignment, and future billing. The mobile
operator then gets a dial tone and initials a call in the usual manner. The call transaction is the
same as any standard telephone call.
During the time of the call, the cell transceiver monitors all of the active channels in its area as well
as in the six surrounding cell areas. It maintains a constant conversation with its six adjacent cells
and the MSO.
Fig.12. Cellular Network

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Together, they control which mobile unit is on what channel, which cell is the controlling cell (by
comparing the strength of the mobile signal in each area), the location of a mobile unit in each cell,
and what action is to be taken when a mobile moves from one cell to another.
The controlling cell uses a form of reverse AGC; that is, it can order the mobile unit to alter its
output so that cell interference is minimized. The mobile output power is maximum at 3W, but it
can be reduced in 1 dB steps (seven steps total) to as little as 0.6 W. All of these control functions
are carried on without the phone operator's knowledge. The cellular radio network is elaborated in
Fig.12.
 In a cellular radio network each cell contains a base station that is connected to a mobile
switching office by a leased terrestrial or microwave circuit. Each base station sends and
receives signals from mobile units located in their cell.
 The MSO handles channel allocation and call switching, and provides a gateway to the
public switched telephone network, thus allowing calls to be switched between the two
networks.
 Because only adjacent cells use different frequencies, a large number of users can be
accommodated by a cellular network even though only a limited frequency bandwidth is
used.
ESTABLISHING A CALL
Fig.13, illustrates the steps involved in a typical call between two mobile users within an area
controlled by a single MTSO:
1. Mobile unit initialization: When a mobile unit is turned on, it scans and selects the
strongest setup control channel used for this system as in Fig.13. (a). Cells with different
frequency bands repetitively broadcast on different setup channels. The receiver selects
the strongest setup channel and monitors that channel. The effect of this procedure is that
the mobile unit has automatically selected the BS antenna of the cell within which it will
operate. Then a handshake takes place between the mobile unit and the MTSO controlling
this cell, through the BS in this cell. The handshake is used to identify the user and register
its location. As long as the mobile unit is on, this scanning procedure is repeated
periodically to account for the motion of the unit. If the unit enters a new cell, then a new BS
is selected. In addition, the mobile unit is monitoring for pages.
2. Mobile originated call: A mobile unit originates a call by sending the number of the called
unit on the preselected setup channel as in Fig.13. (b). the receiver at the mobile unit first
checks that the setup channel is idle by examining information in the forward (from BS)
channel. When an idle is detected, the mobile may transmit on the corresponding reverse
(to BS) channel. The BS sends the request to the MTSO.
3. Paging: The MTSO then attempts to complete the connection to the called unit. The MTSO
sends a paging message to certain BSs depending on the called mobile number as in
Fig.13. (c). Each BS transmits the paging signal on its own assigned setup channel.
4. Call accepted: The called mobile unit recognizes its number on the setup channel being
monitored and responds to that BS, which sends the response to the MTSO. The MTSO
sets up a circuit between the calling and the called BSs. At the same time, the MTSO
selects an available traffic channel within each BS's cell and notifies each BS, which in turn
notifies its mobile unit as in Fig.13. (d). the two mobile units tune to their respective
assigned channels.
5. Ongoing call : While the connection is maintained, the two mobile units exchange voice or
data signals, going through their respective BSs and the MTSO as in Fig.13.(e).

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6. Hand off: If a mobile unit moves out of range of one cell and into the range of another
during a connection, the traffic channel has to change to one assigned to the BS in the new
cell as in Fig.13. (f). the system makes this change without interrupting the call or alerting
the user.
Fig.13. Establishing a mobile cellular call
GSM: (GLOBAL SYSTEM FOR MOBILE)
Global System for Mobile (GSM) is second generation cellular system standard that
was developed to stoke the fragmentation problems of the first cellular systems in Europe. GSM
is the world's first cellular system to specify digital modulation and network level architectures and
services.
A GSM network comprises of many functional units. These functions and interfaces are explained
in this chapter. The GSM network can be broadly divided into:
 The Mobile Station (MS)
 The Base Station Subsystem (BSS)
 The Network Switching Subsystem (NSS)
 The Operation Support Subsystem (OSS)

19. 19
The additional components of the GSM architecture comprise of databases and messaging
systems functions:
 Home Location Register (HLR)
 Visitor Location Register (VLR)
 Equipment Identity Register (EIR)
 Authentication Center (AuC)
 SMS Serving Center (SMS SC)
 Gateway MSC (GMSC)
 Chargeback Center (CBC)
 Transcoder and Adaptation Unit (TRAU)
The following Fig.14 shows the GSM network along with the added elements:
Fig.14.GSM Network
The MS and the BSS communicate across the Um interface. It is also known as the air interface or
the radio link. The BSS communicates with the Network Service Switching (NSS) center across the
A interface.
GSM network areas
In a GSM network, the following areas are defined:
 Cell: Cell is the basic service area; one BTS covers one cell. Each cell is given a Cell Global
Identity (CGI), a number that uniquely identifies the cell.
 Location Area: A group of cells form a Location Area (LA). This is the area that is paged
when a subscriber gets an incoming call. Each LA is assigned a Location Area Identity (LAI).
Each LA is served by one or more BSCs.
 MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service
area.
 PLMN: The area covered by one network operator is called the Public Land Mobile Network
(PLMN). A PLMN can contain one or more MSCs.
The requirements for different Personal Communication Services (PCS) systems differ for each
PCS network. Vital characteristics of the GSM specification are listed below:
Modulation: Modulation is the process of transforming the input data into a suitable format for the
transmission medium. The transmitted data is demodulated back to its original form

