This document provides an overview of computer network technology concepts. It discusses what computer networks are used for, including data sharing, program sharing, and resource sharing. It also reviews key aspects of network components and topology. Specifically, it covers network types (LANs, MANs, WANs), transmission media, and multiplexing techniques. It explains the differences between circuit switching and packet switching, and surveys current network services. The overall purpose is to discuss the basic concepts of computer network technology and emphasize those required to stay informed of new developments in the field.

1. Graduate School of Business S-OIT-14
STANFORD UNIVERSITY 1/96
A Note on Computer Network Technology
The diversity of computer network technologies, standards and products is bewildering. In light of the
growing importance of data communications to modern management, this presents both problems and opportunities.
This note discusses the basic concepts of computer network technology, emphasizing those required to stay abreast
of new developments. Where useful for illustration, specific products or standards are mentioned.
The structure of this note is as follows. The note first considers what computer networks are used for.
Then, it reviews network components and their physical layout and distinguishes among various network types.
Several key network technology concepts are discussed in greater detail. Next, it examines the range of functions
performed by a network and how they are managed through the network architecture. Finally, the service
requirements of network applications and currently available network services are surveyed.
1. Network Uses and Applications
In a broad sense, computer networks are systems of hardware and software used for data, program and
resource sharing, or as a communication medium. One or several of these form the motivation for many computer
network applications, the combinations of tasks performed by the network as viewed by the user.
• Data sharing involves access to remote or distributed databases and files containing data, text, voice, images
or video. Examples of applications where the key function of the network is data sharing include corporate
data warehouses
1
, electronic library catalogs, airline reservation systems, home banking and automated teller
machine networks.
• In program sharing, programs stored on a central server can simultaneously be accessed and loaded for
execution on several local computers. Many application programs in the GSB's MBA lab, including the
Microsoft Office programs you must have heard of by now, are shared in this way.
• Resource sharing includes access to remote computing resources such as computers (e.g., remote login
2
to a
supercomputer to run large-scale weather simulations) and high-speed laser printers, to remote specialized
equipment such as medical imaging instruments in teleradiology and telepathology, and distributed
cooperative computing, where the processing power and memory of multiple computers are joined to solve a
problem.
• Uses of networks as a communication medium include electronic mail (E-mail), video conferencing, electronic
bulletin boards and groupware for distributed collaborative work
3
.
Many computer network applications combine several network uses. A familiar example is office
automation, in which an office network is used for sharing software programs and laser printers, for accessing
centrally stored company data, and as a communication medium for corporate electronic mail. In electronic
commerce, businesses use networks to communicate with their suppliers, partners and customers, exchange product
information, place and check the status of orders, manage inventory levels, make payments, provide product support
etc. In factory automation, manufacturing tools are electronically controlled by programs and data accessed over a
network. A general trend across most computer network applications is the increased use of multimedia documents
combining text, data, voice, images, and video.
2. Network Components
Computer networks consist of communicating devices (or nodes) linked by transmission circuits (also
referred to as circuits, channels, lines, or links), and of network interfaces and networking software.
Prepared by Philipp Afeche under the supervision of Haim Mendelson of the Graduate School of Business, Stanford
University. Comments by Charles Bonini and Anne Korin, as well as partial financial support by the Stanford Computer
Industry Project and the Information Technology Initiative of the Stanford Business School are greatly appreciated.
1
A data warehouse is an enterprise-wide database where a set of data, extracted from a wide variety of information systems,
is centrally stored.
2
In remote login, a user on a local computer (terminal) accesses application programs on a remote computer. In contrast to
program sharing, the programs are executed on the remote computer, and the local computer serves only as a terminal.
3
In a broad sense, the term groupware encompasses any technology that supports interpersonal collaboration through the
computer, ranging from simple electronic mail to more sophisticated products such as Lotus Notes.

2. A Note on Computer Network Technology
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The communicating devices can be grouped into end nodes, where transmissions originate and terminate,
and switches, which are computers used to connect two or more circuits. Examples of end nodes are host computers
(also called hosts) running application programs and storing data, printers, scanners, fax machines, telephones and
automated teller machines. The switches can be bridges, routers, or gateways (see section 6.3.)
Circuits differ in their bandwidth or data rate, which is their capacity, expressed in terms of the maximum
number of bits2
transmitted per second. Current circuits (discussed in section 8.1) have data rates on the order of a
thousand, a million, or a billion bits per second, which is denoted by the abbreviations Kbps (Kilobits per second),
Mbps (Megabits per second), and Gbps (Gigabits per second), respectively.
Devices such as modems or analog-to-digital (A/D) converters serve as network interfaces between the
communicating devices and the transmission circuits.
Networking software plays an important role in managing the communication between the various hardware
devices. Conventions for the many tasks involved in a successful transmission are structured and defined in network
protocols, implemented in hardware and software.
3. Network Topologies
Network topology refers to the physical layout of end nodes, switches and transmission circuits. In
broadcast topologies (see Figure 1), a single communication channel is shared by all end nodes for transmissions,
and all transmissions can be received by all end nodes. In contrast, switches and point-to-point links are used for
interconnecting end nodes in point-to-point networks and transmissions are addressed to specific end nodes. Several
alternative designs are possible (see Figure 2.)
Bus Satellite or Radio Ring
Figure 1: Broadcast topologies. The bus topology is used by Ethernet, the presently dominant protocol for LANs (see
section 4). Satellite or radio networks are based on wireless technology. Data transmitted therefore propagates to any
node within the reach of the sender. The ring topology is frequently used in conjunction with the token ring LAN
protocol, in which a node can only send if it holds the "token", a special message that is passed around the ring.
4. Network Types
Networks are commonly classified by ownership as private networks or public networks, and according to
their geographical scope as Local Area Networks (LANs), Metropolitan Area Networks (MANs) or Wide Area
Networks (WANs).
A private network is built by an organization for its exclusive use, while a public network is established
and operated by a network provider for the specific purpose of providing services to customer organizations and
individuals.
LANs link up to thousands of computers located in the same or in adjacent buildings on a campus and are
typically private. They have a bandwidth of up to over 100 Mbps and a maximum range of a few miles. A familiar
example is the GSB network which connects the personal computers in the MBA, PhD and Sloan computer labs
with those of GSB faculty and staff members. LANs can be interconnected using bridges to form larger LANs that
2
Synonym for binary digit. In binary notation the character 0 or 1.

