The Internet Encyclopedia (Volume 3)

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782 WIDEAREA ANDMETROPOLITANAREANETWORKS

Some form of packet switching is employed in most

core data networks today to move traffic through the net-

work. Various techniques are used to meet customer ex-

pectations for reliable, timely, and effective delivery of

traffic to its intended destination. For example, avirtual

circuitcan be established to approximate the service char-

acteristics available in a circuit-switching environment,

such as guaranteed delivery of packets in the same order

as they were transmitted. However, virtual circuits do not

dedicate resources along the path from source to desti-

nation, so the network must have sufficient intelligence

to keep traffic moving well enough to meet subscriber

expectations.

Choosing the best place to put network intelligence

(at the edge or in the core) has been a subject of ongo-

ing discussion among service providers for many years.

For example, packets could be examined and labeled at

the edge in a way that forwarding decisions in the core

are made by simple, high-speed switches. This approach

would provide very fast core transit, but the cost of many

intelligent edge devices could be high and core switches

must still be smart enough to accommodate and adapt to

changes in network topology or conditions. An alternative

approach makes the edge devices quite simple and inex-

pensive, while requiring the core to have the intelligence

and take the time to understand the characteristics and

accommodate the transport needs of the traffic.

Switching Technologies

In the OSI Reference Model, switching takes place at

Layer 2, the Data Link Layer. However, much of the WAN

switching technology for data networking was developed

from experience with X.25, an ITU-T packet-switching

protocol standard developed in the 1970s to support pub-

lic data networking, and still in use today. X.25 creates

a connection-oriented network out of packet-switching

resources by employing virtual circuits to handle packet

flow, keeping the data link layer simpler but requiring cir-

cuits to be established before packets can be sent. Circuits

that are prebuilt from a source to a particular destina-

tion and then left in place arepermanentvirtual circuits

(PVCs), whileswitchedvirtual circuits (SVCs) are estab-

lished only on demand. SVCs are like dial-up connections,

requiring circuit establishment to the specified destina-

tion for each call before traffic can flow.

X.25

X.25 is a three-layer protocol suite (Figure 6). The OSI net-

work layer equivalent is the packet-layer protocol (PLP),

which has operational modes for call establishment, data

transfer, and call termination, plus idle and restarting op-

erations. These functions are implemented through the

Flag

(frame

delimiter)

Address

(command

or response

indicator)

Control

(frame type,

sequence #,

function)

DATA

FCS

(frame check

sequence)

Flag

(frame

delimiter)

1 byte 1 byte 1 byte variable 2 bytes 1 byte

Figure 7: LAPB frame format.

Application

Presentation

Session

Transport

Network

Data Link

Physical

OSI

Reference

Model

PLP

LAPB

X.21bis,

EIA/TIA-232,

EIA/TIA-449,

EIA-530,

G.703

X.25

Figure 6: X.25 protocol suite.

services of a data link protocol called the Link Access Pro-

cedure, Balanced (LAPB), which is responsible for fram-

ing data and control commands and for basic error check-

ing through use of a frame-check sequence (Figure 7).

During call establishment, the PLP sets up SVCs using

X.121 standard addresses. These include the international

data number (IDN), made up of a four-digit data network

identification code (DNIC, to specify the packet-switching

network containing the destination device) and a national

terminal number (NTN) consisting of as many as 10 digits.

The NTN specifies the exact destination device to which

packets will be forwarded.

Frame Relay

Frame relay is the most widely used packet-switching

WAN technology going into the 21st century. As WAN fa-

cilities became more reliable during the 1980s, interest

rose in streamlining X.25 to improve performance and ef-

ficiency. Frame relay (FR) was thus designed as a Layer-2

protocol suite, with work begun by CCITT in 1984. How-

ever, it was not until 1991, when several major telecommu-

nication equipment manufacturers formed a consortium

called the Frame Relay Forum (FRF) to work out interop-

erability issues and foster acceptance, that frame relay be-

gan to be more widely deployed. In particular, FRF defined

extensions to the CCITT work called the local manage-

ment interface (LMI) to improve service providers’ abili-

ties to provision and manage frame relay services.

Frame relay networks (Figure 8) are based on the con-

cepts of data-terminal equipment (DTE) and data circuit-

terminating equipment (DCE) first defined by X.25. Sub-

scriber hosts, servers, workstations, personal computers,