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Graduate School of Business S-OIT-14STANFORD UNIVERSITY 1/96

A Note on Computer Network Technology

The diversity of computer network technologies, standards and products is bewildering. In light of thegrowing 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 abreastof 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 functionsperformed by a network and how they are managed through the network architecture. Finally, the servicerequirements of network applications and currently available network services are surveyed.

1. Network Uses and ApplicationsIn 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 computernetwork 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, imagesor video. Examples of applications where the key function of the network is data sharing include corporatedata warehouses1, electronic library catalogs, airline reservation systems, home banking and automated tellermachine networks.

• In program sharing, programs stored on a central server can simultaneously be accessed and loaded forexecution on several local computers. Many application programs in the GSB's MBA lab, including theMicrosoft 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 login2 to asupercomputer to run large-scale weather simulations) and high-speed laser printers, to remote specializedequipment such as medical imaging instruments in teleradiology and telepathology, and distributedcooperative computing, where the processing power and memory of multiple computers are joined to solve aproblem.

• Uses of networks as a communication medium include electronic mail (E-mail), video conferencing, electronicbulletin boards and groupware for distributed collaborative work3.

Many computer network applications combine several network uses. A familiar example is officeautomation, in which an office network is used for sharing software programs and laser printers, for accessingcentrally stored company data, and as a communication medium for corporate electronic mail. In electroniccommerce, businesses use networks to communicate with their suppliers, partners and customers, exchange productinformation, place and check the status of orders, manage inventory levels, make payments, provide product supportetc. In factory automation, manufacturing tools are electronically controlled by programs and data accessed over anetwork. A general trend across most computer network applications is the increased use of multimedia documentscombining text, data, voice, images, and video.

2. Network ComponentsComputer 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, StanfordUniversity. Comments by Charles Bonini and Anne Korin, as well as partial financial support by the Stanford ComputerIndustry 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 toprogram 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 thecomputer, ranging from simple electronic mail to more sophisticated products such as Lotus Notes.


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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 andautomated 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 maximumnumber of bits2 transmitted per second. Current circuits (discussed in section 8.1) have data rates on the order of athousand, 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 thecommunicating devices and the transmission circuits.

Networking software plays an important role in managing the communication between the various hardwaredevices. Conventions for the many tasks involved in a successful transmission are structured and defined in networkprotocols, implemented in hardware and software.

3. Network TopologiesNetwork 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 forinterconnecting end nodes in point-to-point networks and transmissions are addressed to specific end nodes. Severalalternative 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 (seesection 4). Satellite or radio networks are based on wireless technology. Data transmitted therefore propagates to anynode within the reach of the sender. The ring topology is frequently used in conjunction with the token ring LANprotocol, in which a node can only send if it holds the "token", a special message that is passed around the ring.

4. Network TypesNetworks 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 AreaNetworks (WANs).

A private network is built by an organization for its exclusive use, while a public network is establishedand operated by a network provider for the specific purpose of providing services to customer organizations andindividuals.

LANs link up to thousands of computers located in the same or in adjacent buildings on a campus and aretypically private. They have a bandwidth of up to over 100 Mbps and a maximum range of a few miles. A familiarexample is the GSB network which connects the personal computers in the MBA, PhD and Sloan computer labswith 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.


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extend over greater distances and support more users. The GSB LAN is linked to other Stanford LANs to form alarge 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 areslower than LANs, although this difference is diminishing due to technological advances. Currently, long distancelinks 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, publicWANs connect up to several million computers. Most large corporations such as IBM, Digital EquipmentCorporation and General Motors have worldwide private WANs to interconnect their geographically dispersedlocations.

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 starnetwork (each pair of end nodes is connected via a central switch) represent the two extremes. Reducing the number oflinks used to interconnect a given number of end nodes reduces the cost, but makes the network more vulnerable to switchfailures 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 areoften internetworks - networks of networks - interconnecting hundreds if not thousands of LANs, MANs, andWANs, part private, part public, based on different topologies and composed of a wide variety of components. Theconnection between these networks is established using bridges, routers, and gateways. Large WANs linking LANsand MANs in internetworks are usually referred to as backbone networks. The best known internetwork is theInternet (see the related note “A Note on the Internet”) It is defined as the network of interconnected andinteroperating networks using the TCP/IP (Transmission Control Protocol / Internet Protocol) protocols and acommon set of network addresses (see section 6.3.)