3. A Note on Computer Network Technology
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extend over greater distances and support more users. The GSB LAN is linked to other Stanford LANs to form a
large campuswide LAN. A MAN is a network that covers an entire city but uses LAN technology.
WANs span large areas such as countries or the entire globe and are either private or public. WANs are
slower than LANs, although this difference is diminishing due to technological advances. Currently, long distance
links of WANs have bandwidths of up to 45 Mbps. Private WANs usually link far fewer computers than LANs,
rarely more than 500, because most companies have offices in a limited number of different cities. In contrast, public
WANs connect up to several million computers. Most large corporations such as IBM, Digital Equipment
Corporation and General Motors have worldwide private WANs to interconnect their geographically dispersed
locations.
LANs are typically based on broadcast topologies, while WANs, except in the case of satellite networks,
consist of point-to-point channels and switches.
Fully Connected Star
Mesh Ring
Figure 2: Point-to-point topologies. The fully connected (one direct channel between each pair of end nodes) and the star
network (each pair of end nodes is connected via a central switch) represent the two extremes. Reducing the number of
links used to interconnect a given number of end nodes reduces the cost, but makes the network more vulnerable to switch
failures and requires mechanisms for establishing end-to-end connections between the nodes and for sharing the links.
The size and complexity of computer networks has grown immensely over the years. Today's networks are
often internetworks - networks of networks - interconnecting hundreds if not thousands of LANs, MANs, and
WANs, part private, part public, based on different topologies and composed of a wide variety of components. The
connection between these networks is established using bridges, routers, and gateways. Large WANs linking LANs
and MANs in internetworks are usually referred to as backbone networks. The best known internetwork is the
Internet (see the related note “A Note on the Internet”) It is defined as the network of interconnected and
interoperating networks using the TCP/IP (Transmission Control Protocol / Internet Protocol) protocols and a
common set of network addresses (see section 6.3.)
5. Key Network Technology Concepts
A few key concepts form the basis for understanding the operation of computer networks and are discussed
in this section. Networks transmit data using analog or digital communication. Its circuits are implemented on
various physical transmission media. Circuits and media are shared among concurrent transmissions using time
division multiplexing or frequency division multiplexing. Circuit switching or packet switching are used for getting

4. A Note on Computer Network Technology
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data from a sending node to the intended receiving node. These and other elements of a network’s functionality are
organized in a network architecture, discussed in detail in section 6.
5.1 Analog and Digital Communication
All forms of data - text, database files, voice, images, video - are represented in communicating devices and
transmitted over circuits using two forms of signals. A signal is the variation of a physical quantity, such as electric
current or a light wave. One can differentiate between analog and digital signals (see Figure 3). For example, the
sound waves generated by speech are converted by the telephone microphone into an analog electrical signal. An
analog signal varies continuously in time and can take on an infinite number of different values. In contrast,
computers and other electronic equipment represent and process information by digital signals, as a sequence of
binary digits, "0" and "1" bits. A digital signal varies discontinuously in time and can take on only a finite
number of different values.
The terms analog and digital circuit or network, and analog and digital communication or transmission,
refer to the form of the transmitted signal, which may differ from its original form in the communicating device. For
example, when transmitting an analog signal over a digital circuit, e.g., voice over long-distance telephone links
(today predominantly digital), an analog-to-digital (A/D) converter is used as a network interface at the source node
to transform the analog voice signal into a digital signal (referred to as digitizing) and a digital-to-analog (D/A)
converter at the destination node to reverse this process. Similarly, when transmitting computer data over analog
circuits, as still prevalent in local telephone networks, a modem (modulator/demodulator) is used as a network
interface between the computer and the transmission circuit to convert the digital computer data into an analog
transmission signal and back.
Time
Voltage
0 1 0 0 0 1 1
Time
Voltage
Analog Digital
Figure 3: Analog vs. Digital. The analog signal shown, which may be generated by a microphone capturing a sound wave,
varies continuously in time and the signal voltage can take on an infinite number of values. The digital signal shown is
used to represent 0 and 1 bits in a computer. It varies discontinuously over time and takes on only two values.
In the past, telephone and television networks were entirely based on analog transmission. Today, digital
networks are quickly replacing analog ones. One of the main advantages of digital compared to analog
communication is its lower error rate. It is therefore best suited for transmitting computer data, which is very error-
sensitive, and video or images with less distortion. Digital switches can also achieve much higher processing rates
than analog ones. Further, digital transmission allows the integration of different types of traffic such as voice, cable
television, video, and computer data on a single network, enabling significant cost reductions.
5.2 Transmission Media
Transmission circuits can be implemented on a number of wire-based (copper, fiber) or wireless (air, free
space) transmission media:
• The twisted pair is the familiar wire used for connecting a telephone to the telephone jack. It consists of two
insulated copper wires twisted together in a helical form. This is the oldest and most widely available
medium. The twisted pair is prevalent in buildings and for local telephone network links (the local loop), and

5. A Note on Computer Network Technology
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is inexpensive to install. However, it can only be used for short links because of significant electrical
interference (created by the close proximity of the two wires) and rapid signal degradation over long distances.
• The coaxial cable is the familiar cable used for connecting a television set to the cable TV jack. It consists of a
cylindrical copper wire and outer conductor, separated by insulating material and protected by a plastic sheath.
It is widely used for LANs, long-distance telephone links, and cable television. Coaxial cable can carry more
data than twisted pair due to lower electrical interference but is more expensive. Due to rapid signal
degradation, coaxial links require amplifiers every few hundred feet to maintain high data rates.
• Data is transmitted by optical signals on fiber optic links, made of very thin glass or plastic fiber, which are
progressively replacing coaxial cable for long-distance telephone transmission and are increasingly used in high-
speed LANs. The optical signals are insensitive to electrical interference and essentially don't degrade, which
leads to data rates and ranges that are orders of magnitude higher than those of any other medium. However,
using fiber also requires substantial investments for hardware components, e.g., switches handling the optical
signals.
• In terrestrial line-of-sight transmission, data is transmitted through the air between ground station antennas
using lasers and infrared signals for LANs and microwave radio transmission over longer distances. This saves
the cost of building physical links, especially to remote areas, and provides mobility. However, microwave
transmission may suffer from atmospheric phenomena and signal interference in high-traffic areas.
• Communication satellites in the sky above the equator relay signals through free space and air from one ground
station to another, thus covering an area hundreds of miles in diameter. Therefore, satellites are excellent for
cost-efficient communication over large areas with little infrastructure. They are used for television networks
and by private networks to bypass the telephone system. As a drawback, transmission via satellite is subject to
a substantial delay due to the long distance traveled.
The attainable bandwidth and range for a medium depends, among other things, on the methods used to represent
data by signals, the medium's sensitivity to electric interference, and the signal degradation properties. Table 1
summarizes the bandwidths and ranges for these media.
Transmission Medium Order of magnitude of
maximum bandwidth [bps]
Order of magnitude of maximum
range between ground stations [m]
Twisted pair 10
6
10
3
Coaxial cable 10
8
10
6
Fiber >10
9
10
6
Terrestrial line-of sight 10
8
10
6
Communication Satellites 10
8
10
8
Table 1: Bandwidths and ranges for common transmission media.
5.3 Time Division Multiplexing and Frequency Division Multiplexing
Multiplexing is used to efficiently share the bandwidth of a network link among the numerous
transmissions in process at any given time. There are two basic multiplexing techniques, frequency division
multiplexing (FDM) and time division multiplexing (TDM).
FDM, used in analog communication, is similar to radio stations having exclusive access to a frequency.
The frequency spectrum of a link, the range of available frequencies, is divided among its users with each having
exclusive access to a frequency band. Fixed TDM (sometimes also referred to as synchronous TDM or Synchronous
Transfer Mode - STM) grants each user exclusive access to the entire link bandwidth for a certain time slot in a fixed
sequence. However, the traffic generated by typical data network applications such as remote login is bursty, i.e.,
periods of high data transfer rates are followed by relatively long periods during which no data is transmitted.
Because both FDM and TDM assign bandwidth to users according to a fixed schedule irrespective of their changing
transmission demands, they don't utilize the available bandwidth efficiently. Asynchronous (or statistical) TDM
mitigates this problem: If the user whose turn it is has no data to transmit, the medium is made available to other
users who need to transmit. Asynchronous Transfer Mode (ATM), an emerging network service discussed in
section 8.2, is based on asynchronous TDM.
5.4 Circuit Switching and Packet Switching

