f TDM and to get a clear view of TDM, we now look at the most common frame structure in telecommunications networks, namely, the primary rate 2,048-Kbps frame used in the European standard areas. This is the basic data stream that carries speech channels and ISDN-B channels through the net- work and it is called E-1. The corresponding North American primary rate is 1.544 Mbps, which carries 24 speech channels and it is known as DS1 or T1. It is also introduced in this section. In the European scheme, the primary rate frame is built up in digital local exchanges that multiplex 30 speech or data channels at bit rate of Introduction to Telecommunications Network Engineering 64 Kbps into the 2,048-Kbps data rate. ITU-T defines this frame structure in Recommendation G.704. 4.6.2.1 The 2-Mbps Frame Structure PCM-coded speech is transmitted as 8-bit samples 8,000 times a second, which makes up a 64-Kbps data rate. These eight-bit words from different users are interleaved into a frame at a higher data rate. The 2,048-Kbps frame in Figure 4.21 is used in the countries imple- menting European standards for telecommunications. It contains 32 time slots, and 30 of them are used for speech or 64-Kbps data. The frame is repeated 8,000 times a second, which is the same as the PCM sampling rate. Each time slot contains an eight-bit sample value and the data rate of each channel is 64 Kbps. These voice channels or data channels are synchro- nously multiplexed into a 2-Mbps data stream, which is often called E1 (first level in European hierarchy). For error-free operation the tributaries (64-Kbps data streams of the users) have to be synchronized with the clock signal of the 2-Mbps multiplexer. The data rate of 2,048 Kbps for the multi- plexer is allowed to vary by 50 parts per million (ppm), and as a conse- quence each user of the network has to take timing from the multiplexer in the network and generate data exactly at the data rate of the multiplexer divided by 32. Each time slot (TS) of a frame contains 8 bits. Frame, 125 mi