3. INTRODUCTION TO GSM
INTRODUCTION
• The Global System for Mobile Communications (GSM) is a set of
recommendations and specifications for a digital cellular telephone
network (known as a Public Land Mobile Network, or PLMN).
• These recommendations ensure the compatibility of equipment from
different GSM manufacturers, and interconnectivity between different
administrations, including operation across international boundaries.
• GSM networks are digital and can cater for high system capacities.
• They are consistent with the world-wide digitization of the telephone
network, and are an extension of the Integrated Services Digital
Network (ISDN), using a digital radio interface between the cellular
network and the mobile subscriber equipment.
4. INTRODUCTION TO GSM
CELLULAR TELEPHONY
• A cellular telephone system links mobile subscribers into the public
telephone system or to another cellular subscriber.
• Information between the mobile unit and the cellular network uses
radio communication. Hence the subscriber is able to move around
and become fully mobile.
• The service area in which mobile communication is to be provided
is divided into regions called cells.
• Each cell has the equipment to transmit and receive calls from any
subscriber located within the borders of its radio coverage area.
Radio
Cell
Mobile subscriber
5. INTRODUCTION TO GSM
GSM FREQUENCIES
• GSM systems use radio frequencies between 890-915 MHz for
receive and between 935-960 MHz for transmit.
• RF carriers are spaced every 200 kHz, allowing a total of 124
carriers for use.
• An RF carrier is a pair of radio frequencies, one used in each
direction.
• Transmit and receive frequencies are always separated by 45 MHz.
UPLINK FREQUENCIES DOWNLINK FREQUENCIES
890 915 935 960
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
6. INTRODUCTION TO GSM
Extended GSM (EGSM)
• EGSM has 10MHz of bandwidth on both transmit and receive.
• Receive bandwidth is from 880 MHz to 890 MHz.
• Transmit bandwidth is from 925 MHz to 935 MHz.
• Total RF carriers in EGSM is 50.
UPLINK FREQUENCIES DOWNLINK FREQUENCIES
880 890 915 925 935 960
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
7. INTRODUCTION TO GSM
DCS1800 FREQUENCIES
• DCS1800 systems use radio frequencies between 1710-1785 MHz
for receive and between 1805-1880 MHz for transmit.
• RF carriers are spaced every 200 kHz, allowing a total of 373
carriers.
• There is a 100 kHz guard band between 1710.0 MHz and 1710.1
MHz and between 1784.9 MHz and 1785.0 MHz for receive, and
between 1805.0 MHz and 1805.1 MHz and between 1879.9 MHz
and 1880.0 MHz for transmit.
• Transmit and receive frequencies are always separated by 95 MHz.
UPLINK FREQUENCIES DOWNLINK FREQUENCIES
1710 MHz 1785 MHz 1805 MHz 1880 MHz
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ
9. FEATURES OF GSM
INCREASED CAPACITY
• The GSM system provides a greater subscriber capacity than analogue
systems.
• GSM allows 25 kHz per user, that is, eight conversations per 200 kHz
channel pair (a pair comprising one transmit channel and one receive
channel).
• Digital channel coding and the modulation used makes the signal
resistant to interference from cells where the same frequencies are re-
used (co-channel interference); a Carrier to Interference Ratio (C/I) level
of 12 dB is achieved, as opposed to the 18 dB typical with analogue
cellular.
• This allows increased geographic reuse by permitting a reduction in the
number of cells in the reuse pattern.
10. FEATURES OF GSM
AUDIO QUALITY
• Digital transmission of speech and high performance digital signal
processors provide good quality speech transmission.
• Since GSM is a digital technology, the signals passed over a digital air
interface can be protected against errors by using better error
detection and correction techniques.
• In regions of interference or noise-limited operation the speech quality
is noticeably better than analogue.
USE OF STANDARDISED OPEN INTERFACES
• Standard interfaces such as C7 and X25 are used throughout the
system. Hence different manufacturers can be selected for different
parts of the PLMN.
• There is a high flexibilty in where the Network components are
situated.
11. FEATURES OF GSM
IMPROVED SECURITY AND CONFIDENTIALITY
• GSM offers high speech and data confidentiality.
• Subscriber authentication can be performed by the system to check if
a subscriber is a valid subscriber or not.
• The GSM system provides for high degree of confidentiality for the
subscriber. Calls are encoded and ciphered when sent over air.
• The mobile equipment can be identified independently from the mobile
subscriber. The mobile has a identity number hard coded into it when
it is manufactured. This number is stored in a standard database and
whenever a call is made the equipment can be checked to see if it has
been reported stolen.
12. FEATURES OF GSM
CLEANER HANDOVERS
• GSM uses Mobile assisted handover techique.
• The mobile itself carries out the signal strength and quality
measurement of its server and signal strength measurement of its
neighbors.
• This data is passed on the Network which then uses sophisticated
algorithms to determine the need of handover.
SUBSCRIBER IDENTIFICATION
• In a GSM system the mobile station and the subscriber are
identified separately.
• The subscriber is identified by means of a smart card known as a
SIM.
• This enables the subscriber to use different mobile equipment
while retaining the same subscriber number.
13. FEATURES OF GSM
ENHANCED RANGE OF SERVICES
• Speech services for normal telephony.
• Short Message Service for point ot point transmission of text
message.
• Cell broadcast for transmission of text message from the cell to all
MS in its coverage area. Message like traffic information or
advertising can be transmitted.
• Fax and data services are provided. Data rates available are 2.4 Kb/
s, 4.8 Kb/s and 9.6 Kb/s.
• Supplementary services like number identification , call barring, call
forwarding, charging display etc can be provided.
14. FEATURES OF GSM
FREQUENCY REUSE
• There are total 124 carriers in GSM ( additional 50 carriers are
available if EGSM band is used).
• Each carrier has 8 timeslots and if 7 can be used for traffic then a
maximum of 868 ( 124 X 7 ) calls can be made. This is not enough
and hence frequencies have to be reused.
• The same RF carrier can be used for many conversations in several
different cells at the same time.
• The radio carriers available are allocated
according to a regular pattern which repeats over 2
the whole coverage area. 1 3
• 4
The pattern to be used depends on traffic
5 7
requirement and spectrum availability.
6 2
• Some typical repeat patterns are 4/12, 7/21 etc. 1
16. NETWORK COMPONENTS
H D
NMC EIR AUC HLR VLR
F C B
A XCDR
OMC-S MSC IWF
UM
BSC
ABIS
OMC-R EC
BTS BTS
PSTN UM
17. NETWORK COMPONENTS
Mobile Switching Centre (MSC)
• The Mobile services Switching Centre (MSC) co-ordinates the setting up
of calls to and from GSM users.
• It is the telephone switching office for MS originated or terminated traffic
and provides the appropriate bearer services, teleservices and
supplementary services.
• It controls a number of Base Station Sites (BSSs) within a specified
geographical coverage area and gives the radio subsystem access to
the subscriber and equipment databases.
• The MSC carries out several different functions depending on its position
in the network.
• When the MSC provides the interface between PSTN and the BSS in
the GSM network it is called the Gateway MSC.
• Some important functions carried out by MSC are Call processing
including control of data/voice call setup, inter BSS & inter MSC
handovers, control of mobility management, Operation & maintenance
support including database management, traffic metering and man
machine interface & managing the interface between GSM & PSTN
19. NETWORK COMPONENTS
Mobile Station (MS)
• The Mobile Station consists of the Mobile Equipment (ME) and the
Subscriber Identity Module (SIM).
Mobile Equipment
• The Mobile Equipment is the hardware used by the subscriber to
access the network.
• The mobile equipment can be Vehicle mounted, with the antenna
physically mounted on the outside of the vehicle or portable mobile
unit, which can be handheld.
• Mobiles are classified into five classes according to their power
rating.
CLASS POWER OUTPUT
1 20W
2 8W
3 5W
4 2W
5 0.8W
20. NETWORK COMPONENTS
SIM
• The SIM is a removable card that plugs into the ME.