6. A Note on Computer Network Technology
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In networks that are based on unswitched circuits the connections between pairs of communicating end
nodes are defined in advance and permanently for repeated use. This is done either using virtual circuits, logical
paths defined between the communicating end nodes without reserving bandwidth, or leased physical circuits.
Virtual circuits defined in advance are also called permanent virtual circuits or private virtual circuits.
Most current WANs are switched, i.e., these connections are not predefined. Two basic switching
techniques, circuit switching and packet switching, are used for getting data from a sending node to the intended
receiver.
In circuit switching (see Figure 4), used for voice communication on the telephone network, a physical
circuit through the network is set up before any data is sent. Bandwidth is reserved for the entire duration of the
connection and all data travels along this physical circuit. If the transmission is bursty, the reserved bandwidth is
used very inefficiently. On the other hand, because circuit switching dedicates bandwidth to an application, it can
guarantee a specified quality of service
1
, in particular a minimal bandwidth and a maximum (transmission) delay
2
Connection requests whose desired quality of service cannot be accommodated are simply blocked (similar to a busy
signal in the telephone network.)
A
B C
D
EF
idle circuit
dedicated physical circuit
Source
Destination
Figure 4: Circuit switching. For the transmission from source A to destination D, a dedicated path is set up using point-
to-point links between the nodes A,C,E, and D (the path is shown by the thick lines in the figure). The other links
(represented by thin lines) are not involved in this transmission.
In packet switching, prevalent in computer networks transmitting data, no bandwidth is dedicated to any
user for the entire duration of a connection. Rather, the data is divided into variable-length packets (so-called
packetization) at the source node and reassembled at the destination. Packets contain in addition to the data
exchanged by users information necessary for delivering packets to the right address, checking and correcting
transmission errors, and so forth. Data is transmitted in packets, and bandwidth is only reserved when needed for a
packet transmission. Cell switching, a recently introduced switching technology, closely resembles packet
switching, with the main difference that data is divided into fixed-length units called cells (see section 8.2). These
are entirely processed in hardware, resulting in very high bandwidth network services such as Asynchronous
Transfer Mode (ATM, see section 8.2 .)
1
The network attributes that characterize its performance in transmitting data.
2
The amount of time elapsed between the time data is transmitted by the sender and the time it is delivered to the receiver

7. A Note on Computer Network Technology
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A
B C
D
EF
Source
of 1, 3
Destination of 1
idle circuit
shared physical circuit
data packet
Destination of 2
Source of 2 Destination of 3
Virtual Circuits
1 ABCD
2 ABE
3 FBC
1
2
2
3
3
1
1
Figure 5: Connection-oriented packet switching based on virtual circuits. Three virtual circuits are currently active along
the following paths: 1, ABCD; 2, ABE; and 3, FBC. Each packet carries the virtual circuit identifier for routing purposes
(represented by the number written in the data packet symbol.) The links between A and B, and between B and C are shared
by the virtual circuits 1 and 2, and 1 and 3, respectively. Virtual circuits only use up bandwidth when a packet is actually
transmitted.
Depending on the method for delivering these packets or cells from source to destination node, one can
distinguish between connection-oriented and connectionless service.
Connection-oriented service (see Figure 5), used for example in X.25 networks (see section 8.2), is modeled
after the telephone system: the service user first establishes, then uses, and finally terminates an end-to-end
connection. The end-to-end connections are implemented as switched virtual circuits, which are only defined on
demand for a specific connection. Each network node keeps a table of all active virtual circuits that pass through it.
Packets travel along the same virtual circuit during the lifetime of the connection. They carry the same virtual
circuit identifier, which is used for routing decisions at each intermediate node.
Connectionless service, used for example in the Internet and other networks based on IP (Internet Protocol,
see section 8.2 and “A Note on the Internet”), is modeled after the postal system. It is typically implemented using
datagrams (see Figure 6), packets that carry the full source and destination address and are routed through the
system independently of all the others. Packets may arrive out of sequence at the destination in which case they
have to be reordered.
The key difference between circuit switching and packet switching lies in the way bandwidth is allocated to
connections. Packet switching utilizes the available bandwidth more efficiently. However, it suffers from the
potential for large delays and lost packets if the network is congested, and requires complex processing for the
routing, packetization and packet reassembly. Note that fixed TDM is best suited to support circuit switching while
asynchronous TDM is natural for packet switching.

8. A Note on Computer Network Technology
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A
B C
D
EF
idle circuit
shared physical circuit
data packet
A,D
F,C
A,D
A,D
F,C
A,E
A,E
Figure 6: Connectionless packet switching based on datagrams. As in figure 5, data is transmitted from A to D, A to E, and
F to C. But in contrast to connection-oriented service, the datagrams belonging to the same source destination pair are
routed independently, using the address information they carry (represented by the letters written in the data packet
symbol.) Therefore, they may have to reordered at their destination (at C, D, and E). As in connection-oriented service,
bandwidth is only tied up for the duration of the packet transmission but can otherwise be shared with other
transmissions.
6. Network Functions and Architecture
6.1 Network Functions
The successful transmission of data between end nodes involves far more than physical links between end
nodes and switches. It also includes network functions (discussed in section 6.3) such as multiplexing and
switching, error control, flow control, routing, congestion control, internetworking, dialog management, common
network representation of data formats, and the provision of application-specific services.
6.2 Network Architectures
The functionality of modern computer networks is typically determined by the network architecture. The
architecture is specific enough to serve as a blueprint for designing the layers in software and hardware, but does not
encompass implementation details. Different architectures share the following features:
• They conceptualize the communication process as a hierarchy of layers, each performing a set of well defined
functions. Peer processes, the software and hardware entities comprising parallel layers on different nodes, carry
on a conversation to perform these functions.
• The conversation rules between peer processes are laid down in protocols, also called peer to peer protocols.
• An interface between each pair of adjacent layers defines which services the lower layer offers to the layer just
above it. Information is not directly exchanged between two peer processes, but first passed across the interfaces
to successively lower layers on the source computer, then transmitted over the physical medium, and finally
passed up on the destination machine layer by layer.
6.3 The OSI Reference Model
The Open Systems Interconnection (OSI) Reference Model (see Figure 7) was developed by the
International Standards Organization (ISO) as a step towards protocol standardization in the early 1980s. It divides
the communication process between two application programs into 7 layers. The ISO and other standards
organizations have also developed protocols for each layer based on the OSI Reference Model.
The protocols of the physical, data link, and network layer specify the communication between adjacent
nodes connected by a circuit. In contrast, the higher transport, session, presentation, and application layers are
end-to-end layers as they contain additional functions required to ensure successful communication from end node to
end node. We briefly describe the function of each layer, starting with the physical layer.
Layer 1: The Physical Layer is responsible for transmitting a stream of “0” and “1” bits over the
physical transmission medium. The issues addressed include transmission medium and connector specifications,

