es (STMs) and they are exact multiples of STM-1 at the 155.52-Mbps data rate, as we can see in Table 4.3. STM-1 data are simply byte interleaved with other STM-1 data streams to make up a higher transmission data rate; no additional framing information is added. Byte interleaving means that, for example, an STM-4 signal contains a byte (8 bits) from the first STM-1 tribu- tary, then from the second, third, and fourth tributaries, and then again from the first one. The demultiplexer receives all STM-1 frames independently. The STM-1 frame is repeated 8,000 times a second, a rate equal to the PCM sampling rate. This makes each 8-bit speech sample visible in a 155.52-Mbps data stream. When PCM coding is synchronized to the same source as SDH systems, we can demultiplex one speech channel just by pick- ing up 1 byte from each STM-1 frame. The frame contains frame alignment information and other information such as management data channels and pointers that tell the location of tributaries in the frame. If tributaries are not synchronous with the STM-1 frame, a pointer (a binary number) in a fixed location in the STM-1 frame tells the location of each tributary. By looking at the value of this pointer, we can easily find the desired tributary signal. This is a great advantage over PDH systems, which require step-by-step demultiplexing (to separate information and stuffing bits) to the level of the tributary that we want to take out from the high- data-rate stream. Multiplexing in SDH is quite a complicated matter because the multi- plexing supports many different PDH and SDH streams to be multiplexed into an STM-1 stream. For example, a single STM-1 may carry 63 E-1 sig- nals or alternatively one E-4 signal. The STM-1 frame structure and how ATM cells are inserted in it are demonstrated in Chapter 6 as an example of SDH framing. A more detailed treatment of the framing subject is not included here. 4.6.4.2 Data Rates of North American SONET The synchronous transport signal level 1 (STS-1) is the basic SONET module tha