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Optical Networks - _6_1 SONET_SDH_74

Optical Networks - _6_1 SONET_SDH_74 - 6.1 SONET/SDH 371...

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6.1 SONET/SDH 371 Table 6.1 Transmission rates for asynchronous and plesiochronous signals, adapted from [SS96]. Level North America Europe Japan 0 0.064 Mb/s 0.064 Mb/s 0.064 Mb/s 1 1.544 Mb/s 2.048 Mb/s 1.544 Mb/s 2 6.312 Mb/s 8.448 Mb/s 6.312 Mb/s 3 44.736 Mb/s 34.368 Mb/s 32.064 Mb/s 4 139.264 Mb/s 139.264 Mb/s 97.728 Mb/s 6.1 SONET/SDH SONET (Synchronous Optical Network) is the current transmission and multiplexing standard for high-speed signals within the carrier infrastructure in North America. A closely related standard, SDH (Synchronous Digital Hierarchy), has been adopted in Europe and Japan and for most submarine links. In order to understand the factors underlying the evolution and standardization of SONET and SDH, we need to look back in time and understand how multiplexing was done in the public network. Prior to SONET and SDH, the existing infrastruc- ture was based on the plesiochronous digital hierarchy (PDH), dating back to the mid-1960s. (North American operators refer to PDH as the asynchronous digital hierarchy.) At that time the primary focus was on multiplexing digital voice circuits. An analog voice circuit with a bandwidth of 4 kHz could be sampled at 8 kHz and quantized at 8 bits per sample, leading to a bit rate of 64 kb/s for a digital voice circuit. This became the widely accepted standard. Higher-speed streams were de- fined as multiples of this basic 64 kb/s stream. Different sets of standards emerged in different parts of the world for these higher-speed streams, as shown in Table 6.1. In North America, the 64 kb/s signal is called DS0 (digital signal-0), the 1.544 Mb/s signal is DS1, the 44.736 Mb/s is DS3, and so on. In Europe, the hierarchy is labeled E0, E1, E2, E3, and so on, with the E0 rate being the same as the DS0 rate. These rates are widely prevalent today in carrier networks and are offered as leased line services by carriers to customers, more often than not to carry data rather than voice traffic. PDH suffered from several problems, which led carriers and vendors alike to seek a new transmission and multiplexing standard in the late 1980s. This resulted in the SONET/SDH standards, which solved many problems associated with PDH. We explain some of the benefits of SONET/SDH below and contrast it with PDH.
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372 Client Layers of the Optical Layer DS4 (140 Mb/s) DS4 (140 Mb/s) DS3s (45 Mb/s) DS3s (45 Mb/s) DS1s (1.5 Mb/s) DS1 drop/add DS1 drop/add SONET 155 Mb/s SONET 155 Mb/s (a) (b) Figure 6.1 Comparison of asynchronous and synchronous multiplexing. (a) In the asyn- chronous case, demultiplexers must be stacked up to extract a lower-speed stream from a multiplexed stream. (b) In the synchronous case, this can be done in a single step using relatively simple circuitry. 1. Multiplexing simplification: In asynchronous multiplexing, each terminal in the network runs its own clock, and while we can specify a nominal clock rate for the signal, there can be significant differences in the actual rates between different clocks. For example, in a DS3 signal, a 20 ppm (parts per million) variation in clock rate between different clocks, which is not uncommon, can produce a difference in bit rate of 1.8 kb/s between two signals. So when lower-speed
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Optical Networks - _6_1 SONET_SDH_74 - 6.1 SONET/SDH 371...

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