20. 20
at the receiving end. The GSM uses Gaussian Minimum Shift Keying (GMSK)
modulation method.
Access Methods: Radio spectrum being a limited resource that is consumed and divided among
all the users, GSM devised a combination of TDMA/FDMA as the method to divide
the bandwidth among the users. In this process, the FDMA part divides the
frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz
bandwidth.
Each BS is assigned with one or multiple frequencies, and each of this frequency is
divided into eight timeslots using a TDMA scheme. Each of these slots is used for
both transmission as well as reception of data. These slots are separated by time so
that a mobile unit doesn’t transmit and receive data at the same time.
Transmission Rate: The total symbol rate for GSM at 1 bit per symbol in GMSK produces
270.833 K symbols/second. The gross transmission rate of a timeslot is 22.8 Kbps.
GSM is a digital system with an over-the-air bit rate of 270 kbps.
Frequency Band: The uplink frequency range specified for GSM is 933 - 960 MHz (basic 900
MHz band only). The downlink frequency band 890 - 915 MHz (basic 900 MHz band
only).
Channel Spacing: Channel spacing indicates the spacing between adjacent carrier frequencies.
For GSM, it is 200 kHz.
Speech Coding: For speech coding or processing, GSM uses Linear Predictive Coding (LPC).
This tool compresses the bit rate and gives an estimate of the speech parameters.
When the audio signal passes through a filter, it mimics the vocal tract. Here, the
speech is encoded at 13 kbps.
Duplex Distance: Duplex distance is the space between the uplink and downlink frequencies. The
duplex distance for GSM is 80 MHz, where each channel has two frequencies that
are 80 MHz apart.
 Frame duration: 4.615 mS
 Duplex Technique: Frequency Division Duplexing (FDD) access mode
previously known as WCDMA.
 Speech channels per RF channel: 8.
The International Mobile Station Equipment Identity (IMEI) looks more like a serial number
which distinctively identifies a mobile station internationally. This is allocated by the equipment
manufacturer and registered by the network operator, who stores it in the Entrepreneurs-in-
Residence (EIR). By means of IMEI, one recognizes obsolete, stolen, or non-functional equipment.
Following are the parts of IMEI:
 Type Approval Code (TAC): 6 decimal places, centrally assigned.
 Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer.
 Serial Number (SNR): 6 decimal places, assigned by the manufacturer.
 Spare (SP): 1 decimal place.
Thus, IMEI = TAC + FAC + SNR + SP. It uniquely characterizes a mobile station and gives clues
about the manufacturer and the date of manufacturing.

21. 21
International Mobile Subscriber Identity (IMSI): Every registered user has an original
International Mobile Subscriber Identity (IMSI) with a valid IMEI stored in their
Subscriber Identity Module (SIM).
IMSI comprises of the following parts:
Mobile Country Code (MCC): 3 decimal places internationally standardized.
Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within
the country.
Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification
number of the subscriber in the home mobile network.
Mobile Subscriber ISDN Number (MSISDN): The authentic telephone number of a mobile station
is the Mobile Subscriber ISDN Number (MSISDN). Based on the SIM, a mobile
station can have many MSISDNs, as each subscriber is assigned with a separate
MSISDN to their SIM respectively.
Listed below is the structure followed by MSISDN categories, as they are defined based on
international ISDN number plan:
 Country Code (CC): Up to 3 decimal places.
 National Destination Code (NDC): Typically 2-3 decimal places.
 Subscriber Number (SN): Maximum 10 decimal places.
Mobile Station Roaming Number (MSRN): Mobile Station Roaming Number (MSRN) is an
interim location dependent ISDN number, assigned to a mobile station by a
regionally responsible Visitor Location Register (VLA). Using MSRN, the incoming
calls are channeled to the MS.
The MSRN has the same structure as the MSISDN.
 Country Code (CC): of the visited network.
 National Destination Code (NDC): of the visited network.
 Subscriber Number (SN): in the current mobile network.
Location Area Identity (LAI): Within a PLMN, a Location Area identifies its own authentic
Location Area Identity (LAI). The LAI hierarchy is based on international standard and structured in
a unique format as mentioned below:
 Country Code (CC): 3 decimal places.
 Mobile Network Code (MNC): 2 decimal places.
 Location Area Code (LAC): maximum 5 decimal places or maximum twice 8 bits coded in
hexadecimal (LAC < FFFF).

