The Internet Encyclopedia (Volume 3)

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778 WIDEAREA ANDMETROPOLITANAREANETWORKS

must provide so that traffic delivery meets user expecta-

tions and application requirements. Convergence is cer-

tainly not new, because in early WANs, digital data were

transformed into analog signals and carried over public

networks that had been designed for voice. Today conver-

gence is available through many more options for what

traffic to combine and how to do it.

FACILITIES AND INFRASTRUCTURE

Digital Transmission

The heritage of digital WANs dates from the early 1960s,

when the Bell System first introduced the T-carrier sys-

tem of physical components to support transport of digi-

tal signals in the United States. The accompanying time-

division multiplexed (TDM) digital signal scheme, called a

digital hierarchy, was based on a standard 64-kilobits per

second (Kbps) signal designed to carry one analog voice

signal transformed by pulse-code modulation (PCM) into

digital form. This basic unit is known asDS0. The Inter-

national Telecommunication Union (ITU) now supports

an entire set of digital signaling standards (Table 1), in-

corporating elements from the North American (United

States/Canada), European, and Japanese standard hierar-

chies.

The traditional U.S. multiplexing hierarchy began with

combining 24 DS0-level signals into one DS1. It is com-

monly called aT1stream, and consists of a sequence of 24

channels combined to create one frame. Each channel is

filled with 8 bits (an octet or byte) representing one PCM

sample. A particular challenge of the time was to ensure

synchronization between transmitter and receiver, which

can be accomplished in several ways. For example, each

frame could be introduced by a unique starting sequence

of 12 bits to allow receiver synchronization to be renewed

on a frame by frame basis. The U.S. designers decided in-

stead to distribute the 12 bits over 12 frames, reducing

transmission overhead at the expense of receiver com-

plexity. The 12-frame sequence was called a superframe.

With improved hardware, synchronization is more easily

maintained over longer periods, and an extended super-

frame (ESF) has replaced the superframe. ESF comprises

24 frames but only needs 6 bits for synchronization, free-

Table 1Digital Signal Hierarchy

Capacity Number

Designation (Mbps) of DS0s

DS1 1.544 24

E1 2.048 32

DS2 6.312 96

E2 8.448 128

J3 32.064

E3 34.368 512

DS3 44.736 672

J4 97.728

E4 139.264 2,048

DS4 274.176 4,032

DS, North America; E, Europe; J, Japan.

ing up 4 Kbps that have been used to improve manage-

ment and control.

In the European scheme (also used by other countries

such as Mexico), the basicE1stream aggregates 32 PCM

channels. Rather than adding synchronization bits, E1

dedicates the first PCM channel for synchronization and

the 17th for management and control signaling.

Optical Fiber Systems

Service providers first used digital multiplexing within

their own networks (e.g., trunking between Central Of-

fices), to improve the return on and extend the life of their

copper cable infrastructure investments. By the 1980s,

however, interest had shifted to fiber optics for longer dis-

tance, higher speed communications. Standards were de-

fined for the Synchronous Optical Network (SONET in the

United States, equivalent to the Synchronous Digital Hier-

archy, SDH, in Europe and elsewhere) to carry TDM traffic

cost-effectively and reliably over metropolitan and wide

area distances. Today SONET specifies both a standard

optical interface signal and a digital signaling hierarchy

tailored to the fiber transmission environment. The hier-

archy is based on an 810-octet frame transmitted every

125 microseconds (μs) to create synchronous transport

signal-level 1 (STS-1) for electrical signals. Each octet is

equivalent to a 64-Kbps PCM channel. For fiber transmis-

sion, the STS-1 equivalent is optical carrier-level 1 (OC-1).

Higher level signals are formed from specific multiples

of OC-1 (Table 2). Each SONET frame is structured into

transport overhead and a synchronous payload envelope

(SPE), which consists of both path overhead and payload.

It is only the payload portion that carries subscriber traffic

to be routed and delivered through the SONET network.

The major building blocks for SONET networks are the

point-to-point multiplexer, and for point-to-multipoint

configurations, the add-drop multiplexer (ADM). In par-

ticular, the ADM allows traffic to be dropped off and the

resultant free capacity to be reused to carry traffic enter-

ing the network at that point. SONET ADMs can also be

employed to create highly survivable networks that max-

imize availability using diverse routing and self-healing,

survivable ring structures. Figure 3a shows a dual-ring

structure where the network accommodates loss of a link

Table 2Basic SONET Levels

Designation Line rate SDH equivalent

OC-1 51.840 Mbps

OC-3 155.250 Mbps STM-1

OC-9 466.560 Mbps STM-3

OC-12 622.080 Mbps STM-4

OC-18 933.120 Mbps STM-6

OC-24 1.24416 Gbps STM-8

OC-36 1.86624 Gbps STM-12

OC-48 2.48832 Gbps STM-16

OC-96 4.97664 Gbps STM-32

OC-192 9.95328 Gbps STM-64

1 Gbps = 1,000 Mbps; 1 Mbps = 10^6 bits per second.