5. Key Network Technology ConceptsA 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 onvarious physical transmission media. Circuits and media are shared among concurrent transmissions using timedivision multiplexing or frequency division multiplexing. Circuit switching or packet switching are used for getting


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data from a sending node to the intended receiving node. These and other elements of a network’s functionality areorganized 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 andtransmitted over circuits using two forms of signals. A signal is the variation of a physical quantity, such as electriccurrent or a light wave. One can differentiate between analog and digital signals (see Figure 3). For example, thesound waves generated by speech are converted by the telephone microphone into an analog electrical signal. Ananalog 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 ofbinary digits, "0" and "1" bits. A digital signal varies discontinuously in time and can take on only a finitenumber 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. Forexample, 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 nodeto 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 analogcircuits, as still prevalent in local telephone networks, a modem (modulator/demodulator) is used as a networkinterface between the computer and the transmission circuit to convert the digital computer data into an analogtransmission 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 isused 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, digitalnetworks are quickly replacing analog ones. One of the main advantages of digital compared to analogcommunication 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 ratesthan analog ones. Further, digital transmission allows the integration of different types of traffic such as voice, cabletelevision, video, and computer data on a single network, enabling significant cost reductions.

5.2 Transmission MediaTransmission 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 twoinsulated copper wires twisted together in a helical form. This is the oldest and most widely availablemedium. The twisted pair is prevalent in buildings and for local telephone network links (the local loop), and


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is inexpensive to install. However, it can only be used for short links because of significant electricalinterference (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 acylindrical 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 moredata than twisted pair due to lower electrical interference but is more expensive. Due to rapid signaldegradation, 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 areprogressively 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, whichleads 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 opticalsignals.

• In terrestrial line-of-sight transmission, data is transmitted through the air between ground station antennasusing lasers and infrared signals for LANs and microwave radio transmission over longer distances. This savesthe cost of building physical links, especially to remote areas, and provides mobility. However, microwavetransmission 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 groundstation to another, thus covering an area hundreds of miles in diameter. Therefore, satellites are excellent forcost-efficient communication over large areas with little infrastructure. They are used for television networksand by private networks to bypass the telephone system. As a drawback, transmission via satellite is subject toa 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 representdata by signals, the medium's sensitivity to electric interference, and the signal degradation properties. Table 1summarizes the bandwidths and ranges for these media.

Transmission Medium Order of magnitude ofmaximum bandwidth [bps]

Order of magnitude of maximumrange between ground stations [m]

Twisted pair 106 103

Coaxial cable 108 106

Fiber >109 106

Terrestrial line-of sight 108 106

Communication Satellites 108 108

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 numeroustransmissions in process at any given time. There are two basic multiplexing techniques, frequency divisionmultiplexing (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 havingexclusive access to a frequency band. Fixed TDM (sometimes also referred to as synchronous TDM or SynchronousTransfer Mode - STM) grants each user exclusive access to the entire link bandwidth for a certain time slot in a fixedsequence. 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 changingtransmission demands, they don't utilize the available bandwidth efficiently. Asynchronous (or statistical) TDMmitigates this problem: If the user whose turn it is has no data to transmit, the medium is made available to otherusers who need to transmit. Asynchronous Transfer Mode (ATM), an emerging network service discussed insection 8.2, is based on asynchronous TDM.

5.4 Circuit Switching and Packet Switching


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In networks that are based on unswitched circuits the connections between pairs of communicating endnodes are defined in advance and permanently for repeated use. This is done either using virtual circuits, logicalpaths 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 switchingtechniques, circuit switching and packet switching, are used for getting data from a sending node to the intendedreceiver.

