30 Channels PCM

30 Channels PCM

History/Background

The 30 Channel PCM communication system developed in 1980 has been manufactured by PTT-ARLA and installed between exchanges in Istanbul. After successful operation of these systems, a design project for a 2 Mbit/s per-channel PCM system has been initiated. Pulse-code modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code. PCM has been used in digital telephone systems and 1980s-era electronic musical keyboards. It is also the standard form for digital audio in computers and the compact disc "red book" format. It is also standard in digital video, for example, using ITU-R BT.601. Uncompressed PCM is not typically used for video in standard definition consumer applications such as DVD or DVR because the bit rate required is far too high.

Frame Structure & Multi-Frame Format

The 2.048 MHz 30 CHANNEL PCM (PCM-30) environment consists of frame structure of 256 bits that is repeated at a 8 KHz rate. Each frame consists of 32-eight-bit time slots that are numbered from 0 to 31. The first eight bits of every frame (time slot 0) are used mainly to provide framing information. Every other time slot 0 contains a fixed seven bit pattern known as the Frame Alignment Signal (FAS). See Figure 1. The FAS pattern (x0011011) identifies time slot 0, from which the other time slots can be readily ascertained. In frames that do not begin with FAS (non-align frames), bit 2 of time slots is fixed at a one to insure that it does not emulate the FAS pattern. Besides framing information, time spare slot 0 contains two sets of spare bits and a alarm. One spare bit per frame that that is intended for international use and it is designated Si in Figure 1. Five spare bits per frame are available for national usage and they are designated Sn in Figure 1. Bit 3 of non-align frames is used to indicate whether the remote unit is in alarm or not. If the Alarm bit is set to a one, then an alarm condition exists at the remote. If it is zero, then no alarm condition exists.

30 CHANNEL PCM Multiframe structure:

Two separate multiframe structure exist in 30 CHANNEL PCM environments, Cyclical Redundancy Check 4 (CRC4) and Channel Associated Signaling (CAS). Both of these multiframes are based on the FAS framing level but they are used independently of each other for different purposes. Although both multiframes consist of a set of 16 consecutive frames, they need not be aligned nor must they both be present in a data stream.

30 CHANNEL PCM CAS MultiFrame Structure:

The CAS multiframe structure is shown in Figure 2. It is made up of 16 frames and it can begin on either an align or non-align frame. The CAS multiframe always resides in time slot 16. The first frame contains a multiframe alignment word (0000) in the upper nibble and a set of spare ...