Table 61 ANSIs Digital Signal Standards

Digital Signal Standard

Bandwidth

Number of Voice Channels

DS-0

64.kbps

1

DS-1

1.544 Mbps

24

DS-1C

3.152 Mbps

48

DS-2

6.312 Mbps

96

DS-3

44.736 Mbps

672

DS-4

274.176 Mbps

4,032

Note DS-0 defines the minimum amount of bandwidth required to transport digitized voice. Given that voice communication typically occurs using an 8 KHz circuit, and digitizing it requires an eight-fold increase in bandwidth, the DS-0 band was defined at 64 kbps. It is not, however, the basic building block of leased lines. That honor goes to the DS-1 specification. DS-0 describes the voice channels that are carved from the other digital signaling specifications.

ANSI's DS standards have been implemented in the telephony carrier system called the T-carrier system. There is a direct, one-to-one correlation between T-carrier circuits and their DS namesakes. For example, the DS-1 standard is embodied in the T-1 transmission facility. T-3 is the physical implementation of the DS-3. For more information on the T-carrier system, refer to the section titled "The T-Carrier System" later in this chapter.

After even a cursory glance through Table 6-1, you can probably recognize at least one of the standards. In North America and Japan, the T-1 has become so prevalent in networking that almost every network-aware technical person recognizes the 1.544 Mbps bandwidth and associates it with that transmission facility. Some people might even recognize the 44.736 Mbps of the T-3. The other standards might not be as readily recognized.

The DS-1C, from a functional perspective, is two DS-1s integrated into a common transmission facility. The DS-2 and DS-4 standards have never achieved widespread utilization. This is primarily due to their cost-versus-performance ratios. DS-3 service is also expensive, but offers a better combination of price and bandwidth than its alternatives.

Today, virtually every situation that requires bandwidth in excess of the DS-3 is satisfied with SONET technology (covered in a following section). Therefore, the T-carrier system is used only for T-1, fractional T-1, and T-3 services. The remaining T-n specifications have fallen into disuse.

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