In circuit switching (see Figure 4), used for voice communication on the telephone network, a physicalcircuit through the network is set up before any data is sent. Bandwidth is reserved for the entire duration of theconnection and all data travels along this physical circuit. If the transmission is bursty, the reserved bandwidth isused very inefficiently. On the other hand, because circuit switching dedicates bandwidth to an application, it canguarantee a specified quality of service1, in particular a minimal bandwidth and a maximum (transmission) delay2

Connection requests whose desired quality of service cannot be accommodated are simply blocked (similar to a busysignal 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 anyuser for the entire duration of a connection. Rather, the data is divided into variable-length packets (so-calledpacketization) at the source node and reassembled at the destination. Packets contain in addition to the dataexchanged by users information necessary for delivering packets to the right address, checking and correctingtransmission errors, and so forth. Data is transmitted in packets, and bandwidth is only reserved when needed for apacket transmission. Cell switching, a recently introduced switching technology, closely resembles packetswitching, with the main difference that data is divided into fixed-length units called cells (see section 8.2). Theseare entirely processed in hardware, resulting in very high bandwidth network services such as AsynchronousTransfer 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


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A

B C

D

EF

Sourceof 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

31

1

Figure 5 : Connection-oriented packet switching based on virtual circuits. Three virtual circuits are currently active alongthe 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 sharedby the virtual circuits 1 and 2, and 1 and 3, respectively. Virtual circuits only use up bandwidth when a packet is actuallytransmitted.

Depending on the method for delivering these packets or cells from source to destination node, one candistinguish between connection-oriented and connectionless service.

Connection-oriented service (see Figure 5), used for example in X.25 networks (see section 8.2), is modeledafter the telephone system: the service user first establishes, then uses, and finally terminates an end-to-endconnection. The end-to-end connections are implemented as switched virtual circuits, which are only defined ondemand 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 virtualcircuit 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 usingdatagrams (see Figure 6), packets that carry the full source and destination address and are routed through thesystem independently of all the others. Packets may arrive out of sequence at the destination in which case theyhave to be reordered.

The key difference between circuit switching and packet switching lies in the way bandwidth is allocated toconnections. Packet switching utilizes the available bandwidth more efficiently. However, it suffers from thepotential for large delays and lost packets if the network is congested, and requires complex processing for therouting, packetization and packet reassembly. Note that fixed TDM is best suited to support circuit switching whileasynchronous TDM is natural for packet switching.


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A

B C

D

EF

idle circuit

shared physical circuit

data packet

A,D

F,C

A,D

A,DF,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, andF to C. But in contrast to connection-oriented service, the datagrams belonging to the same source destination pair arerouted independently, using the address information they carry (represented by the letters written in the data packetsymbol.) 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 othertransmissions.

6. Network Functions and Architecture

6.1 Network FunctionsThe 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 andswitching, error control, flow control, routing, congestion control, internetworking, dialog management, commonnetwork representation of data formats, and the provision of application-specific services.

6.2 Network ArchitecturesThe 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 notencompass implementation details. Different architectures share the following features:

• They conceptualize the communication process as a hierarchy of layers, each performing a set of well definedfunctions. Peer processes, the software and hardware entities comprising parallel layers on different nodes, carryon 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 interfacesto successively lower layers on the source computer, then transmitted over the physical medium, and finallypassed up on the destination machine layer by layer.

6.3 The OSI Reference ModelThe 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 dividesthe communication process between two application programs into 7 layers. The ISO and other standardsorganizations 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 adjacentnodes connected by a circuit. In contrast, the higher transport, session, presentation, and application layers areend-to-end layers as they contain additional functions required to ensure successful communication from end node toend 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 thephysical transmission medium. The issues addressed include transmission medium and connector specifications,


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the representation of bits by analog or digital signals, and multiplexing and switching (see section 5.) However, thephysical layer does not guarantee the reliable transmission and is not concerned with the meaning of bits. It thusonly provides a raw transmission facility.