9. A Note on Computer Network Technology
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the representation of bits by analog or digital signals, and multiplexing and switching (see section 5.) However, the
physical layer does not guarantee the reliable transmission and is not concerned with the meaning of bits. It thus
only provides a raw transmission facility.
Layer 2: The Data Link Layer adds to this raw transmission facility physical reliability to provide an
error-free link to the network layer. This requires functions such as grouping bits into data frames
1
, detecting and
correcting transmission errors (error control), keeping a fast transmitting node from drowning a slower receiving
node in data (flow control), and controlling access to a shared channel in the case of a broadcast network.
Layer 3: The Network Layer’s main task is to determine how data packets are routed through the network
from source to destination (routing). This includes assigning network addresses to all the network nodes, providing
connection-oriented or connectionless service (discussed in section 5.4) to the transport layer (layer 4), managing the
data traffic flows to avoid excessive network congestion (congestion control), and interconnecting two or more
networks (internetworking) using bridges, routers, or gateways (a bridge is typically used to connect two LANs at
the data link layer, routers and gateways are used for WAN interconnection, whereby routers connect networks which
use the same and gateways those using different network layer protocols.) As the routing is very simple in broadcast
networks, their network layer is often thin or nonexistent. The network layer makes the upper layers independent of
the various data transmission and switching technologies used to connect systems. The Internet is based the
Internet Protocol (IP), a connectionless network layer protocol (see section 8.2 and "A Note on the Internet".) The
X.25 network layer protocol
2
used in most traditional public data networks is connection-oriented (see section 8.)
Layer 4: The Transport Layer, whose peer processes reside on the end nodes, guarantees reliable data
transport from source to destination node independent of the underlying network technology. It reassembles data
packets in the right order when they are delivered out of sequence (which is common when the network layer
provides connectionless service), resends packets that were not received due to errors etc., and deals with multiple
deliveries of the same packet. The transport layer also guarantees a certain quality of service in the delivery of data
packets, which may specify the maximum packet delivery delay or the minimum dedicated bandwidth for a
transmission. We discuss the quality of service requirements of network applications in section 7. The
Transmission Control Protocol (TCP) is the transport layer of the Internet.
Layer 5: The Session Layer establishes, manages and terminates sessions between communicating
applications. This includes keeping track of whose turn it is to talk and enforcing it (dialog management), and crash
recovery mechanisms for resuming sessions after a system breakdown.
Layer 6: The Presentation Layer provides a common representation of data formats across the network.
It translates among the various formats used on different nodes for representing data by bits, thus making sure that
applications running on different computer types correctly understand the meaning of the bit streams they exchange.
It is also concerned with other aspects of data representation such as data compression and encryption. The former is
used to reduce the number of bits that have to be transmitted to convey a given amount of data, the latter for network
security and privacy purposes to make data unintelligible to all but their intended recipients.
Layer 7: The Application Layer provides services for application programs and also distributed
information services. For instance, the X.400 application layer protocol specifies standards for all aspects of
electronic mail (E-mail) programs, allowing users to create, edit, exchange, display and store messages. Other
application layer protocols include services for directory lookup (the telephone book’s electronic equivalent), for
transferring files between computers, etc.
1
The name of the data unit exchanged is different at each layer. Generally, the network layer deals with packets, the data
link layer with frames, and the physical layer with bits. The data units exchanged at the upper layers have no standard
name. Frames include all data contained in a network layer packet plus data link layer control information.
2
The X.25 standard includes protocols for the physical, data link, and network layer.

10. A Note on Computer Network Technology
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Application
Presentation
Session
Transport
Network
Data Link
Physical1
2
3
4
5
7
Layers
6
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application Protocol
Presentation Protocol
Session Protocol
Transport Protocol
PhysicalPhysical
Data LinkData Link
NetworkNetwork
End Node A End Node BSwitch Switch
Network Protocol
Data Link Protocol
Physical Protocol
Interface
Interface
P H Y S I C A L T R A N S M I S S I O N M E D I U M
Interface
Interface
Interface
Interface
Figure 7: The 7 layer network architecture based on the OSI Reference Model. Protocols define the rules for
communication between peer processes on the nodes at each layer. Layers 1-3 perform functions across each single link,
from end nodes A and B to their respective neighbor switches and between neighbor switches in the network (the three
long-dashed arrows for the physical, data link and network protocol point to each place where peer process communication
takes place at these layers). Layers 4-7 only perform end node-to-end node functions. Conceptually, peer processes at any
layer talk directly to each other (along the horizontal short-dashed arrows). However, data actually follows the path along
the solid line arrows, i.e., from a given peer process on the originating node down across the layers and interfaces, through
the physical medium to the destination node where it moves up across layers and interfaces to the corresponding peer
process.
7. Service Requirements of Network Applications
Networks provide a range of qualities of service to support the service requirements of network applications.
Among the numerous quality of service parameters, the guaranteed bandwidth and maximum transmission delay are
key. Depending on the type (i.e., text, voice, image, or video) and amount of data transmitted and the user’s needs,
different applications can have different service requirements as expressed by these parameters. In general, one can
distinguish between elastic and real-time applications.