• It identifies the mobile subscriber and provides information about the
service that the subscriber should receive.
• The SIM contains several pieces of information
– International Mobile Subscribers Identity ( IMSI ) - This number
identifies the mobile subscriber. It is only transmitted over the air
during initialising.
– Temporary Mobile Subscriber Identity ( TMSI ) - This number also
identifies the subscriber. It can be alternatively used by the
system. It is periodically changed by the system to protect the
subscriber from being identified by someone attempting to monitor
the radio interface.
– Location Area Identity ( LAI ) - Identifies the current location of the
subscriber.
– Subscribers Authentication Key ( Ki ) - This is used to authenticate
the SIM card.
– Mobile Station International Standard Data Number ( MSISDN ) -
21. NETWORK COMPONENTS
SIM
• Most of the data contained within the SIM is protected against reading
(eg Ki ) or alterations after the SIM is issued.
• Some of the parameters ( eg. LAI ) will be continously updated to
reflect the current location of the subscriber.
• The SIM card can be protected by use of Personal Identity Number
( PIN ) password.
• The SIM is capable of storing additional information such as
accumulated call charges.
FULL SIZE SIM CARD MINI SIM CARD
GSM
22. NETWORK COMPONENTS
Mobile Station International Subscribers Dialling Number ( MSISDN ) :
• Human identity used to call a MS
• The Mobile Subscriber ISDN (MSISDN) number is the telephone
number of the MS.
• This is the number a calling party dials to reach the subscriber.
• It is used by the land network to route calls toward the MSC.
CC NDC SN
98 XXX 12345
CC = Country code
NDC = National Destination Code
SN = Subscriber Number
23. NETWORK COMPONENTS
International Mobile Subscribers Identity ( IMSI ) :
• Network Identity Unique to a MS
• The International Mobile Subscriber Identity (IMSI) is the primary
identity of the subscriber within the mobile network and is
permanently assigned to that subscriber.
• The IMSI can be maximum of 15 digits.
MCC MNC MSIN
404 XX 12345..10
MCC = Mobile Country Code ( 3 Digits )
MNC = Mobile Network Code ( 2 Digits )
MSIN = Mobile Subscriber Identity Number
24. NETWORK COMPONENTS
Temporary Mobile Subscribers Identity ( TMSI ) :
• The GSM system can also assign a Temporary Mobile Subscriber
Identity (TMSI).
• After the subscriber's IMSI has been initialized on the system, the
TMSI can be used for sending messages backwards and forwards
across the network to identify the subscriber.
• The system automatically changes the TMSI at regular intervals, thus
protecting the subscriber from being identified by someone
attempting to monitor the radio channels.
• The TMSI is a local number and is always allocated by the VLR.
• The TMSI is maximum of 4 octets.
25. NETWORK COMPONENTS
Equipment Identity Register ( EIR )
• The Equipment Identity Register (EIR) contains a centralized
database for validating the international mobile station equipment
identity, the IMEI.
• The database contains three lists:
– The white list contains the number series of equipment identities
that have been allocated in the different participating countries.
This list does not contain individual numbers but but a range of
numbers by identifying the beginning and end of the series.
– The grey list contains IMEIs of equipment to be monitored and
observed for location and correct function.
– The black list contains IMEIs of MSs which have been reported
stolen or are to be denied service.
• The EIR database is remotely accessed by the MSC’s in the
Network and can also be accessed by an MSC in a different PLMN.
.
26. NETWORK COMPONENTS
Equipment Identity Register ( EIR )
EIR
White List Grey List Black List
All Valid Service allowed Service denied
assigned ID’s but noted
Range 1 MS IMEI 1 MS IMEI 1
Range 2 MS IMEI 2 MS IMEI 2
Range n MS IMEI n MS IMEI n
27. NETWORK COMPONENTS
International Mobile Equipment Identity ( IMEI ) :
• IMEI is a serial number unique to each mobile
• Each MS is identified by an International Mobile station Equipment
Identity (IMEI) number which is permanently stored in the Mobile
Equipment.
• On request, the MS sends this number over the signalling channel to the
MSC.
• The IMEI can be used to identify MSs that are reported stolen or
operating incorrectly.
TAC FAC SNR SP
6 2 6 1
TAC = Type Approval Code
FAC = Final Assembly Code
SNR = Serial Number
SP = Spare
28. NETWORK COMPONENTS
HOME LOCATION REGISTER( HLR )
• The HLR contains the master database of all subscribers in the PLMN.
• This data is remotely accessed by the MSC´´s and VLRs in the network.
The data can also be accessed by an MSC or a VLR in a different
PLMN to allow inter-system and inter-country roaming.
• A PLMN may contain more than one HLR, in which case each HLR
contains a portion of the total subscriber database. There is only one
database record per subscriber.
• The subscribers data may be accessed by the IMSI or the MSISDN.
• The parameters stored in HLR are
– Subscribers ID (IMSI and MSISDN )
– Current subscriber VLR.
– Supplementary services subscribed to.
– Supplementary services information (eg. Current forwarding
address ).
– Authentication key and AUC functionality.
– TMSI and MSRN
29. NETWORK COMPONENTS
VISITOR LOCATION REGISTER ( VLR )
• The Visited Location Register (VLR) is a local subscriber database,
holding details on those subscribers who enter the area of the network
that it covers.
• The details are held in the VLR until the subscriber moves into the area
serviced by another VLR.
• The data includes most of the information stored at the HLR, as well as
more precise location and status information.
• The additional data stored in VLR are
– Mobile status ( Busy / Free / No answer etc. )
– Location Area Identity ( LAI )
– Temporary Mobile Subscribers Identity ( TMSI )
– Mobile Station Roaming Number ( MSRN )
• The VLR provides the system elements local to the subscriber, with
basic information on that subscriber, thus removing the need to access
the HLR every time subscriber information is required.
30. NETWORK COMPONENTS
Authentication Centre ( AUC )
• The AUC is a processor system that perform authentication function.
• It is normally co-located with the HLR.
• The authentication process usually takes place each time the
subscriber initialises on the system.
• Each subscriber is assigned an authentication key (Ki) which is
stored in the SIM and at the AUC.
• A random number of 128 bits is generated by the AUC & sent to the
MS.
• The authentication algorithm A3 uses this random number and
authentication key Ki to produce a signed response SRES( Signed
Response ).
• At the same time the AUC uses the random number and
Authentication algoritm A3 along with the Ki key to produce a SRES.
• If the SRES produced by AUC matches the one produced by MS is
the same, the subscriber is permitted to use the network.
31. NETWORK COMPONENTS
AUTHENTICATION PROCESS
HLR AUC
Ki, A3, A8 MS
A3 ( RAND, Ki ) = SRES A3 , A8 , A5 , Ki
A8 ( RAND, Ki ) = Kc
Triples RAND
Generated
TRIPLES
VLR
RAND, Kc , SRES
RAND Kc SRES SRES SRES =
A3 (RAND , Ki )
SRES
SRES = SRES
BTS
A5 , AIR INTERFACE
ENCRYPTION Kc =
HYPERFRAME NUM
A8 (RAND , Ki )
Kc
32.
33.
34. NETWORK COMPONENTS
Base Station Sub-System ( BSS ) :
• The BSS is the fixed end of the radio interface that provides control
and radio coverage functions for one or more cells and their
associated MSs.
• It is the interface between the MS and the MSC.
• The BSS comprises one or more Base Transceiver Stations (BTSs),
each containing the radio components that communicate with MSs in
a given area, and a Base Site Controller (BSC) which supports call
processing functions and the interfaces to the MSC.
• Digital radio techniques are used for the radio communications link,
known as the Air Interface, between the BSS and the MS.
• The BSS consists of three basic Network Elements (NEs).
– Transcoder (XCDR) or Remote transcoder (RXCDR) .
– Base Station Controller (BSC).
– Base Transceiver Stations (BTSs) assigned to the BSC. .