22. 22
Temporary Mobile Subscriber Identity (TMSI): Temporary Mobile Subscriber Identity (TMSI) can
be assigned by the VLR, which is responsible for the current location of a
subscriber. The TMSI needs to have only local significance in the area handled by
the VLR. This is stored on the network side only in the VLR and is not passed to the
Home Location Register (HLR).
Local Mobile Subscriber Identity (LMSI): Each mobile station can be assigned with a Local
Mobile Subscriber Identity (LMSI), which is an original key, by the VLR. This key
can be used as the auxiliary searching key for each mobile station within its region.
It can also help accelerate the database access. An LMSI is assigned if the mobile
station is registered with the VLR and sent to the HLR. LMSI comprises of four
octets (4x8 bits).
Cell Identifier (CI): Using a Cell Identifier (CI) (maximum 2 × 8) bit, the individual cells that are
within an LA can be recognized.
GPRS: GPRS architecture works on the same procedure like GSM network, but, has additional
entities that allow packet data transmission. This data network overlaps a second-
generation GSM network providing packet data transport at the rates from 9.6 to
171 kbps. Along with the packet data transport the GSM network accommodates
multiple users to share the same air interface resources concurrently.
Following is the GPRS Architecture diagram:
Fig.15.GPRS
GPRS attempts to reuse the existing GSM network elements as much as possible, but to
effectively build a packet-based mobile cellular network, some new network elements, interfaces,
and protocols for handling packet traffic are required.
Therefore, GPRS requires modifications to numerous GSM network elements as summarized
below:
GSM Network Element: Modification or Upgrade Required for GPRS.
Mobile Station (MS): New Mobile Station is required to access GPRS services. These new
terminals will be backward compatible with GSM for voice calls.
BTS: A software upgrade is required in the existing Base Transceiver Station (BTS).

23. 23
BSC: The Base Station Controller (BSC) requires a software upgrade and the installation of new
hardware called the packet control unit (PCU). The PCU directs the data traffic to the
GPRS network and can be a separate hardware element associated with the BSC.
GPRS Support Nodes (GSNs): The deployment of GPRS requires the installation of new core
network elements called the serving GPRS support node (SGSN) and gateway GPRS
support node (GGSN).
Databases (HLR, VLR, etc.): All the databases involved in the network will require software
upgrades to handle the new call models and functions introduced by GPRS. GPRS
Mobile Stations: New Mobile Stations (MS) are required to use GPRS services
because existing GSM phones do not handle the enhanced air interface or packet data.
A variety of MS can exist, including a high-speed version of current phones to support
high-speed data access, a new PDA device with an embedded GSM phone, and PC
cards for laptop computers. These mobile stations are backward compatible for making
voice calls using GSM.
GPRS Base Station Subsystem: Each BSC requires the installation of one or more Packet
Control Units (PCUs) and a software upgrade. The PCU provides a physical and logical
data interface to the Base Station Subsystem (BSS) for packet data traffic. The BTS
can also require a software upgrade but typically does not require hardware
enhancements.
GPRS Support Nodes: Following two new components, called Gateway GPRS Support Nodes
(GSNs) and, Serving GPRS Support Node (SGSN) are added.
Gateway GPRS Support Node (GGSN): The Gateway GPRS Support Node acts as an interface
and a router to external networks. It contains routing information for GPRS mobiles,
which is used to tunnel packets through the IP based internal backbone to the correct
Serving GPRS Support Node. The GGSN also collects charging information connected
to the use of the external data networks and can act as a packet filter for incoming
traffic.
Serving GPRS Support Node (SGSN): The Serving GPRS Support Node is responsible for
authentication of GPRS mobiles, registration of mobiles in the network, mobility
management, and collecting information on charging for the use of the air interface
Internal Backbone: The internal backbone is an IP based network used to carry packets between
different GSNs. Tunneling is used between SGSNs and GGSNs, so the internal
backbone does not need any information about domains outside the GPRS network.
Signaling from a GSN to a MSC, HLR or EIR is done using SS7.
GPRS LAYERS: The flow of GPRS protocol stack and end-to-end message from MS to the GGSN
is displayed in the below Fig.16. GTP is the protocol used between the SGSN and
GGSN using the Gn interface. This is a Layer 3 tunneling protocol.

24. 24
Fig.16.GPRS LAYERS
 The process that takes place in the application looks like a normal IP sub-network for the
users both inside and outside the network.
 The vital thing that needs attention is, the application communicates via standard IP, that is
carried through the GPRS network and out through the gateway GPRS.
 The packets that are mobile between the GGSN and the SGSN use the GPRS tunneling
protocol, this way the IP addresses located on the external side of the GPRS network do
not have deal with the internal backbone. UDP and IP are run by GTP.
 Sub Network Dependent Convergence Protocol (SNDCP) and Logical Link Control (LLC)
combination used in between the SGSN and the MS.
 The SNDCP flattens data to reduce the load on the radio channel. A safe logical link by
encrypting packets is provided by LLC and the same LLC link is used as long as a mobile is
under a single SGSN.
 In case, the mobile moves to a new routing area that lies under a different SGSN; then, the
old LLC link is removed and a new link is established with the new Serving GSN X.25.
Services are provided by running X.25 on top of TCP/IP in the internal backbone.
QoS: Quality of Service (QoS) requirements of conventional mobile packet data applications are in
assorted forms. The QoS is a vital feature of GPRS services as there are different QoS
support requirements for assorted GPRS applications like real-time multimedia, web
browsing, and e-mail transfer.
GPRS allows defining QoS profiles using the following parameters:
 Service Precedence
 Reliability
 Delay and
 Throughput
These parameters are described below:
Service Precedence: The preference given to a service when compared to another service is
known as Service Precedence. This level of priority is classified into three levels called:
 High
 Normal
 Low

