Link Speeds Offered by Telcos

No mater what you call them—telcos, PTTs, service providers—these companies do not simply let you pick the exact speed of a WAN link. Instead, standards define how fast a point-to-point link can run.

For a long time, the telcos of the world made more money selling voice services. That is no longer the case for any of these companies in the United States, except for the companies that provide local residential telephone service. So, years ago, the telcos of the world developed a standard for sending voice using digital transmissions. Digital signaling inside their networks allowed for the growth of more profitable data services, such as leased lines. It also allowed better efficiencies, making the build-out of the expanding voice networks much less expensive.

The original standard for converting analog voice to a digital signal is called pulse code modulation (PCM). (There are alternatives, but for the exam, you should just be aware of PCM.) PCM defines that an incoming analog voice signal should be sampled 8000 times per second, and each sample should be represented by an 8-bit code. So, 64,000 bits were needed to represent 1 second of voice.

When the telcos of the world built their first digital networks, the baseline transmission speed was 64 kbps because that was the necessary bandwidth for a single voice call. The term digital signal level 0 (DS0) refers to the standard for a single 64-kbps line.

Later the telcos starting selling data services—in other words, leased lines. The phone companies could sell a DS0 service at 64 kbps. However, when it first came out, they typically offered 56-kbps service. Why? Well, it turned out that the telcos needed some bits for some management overhead. They found that if they used a bit inside the actual DS0 channel occasionally, the voice quality did not suffer, so they defined a standard in which a switch regularly could use one of every 8 bits in the DS0 channel for its own purposes. That worked fine for voice. But for data, having something else in the telco network change the bits that you sent does not work very well. At best, it can cause retransmissions; at worst, it doesn't work. So, the telco decided to just sell 7 of every 8 bits that could be sent over a DS0—and 7/8 of 64 kbps is 56 kbps. Today many telcos do not use that bit, so they can offer the full 64-kbps channel.

The telco offers specific increments of the DS0 channel. In the United States, the digital signal level 1 (DS1) standard defines a single line that supports 24 DS0s, plus an 8-kbps overhead channel, for a speed of 1.544 Mbps. (A DS1 is also called a T1 line.) It also defines a digital signal level 3 (DS3) service, also called a T3 line, which holds 28 DS1s. Other parts of the world use different standards, with Europe and Japan using standards that hold 32 DS0s; this type of line often is called an E1.

Table 4-4 lists some of the standards for WAN speeds. Included in the table are the type of line, plus the type of signaling (for example, DS1). The signaling specifications define the electrical signals that encode a binary 1 or 0 on the line. You should be aware of the general idea, and remember the key terms for T1 and E1 lines in particular, for the INTRO exam.

Table 4-4 WAN Speed Summary

Type of Line

Name of Signalling Type

Bit Rate

56

DS0*

56 kbps

64

DS0

64 kbps

T1

DS1

1.544 Mbps (24 DS0s, plus 8 kbps overhead

T3

DS3

44.736 Mbps (28 DS1s, plus management overhead)

E1

ZM

2.048 Mbps (32 DS0s)

E3

M3

34.064 Mbps (16 E1s, plus management overhead)

J1

Y1

2.048 Mbps (32 DS0s; Japanese standard)

*DS0, with 1 robbed bit out of 8

*DS0, with 1 robbed bit out of 8

Later in the chapter, the text explains the Synchronous Optical Network (SONET) standards, which include yet another range of types of WAN lines and speeds.

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