Layer 2: The Data Link Layer adds to this raw transmission facility physical reliability to provide anerror-free link to the network layer. This requires functions such as grouping bits into data frames1, detecting andcorrecting transmission errors (error control), keeping a fast transmitting node from drowning a slower receivingnode 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 networkfrom source to destination (routing). This includes assigning network addresses to all the network nodes, providingconnection-oriented or connectionless service (discussed in section 5.4) to the transport layer (layer 4), managing thedata traffic flows to avoid excessive network congestion (congestion control), and interconnecting two or morenetworks (internetworking) using bridges, routers, or gateways (a bridge is typically used to connect two LANs atthe data link layer, routers and gateways are used for WAN interconnection, whereby routers connect networks whichuse the same and gateways those using different network layer protocols.) As the routing is very simple in broadcastnetworks, their network layer is often thin or nonexistent. The network layer makes the upper layers independent ofthe various data transmission and switching technologies used to connect systems. The Internet is based theInternet Protocol (IP), a connectionless network layer protocol (see section 8.2 and "A Note on the Internet".) TheX.25 network layer protocol2 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 datatransport from source to destination node independent of the underlying network technology. It reassembles datapackets in the right order when they are delivered out of sequence (which is common when the network layerprovides connectionless service), resends packets that were not received due to errors etc., and deals with multipledeliveries of the same packet. The transport layer also guarantees a certain quality of service in the delivery of datapackets, which may specify the maximum packet delivery delay or the minimum dedicated bandwidth for atransmission. We discuss the quality of service requirements of network applications in section 7. TheTransmission Control Protocol (TCP) is the transport layer of the Internet.

Layer 5: The Session Layer establishes, manages and terminates sessions between communicatingapplications. This includes keeping track of whose turn it is to talk and enforcing it (dialog management), and crashrecovery 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 thatapplications 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 isused to reduce the number of bits that have to be transmitted to convey a given amount of data, the latter for networksecurity 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 distributedinformation services. For instance, the X.400 application layer protocol specifies standards for all aspects ofelectronic mail (E-mail) programs, allowing users to create, edit, exchange, display and store messages. Otherapplication layer protocols include services for directory lookup (the telephone book’s electronic equivalent), fortransferring 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 datalink layer with frames, and the physical layer with bits. The data units exchanged at the upper layers have no standardname. 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.


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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 forcommunication 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 threelong-dashed arrows for the physical, data link and network protocol point to each place where peer process communicationtakes place at these layers). Layers 4-7 only perform end node-to-end node functions. Conceptually, peer processes at anylayer talk directly to each other (along the horizontal short-dashed arrows). However, data actually follows the path alongthe solid line arrows, i.e., from a given peer process on the originating node down across the layers and interfaces, throughthe physical medium to the destination node where it moves up across layers and interfaces to the corresponding peerprocess.

7. Service Requirements of Network ApplicationsNetworks 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 arekey. 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 candistinguish between elastic and real-time applications.


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Elastic applications essentially adapt to whatever bandwidth is available and the resulting transmissiondelay. Traditional network applications based on data, such as file transfers and electronic mail, belong in thiscategory. When transmissions slow down because of network congestion, they keep working, just not as quickly asusual. Depending on the desired level of interactivity, applications within this group may differ in their delaysensitivities: transactions initiated during an interactive remote login session are more delay sensitive thaninteractive bulk file transfers, which in turn are more sensitive than asynchronous (non-interactive) bulk file transferssuch as electronic mail and fax.

On the other hand, real-time applications, such as real-time voice conversations or video conferencing, havestrict bandwidth and maximum delay requirements which, if not met, can lead to distortions of the received voice orvideo 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 offerat least two basic qualities of service, best effort service and reserved bandwidth service with guaranteed maximumdelay. Best-effort service means that the network attempts to deliver packets as quickly as possible without makingguarantees about delivery or maximum delays. When the network is overloaded, delays increase and packets aredropped. A number of priority classes of best effort service can be offered to accommodate elastic applications withdifferent delay sensitivities. The Internet presently offers only one class of best effort service. Reserved bandwidthservice with guaranteed maximum delay admits a transmission request to the network only if its bandwidth andmaximum delay requirements can be guaranteed. This service type is appropriate for real-time applications.

8. Currently Available Wide Area Network ServicesBoth 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 thesites 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 relatedtechnology.