25. 25
When there is network congestion, the packets of low priority are discarded as compared to high or
normal priority packets
Reliability: This parameter signifies the transmission characteristics required by an application.
The reliability classes are defined which guarantee certain maximum values for the
probability of loss, duplication, mis-sequencing, and corruption of packets.
Delay: The delay is defined as the end-to-end transfer time between two communicating mobile
stations or between a mobile station and the GI interface to an external packet data
network.
This includes all delays within the GPRS network, e.g., the delay for request and
assignment of radio resources and the transit delay in the GPRS backbone network.
Transfer delays outside the GPRS network, e.g., in external transit networks, are not
taken into account.
Throughput: The throughput specifies the maximum/peak bit rate and the mean bit rate.
Using these QoS classes, QoS profiles can be negotiated between the mobile user and the
network for each session, depending on the QoS demand and the available resources.
The billing of the service is then based on the transmitted data volume, the type of service, and the
chosen QoS profile.
DECT: (DIGITAL ENHANCED CORDLESS TELECOMMUNICATIONS)
This technology is widely used for residential and business cordless phone communications.
Designed for short range use as an access mechanism to the main networks. DECT technology
offers cordless voice, fax, data and multimedia communications, wireless local area networks and
wireless PBX. With the flexibility offered by cordless phone communications, DECT technology has
become the major standard for this application and DECT is now in use in over 100 countries
worldwide.
DECT technology development
 The standard for DECT or Digital Enhanced Telecommunications system was developed by
members of the European Telecommunications Standards Institute (ETSI).
 The first release of the standard was available in 1992 after which much of the work was
focused on inter-working protocols (DECT / GSM, DECT/ISDN, etc.).
 As a result of this work, DECT / GSM inter-working has been standardized and the basic
GSM services can be provided over the DECT air interface.
 This enables DECT terminals to inter-work with DECT systems which are connected to the
GSM infrastructure. All roaming scenarios based on SIM roaming as described in GSM
specifications are applicable.
 Along with requirements arising from the growing use of DECT, this work gave rise to a
number of extensions to the basic DECT standard. This led to a second release of the
standard at the end of 1995.
 This included facilities including: emergency call procedures, definition of the Wireless
Relay Station (WRS), and an optional direct portable to portable communication feature.
DECT codecs
 The basic telephony speech quality offered by DECT is very high compared to many other

26. 26
wireless systems. This is the result of the use of the ITU-T Recommendation G.726 codec
that is employed.
 This is a 32kbit/s ADPCM speech codec and although it uses 32kbps, the quality it affords
is high and there is more than sufficient bandwidth within the system to support it.
TDMA structure
 The DECT TDMA structure enables up to 12 simultaneous basic voice connections per
transceiver. The system is also able to provide widely varying bandwidths by combining
multiple channels into a single bearer.
 For data transmission purposes error protected net throughput rates of integral multiples of
24kbps can be achieved. However the DECT standard defines a maximum data rate of
552kbps with full security.
DECT GAP profile
 All DECT systems are based on a main standard that is the Common Interface (CI), which
is often used in association with the Generic Access Profile (GAP).
 The GAP profile ensures interoperability of equipment from different providers for voice
applications.
 The GAP defines the minimum interoperability requirements including mobility management
and security features. It has different requirements on public and private systems.
 This means that the GAP is effectively the industry standard for a basic fall-back speech
service with mobility management. This basic service is not always used, but instead it
forms the fallback that is always be available, especially when requested by a roaming
phone, etc.
UMTS (UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM)
 3GPP UMTS, the Universal Mobile Telecommunications System is the third generation
(3G) successor to the second generation GSM based cellular technologies which also
include GPRS, and EDGE.
 Although UMTS uses a totally different air interface, the core network elements have been
migrating towards the UMTS requirements with the introduction of GPRS and EDGE.
 In this way the transition from GSM to the 3G UMTS architecture did not require such a
large instantaneous investment.
 UMTS uses Wideband CDMA (WCDMA / W-CDMA) to carry the radio transmissions, and
often the system is referred to by the name WCDMA. It is also gaining a third name.
3GPP UMTS Specifications and Management:
 In order to create and manage a system as complicated as UMTS or WCDMA it is
necessary to develop and maintain a large number of documents and specifications.
 For UMTS or WCDMA, these are now managed by a group known as 3GPP - the Third
Generation Partnership Programme. This is a global co-operation between six
organizational partners - ARIB, CCSA, ETSI, ATIS, TTA and TTC.
 The scope of 3GPP was to produce globally applicable Technical Specifications and
Technical Reports for a 3rd Generation Mobile Telecommunications System.
 This would be based upon the GSM core networks and the radio access technologies that
they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division
Duplex (FDD) and Time Division Duplex (TDD) modes).