35. NETWORK COMPONENTS
Transcoder( XCDR )
• The speech transcoder is the interface between the 64 kbit/s PCM
channel in the land network and the 13 kbit/s vocoder (actually 22.8
kbit/s after channel coding) channel used on the Air Interface.
• This reduces the amount of information carried on the Air Interface and
hence, its bandwidth.
• If the 64 kbits/s PCM is transmitted on the air interface without
occupation, it would occupy an excessive amount of radio bandwidth.
This would use the available radio spectrum inefficiently.
• The required bandwidth is therefore reduced by processing the 64
kbits/s PCM data so that the amount of information required to transmit
digitized voice falls to 13kb/s.
• The XCDR can multiplex 4 traffic channels into a single 64 kbit/s
timeslot. Thus a E1/T1 serial link can carry 4 times as many channels.
• This can reduce the number of E1/T1 leased lines required to connect
remotely located equipment.
• When the transcoder is between the MSC and the BSC it is called a
remote transcoder (RXCDR).
37. NETWORK COMPONENTS
TRANSCODING
30 Timeslots
1 traffic channel / TS Each Timeslot =16 X 4
64 Kbps / TS = 64 Kb/s
4 E1 lines = 30 X 4 30 timeslots = 30 x 4
=120 Timeslots
=120 traffic channels
MSC XCDR BSC
Transcoded information from four calls
0 1 2 16 31
38. NETWORK COMPONENTS
Base Station Controller (BSC)
• The BSC network element provides the control for the BSS.
• It controls and manages the associated BTSs, and interfaces with
the Operations and Maintenance Centre (OMC).
• The purpose of the BSC is to perform a variety of functions. The
following comprise the functions provided by the BSC:
– Controls the BTS components.-
– Performs Call Processing.
– Performs Operations and Maintenance (O & M).
– Provides the O & M link (OML) between the BSS and the OMC.
– Provides the A Interface between the BSS and the MSC.
– Manages the radio channels.
– Transfers signalling information to and from MSs.
40. NETWORK COMPONENTS
Base Transceiver Station (BTS)
• The BTS network element consists of the hardware components,
such as radios, interface modules and antenna systems that
provide the Air Interface between the BSS and the MSs.
• The BTS provides radio channels (RF carriers) for a specific RF
coverage area.
• The radio channel is the communication link between the MSs
within an RF coverage area and the BSS.
• The BTS also has a limited amount of control functionality which
reduces the amount of traffic between the BTS and BSC.
42. NETWORK COMPONENTS
BTS Connectivity
Open ended Daisy Chain
MSC BSC BTS12 BTS13 BTS14
Star
BTS11 BTS5
Daisy Chain with
BTS1 a fork. Fork has a
return loop back
Daisy Chain with
BTS6 to the chain
a fork. Fork has a
BTS4
return loop back
to the chain
BTS2 BTS7
BTS9
BTS11 BTS3 BTS8
43. NETWORK COMPONENTS
Operation And Maintenance Centre For Radio (OMC-R)
• The OMC controls and monitors the Network elements within a
region.
• The OMC also monitors the quality of service being provided by the
Network.
• The following are the main functions performed by the OMC-R
– The OMC allows network devices to be manually removed for or
restored to service. The status of network devices can be
checked from the OMC and tests and diagnostics invoked.
– The alarms generated by the Network elements are reported
and logged at the OMC. The OMC-R Engineer can monitor and
analyze these alarms and take appropriate action like informing
the maintenance personal.
– The OMC keeps on collecting and accumulating traffic statistics
from the network elements for analysis.
– Software loads can be downloaded to network elements or
uploaded to the OMC.
45. NETWORK COMPONENTS
Base Station Identity Code
• BSIC allows a mobile station to distinguish between neighboring base
stations.
• It is made up of 8 bits.
7 6 5 4 3 2 1 0
0 0 NCC BCC
BCC
NCC = National Colour Code( Differs from operator to operator )
BCC = Base Station Colour Code, identifies the base station to help
distinguish between Cell’s using the same BCCH frequencies
46. NETWORK COMPONENTS
MS Class Mark
• The MS is identified by it’s classmark which the mobile sends during
it’s initial message.
• The classmark contains the following information
– Revision level - Identifies the phase of the GSM specifications the
mobiles complies with.
– RF Power Capabilities - The maximum power the mobile can
transmit. This information is held in the MS Power Class Number.
– Ciphering Algorithm - Indicates the ciphering algorithm
implemented in the mobile. There is only one algorithm (A5 ) in
GSM phase 1, however GSM phase 2 specifies different
algorithms (A5/0 to A5/7 )
– Frequency Capability - Indicates the frequency bands the MS can
receive and transmit on.
– Short Message Capability- Indicates whether the MS is able to
receive short messages or not.
47. MOBILE MAXIMUM RANGE
RANGE= TIMIMG ADVANCE * BIT PERIOD* VELOCITY
2
TIMING ADVANCE = DELAY OF BITS (0-63)
BIT PERIOD= 577/156.25 = 3.693µsec =3.693 * 10e-6 sec
VELOCITY= 3 * 10e5 Km/sec
RANGE= 34.9 Km
48. MULTIPLE ACCESS TECHNIQUES
• In order for several links to be in progress simultaneously in the
same geographical area without mutual interference , multiple
access techniques are deployed.
• The commonly used multiple access techniques are
– Frequency Division Multiple Access (FDMA )
– Time Division Multiple Access (TDMA )
– Code Division Multiple Access (CDMA )
49. TERRESTERIAL INTERFACE
• The terrestrial interfaces comprises all the connections between
the GSM system entities ,apart from the Um or air interface.
• The terrestrial interfaces transport the traffic across the system
and allows the passage of thousands of data messages to make
the system function.
• The standard interfaces used are
– 2 Mb/s
– Signalling System (C7 or SS7
– Packet Switched Data
– A bis using the LAPD protocol (Link Access Procedure D )
•
50. INTERFACE NAMES
Each interface specified in GSM has a name associated with it.
NAME INTERFACE
Um MS ----- BTS
Abis BTS ----- BSC
A MSC ------ BSC
B MSC ------ VLR
C MSC ------ HLR
D VLR ----- HLR
E MSC ------ MSC
F MSC ------ EIR
G VLR ------ VLR
H HLR ------ AUC
51. 2 Mbits/s Trunk 30- channel PCM
This interface carries the traffic from the PSTN to the MSC,
between MSC’s, from the MSC to the BSC’s and from the BSC’s to
the BTS’s.
It represents the physical layer in the OSI model.
Each 2 Mb/s link provides 30 traffic channels available to carry
speech ,data or control information.
Typical Configuration
TS 0 TS 1-15 TS 16 TS 17 - 31
TS 0 - Frame allignment/ Error checking/ Signalling/ Alarms
TS 1-15 , 17-31 - Traffic
TS 16 - Siganlling
53. Cell Global Identity ( CGI ) :
LAI
MCC MNC LAC CI
CGI
MCC = Mobile Country Code
MNC = Mobile Network Code
LAC = Location Area Identity
CI = Cell Identity
54. CHANNEL CONCEPT
CHANNELS Downlink
Uplink
Physical channel - Each timeslot on a carrier is referred to as a physical
channel. Per carrier there are 8 physical channels.
Logical channel - Variety of information is transmitted between the MS and
BTS. There are different logical channels depending on the information
sent. The logical channels are of two types
• Traffic channel
• Control channel
56. CHANNEL CONCEPT
GSM Control Channels
Control Channels
BCH ( Broadcast channels ) CCCH(Common Control Chan) DCCH(Dedicated Channels)
Downlink only Downlink & Uplink Downlink & Uplink
BCCH Synch. RACH CBCH SDCCH ACCH
Broadcast
control channel
Channels Random
Access Channel Cell Broadcast Standalone
dedicated Associated
Channel Control Channels
control channel
SCH FCCH PCH/
Synchronisation Frequency
AGCH FACCH SACCH
Correction channel Fast Associated Slow associated
channel Paging/Access grant Control Channel Control Channel
57. CHANNEL CONCEPT
BCH Channels
BCCH( Broadcast Control Channel )
• Downlink only
• Broadcasts general information of the serving cell called System
Information
• BCCH is transmitted on timeslot zero of BCCH carrier
• Read only by idle mobile at least once every 30 secs.