8.1 Digital Circuits56/64 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 controlinformation. The T lines currently in use are fractional T1 (fT1), T1 and T3. fT1 lines come typically with abandwidth on the order of a few hundred Kbps in increments of 128 Kbps. T1 lines, which have a bandwidth of1.544 Mbps, are made up of twenty-four 56/64 Kbps circuits, and multiplexing 29 T1 lines results in a T3 linewith 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 theintegration of voice and other data services over a single link. Two channel types, “B” and “D”, have been definedand 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 controlinformation and can also be used for transmitting data. An ISDN Basic Rate Interface thus provides a totalbandwidth of 144 Kbps over an existing local telephone line. The main advantages of Basic Rate ISDN comparedto a regular analog telephone line include its ability to provide up to 2 simultaneous voice or data conversationsover one physical line, a higher quality and reliability due to the use of digital transmission, and more bandwidthfor applications such as videoconferencing.

SONET establishes a hierarchy of transmission rates and formats to be used for very high speed digitaltransmission 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 andextends up to SONET OC-48 at 2.5 Gbps.


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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 digitalcircuits only provide the functionality of the physical layer.

8.2 Packet Switching, Frame Relay and Cell Switching ServicesThese 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 intofixed-size cells, in the way they manage connections (i.e. using virtual circuits or datagrams) and in the bandwidththey 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 thecarrier network.

X.25 is a connection-oriented packet switching service and widely used by many major public datanetworks since the 1970s. It contains extensive error correction procedures which are performed at the networkswitches and significantly slow transmission speeds. At speeds above 256 Kbps, the network overhead seriouslyaffects the data throughput. X.25 is therefore only suited for bursty data at these speeds, and is not suited for voiceand full motion video, which require low delays. Also, the X.25 bandwidth is not satisfactory for the transfer oflarge files.

Frame Relay, as currently implemented, is an unswitched service based on private virtual circuits (seesection 5.4). Data is transmitted in variable-length blocks called frames. While this service is unswitched, itresembles packet switching in that bandwidth is only tied up when data is actually transmitted. The private virtualcircuits appear like physical leased lines to the network customers, but share in effect the network provider's physicalcircuits 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 excesscapacity 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, andnetwork access is typically established using fT1 or T1 leased lines. This high data rate compared to X.25 mainlyresults from the design assumption that the network facilities are reliable and hence no extensive error control (as inX.25 networks) is performed.

IP, a connectionless packet switching service, is the network layer protocol currently used in the Internetand was introduced in the early 1980s. In principle it can be implemented on top of very fast switches and circuitsof any technology, resulting in very high bandwidths in excess of T3 rates. However, since it only provides besteffort service it is not well suited for real-time applications (see section 7.) For a detailed discussion of IP, see “ANote on the Internet.”

SMDS is a connectionless service based on cell switching. Its operation principle resembles that of aconnectionless packet switching service like IP, except that SMDS groups data into fixed-length cells, which can beprocessed more quickly by switches than the variable-length data packets used in X.25 and IP networks. The cellsare dynamically routed through the best available route. T1 or T3 leased lines are used for network access, andbandwidths up to 45 Mbps are supported.

ATM is a connection-oriented cell switching service1. Like SMDS, it uses fixed cell sizes. But incontrast to SMDS, all cells belonging to one transmission are routed through the same virtual circuit. ATM differsfrom conventional X.25 packet switching mainly in that it uses fixed length cells, highly simplified protocols andonly does error correction at the end nodes. Compared to circuit switching, connection setup delays are minimal inATM. It uses asynchronous time-division multiplexing (see section 5.3) and combines the main advantages ofcircuit switching, i.e., low transmission delays and guaranteed bandwidth, with the efficient bandwidth utilization ofpacket switching. Today, ATM is seen as the technology of choice, which will enable the development of highspeed integrated networks offering a range of services tailored to all kinds of data, voice, image and videoapplications. It will be available at speeds of 155 Mbps (SONET OC-3) up to several Gbps. While the definitionof 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.


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2 . Variable bit rate (VBR) is intended for bursty traffic, as generated by transaction processing applications orLANs, and resembles Frame Relay's CIR service: Users can sporadically send data at higher rates as long asthey 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 stationsto slow down their transmission when the network is congested, thereby preventing data loss (which can occurif data is sent into a congested network.)

