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TDM over T1
When an ordinary voice call is made, analog signals representing the voice are sent to the central office (CO) where they are converted to digital format by sampling. This sampling is performed by using some form of pulse code modulation (PCM), so the signals can be switched and routed in the telephone network. The digital signals are then converted back to analog before being presented at the call's destination. The PCM is carried in a particular time slot of a protocol frame through a particular physical connection for the duration of the call, lasting from the instant the originator picks up the phone (goes off-hook) until both parties hang up. The time-slot allocation and assignment allows multiple calls to be placed or multiplexed into a single protocol frame. This is called time division multiplexing (TDM).
The traditional TDM network is constructed by channelized T1 circuits. A 1.544–Mbps channelized T1 has twenty-four 64–kbps trunk circuits. The efficient utilization of copper outside plant facilities was one of the benefits and impetuses for mass deployment of T1s. Twenty-four simultaneous conversations were conducted over two copper pairs, one of which was for transmission and the other for receive use. Figure 1 depicts N frames, where the order of transmission is N, 1, 2, 3, etc. This is synchronous TDM because each time slot is assigned statically. Each slot is dedicated to the user for the duration of the call, even when the user has nothing to transmit. This creates an inefficient utilization of bandwidth.
Figure 1. Basic TDM Transmission
Sampled Voice and Noise from T1 Line
The analog signals that make up a voice signal are subject to electromagnetic interference (EMI) during transmission, so any noise that is picked up in transit at the originating or terminating end of the call also gets digitized and sent to the other party. The PSTN is not smart technology. It cannot recognize what parts of the analog signal represent actual content and simply converts the entire signal presented to it at the origin point, reconstructing it exactly—including any noise it picked up before being encoded—for delivery at the call-destination point. The reconstructed analog signal at the receiving end is subject to pure analog noise injection that might come from nearby power lines or radio transmitters, too.
Circuit Fails: The Call Drops
TDM is based on using time slots in protocol frames being sent over a physical circuit for carrying a call. Various regulatory bodies have mandated that telephone service achieves certain levels of reliability and availability, forcing the use of redundant or backup circuits to satisfy the requirements. During periods in which TDM–based switching equipment is near its maximum call-processing capacity. If a circuit fails, there may not be another one available as a backup to handle the in-progress calls. Thus, all of those calls will be dropped, and angry customers will be the result.