SCH( Synchronisation Channel )
• Downlink only
• Carries information for frame synchronisation. Contains TDMA
frame number and BSIC.
FCCH( Frequency Correction Channel )
• Downlink only.
• Enables MS to synchronise to the frequency.
• Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
58. CHANNEL CONCEPT
CCCH Channels
RACH( Random Access Channel )
• Uplink only
• Used by the MS to access the Network.
AGCH( Access Grant Channel )
• Downlink only
• Used by the network to assign a signalling channel upon
successfull decoding of access bursts.
PCH( Paging Channel )
• Downlink only.
• Used by the Network to contact the MS.
59. CHANNEL CONCEPT
DCCH Channels
SDCCH( Standalone Dedicated Control Channel )
• Uplink and Downlink
• Used for call setup, location update and SMS.
SACCH( Slow Associated Control Channel )
• Used on Uplink and Downlink only in dedicated mode.
• Uplink SACCH messages - Measurement reports.
• Downlink SACCH messages - control info.
FACCH( Fast Associated Control Channel )
• Uplink and Downlink.
• Associated with TCH only.
• Is used to send fast messages like handover messages.
• Works by stealing traffic bursts.
60. CHANNEL CONCEPT
NORMAL BURST
FRAME1(4.615ms)
FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3 57 bits 26 bits 57 bits 3
Guard Tail Flag Training Flag Tail Guard
Data Data
Period Bits Bit sequence Bit Bits Period
Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH,
SACCH and FACCH.
61. CHANNEL CONCEPT
NORMAL BURST
Data - Two blocks of 57 bits each. Carries speech, data or control info.
Tail bits - Used to indicate the start and end of each burst. Three bits always
000.
Guard period - 8.25 bits long. The receiver can only receive and decode if
the burst is received within the timeslot designated for it.Since the MS are
moving. Exact synchronization of burst is not possible practically. Hence
8.25bits corresponding to about 30us is available as guard period for a
small margin of error.
Flag bits - This bit is used to indicate if the 57 bits data block is used as
FACCH.
Training Sequence - This is a set sequence of bits known by both the
transmitter and the receiver( BCC of BSIC). When a burst of information is
received the equaliser searches for the training sequence code. The
receiver measures and then mimics the distortion which the signal has been
subjected to. The receiver then compares the received data with the
distorted possible transmitted sequence and chooses the most likely one.
62. CHANNEL CONCEPT
FREQUENCY CORRECTION BURST
FRAME1(4.615ms) FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3 142 bits 3
Guard Tail Tail Guard
Fixed Data
Period Bits Bits Period
• Carries FCCH channel.
• Made up of 142 consecutive zeros.
• Enables MS to correct its local oscillator locking it to that of the BTS.
63. CHANNEL CONCEPT
SYNCHRONISATION BURST
FRAME1(4.615ms) FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3 39 bits 64 bits 39 bits 3
Guard Tail Encrypted Synchronisation Encrypted Tail Guard
Period Bits Bits Sequence Bits Bits Period
• Carries SCH channel.
• Enables MS to synchronise its timings with the BTS.
• Contains BSIC and TDMA Frame number.
64. CHANNEL CONCEPT
DUMMY BURST
FRAME1(4.615ms) FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3 57 bits 26 bits 57 bits 3
Guard Tail Flag Training Flag Tail Guard
Data Data
Period Bits Bit sequence Bit Bits Period
• Transmitted on the unused timeslots of the BCCH carrier in the
downlink.
65. CHANNEL CONCEPT
ACCESS BURST
FRAME1(4.615ms) FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
8 41 bits 36 bits 3 68.25 bits
Tail Synchronisation Encrypted Tail Guard
Bits Sequence Bits Bits Period
• Carries RACH.
• Has a bigger guard period since it is used during initial access and
the MS does not know how far it is actually from the BTS.
66. CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
MS Uplink
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
• If Uplink and Downlink are aligned exactly, then MS will have to
transmit and receive at the same time. To overcome this problem a
offset of 3 timeslots is provided between downlink and uplink
67. CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
MS Uplink
5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5
3 timeslot
offset
• As seen the MS does not have to transmit and receive at the same
time. This simplifies the MS design which can now use only one
synthesizer.
68. CHANNEL CONCEPT
26 FRAME MULTIFRAME STRUCTURE
4.615 msec
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
120 msec
• MS on dedicated mode on a TCH uses a 26-frame multiframe
structure.
• Frame 0-11 and 13-24 used to carry traffic.
• Frame 12 used as SACCH to carry control information from and to MS
to BTS.
• Frame 25 is idle and is used by mobile to decode the BSIC of neighbor
cells.
74. CODING
SPEECH CODING
• The transmission of speech is one of the most important service of a
mobile cellular system.
• The GSM speech codec, which will transform the analog signal(voice)
into a digital representation, has to meet the following criterias
• A good speech quality, at least as good as the one obtained with
previous cellular systems.
• To reduce the redundancy in the sounds of the voice. This
reduction is essential due to the limited capacity of transmission of
a radio channel.
• The speech codec must not be very complex because complexity is
equivalent to high costs.
• The final choice for the GSM speech codec is a codec named RPE-
LTP (Regular Pulse Excitation Long-Term Prediction).
75. CODING
SPEECH CODING
• This codec uses the information from previous samples (this
information does not change very quickly) in order to predict the
current sample.
• The speech signal is divided into blocks of 20 ms. These blocks are
then passed to the speech codec, which has a rate of 13 kbps, in order
to obtain blocks of 260 bits.
76. CODING
CHANNEL CODING
• Channel coding adds redundancy bits to the original information in
order to detect and correct, if possible, errors ocurred during the
transmission.
• The channel coding is performed using two codes: a block code and
a convolutional code.
• The block code receives an input block of 240 bits and adds four
zero tail bits at the end of the input block. The output of the block
code is consequently a block of 244 bits.
• A convolutional code adds redundancy bits in order to protect the
information. A convolutional encoder contains memory. This property
differentiates a convolutional code from a block code.
• A convolutional code can be defined by three variables : n, k and K.
• The value n corresponds to the number of bits at the output of the
encoder, k to the number of bits at the input of the block and K to the
memory of the encoder.
77. CODING
CHANNEL CODING ( Cont )
• The ratio, R, of the code is defined as R = k/n.
Example - Let's consider a convolutional code with the following
values: k is equal to 1, n to 2 and K to 5. This convolutional code uses
then a rate of R = 1/2 and a delay of K = 5, which means that it will
add a redundant bit for each input bit. The convolutional code uses 5
consecutive bits in order to compute the redundancy bit. As the
convolutional code is a 1/2 rate convolutional code, a block of 488 bits
is generated. These 488 bits are punctured in order to produce a block
of 456 bits. Thirty two bits, obtained as follows, are not transmitted :
C (11 + 15 j) for j = 0, 1, ..., 31
k=1 Convolution code R = k/n = 1/2 n=2
1 bit input 2 bit input
• The block of 456 bits produced by the convolutional code is then
passed to the interleaver
78. CODING
CHANNEL CODING FOR GSM SPEECH CHANNELS
• Before applying the channel coding, the 260 bits of a GSM speech
frame are divided in three different classes according to their function
and importance.
• The most important class is the class 1a containing 50 bits.Next
important is the class 1b, which contains 132 bits.The least important
is the class 2, which contains the remaining 78 bits.
• The different classes are coded differently.
• First of all, the class 1a bits are block-coded. Three parity bits, used
for error detection, are added to the 50 class 1a bits.The resultant 53
bits are added to the class 1b bits.
