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call arrives in the U.S. from a European country, the receiving American carrier must convert the incoming E-1 signal to T-1. If a call originates from Canada and is terminated in Australia, the Canadian originating carrier must convert the call to E-1 before transmitting it to Australia.
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Up the Food Chain: From T-1 to DS3 . . . and Beyond
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When T-1 and E-1 first emerged on the telecommunications scene, they represented a dramatic step forward in terms of the bandwidth that service providers now had access to. In fact, they were so bandwidth rich that there was no concept that a customer would ever need access to them. What customer, after all, could ever have a use for 1.5 million bits per second of bandwidth Of course, that question was rendered moot in short order as increasing requirements for bandwidth drove demand that went well beyond the limited capabilities of low-speed transmission systems. As T-1 became more mainstream, its usage went up, and soon requirements emerged for digital transmission systems with capacity greater than 1.544 Mbps. The result was the creation of what came to be known as the North American Digital Hierarchy, shown in Figure 1-27. The table also shows the European and Japanese hierarchy levels.
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Figure 1-27 International Multiplexing hierarchies.
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Hierarchy Level DS-0 DS-1 E-1 DS-1c DS-2 E-2 DS-3 DS-3c E-3 DS-4
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Europe 64 Kbps 2.048 Mbps
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United States 64 Kbps 1.544 Mbps 3.152 Mbps 6.312 Mbps
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Japan 64 Kbps 1.544 Mbps 3.152 Mbps 6.312 Mbps 32.064 Mbps
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8.448 Mbps 34.368 Mbps 44.736 Mbps 91.053 Mbps 139.264 Mbps 274.176 Mbps
397.2 Mbps
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
Beginnings
From DS-1 to DS-3
1
We have already seen the process employed to create the DS-1 signal from 24 incoming DS-0 channels and an added frame bit. Now we turn our attention to higher bit rate services. As we wander our way through this explanation, pay particular attention to the complexity involved in creating higher rate payloads. This is one of the great advantages of SONET and SDH. The next level in the North American Digital Hierarchy is called DS-2. Although it is rarely seen outside of the safety of the multiplexer in which it resides, it plays an important role in the creation of higher bit rate services. It is created when a multiplexer bit interleaves four DS-1 signals, inserting as it does so a control bit, known as a C-bit, every 48 bits in the payload stream. Bit interleaving is an important construct because it contributes to the complexity of the overall payload. In a bit interleaved system, multiple bit streams are combined on a bit-by-bit basis, as shown in Figure 1-28. When payload components are bit-interleaved to create a higher rate multiplexed signal, the system first selects bit one from channel one, bit one from channel two, bit one from channel three, and so on. Once it has selected and transmitted all of the first bits, it goes on to the second bits from each channel, then the third, until it has created the super-rate frame. Along the way it intersperses C-bits, which are used to perform certain control and management functions within the frame. Once the 6.312 Mbps DS-2 signal has been created, the system shifts into high gear to create the next level in the transmission hierarchy. Seven DS2 signals are then bit-interleaved along with C-bits after every 84 payload bits to create a composite 44.736 Mbps DS-3 signal. The first part of this
Bit 1, frame 4
Figure 1-28 Bit interleaving.
Bit 1, frame 2
Bit 1, frame 1 Bit 1, frame 3
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
Beginnings
Beginnings
process, the creation of the DS-2 payload, is called M12 multiplexing; the second step, which combines DS-2s to form a DS-3, is called M23 multiplexing. The overall process is called M13, and is illustrated in Figure 1-29. The problem with this process is the bit-interleaved nature of the multiplexing scheme. Because the DS-1 signal components arrive from different sources, they may be (and usually are) slightly off from one another in terms of the overall phase of the signal; in effect, their speeds differ slightly. This is unacceptable to a multiplexer, which must rate-align them if it is to properly multiplex them, beginning with the head of each signal. In order to do this, the multiplexer inserts additional bits, known as stuff bits, into the signal pattern at strategic places that serve to rate align the components. The structure of a bit-stuffed DS-2 frame is shown in Figure 1-30; a DS-3 frame is shown in Figure 1-31.
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