27. 27
 Since it was originally formed, 3GPP has also taken over responsibility for the GSM
standards as well as looking at future developments including LTE (Long Term Evolution)
and the 4G technology known as LTE Advanced.
3G UMTS capabilities
 UMTS uses Wideband CDMA - WCDMA - as the radio transmission standard. It employs a
5MHz channel bandwidth. Using this bandwidth it has the capacity to carry over 100
simultaneous voice calls, or it is able to carry data at speeds up to 2Mbps in its original
format.
 However with the later enhancements of HSDPA and HSUPA included in later releases of
the standard the data transmission speeds have been increased to 14.4Mbps.
 Many of the ideas that were incorporated into GSM have been carried over and enhanced
for UMTS. Elements such as the SIM have been transformed into a far more powerful
USIM (Universal SIM).
 In addition to this, the network has been designed so that the enhancements employed for
GPRS and EDGE can be used for UMTS. In this way the investment required is kept to a
minimum.
 A new introduction for UMTS is that there are specifications that allow both Frequency
Division Duplex (FDD) and Time Division Duplex (TDD) modes.
 The first modes to be employed are FDD modes where the uplink and downlink are on
different frequencies. The spacing between them is 190 MHz for Band 1 networks being
currently used and rolled out.
 However the TDD mode where the uplink and downlink are split in time with the base
stations and then the mobiles transmitting alternately on the same frequency is particularly
suited to a variety of applications.
 Obviously where spectrum is limited and paired bands suitably spaced are not available. It
also performs well where small cells are to be used. As a guard time is required between
transmit and receive, this will be smaller when transit times are smaller as a result of the
shorter distances being covered.
 A further advantage arises from the fact that it is found that far more data is carried in the
downlink as a result of internet surfing, video downloads and the like.
 This means that it is often better to allocate more capacity to the downlink. Where paired
spectrum is used this is not possible.
 However when a TDD system is used it is possible to alter the balance between downlink
and uplink transmissions to accommodate this imbalance and thereby improve the
efficiency. In this way TDD systems can be highly efficient when used in Pico cells for
carrying Internet data.
 The TDD systems have not been widely deployed, but this may occur more in the future. In
view of its character, it is often referred to as TD-CDMA (Time Division CDMA).
3G UMTS / WCDMA technologies
There are several key areas of 3G UMTS / WCDMA. Within these there are several key
technologies that have been employed to enable UMTS / WCDMA to provide a leap in
performance over its 2G predecessors.
Some of these key areas include:
Radio interface: The UMTS radio interface provides the basic definition of the radio signal. W-
CDMA occupies 5 MHz channels and has defined formats for elements such as

28. 28
synchronization, power control and the like Read more about the UMTS / W-CDMA radio
interface.
CDMA technology: 3G UMTS relies on a scheme known as CDMA or code division multiple
accesses to enable multiple handsets or user equipment’s to have access to the base
station. Using a scheme known as direct sequence spread spectrum, different UEs have
different codes and can all talk to the base station even though they are all on the same
frequency Read more about the code division multiple access.
UMTS network architecture: The architecture for a UMTS network was designed to enable
packet data to be carried over the network, whilst still enabling it to support circuit
switched voice. All the usual functions enabling access to the network, roaming and the
like are also supported.
UMTS modulation schemes: Within the CDMA signal format, a variety of forms of modulation are
used. These are typically forms of phase shift keying. Read more about the modulation
schemes.
UMTS channels: As with any cellular system, different data channels are required for passing
payload data as well as control information and for enabling the required resources to be
allocated.
UMTS TDD: There are two methods of providing duplex for 3G UMTS. One is what is termed
frequency division duplex, FDD. This uses two channels spaced sufficiently apart so that
the receiver can receive whilst the transmitter is also operating. Another method is to use
time vision duplex, TDD where short time blocks are allocated to transmissions in both
directions. Using this method, only a single channel is required
Handover: One key area of any cellular telecommunications system is the handover (handoff)
from one cell to the next. Using CDMA there are several forms of handover that are
implemented within the system.
3G UMTS enhancements
 The basic 3G UMTS cellular system enabled data rates up to 2048kbps to be achieved.
However as the use of data rapidly increased, this Fig., were no longer sufficient and
further data rate increases were required.
 A scheme known as HSDPA, high speed packet download access was first introduced to
enable the downlink speed to be increased. This was followed with HSUPA; high speed
packet uplink access was introduced. The combined suite was then known as HSPA, high
speed packet access.
 The UMTS WCDMA system offered a significant improvement in capability over the
previous 2G services.
IMT2000
 International Mobile Telecommunications for the year 2000 (IMT-2000) is a worldwide set of
requirements for a family of standards for the 3rd
generation of mobile communications. The
IMT-2000 "umbrella specifications" are developed by the International Telecommunications
Union (ITU).
 Originally it was the intention to have only one truly global standard but that turned out to be
impossible. IMT-2000 should provide worldwide mobile broadband multimedia services via a
single global frequency band. The frequency range should be around 2000 MHz

29. 29
Fig.17.IMT2000
In the umbrella specification a number of characteristics are defined which the underlying
technologies should meet. The main characteristics are:
 Worldwide usage, integration of satellite and terrestrial systems to provide global coverage;
 Used for all radio environments, (LAN, cordless, cellular, satellite);
 Wide range of telecommunications services,(voice, data, multimedia, internet);
 Support both packet-switched (PS) and circuit-switched (CS) data transmission;
 Offer high data rates up to 2 Mbps,
 144 kbps for high mobility,
 384 kbps with restricted mobility and,
 2 Mbps in an indoor office environment;
 Offer high spectrum efficiency. For the terrestrial mobile network, there are six family
members identified as being IMT-2000 compatible:
 IMT Direct Spread (IMT-DS; also known as UMTS/UTRA-FDD);
 IMT Multicarrier (IMT-MC; also known as CDMA2000);
 IMT Time Code (IMT-TC; also known as UMTS/UTRA-TDD, TD-CDMA and TD- SCDMA
(“narrowband TDD”);
 IMT Single Carrier (IMT-SC; also known as UWC-136 or EDGE);
 IMT Frequency Time (IMT-FT; also known as DECT).
 IMT OFDMA TDD WMAN (also known as mobile WiMAX)
 Frequency bands: The frequency bands 1885-2025 MHz and 2110-2200 MHz were
identified for IMT-2000 by the ITU in 1992. Terrestrial IMT-2000 networks will operate in the
following bands:
 1920 – 1980 MHz paired with 2110 - 2170 MHz, FDD with mobile stations transmitting in the
lower sub-band.
 1885 - 1920 MHz and 2010 - 2025 MHz, unpaired for TDD operation.

