## Frame structure and multiplexing hierarchy for European ITU (CCITT) telephony

#### In chapter 7, the method by which individual voice channels are digitized at the local telephone exchange or switching centre is discussed. Each incoming analogue phone line is sampled at a rate of 8000 samples per second, and each sample is represented as an 8-bit word.
In order to send these digitized voice samples between switching centres, high capacity data links are used, often optical fibres, capable of supporting several Gigabits/second. The individual 8-bit words are grouped into frames for transmission as shown below, with the addition of an 8-bit signalling word and an 8-bit framing/synchronisation word. Thirty-two 8-bit words are used in the E-series ITU standard, and twenty-four 8-bit words in the American T-series standard.
An E-series frame is thus 8 x 32 = 256 bits wide, and is sent at a rate of 8000 frames per second, to match the voice channel sampling rate. The bit rate on the basic E1 channel is thus 256 x 8000 = 2048 Mbps.
E1 channels can be grouped into super frames to give E2 channels, E2 into E3 channels and so on. Additional framing and signalling words are added at each stage to control routing within the switching network.