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Broadband networks are configured through the use of several types of digital communications links. The most common of these, which have been in use by communications carriers for approximately 20 years and available to users for approximately a decade, are T1 and T3 links. These will be superseded eventually by the synchronous optical network (SONET), a set of standards for broadband networks that operate over fiber-optic links and that will provide more versatile bandwidth to both carriers and users.

T1 is a digital communications system for simultaneously transmitting 24 voice, data, and video signals over the same circuit. The original users of T1 were telephone companies and government agencies. Because each T1 is equivalent to 24 voicegrade circuits, the telephone companiesi?? savings from using T1 were substantial. At first the cost of T1 to end users was prohibitive; since T1 services were tariffed during the 1980s, however, competition among major carriers has brought the prices down significantly.

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In addition, at first the need for higher bandwidth and additional services and capabilities increased the demand for T1 circuits such that the waiting time for installation was several months. Each T1 is equivalent to 24 separate circuits between two sites, an advantage for T1 because multiple lines and multiple-site networks are expensive and hard to manage. The end-user equipment at each site can be PBXs, videoconferencing systems, imaging systems, or host computers. T1s connecting two PBXs are called tie trunks.

For some old PBXs that cannot directly receive a T1 signal, a T1 line must first be terminated at a channel bank. Analog lines are subject to noise and signal deterioration, and equipment that is used for reconstructing the analog signal cannot make them suitable for data transmission, because analog lines amplify both original signal and noise. T1 circuits are more immune to noise and distortion than multiple lines. T1 signals can also be corrupted, but because they are digital, processing will regenerate the original signal without amplifying the noise.

Because T1s are dedicated point-to-point circuits, each customer pays a fixed rate for that service. The T1 charges depend on the distance between the two locations and each locationi?? s distance to the carrieri?? s closest point of presence or local central office. However, the closest connection may not be the least expensive one. Recently, interexchange carriers have started offering switched T1 services. With switched T1, customers pay for the portion of bandwidth they use. In either case, the connection between the customeri?? s site and the closest central office is a dedicated circuit and has a fixed cost.

U. S. telephone serving areas are divided into about 200 local access and transport areas (LATAs). A LATA can include part or all of one state. The transmission between LATAs is done by long-distance or interexchange carriers even if both LATAs are part of the same local exchange carrieri?? s serving area, so a T1 circuit may involve more than the local exchange carrier. Channel Service Unit. The channel service unit is the first interface between the user and the network and the last regeneration point for the incoming signal before it is delivered to customer premises equipment.

(In the past, channel service units were part of the network, but now they also are considered as customer premises equipment. ) The channel service unit is responsible for general line monitoring functions, discussed later in this chapter. If a T1 multiplexer or piece of customer premises equipment fails, the channel service unit sends streams of 1s to the network to keep the connection alive until the customer premises equipment recovers. Some T1 multiplexers and PBXs have built-in channel service units.

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