30. 30
 In Europe is the TDD band from 1885-1900 MHz not available for licenses use of IMT-2000,
this is used by cordless telephony (DECT).
 In addition to this core-band the frequency band 2500 to 2690 MHz was identified in 2000, of
which the edges, ranging from 2500-2520 and 2670-2690 MHz, are at first identified for
satellite communications.
 Existing second generation bands (including GSM bands) 806 to 960 MHz, 1429 to 1501
MHz and 1710 to 1885 MHz are also identified for IMT-2000 in the long term.
LIMITED RANGE CORDLESS PHONES
 Many PBX and key system suppliers provide lower priced home type or proprietary 900
MHz wireless phones.
 The phones only work within range of the antenna in the phone and the phone's base unit.
The 900 MHz phones have a range of about 125 to 150 feet depending on building
conditions. There also is an upper limit of about 20 cordless phones supported in the same
building.
 Proprietary cordless phones have features powered by the phone system to which they are
connected.
 These features, voice mail message lights, multiple call appearances and hold buttons,
make the phones easier to use and more functional. In building wireless phones are used
for the following personnel who often use headsets with their phones:
 Console operators, to be able to take calls when they step away from their desk for
functions such as making copies.
 Warehouse employees
 Retail store personnel who can take calls from anywhere in the store.
 Call centre agents
 People who work at home
Fig.18. Cordless telephone base phone and handset

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CT1:
 The first generation of cordless telephone, still very popular, is designed to serve the
domestic environment with a range of 100m.
 The base station transmits on one of eight frequencies between 1.6MHz and 1.8MHz, and
the handset one of eight frequencies in the 47MHz band. Frequency modulation is used
with a deviation of ±4kHz at the base station and 2.5 kHz at the handset.
 Permitted effective radiated power at both the base station and the handset is 10mW.
CT2:
 The second generation of cordless telephone. The services, under various names,
telephonic, phone zone, etc., were planned to provide for the general public a lower cost
alternative to the cellular radiotelephone networks which were seen at the time as a
businessman's preserve.
 However, the low market uptake caused their demise in the UK, although services are
thriving in some European and far eastern countries.
 One hand-portable transmitter/receiver can operate to a local base station installed in the
home or office, or through a number of multi-channel base stations with a range of
approximately 200 m installed in public places.
 While away from the local base station the subscriber must initiate a call: calls cannot be
made from the PSTN to a CT2 subscriber's handset.
 The operational frequencies are in the band 864.1 to 868.1 MHz and employ time division
multiple access (TDMA). Speech is digitized at 32kbit/s, stored, and then transmitted at
64kbits/s in 1ms slots.
 This leaves the alternate 1ms slots available for the digitized and stored speech of a reply.
Duplex operation is achieved in this way on a single radio frequency.
CT3:
 The third generation of cordless telephone, Digital European Cordless Telephone (DECT)
is a pan-European system.
 DECT operates in the 1880 to 1900 MHz band. It offers data handling facilities and the
ability for a subscriber to receive calls while away from the local base station.
 The techniques are similar to those used for GSM (Global System for Mobile
Communications) although, because the mobile is virtually stationary, the constraints on
data transmission are less severe and no hand-off is required.
 The 20 MHz RF bandwidth is divided into 13 carriers spaced at 1.7 MHz intervals, each
carrier containing 12 TDMA channels with GMSK (Gaussian Minimum Phase Shift Keying)
modulation.
 CT3 is developed from DECT and operates in the band 800 to 900 MHz. Each 8 MHz
section of that band is divided into 1 MHz blocks, each containing sixteen 1ms time slots.
FACSIMILE (FAX)
 In addition to basic signals consisting of speech, music, or telegraph codes, a
telecommunication system is often required to transmit signals of a visual nature.
 Facsimile means an exact reproduction, and in facsimile transmission an exact
reproduction of a document or picture is provided at the receiving end.
 Television means visually at a distance and a television system is used to reproduce any
scene at the receiving end. It differs from facsimile in that the scene may be live (i.e.
include movement).

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 Information is transmitted at a much faster rate in television transmission than it is in
facsimile transmission. As a result, television transmission requires a much larger
bandwidth, and special wideband circuits are required. The small bandwidth required for
facsimile makes it suitable for transmission over normal telephone lines.
FACSIMILE MACHINE
 An input scanner can be used to transmit images over a telecommunications link to a
remote printer. This principle has been in use commercially for news photograph
transmission since 1930s.
 Combined send/receive machines suitable for office use became widely available in 1960s,
and between 1980 and 1987 the number of machines connected to the world-wide
telephone network grew from a quarter of a million to two million.
 Facsimile provides very fast transmission of almost any documentary material without
specialist preparation.
 However, between 2 million and 10 million bits are required for a raster scan of an A4 page;
this has to be compared with 20000 bits for a similar-sized page of ASCII-coded characters.
This added transmission burden is somewhat alleviated by the ability to tolerate very high
error rates.
Fig.19. Personal facsimile machine
 The design of facsimile machines has been strongly influenced by the use of the PSTN
(public switched telephone network).
 Firstly, the network was designed for speech, not data. Therefore the power/frequency/time
characteristics of the transmitted facsimile signal must be chosen to suit the network.
 Secondly, transmission time is expensive, so effective data compression and channel
modulation methods must be used.
 Thirdly, since the public network is switched, every facsimile machine can in principle be
connected to every other. Rigorous development and application of international standards
is therefore necessary to ensure that this potential for interconnection is not wasted.
Fig.20. shows the block diagram of a typical facsimile machine. When data is read from an input
document it is first compressed and then modulated on to an audio-frequency carrier prior to being
coupled to the line. The receive path is the reverse of this.