11. A Note on Computer Network Technology
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Elastic applications essentially adapt to whatever bandwidth is available and the resulting transmission
delay. Traditional network applications based on data, such as file transfers and electronic mail, belong in this
category. When transmissions slow down because of network congestion, they keep working, just not as quickly as
usual. Depending on the desired level of interactivity, applications within this group may differ in their delay
sensitivities: transactions initiated during an interactive remote login session are more delay sensitive than
interactive bulk file transfers, which in turn are more sensitive than asynchronous (non-interactive) bulk file transfers
such as electronic mail and fax.
On the other hand, real-time applications, such as real-time voice conversations or video conferencing, have
strict bandwidth and maximum delay requirements which, if not met, can lead to distortions of the received voice or
video signal to the point where it becomes incomprehensible.
In order to satisfy the service requirements of both elastic and real-time applications, a network has to offer
at least two basic qualities of service, best effort service and reserved bandwidth service with guaranteed maximum
delay. Best-effort service means that the network attempts to deliver packets as quickly as possible without making
guarantees about delivery or maximum delays. When the network is overloaded, delays increase and packets are
dropped. A number of priority classes of best effort service can be offered to accommodate elastic applications with
different delay sensitivities. The Internet presently offers only one class of best effort service. Reserved bandwidth
service with guaranteed maximum delay admits a transmission request to the network only if its bandwidth and
maximum delay requirements can be guaranteed. This service type is appropriate for real-time applications.
8. Currently Available Wide Area Network Services
Both private and public WANs are typically based on a combination of circuits. An organization wanting
to interconnect two or more geographically distributed sites has two basic options: Establish private (unswitched)
circuits leased from a telecommunications carrier, between sites, or connect to a carrier's public network from the
sites using private or dial-up (circuit-switched) access circuits.
Using a local analog telephone line with a fast modem, one can achieve today a bandwidth of 28.8 Kbps.
For digital circuits, current standards (see section 8.1 below) are 56/64 Kbps circuits, the T system, ISDN
(Integrated Services Digital Network) and SONET (Synchronous Optical Network.)
In addition to providing their lines to customers directly on an unswitched or circuit switched basis,
carriers also offer X.25, IP (Internet Protocol), Frame Relay, SMDS (Switched Multimegabit Data Service) and ATM
(Asynchronous Traffic Mode) services (see section 8.2 below), which are based on packet switching or related
technology.
8.1 Digital Circuits
56/64 Kbps circuits and the T system form a family of digital transmission circuits which differ mainly in
the data rates they provide (see Table 3). A 56/64 Kbps circuit provides 56 Kbps for data plus 8 Kbps for control
information. The T lines currently in use are fractional T1 (fT1), T1 and T3. fT1 lines come typically with a
bandwidth on the order of a few hundred Kbps in increments of 128 Kbps. T1 lines, which have a bandwidth of
1.544 Mbps, are made up of twenty-four 56/64 Kbps circuits, and multiplexing 29 T1 lines results in a T3 line
with a data rate of 44.736 Mbps. In all T circuits, channels are multiplexed using fixed time division multiplexing
(see section 5.3).
ISDN is a set of standards developed for digital dial-up network access. Its primary objective is the
integration of voice and other data services over a single link. Two channel types, “B” and “D”, have been defined
and combined into a Basic Rate Interface (2B + 1 D channel) and a Primary Rate Interface (23 B + 1 D channel).
Each “B” channel can carry voice or data at up to 64 Kbps, the “D” channel provides 16 Kbps for control
information and can also be used for transmitting data. An ISDN Basic Rate Interface thus provides a total
bandwidth of 144 Kbps over an existing local telephone line. The main advantages of Basic Rate ISDN compared
to a regular analog telephone line include its ability to provide up to 2 simultaneous voice or data conversations
over one physical line, a higher quality and reliability due to the use of digital transmission, and more bandwidth
for applications such as videoconferencing.
SONET establishes a hierarchy of transmission rates and formats to be used for very high speed digital
transmission over fiber optic networks. The ATM networks of the future will be implemented over SONET lines.
The SONET standard is set at increments of 51.48 Mbps. It currently starts at 51.48 Mbps with SONET OC-1 and
extends up to SONET OC-48 at 2.5 Gbps.

12. A Note on Computer Network Technology
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Current carrier networks are implemented over T lines. SONET lines are not yet deployed on a large scale,
but are expected to become increasingly important. Notice that in terms of the OSI Reference Model, the digital
circuits only provide the functionality of the physical layer.
8.2 Packet Switching, Frame Relay and Cell Switching Services
These include the traditional packet services X.25 and IP, as well as the emerging Frame Relay, SMDS,
and ATM. They essentially differ in whether they group bits into variable-length data packets or frames or into
fixed-size cells, in the way they manage connections (i.e. using virtual circuits or datagrams) and in the bandwidth
they support. These services are generally implemented on top of the carrier networks which are based on T lines
(SONET lines in the future). Some of them are also available on the access links from the customer site to the
carrier network.
X.25 is a connection-oriented packet switching service and widely used by many major public data
networks since the 1970s. It contains extensive error correction procedures which are performed at the network
switches and significantly slow transmission speeds. At speeds above 256 Kbps, the network overhead seriously
affects the data throughput. X.25 is therefore only suited for bursty data at these speeds, and is not suited for voice
and full motion video, which require low delays. Also, the X.25 bandwidth is not satisfactory for the transfer of
large files.
Frame Relay, as currently implemented, is an unswitched service based on private virtual circuits (see
section 5.4). Data is transmitted in variable-length blocks called frames. While this service is unswitched, it
resembles packet switching in that bandwidth is only tied up when data is actually transmitted. The private virtual
circuits appear like physical leased lines to the network customers, but share in effect the network provider's physical
circuits using statistical time division multiplexing. Network users subscribe to a Committed Information Rate
(CIR), the average bandwidth they expect to need. Users may temporarily exceed their CIR by using excess
capacity up to the speed of their access link, but the traffic in excess of a user's CIR is so-called discard eligible, i.e.,
frames can be dropped in the case of network congestion. Frame Relay can support speeds of up to 1.544 Mbps, and
network access is typically established using fT1 or T1 leased lines. This high data rate compared to X.25 mainly
results from the design assumption that the network facilities are reliable and hence no extensive error control (as in
X.25 networks) is performed.
IP, a connectionless packet switching service, is the network layer protocol currently used in the Internet
and was introduced in the early 1980s. In principle it can be implemented on top of very fast switches and circuits
of any technology, resulting in very high bandwidths in excess of T3 rates. However, since it only provides best
effort service it is not well suited for real-time applications (see section 7.) For a detailed discussion of IP, see “A
Note on the Internet.”
SMDS is a connectionless service based on cell switching. Its operation principle resembles that of a
connectionless packet switching service like IP, except that SMDS groups data into fixed-length cells, which can be
processed more quickly by switches than the variable-length data packets used in X.25 and IP networks. The cells
are dynamically routed through the best available route. T1 or T3 leased lines are used for network access, and
bandwidths up to 45 Mbps are supported.
ATM is a connection-oriented cell switching service
1
. Like SMDS, it uses fixed cell sizes. But in
contrast to SMDS, all cells belonging to one transmission are routed through the same virtual circuit. ATM differs
from conventional X.25 packet switching mainly in that it uses fixed length cells, highly simplified protocols and
only does error correction at the end nodes. Compared to circuit switching, connection setup delays are minimal in
ATM. It uses asynchronous time-division multiplexing (see section 5.3) and combines the main advantages of
circuit switching, i.e., low transmission delays and guaranteed bandwidth, with the efficient bandwidth utilization of
packet switching. Today, ATM is seen as the technology of choice, which will enable the development of high
speed integrated networks offering a range of services tailored to all kinds of data, voice, image and video
applications. It will be available at speeds of 155 Mbps (SONET OC-3) up to several Gbps. While the definition
of ATM standards is still in progress, four distinct qualities of service have so far been defined:
1 . Constant bit rate (CBR) provides a virtual fixed-bandwidth circuit, primarily aimed at real-time applications.
1
The term ATM also refers to the fast switches used for processing data cells. ATM switches are not considered here.