• Four zero bits are added to this block of 185 bits (50+3+132). A
convolutional code, with r = 1/2 and K = 5, is then applied, obtaining
an output block of 378 bits.
• The class 2 bits are added, without any protection, to the output
block of the convolutional coder. An output block of 456 bits is finally
obtained.
79. CODING
Speech Channel Coding
260 bits
Class 1a Class 1b
50 bits 132 bits
Parity Tail
check Class 1a 3 Class 1b 4 bits
50 bits 132 bits
Class 2
78 bits
Convolution coding
378 bits
456 bits
80. CODING
CHANNEL CODING FOR CONTROL CHANNELS
• In GSM the signalling information is just contained in 184 bits.
• Forty parity bits, obtained using a fire code, and four zero bits are
added to the 184 bits before applying the convolutional code (r = 1/2
and K = 5). The output of the convolution code is then a block of 456
bits which does not need to be punctured.
Parity
184 bits bits
Fire Tail
code bits
184 bits 40 bits 4
Convolution coding
456 bits
81. CODING
CHANNEL CODING FOR DATA CHANNELS
• In data information is contained in 240 bits.
• Four tails bits are added to the 240 bits before applying the
convolutional code (r = 1/2 and K = 5). The output of the
convolutional code is then a block of 488 bits which when punctuated
yields 456 bits.
240 bits
Tail
bits
240 bits 4
Convolution coding
488 bits
Punctuate
456 bits
82. INTERLEAVING
INTERLEAVING
• An interleaving rearranges a group of bits in a particular way.
• It is used in combination with FEC codes( Forward Error Correction
Codes ) in order to improve the performance of the error correction
mechanisms.
• The interleaving decreases the possibility of losing whole bursts
during the transmission, by dispersing the errors.
• As the errors are less concentrated, it is then easier to correct them.
83. INTERLEAVING
GSM SPEECH CHANNEL INTERLEAVING
• A burst in GSM transmits two blocks of 57 data bits each.
• Therefore the 456 bits corresponding to the output of the channel coder
fit into 8 ‘57 data’ bits (8 * 57 = 456). The 456 bits are divided into eight
blocks of 57 bits.
• The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the
second one the bit numbers (1, 9, 17, .....449), etc.
• The last block of 57 bits will then contain the bit numbers (7, 15, .....455).
• The first four blocks of 57 bits are placed in the even-numbered bits of
four consecutive bursts.
• The other four blocks of 57 bits are placed in the odd-numbered bits of
the next four bursts.
• The interleaving depth of the GSM interleaving for speech channels is
eight.
• A new data block also starts every four bursts. The interleaver for
speech channels is called a block interleaver.
85. INTERLEAVING
CONTROL CHANNEL INTERLEAVING
• A burst in GSM transmits two blocks of 57 data bits each.
• Therefore the 456 bits corresponding to the output of the channel coder
fit into four bursts (4*114 = 456).
• The 456 bits are divided into eight blocks of 57 bits. The first block of 57
bits contains the bit numbers (0, 8, 16, .....448), the second one the bit
numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain
the bit numbers (7, 15, .....455).
• The first four blocks of 57 bits are placed in the even-numbered bits of
four bursts.
• The other four blocks of 57 bits are placed in the odd-numbered bits of
the same four bursts.
• Therefore the interleaving depth of the GSM interleaving for control
channels is four and a new data block starts every four bursts.
• The interleaver for control channels is called a block rectangular
interleaver.
86. INTERLEAVING
DATA INTERLEAVING
• A particular interleaving scheme, with an interleaving depth equal to
22, is applied to the block of 456 bits obtained after the channel coding.
• The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits
each, 2 blocks of 12 bits each and 2 blocks of 6 bits each.
• It is spread over 22 bursts in the following way :
• the first and the twenty-second bursts carry one block of 6 bits
each
• the second and the twenty-first bursts carry one block of 12 bits
each
• the third and the twentieth bursts carry one block of 18 bits each
• from the fourth to the nineteenth burst, a block of 24 bits is placed
in each burst
• A burst will then carry information from five or six consecutive data
blocks. The data blocks are said to be interleaved diagonally.
87. MODULATION
CIPHERING
• Ciphering is used to protect signaling and user data.
• A ciphering key is computed using the algorithm A8 stored on the
SIM card, the subscriber key and a random number delivered by the
network (this random number is the same as the one used for the
authentication procedure).
• A 114 bit sequence is produced using the ciphering key, an algorithm
called A5 and the burst numbers.
• This bit sequence is then XORed with the two 57 bit blocks of data
included in a normal burst.
• In order to decipher correctly, the receiver has to use the same
algorithm A5 for the deciphering procedure.
MODULATION
• Modulation is done using 0.3 GMSK
89. SIGNALLING SYSTEM
WHAT IS SIGNALLING ?
• The term signaling is used in many contexts.
• In technical systems, it very often refers to the control of different
procedures.
• With reference to telephony, signaling means the transfer of
information and the instructions relevant to control and monitor
telephony connections.
90. SIGNALLING SYSTEM C7
GENERAL INTRODUCTION
• Today’s global telecom networks are included in very complex
technical systems.
• Naturally, a system of this type requires extensive signaling, both
internally in different nodes (for example, exchanges) and externally
between different types of network nodes.
• During this training we will focus on external signaling.
• Thus, the term signaling in the following slides always refers to
external signaling traffic.
• The main purpose of using signaling in modern telecom networks –
where different network nodes must cooperate and communicate with
each other – is to enable transfer of control information between
nodes in connection with:
–Traffic control procedures as set-up, supervision, and release
of telecommunication connections and services
91. GENERAL INTRODUCTION
• Database communication, for example, database queries concerning
specific services, roaming in cellular networks, etc.
• Network management procedures, for example, blocking or
deblocking trunks.
• Traditionally, external signaling has been divided into two basic types
– Access signaling (for example, Subscriber Loop Signaling) This
means signaling between a subscriber terminal (telephone) and
the local exchange.
– Trunk signaling (that is, Inter-Exchange Signaling) This is used
for signaling between exchanges.
92. SIGNALING IN TELECOMMUNICATION NETWORK
SIGNALLING
ACCESS SIG TRUNK SIGNALLING
SUBSCRIBER LINE SIG. CHANNEL ASSOCIATED SIG.
DIGITAL SUBSCRIBER SIG. COMMON CHANNEL SIG.
93. Access Signaling
• There are many types of access signaling, for example, PSTN
analogue subscriber line signaling, ISDN Digital Subscriber Signaling
System (DSS1), and signaling between the MS and the network in the
GSM system.
• Signaling on the analogue subscriber line between a telephony
subscriber and the Local Exchange (LE) is performed by means of
on/off hook signals, dialed digits, information tones (dial tone, busy
tone, etc.), recorded announcements, and ringing signals.
• The dialed digits can be sent in two different ways: as decadic pulses
(used for old-type rotary-dial telephones), or as a combination of two
tones (used for modern pushbutton telephones). The latter system is
known as the Dual Tone Multi Frequency (DTMF).
• The information tones (dial tone, ringing tone, busy tone, etc.) are audio
signals used to keep the calling party (the A-subscriber) informed about
what is going on in the network during the set-up of a call.
94. Access Signaling
• Digital Subscriber Signaling System No. 1 (DSS1) is the standard
access signaling system used in ISDN. It is also called a D-channel
signaling system
• D-channel signaling is defined for digital access lines only.
• The signaling protocols are based on the OSI (Open System
Interconnection) reference model, layers 1 to 3.
• Consequently, the signaling messages are transferred as data
packets between the user terminal and the local exchange.
• Due to the much more complex service environment at the ISDN
user’s site, the amount of signaling information and the number of
variations
95. Trunk Signaling
• The Inter-exchange Signaling information is usually transported on
one of the time slots in a PCM link, either in association with the
speech channel or independently.
• There are two commonly used methods for Inter Exchange Signaling.
Channel Associated Signaling (CAS)
– In CAS, the speech channel (in-band), or a channel closely
associated with a speech channel (out-band), is used for
signaling.