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Fig.20. Block diagram of a typical facsimile machine
BASIC FAX MACHINE OPERATIONS
 Essentially, a fax machine scans original documents, converts the scanned images into
electrical signals, and transmits them over telephone lines to a receiving fax machine.
 The receiving fax machine in turn converts the received signals back into the graphical
images of the original document and prints them. The basic Group 3 fax machine
operations for transmitting a page are presented in Fig.21.
The handshake process: The sending and receiving fax-modems set up the transmission
protocols, transmission speed, and other settings between them in a handshake process. If one
modem cannot transmit at the highest speed of the other, both modems agree to fall back to the
next highest speed at which both modems can transmit on the line.
Fig.21. Basic fax machine operations

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At the transmitting ends:
1. Scanning: The images on the page are scanned and transformed into analog signals to
begin the transmission process. Either a charge-coupled device or contact image sensor
scanner scans the page being sent. A photo sensor array of 1728 tiny sensors for A4 paper
size (or 2048 for B4) targets very small picture elements (pixels) on a line of page, one
sensor per pixel, resulting in 1728 (or 2048) bits per line. The array determines whether
each pixel is black or white and accordingly generates a strong or weak electronic signal for
that pixel.
A page is scanned line-by-line with all the pixels in a thin strip from 0.13 to 0.25 mm high
across the top of the page, or between 10 and 12 scan lines per line of text. Successive
strips are scanned until the whole page is converted into a series of electrical pulses. A/D
Conversion: The scanner signals are converted from analog to digital with typically from
one to six bits per pixel. After image processing is complete, one bit per pixel is produced.
2. Video Processing: The processing of the scanner data can be done on the analog
scanner signal, the digital data, or both. It accommodates for the shading, distortions, and
other aspects of the original image so that reproduction can be as accurate as possible.
Shading compensation checks for non-uniformity in the scanner optical system and
corrects distortions due to both, light sources as well as non-uniformity in the scanner
element.
3. Threshold: The conversion of the scanner output from grey level to a black-and-white level
must also be performed. It may include dithering (or half-toning), a method of generating
pseudogrey scales.
Other video processing techniques include automatic background correction, automatic
contrast control, edge enhancement and MTF (modulation transfer function) correction.
These can be performed in one or two dimensions. Images may also be reduced or
enlarged.
4. Compressing the digital signals: The data compression operation can reduce the picture
information by a factor of from 5 to 20, depending on the characteristics of the image. This
operation generates code words containing the pixel information in compressed digital
signals.
5. Modulation: The compressed digital signals are modulated by the modem into analog
signals (a tone series) that can be sent over regular telephone lines. Group 3 fax machines
are half duplex and can either send or receive at any time.
6. Transmission: The analog signals are then transmitted over the phone lines from the
sending modem to the receiving modem.
At the receiving end:
1. Demodulation: A modem demodulates or decodes the received analog tone signals
regenerating the digital signals (bit streams) sent.
2. Decompression: The next step is to expand the digital signals and reconstruct the page's
images into black-and-white pixels which represent the pixels of the page's image.
3. Thermal printing: The thermal printer converts the expanded bit stream into a copy of the
original page. The printer's wires are spaced 203 to the inch, touching the temperature-
sensitive recording paper. For black marking, the wires heat up when high current passes
through them. The wires go from non-marking (white) to marking (black) temperature, and
back again in a few milliseconds.

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WiFi
4. Resolution: Standard resolution is 203 lines per inch across and 98 lines per inch down
the page. Fine resolution requires twice the number of lines (196 lines per inch) down the
page. Most group 3 fax machines include a high resolution option.
 Wi-Fi is the name of a popular wireless networking technology that uses radio waves to
provide wireless high-speed Internet and network connections.
 A common misconception is that the term Wi-Fi is short for "wireless fidelity," however this
is not the case. Wi-Fi is simply a trademarked phrase that means IEEE 802.11x.
The Wi-Fi Alliance
 The Wi-Fi Alliance, the organization that owns the Wi-Fi registered trademark term
specifically defines Wi-Fi as any "wireless local area network (WLAN) products that are
based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards."
 Initially, Wi-Fi was used in place of only the 2.4GHz 802.11b standard, however the Wi-Fi
Alliance has expanded the generic use of the Wi-Fi term to include any type of network or
WLAN product based on any of the 802.11 standards, including 802.11b, 802.11a, dual-
band, and so on, in an attempt to stop confusion about wireless LAN interoperability.
Wi-Fi Working Principle
 Wi-Fi works with no physical wired connection between sender and receiver by using radio
frequency (RF) technology, a frequency within the electromagnetic spectrum associated
with radio wave propagation.
 When an RF current is supplied to an antenna, an electromagnetic field is created that then
is able to propagate through space.
 The cornerstone of any wireless network is an access point (AP). The primary job of an
access point is to broadcast a wireless signal that computers can detect and "tune" into. In
order to connect to an access point and join a wireless network, computers and devices
must be equipped with wireless network adapters.
WiFi
 Wi-Fi is the name of a popular wireless networking technology that uses radio waves to
provide wireless high-speed Internet and network connections.
 A common misconception is that the term Wi-Fi is short for "wireless fidelity," however this
is not the case.
 Initially, Wi-Fi was used in place of only the 2.4GHz 802.11b standard, however the Wi-Fi
Alliance has expanded the generic use of the Wi-Fi term to include any type of network or
WLAN product based on any of the 802.11 standards, including 802.11b, 802.11a, dual-
band, and so on, in an attempt to stop confusion about wireless LAN interoperability.
Working of WiFi
 Wi-Fi works with no physical wired connection between sender and receiver by using radio
frequency (RF) technology, a frequency within the electromagnetic spectrum associated
with radio wave propagation.
 When an RF current is supplied to an antenna, an electromagnetic field is created that then
is able to propagate through space.
 The cornerstone of any wireless network is an access point (AP). The primary job of an
access point is to broadcast a wireless signal that computers can detect and "tune" into.

