Absolute Beginners Guide to Networking (4th Edition)

The T-Carrier system is a telecommunication technology that consists of high-speed digital lines made up of multiple channels. It is another one of the communication possibilities provided by the POTS. The T-Carrier system can be used for the high-speed transfer of voice, video, and data. Each channel on a line can provide 64Kbps of throughput. By combining a number of channels into one line, different elements, such as voice and data, can be moved at the same time over the same line (with each carried by a different channel).

The device that combines signals carried on these separate channels (when data must be sent over the digital line as a single data stream) is called a multiplexer or MUX . A demultiplexer is typically built into a MUX to split a received data stream into the appropriate channels. A demultiplexer actually works along the same lines as a cable-ready television or VCR. A single data signal comes into your home from the local cable television provider, and your cable-ready television or VCR contains a multiplexer that breaks the data feed down into the 200+ television channels that you are constantly surfing (no one ever said that more channels would provide better programming). That's what broadband transmission is all aboutmultiple channels on a single feed.

The number of channels on a T-Carrier line obviously defines the amount of bandwidth that the line supplies , and different T-Carrier lines are available. The T-1 line is the basic unit of the T-Carrier system. It provides 24 64Kbps channels, which can be combined to provide a total transmission bandwidth of 1.544Mbps. Several other T-Carrier lines exist that can provide a large number of channels and extremely high data rates. Table 13.2 provides a summary of the different T-Carrier lines available.

Table 13.2. The T-Carrier Lines

Carrier Line

Channels

Total Data Rate

T-1

24

1.544Mbps

T-2

96

6.312Mbps

T-3

672

44.736Mbps

T-4

4,032

274.760Mbps

The cost of leasing T-Carrier lines has actually dropped in recent times. However, for most companies, even a T-1 line can seem a bit pricey. Many companies lease one T-1 and use some of the channels for data and others for the company's phone systems. Many local phone providers also have begun offering Fractional-T connections , where a company only leases a specified number of channels on a T-1 line.

Tip

The T-1 line is the basic unit of the T-Carrier system. All the other T-Carrier lines available can actually be thought of as simply particular multiples of T-1 lines. A T-2 line, for example, consists of four T-1 lines, a T-3 line consists of 28 T-1 lines, and a T-4 line consists of 168 T-1 lines.

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