PhysicalCircuits

VirtualCircuits

Datagrams

Unswitched56/64 Kbps, fT1,T1,T3,SONET(leased physical circuits,static bandwidth allocation)

Frame Relay(private virtual circuits,dynamic bandwidth allocation)

Circuit switched56/64 Kbps, fT1,T1,T3,ISDN(dial-up physical circuits,static bandwidth allocation)

Packet switchedX.25, ATM(switched virtual circuits,dynamic bandwidth allocation)

IP, SMDS(datagrams, dynamicbandwidth 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 belowsummarizes 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 ApplicationsThe 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 canonly be guaranteed by services which reserve bandwidth for the duration of the transmission. Circuit switchedservices as well as the fast-packet services ATM and SMDS are therefore best-suited, with the specific choicedepending mainly on the application’s bandwidth requirement.


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• Constant data applications (such as bulk file transfers) require higher bandwidths for an extended period of timebut don't rely on small connection setup delays. Thus, they can be suitably operated using circuit-switchedservices.

• Bursty data applications, characteristic of LAN interconnections, typically exhibit high peak load to averagedata rates. Frame Relay was specifically designed for such applications while circuit switched services are notwell-suited, since they use the reserved bandwidth very inefficiently. X.25, IP, SMDS, and ATM are alsosuited for bursty traffic with the specific choice depending on the application’s bandwidth requirement. Forexample, X.25 is not and Frame Relay only rarely sufficient to support full motion video or voice for multipleusers sharing the same link.

• ATM offers a set of specifically tailored service classes to suit all application types.


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Glossaryanalog 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 forapplication 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 switchingand statistical (asynchronous) time division multiplexing (TDM).

automated teller machine (ATM) network. An application making use of networks for data sharing, in whichspecialized computer terminals - automated teller machines - are linked to a bank's computers and databasesand 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 asynchronoustransfer mode (ATM) network.

backbone network. In internetworks, a wide area network (WAN) forming the backbone by linking together localarea networks (LAN) and metropolitan area networks (MANs).

bandwidth. The term used for the circuit capacity, expressed in terms of the maximum number of bits transmittedper 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 ormaximum 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 bustopology, 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 periodsduring 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 isvery 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 aphysical circuit through the network is reserved before any data is sent, and bandwidth is tied up for theentire duration of the connection.

coaxial cable. A transmission medium made of copper wire. The cable used for connecting a television set to thecable 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 bulletinboards, groupware, and video conferencing. See network uses.

communication satellite. Serves in a satellite network for relaying signals between ground stations using wirelesstransmission through the air and free space.


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computer network. A system of hardware and software used for data, program and resource sharing, or as acommunication medium. See network applications, network architecture, network components, networkfunctions, 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 networkcongestion.

connectionless. An approach to packet switching in which each packet is individually routed from source todestination. It is typically implemented with datagrams.

connection-oriented. An approach to packet switching in which a path is set up between communicating endnodes 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 asynchronoustransfer mode (ATM) network.

corporate data warehouse. It is an enterprise-wide database where a set of data, extracted from a wide variety ofoperational 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 toconvey a given amount of data.

data link layer. Layer 2 in the open systems interconnection (OSI) reference model. It provides physicalreliability 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, orvideo. Examples include corporate data warehouses and automated teller machine (ATM) networks. Seenetwork uses.

datagram. In connectionless packet or cell switching, a self-contained packet or cell carrying enough information tobe 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 ofwhose 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 theresulting transmission delay.

electronic commerce. A summary term for network applications used by companies to exchange businessinformation with their suppliers, partners and customers. These applications often combine several networkuses, 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. Seecommunication 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 accessto the link's entire bandwidth for a certain time slot in a fixed sequence, irrespective of their changingtransmission demands. Also called synchronous time division multiplexing (TDM) or synchronoustransfer mode (STM).

flow control. The network functions involved in keeping a fast transmitting node from drowning a slowerreceiving node in data.

frame. In the variable-length sequence of bits grouped together at the source node's data link layer. A frameincludes data contained in a packet plus data link layer control information.