Common Channel Signaling (CCS)
– In this case a dedicated channel, completely separate from the
speech channel, is used for signaling. Due to the high capacity,
one signaling channel in CCS can serve a large number of
speech channels.
• In a GSM network, CCITT Signaling System No. 7 is used.
• Signaling System No. 7 is a Common Channel Signaling system.
96. CHANNEL ASSOCIATED SIGNALING (CAS)
• Channel Associated Signaling (CAS) means that the signaling is
always sent on the same connection (PCM link) as the traffic.
• The signaling is associated with the traffic channel.
• In a 2 Mb/s PCM link, 30 time slots are used for speech, whereas TS
0 is used for synchronization and TS 16 is used for the line signaling.
• All 30 traffic connections share TS 16 in a multiframe consisting of
16 consecutive frames.
• On TS 16, each traffic channel has a permanently allocated recurring
location for line signaling, where two traffic channels share TS 16 in
one frame.
97. COMMON CHANNEL SIGNALING (CCS)
• In CCS, signaling messages (or data packets) are transmitted over time
slots in a PCM link reserved for the purpose of signaling.
• The system is designed to use a common data channel (or signaling
link) as the carrier of all signals, required by a large number of traffic
channels.
• In 1968, CCITT specified a Common Channel Signaling system called
CCS System No. 6, which was designed especially for international
analogue telephony networks.
• However, very few installations of this system remain today. It has, as
already mentioned, been replaced by Signaling System No. 7.
• The first version of SS7 (1980) was designed for telephony and data.
• In the 80’s the demand for new services dramatically increased and the
SS7 was therefore developed to meet the signaling requirements,
specified for all these new services.
• Today SS7 is used in many different networks and related services
typically betn PSTN, ISDN, PLMN & IN services throughout the world.
98. OSI REFERENCE MODEL
• The Signaling System No. 7, which is a type of packet switched data
communication system, is structured in a modular and layered way.
• Such a design of SS7 is similar to the Open System Interconnection
model.
• Open Systems are systems that use standardized communication
procedures developed from the reference model.
• Thus, all such open systems are able to communicate with each
other.
• The word “system” can refer to computers, exchanges, data
networks, etc.
99. OSI MODEL REFERENCE DIAGRAM
APPLICATION APPLICATION
PRESENTATION PRESENTATION
SESSION SESSION
TRANSPORT TRANSPORT
NETWORK NETWORK
LINK LINK
PHYSICAL PHYSICAL
100. COMMUNICATION PROCESS
• Each layer has its own specified functions and provides specific
services for the layers above.
• It is important to define the interfaces between different layers and the
functions within each layer.
• The way a function is realized within a layer is not predicted.
• Logically, the communication between functions always takes place
on the same level according to the protocols for that level.
• Only functions on the same level can “talk to each other”.
• In the transmitting system, the protocol for each layer adds
information to the data received from the layer above.
• The addition usually consists of a header and/or a trailer.
• In the receiving system, the additions are used, for example, to
identify bits or data fields carrying information for that specific layer
only.
• These fields are decoded by layer functionality and are removed
when delivering the message to the applications orlayers above.
101. • When the data reaches the application layer on the receiving side,
it consists of only the data that originated in the application layer
of the sending system.
• Logically, each layer communicates with the corresponding layer
in the other system.
• This communication is called Peer-to-Peer communication and is
controlled by the layer’s protocol.
DESCRIPTION OF LAYERS
Application Layer
• This layer provides services for support of the user’s application
process and for control of all communication between
applications.
• Examples of layer 7 functions are file transfer, message handling,
directory services, and operation and maintenance.
102. Presentation Layer
• This layer defines how data is to be represented, that is, the syntax.
• The presentation layer transforms the syntax used in the application
into the common syntax needed for the communication between
applications.
• Layer 6 contains data compression.
Session Layer
• This layer establishes connections between presentation layers in
different systems.
• It also controls the connection, the synchronization and the
disconnection of the dialogue.
• It allows the presentation layer to determine checkpoints, from which
the retransmission will start when the data transmission has been
interrupted.
103. Transport Layer
• This layer guarantees that the bearer service has the quality
required by the application in question.
• Examples of functions are error detection and correction (end-to-
end), and flow control.
• The transport layer optimizes the data communication, for example
by multiplexing or splitting data streams before they reach the
network.
104. Network Layer
• The basic network layer service is to provide a transparent channel.
• This means that the application requesting a channel ignores network
problems and the related signal exchange because that is the task of
the lower levels.
• It just requires an open channel, transparent for the transmission of
data, between transport layers in different systems.
• The Network Layer establishes, maintains, and releases connections
between the nodes in the network and handles addressing and
routing of circuits.
Data Link Layer
• This layer provides an essentially error-free point-to-point circuit
between network layers.
• The layer contains resources for error detection, error correction, flow
control, and retransmission.
105. Physical Layer
• This layer provides mechanical, electrical, functional, and
procedural resources for activating, maintaining, and blocking
physical circuits for the transmission of bits between data link
layers.
• The physical layer contains functions for converting data into
signals compatible with the transmission medium.
• For the communication between only two exchanges, layers 1 and
2 are sufficient.
• For the communication between all exchanges in the network, layer
3 must be added because it provides addressing and routing.
106. SIGNALING SYSTEM NO. 7 INTRODUCTION
• The Signaling System (SS)No. 7 is an elaborate set of
recommendations defining protocols for the internal management of
digital networks.
• These recommendations were introduced in 1980 and revised in
1984 and 1988 in different-colored books (yellow, red, and blue).
• CCITT SS No. 7 is intended primarily for digital networks, both
national and international, where the high transmission rates (64
kbps) can be exploited.
• It may also be used on analogue lines especially on international
trunks (CCITT SS No 6).
• CCS was initially meant for telephony only, but has now evolved into
non-telephony and non-connection related applications (for example,
location updating of a mobile subscriber).
• A dialogue with a database or between two databases is a typical
application for CS in GSM.
107. • Thus, there is a need for a generic system that is able to support a
wide variety of applications in telecommunication.
• The variety of applications is increasing as new types of telephony
systems and a wider use of databases in the network become
necessary (mobile telephony networks, ISDN, IN, etc.).
• Even though the standardization of SS7 is now the responsibility of
ITU-T, for traditional and historical reasons, the system is often
called “CCITT No. 7 signaling system”.
• The signaling system used in GSM follows the CCITT
recommendations.
• The modular layer structure allows flexible usage of the
specifications.
108. USER PARTS
• The User Parts (UPs) contain functions dealing with the processing of
signal information before and after it is transmitted through the
signaling network.
• The MTP provides the means of reliable transport and delivery of UP
information across the SS7 network.
• It also has the ability to react to system and network failures that
affect the information from the UPs and take necessary action to
ensure that the information is safely conveyed.
• The User does not mean the subscriber involved in the call, but the
user of the MTP.
• The MTP is a common transport system developed to serve one or
more User Parts in the same node.
• Every Signaling Point(SP) consists of MTP & a number of its users.
• Only UPs of the same type can communicate with each other.
• To forward signaling messages between UPs, located in different
nodes, the MTP is used.
109. USERS OF SIGNALING SYSTEM CCITT NO 7
MAP CAP BSSAP ISUP TUP
TCAP
SCCP
MTP
CCITT SS NO. 7 PROTOCOLS IN GSM
110. MTP user parts
ISUP (ISDN User Part)
• It provides control-functions and signaling, needed in an ISDN, to
deal with ISDN subscriber calls and related functions.
TUP (Telephony User Part)
• It provides all necessary functions and signaling for dealing with a
telephony user.
• TUP is being replaced by ISUP in telecommunication networks.
DUP (Digital User Part)
• This UP is used for purposes such as file transfer and related
signaling.
111. SCCP
• The MTP was designed for the real-time applications of telephony.
• The connectionless nature of the MTP provides a low-overhead facility
suiting the requirements of telephony.
