The Internet Encyclopedia (Volume 3)

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324 STANDARDS ANDPROTOCOLS INDATACOMMUNICATIONS

Frame

Header

Datagram Datagram Data Area

Header

Frame Data Header

Figure 4: The original data are encapsulated within a

datagram enclosed within a frame.

more than one user may access the same line a variety of

multiplexing techniques are used. The following sections

discuss some common protocols associated with delivery

across nonproprietary networks.

Asynchronous Transfer Mode (ATM)

The ATM design was a response from the integrated ser-

vices digital network (ISDN) design for a broadband mode

to offer packet services to service providers across the dig-

ital subscriber networks. As such it is the delivery mech-

anism for B-ISDN services. It uses small packets or cells

of 53 bytes for delivery: 48 bytes for data, 5 bytes for the

header. This allows faster routers to be implemented but

in order to reduce overhead a virtual connection approach

is used.

The ATM reference model defined in CCITT I.321 in-

troduces the concept of planes for different types of func-

tionality, e.g., user, control, and management. Each plane

is divided into layers where higher layers follow the OSI

model but the lower layers, data link and physical, are re-

placed by the ATM adaptation layer (AAL), the ATM layer,

and the physical layer.

The physical layer converts the ATM cell stream into

bits for transfer across the chosen medium. Since ATM

was aimed at broadband delivery it is usual to operate at

155.52 Mbps or the higher 622.08-Mbps rates delivered

across fiber-optic cable.

The ATM layer provides multiplexing and demulti-

plexing of cells from different connections into a single

stream, implementation of flow control, and management

functions.

The ATM adaptation layer segments higher layer data

into ATM size cells and vice versa. Several types of AAL

have been defined to support different levels of service.

Fibre Distributed Data Interface (FDDI)

The fibre distributed data interface was designed as a very

reliable high bandwidth fiber optic LAN. It is configured as

a dual ring, thus ensuring continuity of service should one

cable fail. Based on the original token ring protocol, IEEE

802.5, it was originally developed by the American Stan-

dards Institute (ANSI) working group X3T9.5 in 1982. It

was not until 1988 that implementations first appeared.

The FDDI reference model replaces the data link and phys-

ical link layers of the ISO model and connects to the IEEE

802.2 model logical link layer (LLC). Figure 5 shows the

four layers peculiar to the FDDI, e.g.,the medium access

control (MAC), the Physical, the Physical Layer Medium

Dependent, and the Station Management layer.

To

Management

application

LLC (IEE802.2)

Medium Access Control

(MAC) layer

Physical (PHY) Layer

Physical Layer Medium

Dependent(PMD)

Station

Management

(SMT)

ISO DLL

ISO PHY

Figure 5: The FDDI reference model.

The following is a brief description of the relevant

layers:

The MAC layer controls network access and is defined

in ISO 9314-2 “Information Processing Systems: Fiber

Distributed Data Interface Part 2: Token Ring MAC.”

The physical layer determines encoding and decoding and

the clocking of the data stream to the line. Simply, it is

responsible for the moving of the actual stream of 1’s

and 0’s from host to host. It is defined by ISO 9314-1

“Information Processing Systems: Fiber Distributed

Data Interface Part 1: Token Ring Physical Layer.”

The physical layer medium dependent layer provides the

actual connection to the ring and is defined in ISO

9314-3 “Information Processing Systems: Fiber Dis-

tributed Data Interface Part 3: Token Ring Physical

layer Medium Dependent.”

The station management layer functions are necessary

for the control, supervision, and management of the

connected stations. Its purpose at the network level is

addressing and configuration.

Integrated Services Digital Network (ISDN)

The ISDN services were introduced in the 1980s to pro-

vide PSTN users with the opportunity to access services

other than telephony. Initially converting analog service

to digital it soon became apparent that a broadband ser-

vice would be required (ISDN originally provided up to

2 Mbps, or 30×64 kbps channels). This was introduced

as B-ISDN and initially used synchronous transfer mode

due to technology limitations; in 1988 ATM was chosen

as the carrier (see Asynchronous Transfer Mode (ATM)).

It is the responsibility of the ITU CCITT group XV111.

TCP/IP PROTOCOL SUITES

The TCP/IP protocol is covered in detail elsewhere within

this chapter. Also see Black (1995). Here we will briefly

look at its origins and pay particular attention to its use

and that of associated protocols on the Internet and in

e-commerce. Although now universally known as two sep-

arate entities, TCP/IP began life in 1974 when Vint Cerf

and Bob Kahn published “A Protocol for Packet Net-

work Interconnection,” which specified the design of a

transmission control program (TCP) (Cerf & Kahn, 1974).

TCP/IP were split in 1978 and recognized as sepa-

rate entities in 1982 when formally established as the

ARPANET protocol. The IP has become the de facto stan-

dard for packet-switched transport across the Internet.