13. A Note on Computer Network Technology
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2 . Variable bit rate (VBR) is intended for bursty traffic, as generated by transaction processing applications or
LANs, and resembles Frame Relay's CIR service: Users can sporadically send data at higher rates as long as
they don’t exceed a specified average.
3 . Unspecified bit rate (UBR) is the ATM equivalent of best effort service, as realized in IP.
4 . Available bit rate provides minimum bandwidth guarantees to applications and also gives access to any
available bandwidth in excess of this minimum. Intelligence built into the network instructs sending stations
to slow down their transmission when the network is congested, thereby preventing data loss (which can occur
if data is sent into a congested network.)
Physical
Circuits
Virtual
Circuits
Datagrams
Unswitched
56/64 Kbps, fT1,T1,T3,
SONET
(leased physical circuits,
static bandwidth allocation)
Frame Relay
(private virtual circuits,
dynamic bandwidth allocation)
Circuit switched
56/64 Kbps, fT1,T1,T3,
ISDN
(dial-up physical circuits,
static bandwidth allocation)
Packet switched
X.25, ATM
(switched virtual circuits,
dynamic bandwidth allocation)
IP, SMDS
(datagrams, dynamic
bandwidth allocation)
Table 2: Switching and connection management in currently available network services.
Table 2 above classifies these network services based on switching and connection management. Table 3 below
summarizes the bandwidth support they provide.
≤ 9.6 Kbps > 9.6 Kbps
≤ 64 Kbps
> 64 Kbps ≤
1.544 Mbps
> 1.544 Mbps
≤ 45 Mbps
> 45 Mbps
56/64 Kbps X X
fT1/T1 X
T3 X
ISDN X X X
SONET X
X.25 X X X
IP X X X X X
Frame Relay X X
SMDS X X
ATM X
Table 3: Bandwidths supported by currently available network services.
8.3 Match between Network Services and Applications
The above discussion of network services and the bandwidth support they offer (see Table 3) suggests the
following guidelines for assigning network applications to available services:
• Real-time applications: The required bandwidth and maximum delay allowed for real-time applications can
only be guaranteed by services which reserve bandwidth for the duration of the transmission. Circuit switched
services as well as the fast-packet services ATM and SMDS are therefore best-suited, with the specific choice
depending mainly on the application’s bandwidth requirement.

14. A Note on Computer Network Technology
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• Constant data applications (such as bulk file transfers) require higher bandwidths for an extended period of time
but don't rely on small connection setup delays. Thus, they can be suitably operated using circuit-switched
services.
• Bursty data applications, characteristic of LAN interconnections, typically exhibit high peak load to average
data rates. Frame Relay was specifically designed for such applications while circuit switched services are not
well-suited, since they use the reserved bandwidth very inefficiently. X.25, IP, SMDS, and ATM are also
suited for bursty traffic with the specific choice depending on the application’s bandwidth requirement. For
example, X.25 is not and Frame Relay only rarely sufficient to support full motion video or voice for multiple
users sharing the same link.
• ATM offers a set of specifically tailored service classes to suit all application types.

15. A Note on Computer Network Technology
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Glossary
analog circuit. A circuit over which analog signals are transmitted.
analog communication. See analog transmission.
analog network. A network over which analog signals are transmitted.
analog signal. A signal that varies continuously in time and can take on an infinite number of different values.
analog transmission. The transmission of data using analog signals. Also called analog communication.
analog-to-digital (A/D) converter. A network interface used to convert an analog signal into a digital signal.
application layer. Layer 7 in the open systems interconnection (OSI) reference model. It provides services for
application programs and also distributed information services.
asynchronous time division multiplexing (TDM). See statistical time division multiplexing (TDM).
asynchronous transfer mode (ATM). A wide area network service based on connection-oriented cell switching
and statistical (asynchronous) time division multiplexing (TDM).
automated teller machine (ATM) network. An application making use of networks for data sharing, in which
specialized computer terminals - automated teller machines - are linked to a bank's computers and databases
and used to conduct banking transactions without the assistance of a human teller.
available bit rate (ABR). A standard specifying one of the qualities of service provided by an asynchronous
transfer mode (ATM) network.
backbone network. In internetworks, a wide area network (WAN) forming the backbone by linking together local
area networks (LAN) and metropolitan area networks (MANs).
bandwidth. The term used for the circuit capacity, expressed in terms of the maximum number of bits transmitted
per second. Also called data rate.
basic rate interface. An integrated services digital network (ISDN) service with a total bandwidth of 144 Kbps
(Kilobits per second.)
best effort service. A network service attempting to deliver data as quickly as possible without making delivery or
maximum delay guarantees.
bit. Synonym for binary digit, in binary notation either the character 0 or 1.
bridge. A type of switch used to connect two local area networks (LANs) at the data link layer.
broadcast circuit. The link shared by all end nodes in a broadcast topologies.
broadcast topologies. In broadcast topologies, a single circuit is shared by all end nodes. Examples are the bus
topology, ring topology, and the wireless satellite or radio networks.
bursty. Refers to communication in which periods of high data transfer rates are followed by relatively long periods
during which no data is transmitted.
bus topology. A form of broadcast topology used in Ethernet local area networks (LANs).
cell. In cell switching, the fixed-length sequence of bits grouped together for transmission. It contains user data and
control information. See also packet, frame.
cell switching. A switching technique in which data is divided into cells that are individually transmitted. It is
very similar to packet switching, except that cells are of fixed length and are entirely processed in hardware,
resulting in much higher bandwidths.
channel. See transmission circuit.
circuit. See transmission circuit.
circuit-switched circuit. A circuit established using circuit switching. Also called a dial-up circuit.
circuit switching. A switching technique, used for voice communication in the telephone network, in which a
physical circuit through the network is reserved before any data is sent, and bandwidth is tied up for the
entire duration of the connection.
coaxial cable. A transmission medium made of copper wire. The cable used for connecting a television set to the
cable TV jack is a familiar example.
committed information rate (CIR). The average data rate a frame relay user subscribes to.
communicating device. See node.
communication medium. Examples of network applications include electronic mail (E-mail), electronic bulletin
boards, groupware, and video conferencing. See network uses.
communication satellite. Serves in a satellite network for relaying signals between ground stations using wireless
transmission through the air and free space.

16. A Note on Computer Network Technology
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computer network. A system of hardware and software used for data, program and resource sharing, or as a
communication medium. See network applications, network architecture, network components, network
functions, network topologies, network types, network uses, and wide area network services.
congestion control. The network functions involved in managing the data traffic flows to avoid excessive network
congestion.
connectionless. An approach to packet switching in which each packet is individually routed from source to
destination. It is typically implemented with datagrams.
connection-oriented. An approach to packet switching in which a path is set up between communicating end
nodes before any data is sent. All packets are routed through the same path from source to destination.
Connection-oriented service is typically implemented with virtual circuits.
constant bit rate (CBR). A standard specifying one of the qualities of service provided by an asynchronous
transfer mode (ATM) network.
corporate data warehouse. It is an enterprise-wide database where a set of data, extracted from a wide variety of
operational management information systems, is centrally stored. See data sharing.
data compression. The network functions involved in reducing the number of bits that have to be transmitted to
convey a given amount of data.
data link layer. Layer 2 in the open systems interconnection (OSI) reference model. It provides physical
reliability to present an error-free link to the network layer.
data rate. See bandwidth.
data sharing. It involves access to remote or distributed databases and files containing data, text, voice, images, or
video. Examples include corporate data warehouses and automated teller machine (ATM) networks. See
network uses.
datagram. In connectionless packet or cell switching, a self-contained packet or cell carrying enough information to
be independently routed from source to destination.
delay. See transmission delay.
dialog management. The network functions involved in a conversation between two end nodes for keeping track of
whose turn it is to talk and enforcing it.
dial-up circuit. See circuit-switched circuit.
digital circuit. A circuit over which digital signals are transmitted.
digital communication. See digital transmission.
digital network. A network over which digital signals are transmitted.
digital signal. A signal that varies discontinuously in time and can take on only a finite number of different values.
digital transmission. The transmission of data using digital signals. Also called digital communication.
digital-to-analog (D/A) converter. A network interface used to convert a digital signal into an analog signal.
elastic application. A network application that essentially adapts to whatever bandwidth is available and the
resulting transmission delay.
electronic commerce. A summary term for network applications used by companies to exchange business
information with their suppliers, partners and customers. These applications often combine several network
uses, such as data sharing, resource sharing, and networks as a communication medium.
electronic mail (E-mail). A network application used for exchanging messages between network users. See
communication medium.
encryption. The network functions involved in making data unintelligible for all but their intended recipients.
end node. A node where transmissions originate or terminate.
error control. The network functions involved in detecting and correcting transmission errors.
Ethernet. A widespread local area network (LAN) protocol based on a bus broadcast topology.
fiber optic cable. A transmission medium made of very thin glass or plastic fiber that conducts optical signals.
fixed time division multiplexing (TDM). Time division multiplexing in which users are granted exclusive access
to the link's entire bandwidth for a certain time slot in a fixed sequence, irrespective of their changing
transmission demands. Also called synchronous time division multiplexing (TDM) or synchronous
transfer mode (STM).
flow control. The network functions involved in keeping a fast transmitting node from drowning a slower
receiving node in data.
frame. In the variable-length sequence of bits grouped together at the source node's data link layer. A frame
includes data contained in a packet plus data link layer control information.