36. 36
 In order to connect to an access point and join a wireless network, computers and devices
must be equipped with wireless network.
Wi-Fi Support
 Wi-Fi is supported by many applications and devices including video game consoles, home
networks, PDAs, mobile phones, major operating systems, and other types of consumer
electronics.
 Any products that are tested and approved as "Wi-Fi Certified" (a registered trademark) by
the Wi-Fi Alliance are certified as interoperable with each other, even if they are from
different manufacturers. For example, a user with a Wi-Fi Certified product can use any
brand of access point with any other brand of client hardware that also is also "Wi-Fi
Certified".
 Products that pass this certification are required to carry an identifying seal on their
packaging that states "Wi-Fi Certified" and indicates the radio frequency band used
(2.5GHz for 802.11b, 802.11g, or 802.11n, and 5GHz for 802.11a).
BLUETHOOTH
Compared to the WLAN technologies presented in sections 7.3 and 7.4, the Bluetooth technology
discussed here aims at so-called ad-hoc Pico nets, which are local area networks with a very
limited coverage and without the need for an infrastructure. This is a different type of network is
needed to connect different small devices in close proximity (about 10 m) without expensive wiring
or the need for a wireless infrastructure
User scenarios
Many different user scenarios can be imagined for wireless Pico nets or WPANs:
 Connection of peripheral devices: Today, most devices are connected to a desktop
computer via wires (e.g., keyboard, mouse, joystick, headset, speakers).
 This type of connection has several disadvantages: each device has its own type of cable,
different plugs are needed, and wires block office space. In a wireless network, no wires
are needed for data transmission.
 However, batteries now have to replace the power supply, as the wires not only transfer
data but also supply the peripheral devices with power.
 Support of ad-hoc networking: Imagine several people coming together, discussing issues,
exchanging data (schedules, sales Fig. etc.).
 For instance, students might join a lecture, with the teacher distributing data to their
personal digital assistants (PDAs). Wireless networks can support this type of interaction;
small devices might not have WLAN adapters following the IEEE 802.11 standard, but
cheaper Bluetooth chips built in.

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Fig.22. Bluetooth con Figurations
Example con Figurations with a Bluetooth-based Pico nets
 Bridging of networks: Using wireless Pico nets, a mobile phone can be connected to a PDA
or laptop in a simple way. Mobile phones will not have full WLAN adapters built in, but could
have a Bluetooth chip.
 The mobile phone can then act as a bridge between the local Pico nets and, e.g., the global
GSM network as in Fig. 22.
 For instance, on arrival at an airport, a person’s mobile phone could receive e-mail via GSM
and forward it to the laptop which is still in a suitcase. Via a Pico nets, a fileserver could
update local information stored on a laptop or PDA while the person is walking into the
office.
 Architecture like IEEE 802.11b, Bluetooth operates in the 2.4 GHz ISM band. However,
MAC, physical layer and the offered services are completely different. After presenting the
overall architecture of Bluetooth and its specialty, the Pico nets, the following sections
explain all protocol layers and components in more detail.
Networking
 To understand the networking of Bluetooth devices a quick introduction to its key features is
necessary. Bluetooth operates on 79 channels in the 2.4 GHz band with 1 MHz carrier
spacing.
 Each device performs frequency hopping with 1,600hops/s in a pseudo random fashion.
Bluetooth applies FHSS for interference mitigation (and FH-CDMA for separation of
networks).
 A very important term in the context of Bluetooth is a Pico nets. A Pico nets is a collection
of Bluetooth devices which are synchronized to the same hopping sequence. Fig. 23,
shows a collection of devices with different roles.
 One device in the Pico nets can act as master (M), all other devices connected to the
master must act as slaves (S).
 The master determines the hopping pattern in the Pico nets and the slaves have to
synchronize to this pattern.
 Each Pico nets has a unique hopping pattern. If a device wants to participate it has to
synchronize to this.

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Fig.23. collection of devices with different roles
 Two additional types of devices are shown: parked devices (P) cannot actively participate in
the Pico nets (i.e., they do not have a connection), but are known and can be reactivated
within some milliseconds (see section 7.5.5).
 Devices in stand-by (SB) do not participate in the Pico nets. Each Pico nets has exactly one
master and up to seven simultaneous slaves.
 More than 200 devices can be parked. The reason for the upper limit of eight active devices
is the 3-bit address used in Bluetooth. If a parked device wants to communicate and there
are already seven active slaves, one slave has to switch to park mode to allow the parked
device to switch to active mode.