The Internet Encyclopedia (Volume 3)

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784 WIDEAREA ANDMETROPOLITANAREANETWORKS

popular interfaces tend to be OC-3, OC-12, and OC-48

(Table 2), according to their application in WANs, MANs,

or LANs.

An important feature of ATM is the definition of service

categories for traffic management:

Constant Bit Rate(CBR) was designed to emulate tradi-

tional circuit-switched connections. It is characterized

by minimum and maximum cell rates specified at the

same, constant value. Typical CBR applications include

uncompressed voice and video, or television, all sensi-

tive to both delay and delay variation.

Variable Bit Ratereal-time (VBR-rt) and non-real-time

(VBR-nrt) are characterized by specified minimum and

maximum cell rates, much like frame relay. Typical ap-

plications include compressed voice or video, and mul-

timedia e-mail. VBR-rt handles applications sensitive

to delay variation, while VBR-nrt is suitable for bursty

traffic.

Unspecified Bit Rate(UBR) handles traffic on a best-effort

basis, without guaranteeing delivery or any particu-

lar rate. This is used to carry data (such as store-and-

forward e-mail) not sensitive to delay. In a highly con-

gested network situation, UBR cells may be discarded

so that the network can meet its traffic contracts for

the other types.

Available Bit Rate(ABR) is characterized by a guaranteed

minimum cell rate, but may offer additional bandwidth

when network resources are available. Rate-based flow

control provides the adjustment mechanism. When it

is offered, ABR is often preferred for data traffic.

ATM’s service categories are crucial to meeting user de-

mands forquality of service(QoS), which generally means

guaranteed, timely delivery of traffic to match the needs

of particular applications. An ATM end system will re-

quest a particular level of service for traffic entering the

network, forming a traffic contract with the network. The

ATM switches throughout the network are responsible for

meeting the terms of the contract by traffic shaping (using

queues to smooth out traffic flow) and by traffic policing

to enforce the limits of the contract. The capabilities of

ATM to provide QoS end to end across a network for mul-

tiple types of traffic simultaneously are the most sophis-

ticated to date, and distinguish ATM from other packet-

switching technologies. Its suitability for LAN, MAN, and

WAN applications makes ATM especially popular with

service providers, because they can use one technology

throughout to manage their own infrastructure and to

support a large variety of service offerings to their cus-

tomers.

Fiber Distributed Data Interface (FDDI)

Fiber distributed data interface (FDDI) was developed

by the American National Standards Institute (ANSI) in

the mid-1980s as a 100-Mbps standard for ring-based

networks that had outgrown their capacity to handle

high-speed workstations or provide nonblocking back-

bone connections. It was designed originally to expand

the typical LAN environment, using a timed token access

method for sharing bandwidth at OSI Layer 2 and read-

start of data flow

end of data flow

(head of

Bus A)

(head of

Bus B Bus B)

Bus A

node

Figure 10: DQDB architecture (ANSI/IEEE Std 802.6, 1994

edition).

ily available optical components and fibers for the Phys-

ical Layer. Ring management functions are distributed,

with single- and dual-ring topologies supported, to create

a highly reliable network with deterministic, predictable

performance. FDDI was the first LAN technology suitable

for distances beyond a building or small campus, and was

used by some to cover the geographic scope of a MAN.

Distributed Queue Dual Bus (DQDB)

Distributed queue dual bus (DQDB) was also developed

during the 1980s, specifically to address the needs of

metropolitan area networking for integrated services such

as voice, data, and video. The IEEE 802.6 working group

finally ratified it as a Layer-2 standard in 1990. As its

name suggests, DQDB specifies a network topology of two

unidirectional buses that are able to interconnect multi-

ple nodes (Figure 10). The supporting physical layer for

DQDB initially offered various transmission interfaces

and speeds from DS3 (45 Mbps) to STM-1 (155 Mbps).

The idea of DQDB was that multiple subnetworks could be

interconnected to form a MAN, with the goal of support-

ing connectionless and connection-oriented data trans-

fers, along with isochronous traffic, sharing the total com-

munication capacity available.

DQDB may be most familiar as the basis for defini-

tion ofswitched multimegabit data service(SMDS) packet-

switched public data networks. SMDS was designed by

Bell Communications Research (Bellcore) for high-speed,

connectionless delivery of data beyond the LAN. Its vari-

able frame size up to 9188 octets is large enough to en-

compass as payload any of the popular LAN technology

frames (i.e., Ethernet, token ring, and FDDI). The SMDS

interface protocol was defined as a three-level protocol

that specifies how subscribers access the network. As a

service, SMDS was intended to be independent from any

underlying transport technology. Thus it was first offered

at DS1 to DS3 access speeds, with a goal of increasing

later to OC-3.

Ethernet

Ethernet became the dominant LAN technology in the lat-

ter 1990s, as extensions from the original 10 Mbps were

defined for 100 Mbps, then 1,000 Mbps(=1Gbps), and be-

came widely deployed. In the same time period, new com-

munication companies with no telephony heritage began

laying optical fiber, and leasing capacity for short-haul

(i.e., MAN) or long-haul (i.e., WAN) connections rather