17. A Note on Computer Network Technology
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frame relay. An unswitched wide area network service based on permanent virtual circuits.
frequency division multiplexing (FDM). A multiplexing technique used in analog communication, in which the
link's frequency spectrum is divided among its users with each having access to a part of it.
fully connected topology. A point-to-point topology in which all end node pairs are directly connected through a
circuit.
gateway. A type of switch used for wide area network (WAN) interconnection for networks using different network
layer protocols.
groupware. An application making use of networks as a communication medium. Groupware encompasses any
technology that supports interpersonal collaboration through the computer, ranging from simple electronic
mail (E-mail) to more sophisticated products such as Lotus Notes.
host. See host computer.
host computer. In a network, a computer running application programs and storing data.
integrated services digital network (ISDN). A digital circuit for dial-up network access that offers a basic rate
interface and a primary rate interface. Its main objective is the integration of voice and other data services
over a single circuit. See also wide area network services.
interface. In a network architecture, a set of definitions specifying the boundary between adjacent layers and the
services that the lower layer offers to the one just above it.
Internet. The best known internetwork. It is defined as the network of interconnected and interoperating networks
using the internet protocol (IP) as their network layer protocol, the transmission control protocol (TCP)
as their transport layer protocol, and a common set of network addresses.
internet protocol (IP). 1) A wide area network service based on connectionless packet switching and used in the
Internet. 2) The network layer protocol implementing this service.
internetwork. A network of interconnected local area networks (LANs), metropolitan area networks (MANs),
and/or wide area networks (WANs).
internetworking. The network functions involved in the joint operation of several networks as an internetwork.
layer. A part of a network architecture that is assigned a set of well selected network functions.
leased circuit. Also called private circuit or unswitched circuit. A circuit leased from a telecommunications
carrier.
line. See transmission circuit.
link. See transmission circuit.
local area network (LAN). A network linking computers in the same or in adjacent buildings on a campus with a
range of a few miles. It is typically a private network and uses a broadcast topology.
local loop. A term often used for local telephone network links.
mesh topology. A point-to-point topology in which some end node pairs are directly connected through a circuit
and others only indirectly through circuits and other nodes.
metropolitan area network (MAN). A network that covers and entire city or urban area and uses local area
network (LAN) technology.
modem (modulator-demodulator). A network interface used to convert a digital signal for transmission in the
form of an analog signal and afterwards back into a digital signal.
multimedia document. A document combining several forms of data such as text, voice, images, and video.
multiplexing. A technique used for sharing the bandwidth of a link among the numerous transmissions in process
at any given time. There are two basic techniques, time division multiplexing (TDM) and frequency
division multiplexing (FDM). See network functions.
network. See computer network.
network address. An identifier permanently assigned to each network node. Network addresses are required,
among other things, for ensuring the delivery of data to the right destination.
network applications. The combinations of tasks performed by the network, viewed from the users' perspective.
Examples include corporate data warehouses, electronic commerce, and electronic mail. Network
applications are motivated by one or several network uses.
network architecture. The structural organization of a modern network's functionality in a hierarchy of layers and
protocols.
network components. These include communicating devices (or nodes), which can be end nodes or switches, and
transmission circuits (also called circuits, channels, lines, or links), network interfaces, networking
software, and network protocols.

18. A Note on Computer Network Technology
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network functions. The tasks a network has to perform in order to successfully transmit data. These include
congestion control, data compression, dialog management, encryption, error control, flow control,
internetworking, routing, switching, multiplexing etc.
network interface. A network component used for attaching a communicating device to a transmission circuit and
performing additional functions such as converting between an analog and a digital signal. Examples
include modems, analog-to-digital (A/D) and digital-to-analog (D/A) converters.
network layer. Layer 3 in the open systems interconnection (OSI) reference model. Its main task is to determine
how data is routed through the network from source to destination (routing.)
network protocol. See protocol.
networking software. Plays an important role for managing the communication between the various hardware
devices. See network components.
network topologies. Network topology refers to the physical layout of end nodes, switches and transmission
circuits. One distinguishes broadcast topologies and point-to-point topologies.
network types. Networks are commonly classified into different types. Based on ownership, one distinguishes
private or public networks, and according to their geographical scope local area networks (LANs),
metropolitan area networks (MANs), and wide area networks (WANs).
network uses. Networks are used for data, program and resource sharing, or as a communication medium. Many
computer network applications, e.g., electronic commerce, combine several network uses.
node. Can be an end node, where transmissions originate or terminate, or a switch, an intermediate node. Also
called a communicating device. See network components.
open systems interconnection (OSI) reference model. A reference model for network architectures that divides
the communication process into 7 layers, the physical, data link, network, transport, session,
presentation, and application layer.
packet. In packet switching, the variable-length sequence of bits grouped together at the source node's network
layer. It contains user data and control information. See also cell, frame.
packet reassembly. The process of assembling the received data packets into the original message intended for the
recipient.
packet switching. A switching technique, prevalent in computer networks, in which data is divided into variable-
length packets that are individually transmitted. Bandwidth is only reserved when needed for a packet
transmission. Packets are routed from source to destination using either connection-oriented or
connectionless service. Cell switching service is similar to packet switching.
packetization. The process of grouping bits into packets at the source node.
peer processes. In a network architecture, the hardware and software comprising parallel layers on communicating
computers.
peer to peer protocol. See protocol.
permanent virtual circuit. See private virtual circuit.
physical layer. Layer 1 in the open systems interconnection (OSI) reference model. It is responsible for
transmitting a stream of "0" and "1" bits over the physical transmission medium.
physical transmission medium. The medium on which a transmission circuit is implemented. It can be wire-
based, as in the twisted pair, coaxial cable and fiber optic cable, or wireless such as air as in terrestrial
line-of-sight transmission or free space and air when using communication satellites. Also called
transmission medium.
point-to-point link. A link connecting a pair of communicating devices.
point-to-point network. A network based on point-to-point links, i.e., using a point-to-point topology.
point-to-point topologies. In a point-to-point topology, switches and point-to-point links are used for physically
interconnecting end nodes. Connections between end nodes are established using circuit switching or
packet switching. Examples include the fully connected, ring, mesh, and star topologies.
presentation layer. Layer 6 in the open systems interconnection (OSI) reference model. It provides a common
representation of data formats across the network.
primary rate interface. An integrated services digital network (ISDN) service with a total bandwidth of 1.488
Mbps (Megabits per second.)
private circuit. See leased circuit.
private virtual circuit. A virtual circuit defined in advance to be repeatedly used by many transmissions between
the same end nodes. Also called permanent virtual circuit.
private network. A network built by an organization for its exclusive use.