• Regarding GSM, other applications such as network management
need services such as expanded addressing capability and reliable
message transfer.
• The SCCP was developed to meet these requirements.
• The SCCP also sends its messages through the MTP.
• The SCCP provides functions for completely new services, for
example, non-circuit-related signaling.
• Some functions, not directly related to users, but necessary for
network control, are used.
• The main reason is that they are necessary for serving applications in
higher layers and for maintenance purposes.
112. SCCP
• These functions use SCCP services:
Transaction Capabilities (TC)
– First introduced in 1984, TC provides the mechanisms for
transaction-oriented applications and functions.
Operation and Maintenance Application Part (OMAP)
– Specifies network management functions and messages related
to operation and maintenance.
113. OSI Model CCITT SS NO 7 Model
ASE
APPLICATION USER PARTS
TCAP
PRESENTATION
SESSION
TRANSPORT
SCCP
NETWORK
SIGNALLING NETWORK
NSP
MTP
LINK SIGNALLING LINK
PHYSICAL SIGNALLING DATA LINK
118. RF POWER CONTROL
• RF power control is employed to minimise the transmit power
required by MS or BS while maintaining the quality of the radio
links.
• By minimising the transmit power levels, interference to co-channel
users is reduced.
• Power control is implemented in the MS as well as the BSS.
• Power control on the Uplink also helps to increase the battery life.
119. POWER CONTROL IN THE MS
• The RF power level employed by the MS is indicated by means of
the 5 bit TXPWR field sent either in the layer 1 header of each
downlink SACCH message block, or in a dedicated signalling block.
• The MS confirms the power level that it is currently employing by
setting the MS_TXPWR_CONF field in the uplink SACCH L1 header
to its current power setting. The value of this field is the power setting
actually used by the mobile for the last burst of the previous SACCH
period.
• The MS employs the most recently commanded RF power level
appropriate to the channel for all transmitted bursts on either a TCH
(including handover access burst), FACCH,SACCH or SDCCH.
• When accessing a cell on the RACH (random access) and before
receiving the first power command during a communication on a
DCCH or TCH (after an IMMEDIATE ASSIGNMENT), the MS uses
either the power level defined by the MS_TXPWR_MAX_CCH
parameter broadcast on the BCCH of the cell, or the maximum
TXPWR of the MS as defined by its power class, whichever is the
120. 1111111 indicates this field does not have any TA value
8 7 6 5 4 3 2 1
Spare Ordered MS Power Level Octet 1
Spare Ordered Timing Advance Octet 2
POWER CONTROL MS
• The range over which a MS is capable of varying its RF output
power is from its maximum output down to 20mW, in steps of
nominally 2dB.
• 0 - 43dBm…….15 - 13dBm.
121. TIMING OF POWER CHANGE BY MS
• Upon receipt of a command on the SACCH to change its RF power
level (TXPWR field) the MS changes to the new level at a rate of
one nominal 2dB power step every 60ms (13 TDMA frames), i.e. a
full range change of 15 steps should take about 900ms .
• The change commences at the first TDMA frame belonging to the
next reporting period . The MS changes the power one nominal 2
dB step at a time, at a rate of one step every 60 ms following the
initial change, irrespective of whether actual transmission takes
place or not.
• In case of channel change the commanded power level is applied
on the new channel immediately.
122. BSS POWER CONTROL
• Power control at BSS is optional.
• The range over which the BS is capable of reducing its RF output
power from its maximum level is nominally 30dB, in 15 steps of
nominally 2dB.
123. RADIO LINK FAILURE
• The criterion for determining Radio Link Failure in the MS is based on
the success rate of decoding messages on the downlink SACCH.
• The radio link failure criterion is based on the radio link counter S.
• If the MS is unable to decode a SACCH message, S is decreased by 1.
• If a SACCH message is decoded successfully, S is increased by 2.
• If S reaches 0 a radio link failure is assumed & the MS aborts the conn.
• The RADIO_LINK_TIMEOUT parameter is transmitted by each BS in the
BCCH data.
4
Decoded
3
Not Decoded
2
1
0
SACCH Blocks
124. RADIO LINK FAILURE
• The MS continues transmitting as normal on the uplink until S
reaches 0.
• The algorithm will start after the assignment of a dedicated channel
and S is initialized to RADIO_LINK_TIMEOUT.
• The aim of determining radio link failure in the MS is to ensure that
calls with unacceptable voice/data quality, which cannot be
improved either by RF power control or handover, are either re-
established or released in a defined manner.
• In general the parameters that control the forced release should be
set such that the forced release will not normally occur until the call
has degraded to a quality below that at which the majority of
subscribers would have manually released. This ensures that, for
example, a call on the edge of a radio coverage area, although of
bad quality, can usually be completed if the subscriber wishes.
125. CELL SELECTION AND RE-SELECTION
• In Idle mode (i.e. not engaged in communicating with a BS), an MS will
do the cell selection and re-selection procedures .
• The procedures ensure that the MS is camped on a cell from which it
can reliably decode downlink data and with which it has a high
probability of communications on the uplink. The choice of cell is
determined by the path loss criterion. Once the MS is camped on a
cell, access to the network is allowed.
• An MS is said to be camped on a cell when it has determined that the
cell is suitable and stays tuned to a BCCH + CCCH of that cell. While
camped on a cell, an MS may receive paging messages or under
certain conditions make random access attempts on a RACH of that
cell, and read BCCH data from that cell.
• The MS will not use the discontinuous reception (DRX) mode of
operation (i.e. powering itself down when it is not expecting paging
messages from the network) while performing the selection and
reselection algorithm. However use of powering down is permitted at
all other times in idle mode.
126. CELL SELECTION AND RE-SELECTION
• For the purposes of cell selection and reselection, the MS is required
to maintain an average of received signal strengths for all monitored
frequencies. These quantities termed the "receive level averages” is
the averages of the received signal strengths measured in dBm.
• The cell selection and reselection procedures make use of the "BCCH
Allocation" (BA) list. There are in two BA lists which may or may not be
identical, depending on choices made by the PLMN operator.
• (i) BA (BCCH) - This is the BA sent in System Information Messages
on the BCCH. It is the list of BCCH carriers in use by a given PLMN in
a given geographical area. It is used by the MS in cell selection and
reselection.
• (ii) BA (SACCH) - This is the BA sent in System Information
Messages on the SACCH and indicates to the MS which BCCH
carriers are to be monitored for handover purposes.
• When the MS goes on to a TCH or SDCCH, it starts monitoring BCCH
carriers in BA (BCCH) until it gets its first BA (SACCH) message.
127. CELL SELECTION - NO BCCH DATA AVAILABLE
• The MS searches all 124 RF channels in the GSM system, takes
readings of RSS on each RF channel, and calculate the received
level average for each.
• The averaging is based on at least five measurement samples per
RF carrier spread over 3 to 5 secs.
• The MS tunes to the carrier with the highest average RSS &
determines whether or not this carrier is a BCCH carrier.
• If it is a BCCH carrier, the MS attempts to synchronise to this carrier
and read the BCCH data. The MS camps on the cell provided it can
successfully decode the BCCH data and this data indicates that it is
part of the selected PLMN, that the cell is not barred
(CELL_BAR_ACCESS = 0) & that the parameter C1 is greater than
0.
• If the cell is part of the selected PLMN but is barred or C1 is less than
zero, the MS uses the BCCH Allocation obtained from this cell and
subsequently only searches these BCCH carriers. Otherwise the MS
tune to the next highest carrier and so on.
128. CELL SELECTION - NO BCCH DATA AVAILABLE
• CELL_BAR_ACCESS may be employed to bar a cell that is only
intended to handle handover traffic etc. For example of this could
be an umbrella cell which encompasses a number of microcells.
• If at least the 30 strongest RF channels have been tried, but no
suitable cell has been found, provided the RF channels which have
been searched include at least one BCCH carrier, the available
PLMN's shall be presented to the user, otherwise more RF
channels shall be searched until at least one BCCH carrier is found.
