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The most common form of multiplexed access and transport is T-carrier, or E-carrier outside the United States. Let s take a few minutes to describe them.
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Framing and Formatting in T1
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The standard T-Carrier multiplexer, shown in Figure 3-38, accepts inputs from 24 sources, converts the inputs to PCM bytes, then time-division multiplexes the samples over a shared four-wire facility, as shown in Figure 3-39. Each of the 24 input channels yields an eight-bit sample, in
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Figure 3-38 T-Carrier multiplexer channel banks, showing DS0 cards
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Figure 3-39 A time-division multiplexer in action
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round-robin fashion, once every 125 microseconds (8,000 times per second). This yields an overall bit rate of 64 Kbps for each channel (eight bits per sample 8,000 samples per second). The multiplexer gathers one eight-bit sample from each of the 24 channels, and aggregates them into a 192-bit frame. To the frame it adds a frame bit, which expands the frame to a 193-bit entity. The frame bit is used for a variety of purposes that will be discussed in a moment. The 193-bit frames of data are transmitted across the four-wire facility at the standard rate of 8,000 frames per second, for an overall T1 bit rate of 1.544 Mbps. Keep in mind that 8 Kbps of the bandwidth consists of frame bits (one frame bit per frame, 8,000 frames per second); only 1.536 Mbps belongs to the user.
Beginnings: D1 Framing
The earliest T-Carrier equipment was referred to as D1 and was considerably more rudimentary in function than modern systems (see Figure 3-40). In D1, every eight-bit sample carried seven bits of user information (bits 1 through 7) and one bit for signaling (bit 8). The signaling bits were used for exactly that: indications of the status of the line (on-hook, offhook, busy, high and dry, etc.), while the seven user bits carried encoded voice information. Since only seven of the eight bits were available to the user, the result was considered to be less than toll quality (128 possible values, rather than 256). The frame bits, which in modern systems indicate the beginning of the next 192-bit frame of data, toggled back and forth between zero and one.
Figure 3-40 D1 framing
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Evolution: D4
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As time went on and the stability of network components improved, an improvement on D1 was sought after and found. Several options were developed, but the winner emerged in the form of the D4 or superframe format. Rather than treat a single 193-bit frame as the transmission entity, superframe gangs together twelve 193-bit frames into a 2,316bit entity shown in Figure 3-41 that obviously includes twelve frame bits. Please note that the bit rate has not changed: We have simply changed our view of what constitutes a frame. Since we now have a single (albeit large) frame, we clearly don t need twelve frame bits to frame it; consequently, some of them can be redeployed for other functions. In superframe, the six odd-numbered frame bits are referred to as terminal framing bits, and are used to synchronize the channel bank equipment. The even-numbered framing bits, on the other hand, are called signal framing bits and are used to indicate the receiving device where robbed-bit signaling occurs. In D1, the system reserved one bit from every sample for its own signaling purposes, which succeeded in reducing the user s overall throughout. In D4, that is no longer necessary: Instead, we signal less frequently, and only occasionally rob a bit from the user. In fact, because the system operates at a high transmission speed, network designers determined that signaling can occur relatively infrequently and still convey adequate information to the network. Consequently, bits are robbed from the sixth and eighth iteration of each channel s samples, and then only the least significant bit from each sample. The resulting change in voice quality is negligible.
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