19. A Note on Computer Network Technology
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program sharing. One of the network uses in which programs stored on a central server can simultaneously be
accessed and loaded for execution on several local computers.
protocol. A set of rules for the conversation between peer processes comprising a layer in a network architecture.
Also called peer to peer protocol. For example, protocols have been defined for each of the 7 layers of the
OSI Reference Model. See network components.
public network. A network built and operated by a network provider for the specific purpose of providing services
to customer organizations and individuals.
quality of service. The network attributes that characterize its performance in transmitting data, such as the
minimum bandwidth and the maximum transmission delay.
real-time application. A network application that has strict minimum bandwidth and maximum delay
requirements, which, if not met, can lead to the received data becoming incomprehensible.
remote login. A form of resource sharing, in which a user on a local computer (terminal) accesses application
programs on a remote computer. In contrast to program sharing, the programs are executed on the remote
computer.
reserved bandwidth service with guaranteed maximum delay. A network service admitting a transmission
request to the network only after checking that its bandwidth and maximum delay requirements can be met.
resource sharing. A category of network uses including access to remote computing resources (e.g., remote login),
to remote specialized equipment such as medical imaging instruments, and distributed cooperative
computing, where the processing power and memory of multiple computers are joined to solve a problem.
ring topology. A form of broadcast topology used in token ring local area networks (LANs). Less frequently used
as a point-to-point topology.
router. A type of switch used for wide area network (WAN) interconnection for networks using the same network
layer protocols.
routing. The mechanisms for determining the path of packets or cells through the network from source to
destination. See network functions.
session layer. Layer 5 in the open systems interconnection (OSI) reference model. It establishes, manages and
terminates sessions between communicating applications.
signal. A variation of a physical quantity, such as electric current or a light wave, used to represent data.
star topology. A point-to-point topology in which circuits emanating from each end node pass through a central
switch.
statistical time division multiplexing (TDM). Time division multiplexing in which users are granted exclusive
access to the link's entire bandwidth based on their individual transmission demands and fairness
considerations. Also called asynchronous time division multiplexing (TDM) or asynchronous transfer
mode (ATM.)
switch. A communicating device used to connect two or more circuits. Can be a bridge, a router, or a gateway.
switched multimegabit data service (SMDS). A wide area network service based on connectionless cell
switching.
switched virtual circuit. A virtual circuit defined on demand for a specific connection.
switching. A technique used for getting data from a sending node to the indented receiver in point-to-point
networks. There are two basic techniques, circuit switching and packet switching. See network functions.
synchronous optical network (SONET). A family of digital circuits and formats for high speed transmission over
fiber optic networks, providing bandwidths in increments of 51.48 Mbps (Megabits per second) up to 2.5
Gbps (Gigabits per second.) See wide area network services.
synchronous time division multiplexing (TDM). See fixed time division multiplexing (TDM).
synchronous transfer mode (STM). See fixed time division multiplexing (TDM).
T system and the 56/64 Kbps circuit. A family of digital circuits (including fractional T1, T1, and T3) in which
the combination of 56/64 Kbps (Kilobits per second) circuits by fixed time division multiplexing (TDM)
results in successively higher bandwidth lines ranging from 128 Kbps to 44.736 Mbps (Megabits per
second.) See wide area network services.
terrestrial line-of-sight transmission. Wireless transmission through the air between ground station antennas.
time division multiplexing (TDM). A multiplexing technique in which users are granted exclusive access to the
link's entire bandwidth for a limited time.
token ring. A widespread local area network (LAN) protocol, based on a ring topology, in which a node can only
send if it holds the "token", a special message that is passed around the ring.

20. A Note on Computer Network Technology
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transmission circuit. A network component used to link communicating devices for exchanging data. Also called
circuit, channel, line, or link.
transmission control protocol (TCP). The transport layer protocol used in the Internet.
transmission delay. The amount of time elapsed between the time data is transmitted by the sender and the time it
is delivered to the receiver.
transmission medium. See physical transmission medium.
transport layer. Layer 4 in the open systems interconnection (OSI) reference model. It guarantees reliable data
transport from source to destination node.
twisted pair. The copper cable used for connecting a telephone to the telephone jack is a familiar example. See
transmission medium.
unspecified bit rate (UBR). A standard specifying one of the qualities of service provided by an asynchronous
transfer mode (ATM) network.
unswitched circuit. A circuit defined in advance and permanently to be used repeatedly between two end nodes. It
may be either a private (permanent) virtual circuit or a leased physical circuit.
variable bit rate (VBR). A standard specifying one of the qualities of service provided by an asynchronous
transfer mode (ATM) network.
virtual circuit. In connection-oriented packet or cell switching, a logical path defined between two end nodes
without reserving bandwidth. It can be either a switched virtual circuit or a permanent virtual circuit.
wide area network (WAN). A network spanning large areas such as countries or the entire globe. It is either a
private or a public network and typically uses a point-to-point topology.
wide area network services. The wide area data transmission services offered by network providers and
telecommunications carriers. These include the T system and 56/64 Kbps circuits, Integrated Service
Digital Network (ISDN) and Synchronous Optical Network (SONET) circuits, X.25, internet protocol
(IP), frame relay, switched multimegabit data service (SMDS) and asynchronous transfer mode (ATM).
X.25. 1) A wide area network service based on connection-oriented packet switching and widely used by many
public networks. 2) The set of protocols (specifying the physical, data link, and network layer)
implementing this service.
X.400. An application layer protocol that specifies standards for all aspects of electronic mail (E-mail) programs.

21. A Note on Computer Network Technology
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References
[1] The Computer Science and Telecommunications Board of the National Research Council, Realizing The
Information Future: The Internet and Beyond. National Academy Press, Washington, D.C., 1994.
[2] Frost & Sullivan Market Intelligence, Public Data Service Markets (U.S.)1994, New York, 1994.
[3] Hughes, D., and K. Hooshmand, ABR Stretches ATM Network Resources. Data Communications, April
1994, v24n5, p.123-128.
[4] McQuillan, J., Why Can't a WAN Be More Like a LAN?. Business Communications Review, August
1994, v24n8, p.10-12.
[5] Misra, J., and B.Belitsos, Business Telecommunications. Irwin, Homewood, Illinois, 1987.
[6] Office of Technology Assessment, Advanced Network Technology. Wasington,D.C., 1993.
[7] Rowe, S.H., Business Telecommunications. Science Research Associates, Chicago, Ill., 1988.
[8] Shenker, S., Service Models and Pricing Policies for an Integrated Services Internet. in:
Proceedings of “Public Access to the Internet”, Harvard University, 1993.
[9] Tanenbaum, A.S., Computer Networks. Prentice Hall, Englewood Cliffs, N.J., 1989.