• 30 RF channels are specified to give a high probability of finding all
suitable PLMN's, without making the process take too long.
129. CELL SELECTION - BCCH INFORMATION AVAIL.
• The MS stores the BCCH carriers in use by the PLMN selected when it
was last active in the GSM network. A MS may also store BCCH
carriers for more than one PLMN which it has selected previously (e.g.
at national borders or when more than one PLMN serves a country).
• If an MS includes a BCCH carrier storage option it searches only for
BCCH carriers in the list.
• If an MS decodes BCCH data from a cell of the selected PLMN but is
unable to camp on that cell, the BA of that cell is examined. Any BCCH
carriers in the BA which are not in the MS's list of BCCH carriers to be
searched is added to the list.
• If no suitable cell has been found after all the BCCH carriers in the list
have been searched, the MS acts as if there were no stored BCCH
carrier information. Since information concerning a number of channels
is already known to the MS, it may assign high priority to
measurements on those of the 30 strongest carriers from which it has
not previously made attempts to obtain BCCH information, and omit
repeated measurements on the known ones.
130. PATH LOSS CRITEREON( C1)
• This parameter is used to ensure that the MS is camped on the cell
with which it has the highest probability of successful communication
on uplink and downlink.
• The path loss criterion parameter C1 used for cell selection and
reselection is defined by:
C1 = (A - Max(B,0))
where A = Received Level Average - RXLEV_ACCESS_MIN
B = MS_TXPWR_MAX_CCH - P
RXLEV_ACCESS_MIN =Minimum received level at the MS
required for access to the system.
MS_TXPWR_MAX_CCH = Maximum TXPWR level an MS may
use when accessing the system.
P = Maximum RF output power of the MS.
• All values are expressed in dBm.
131. PATH LOSS CRITEREON( C1)
A = + Good Downlink
- Poor Downlink
B = - Good Downlink
+ Poor Downlink
Monitoring of Received Level and BCCH data
• In Idle Mode an MS continues to monitor all BCCH carriers as
indicated by the BCCH Allocation .
• A running average of received level in the preceding 5 to 60
seconds is be maintained for each carrier in the BCCH Allocation.
• For the serving cell receive level measurement samples is taken at
least for each paging block of the MS and the receive level average
is determined using samples collected over a period of 5 s or five
consecutive paging blocks of that MS, whichever is the greater
period.
132. Monitoring of Received Level and BCCH data
• At least 5 received level measurement samples are required per
receive level average value. New sets of receive level average
values is calculated as often as possible.
• The same number of measurement samples is taken for all non
serving cell BCCH carriers, and the samples allocated to each carrier
is as far as possible uniformly distributed over each evaluation
period.
• The list of the 6 strongest carriers is updated at least every minute
and may be updated more frequently.
• In order to minimise power consumption, MSs that employ DRX (i.e.
power down when paging blocks are not due) monitor the signal
strengths of non-serving cell BCCH carriers during the frames of the
Paging Block that they are required to listen to. Received level
measurement samples can thus be taken on several non-serving
BCCH carriers and on the serving carrier during each Paging Block.
• The MS includes the BCCH carrier of the current serving cell (i.e. the
cell the MS is camped on) in this measurement routine.
133. Monitoring of Received Level and BCCH data
• The MS has to decode the full BCCH data of the serving cell at least
every 30 seconds.
• The MS attempts to decode the BCCH data block that contains the
parameters affecting cell reselection for each of the 6 strongest non-
serving cell BCCH carriers at least every 5 minutes.
• When the MS recognizes that a new BCCH carrier has become one of
the 6 strongest, the BCCH data shall be decoded for the new carrier
within 30 seconds.
• The MS attempts to check the BSIC for each of the 6 strongest non
serving cell BCCH carriers at least every 30 seconds, to confirm that it
is monitoring the same cell.
• If a change of BSIC is detected then the carrier is treated as a new
carrier and the BCCH data redetermined.
• When requested by the user, the MS monitors the 30 strongest GSM
carrier to determine, within 15 seconds, which PLMN's are available.
This monitoring is done so as to minimise interruptions to the
monitoring of the PCH.
134. CALL RE-ESTABLISHMENT
• In the event of a radio link failure, call re-establishment may be
attempted if it is enabled in the database.
• The received level measurement samples taken on surrounding cells
and on the serving cell BCCH carrier in the last 5 seconds is
averaged, and the carrier with the highest average received level
which is part of a permitted PLMN is taken.
• A BCCH data block containing the parameters affecting cell selection
is read on this carrier.
• If the parameter C1 is greater than zero, it is part of the selected
PLMN, the cell is not barred, and call re-establishment is allowed, call
re-establishment is attempted on this cell.
• If the above conditions are not met, the carrier with the next highest
average received level is taken, and the MS repeats the above
procedure.
• If the cells with the 6 strongest average received level values are tried
but cannot be used, the call re-establishment attempt is abandoned.
135. bs_ag_blk_res
• To ensure that some of the blocks are always left clear for access
grant messages the parameter bs_ag_blk_res is used to input the
number of blocks to be reserved for this purpose.
• The reserved blocks is not be used for paging whatever the demand.
• If more than one timeslot exists within a cell, this parameter will
reserve the indicated number of blocks on each timeslot.
• This parameter is broadcast on the BCCH.
• This parameter is used to calculate the number of paging groups
available.
COMBINED CCCH BLOCKS AGCH BLOCKS PCH BLOCKS
No 9 0 9
No 9 1 8
No 9 2 7
No 9 3 6
No 9 4 5
No 9 5 4
No 9 6 3
No 9 7 2
Yes 3 0 3
Yes 3 1 2
Yes 3 2 1
136. Bs_pa_mfrms
• Used to indicate the number of 51 frame multiframes between
transmission of paging messages to MS of the same group.
• Is transmitted on BCCH.
• Used by the MS to calculate its paging group.
8 16 24 32 8
Value 7 15 23 31 7
0 = 2 multiframes 6 14 22 30 6
1 = 3 multiframes 5 13 21 29 5
2 = 4 multiframes 4 12 20 28 4
3 11 19 27 3
3 = 5 multiframes
2 10 18 26 2
4 = 6 multiframes 1 9 17 25 1
5 = 7 multiframes AGCH AGCH AGCH AGCH AGCH
6 = 8 multiframes BCCH BCCH BCCH BCCH BCCH
7 = 9 multiframes
137. PAGING
Example
cch_conf = 0
bs_ag_blk_res = 1
bs_pa_mfrms = 2
If cch_conf = 1
minimum = 2
If cch_conf = 6
Maximum = 81 * 4
Min time between pages = 2 * 235.5 = 471ms
Max time between pages = 9 * 235.5 =2.1195 sec
138. max_retran
• An MS requests resources from the network by transmitting an
``access burst´´ containing the channel request message.
• For a single request, channel request will be repeated upto M +
1 times where M = max_retran.
max_retrans M
1 1
2 2
3 4
4 7
139. tx_integer
• To reduce the chances of collision the wait period is
randomised for each MS.
• After the first channel request is sent the next is repeated after
a random wait period in the set
(S, S+1,….., S+T-1)
• Wait period from this set is chosen randomly from this set.
TX INTEGER S FOR NON- S FOR COMB
RACH SLOTS COMB CCCH CCCH
3, 8, 14 55 41
4, 9, 16 76 52
5, 10, 20 109 58
6, 11, 25 163 86
7, 12, 32 127 115
140. AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP
Maximum AGCH reservation for non-combined multiframe = 7
Available paging blocks = 2
Maximum AGCH reservation for combined multiframe = 1
Available paging blocks = 2
Minimum AGCH reservation for non-combined multiframe = 0
Available paging blocks = 9
Minimum AGCH reservation for combined multiframe = 0
Available paging blocks = 3
No of paging blocks will have a range of 2 - 9
141. CALCULATION OF CCCH AND PAGING GROUP NO
CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]
div N
Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS *
N ) ] mod N
