The Internet Encyclopedia (Volume 3)

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SWITCHING,ROUTING,ANDSIGNALING 783

FR network

DTE

DTE

DCE

PSE

Figure 8: Frame relay network elements.

and terminals connected to a frame relay network are all

considered to be DTE. The DCE is usually built as an in-

terface into the service provider’s packet-switching equip-

ment (PSE) rather than just being a modem at the edge

of an X.25 network. Frame relay also uses virtual circuits

to create a bidirectional communication path between a

pair of DTE devices. FR virtual circuits are distinguished

by data link connection identifiers (DLCIs), which may

have local significance only, meaning that each end of a

single virtual circuit could have a different DLCI assigned

by the FR service provider.

The format for frame relay data combines LAPB’s ad-

dress and control fields into one 16-bit address field that

contains the 10-bit DLCI, an extended addressing indica-

tor bit (for future use), a command/response bit that is

not used, and congestion control information. To mini-

mize network overhead, the congestion control mecha-

nisms are quite simple:

one forward-explicit congestion notification (FECN) bit

that tells a DTE that congestion occurred along the

path in the directionfromthe sourcetothe destina-

tion;

one backward-explicit congestion notification (BECN) bit

that tells a DTE that congestion occurred along the

path in the directionoppositeto the transmission from

the source to the destination; and

one discard-eligibility (DE) bit to indicate whether this is

a lower priority frame that may be discarded before

others in a congested situation.

As a packet-switching technology, frame relay also de-

pends on the bursty nature of data traffic to make effi-

cient use of its transmission facilities for larger numbers

of subscribers than could be served with physically ded-

icated connections. The ability to overbook resources is

fundamental to the service provider’s business model, as

well as being a benefit to subscribers, who may be able to

insert traffic occasionally at a higher rate than nominal

for their access link (called bursting).

Integrated Services Digital Network (ISDN)

Integrated services digital network (ISDN) is a set of

telecommunication standards first developed from the

perspective of telephony networks to accommodate multi-

ple types of traffic such as voice, fax, data, alarm systems,

and video, all in digital format, over a single network. The

goal was to develop standard interfaces, both for access

and within the network, that would allow all types of dig-

ital traffic to be transported end to end, reliably, and in

a timely fashion according to the needs of its applica-

tion. The best-known elements of ISDN are the user in-

terface definitions for connecting subscriber equipment

to the network: the primary rate interface (PRI), intended

to replace T1 and E1 services, and the basic rate interface

(BRI), designed with multiple channels for voice or data

traffic from an individual subscriber.

Asynchronous Transfer Mode (ATM)

Asynchronous transfer mode (ATM) was selected as the

OSI Layer-2 transport technology for broadband ISDN

(B-ISDN) in 1988. It was designed to be useful across

WAN, MAN, and LAN communications, as well as to ac-

commodate multiple types of traffic in a single network

(voice, data, video, etc.) and scale for very large networks.

Other design goals included the abilities to support a

variety of media types (e.g., fiber and copper), leverage

signaling standards already developed for other tech-

nologies, promote low-cost switching implementations

(potentially one-tenth the cost of routing), adapt readily to

future network requirements, and enable new, large-scale

applications. The challenges inherent in such a diverse set

of goals brought together designers from many different

backgrounds, and resulted in a rather complex architec-

ture (Figure 9).

Basically, ATM is a connection-oriented, packet-

switching technology that uses fixed-length packets called

cells. The 53-byte cell size (5 bytes of header information

and 48 bytes for the payload) was chosen as a compromise

between the optimal size for voice traffic and the larger

size preferred for data applications. The fixed size and

format mean that very fast switches can be built across

a broad range of transmission rates, from megabits to

gigabits per second and beyond. ATM interfaces are of-

ten characterized by their equivalent optical-carrier lev-

els whether they employ fiber or copper media. The most

Application

Presentation

Session

Transport

Network

Data Link

Physical

OSI Reference

Model

ATM Adaptation Layer

ATM Layer

Physical Layer

Higher Layers Higher Layers

Management Plane

Control Plane Control Plane

Figure 9: ATM reference model.