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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 thelink'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 acircuit.

gateway. A type of switch used for wide area network (WAN) interconnection for networks using different networklayer protocols.

groupware. An application making use of networks as a communication medium. Groupware encompasses anytechnology that supports interpersonal collaboration through the computer, ranging from simple electronicmail (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 servicesover 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 theservices 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 networksusing 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 theInternet. 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 arange 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 circuitand 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 areanetwork (LAN) technology.

modem (modulator-demodulator). A network interface used to convert a digital signal for transmission in theform 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 frequencydivision 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. Networkapplications are motivated by one or several network uses.

network architecture. The structural organization of a modern network's functionality in a hierarchy of layers andprotocols.

network components. These include communicating devices (or nodes), which can be end nodes or switches, andtransmission circuits (also called circuits, channels, lines, or links), network interfaces, networkingsoftware, and network protocols.


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network functions. The tasks a network has to perform in order to successfully transmit data. These includecongestion 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 andperforming additional functions such as converting between an analog and a digital signal. Examplesinclude 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 determinehow 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 hardwaredevices. See network components.

network topologies. Network topology refers to the physical layout of end nodes, switches and transmissioncircuits. One distinguishes broadcast topologies and point-to-point topologies.

network types. Networks are commonly classified into different types. Based on ownership, one distinguishesprivate 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. Manycomputer 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. Alsocalled a communicating device. See network components.

open systems interconnection (OSI) reference model. A reference model for network architectures that dividesthe 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 networklayer. 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 therecipient.

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 packettransmission. Packets are routed from source to destination using either connection-oriented orconnectionless 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 fortransmitting 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 terrestrialline-of-sight transmission or free space and air when using communication satellites. Also calledtransmission 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 physicallyinterconnecting end nodes. Connections between end nodes are established using circuit switching orpacket 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 commonrepresentation of data formats across the network.

primary rate interface. An integrated services digital network (ISDN) service with a total bandwidth of 1.488Mbps (Megabits per second.)

private circuit. See leased circuit.

private virtual circuit. A virtual circuit defined in advance to be repeatedly used by many transmissions betweenthe same end nodes. Also called permanent virtual circuit.

private network. A network built by an organization for its exclusive use.


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program sharing. One of the network uses in which programs stored on a central server can simultaneously beaccessed 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 theOSI Reference Model. See network components.

public network. A network built and operated by a network provider for the specific purpose of providing servicesto customer organizations and individuals.

quality of service. The network attributes that characterize its performance in transmitting data, such as theminimum bandwidth and the maximum transmission delay.

real-time application. A network application that has strict minimum bandwidth and maximum delayrequirements, 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 applicationprograms on a remote computer. In contrast to program sharing , the programs are executed on the remotecomputer.

reserved bandwidth service with guaranteed maximum delay. A network service admitting a transmissionrequest 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 cooperativecomputing, 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 usedas a point-to-point topology.

router. A type of switch used for wide area network (WAN) interconnection for networks using the same networklayer protocols.

routing. The mechanisms for determining the path of packets or cells through the network from source todestination. See network functions.

session layer. Layer 5 in the open systems interconnection (OSI) reference model. It establishes, manages andterminates 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 exclusiveaccess to the link's entire bandwidth based on their individual transmission demands and fairnessconsiderations. Also called asynchronous time division multiplexing (TDM) or asynchronous transfermode (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 cellswitching.

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 overfiber optic networks, providing bandwidths in increments of 51.48 Mbps (Megabits per second) up to 2.5Gbps (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 whichthe 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 persecond.) 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.


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transmission circuit. A network component used to link communicating devices for exchanging data. Also calledcircuit, 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 itis 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 datatransport from source to destination node.

twisted pair. The copper cable used for connecting a telephone to the telephone jack is a familiar example. Seetransmission medium.

unspecified bit rate (UBR). A standard specifying one of the qualities of service provided by an asynchronoustransfer mode (ATM) network.

unswitched circuit. A circuit defined in advance and permanently to be used repeatedly between two end nodes. Itmay 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 asynchronoustransfer mode (ATM) network.

virtual circuit. In connection-oriented packet or cell switching, a logical path defined between two end nodeswithout 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 aprivate 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 andtelecommunications carriers. These include the T system and 56/64 Kbps circuits, Integrated ServiceDigital 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 manypublic 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.


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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.

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