The digital baseband signal adopted in Europe, China and North America is at a 64kbit/s rate. Chapter 4 describes how a 4 kHz speech channel is converted into digitized samples; each sample is allocated an 8-bit code corresponding to one of 256 (28) quantizing levels, and the sampling rate is 8 kHz. The same 8-bit word or octet may also be obtained from the IA5 alphabet (7 bits) plus a parity bit. A signaling bit is sometimes added to each octet (increasing the rate to 72 kbit/s), but this is normally removed at the multiplexing stage to restore the bit rate to 64 kbit/s, the signaling information being transferred to the signaling channel in the multiplex stream.
The bit duration in the 30 channel PCM system is 488 ns, so an octet occupies 8 X 0.488=3.9 |is. The interval between successive samples of a voice channel is 125 |is (i.e., 1/8000), so each sample occupies only a small fraction of the sampling time, which in this context is called a frame. The remaining time within the 125 |is frame is allocated to other voice channels as described as follows. The PCM 30-channel system is a TDM-Level 1 (primary) multiplexing system with a rate of 2048 kbit/s.
In time division multiplexing the remainder of the frame contains digitized samples from other voice channels and since 125^3.9=32, that is the total number of channels that can be contained in one frame. Each frame is therefore divided into 32 time slots (TS) 30 of which are used for voice channels and 2 for control purposes. The time slots are designated TS0-TS31; TS0 contains a frame-alignment signal, and TS16 is reserved for interexchange signaling.
Each TS contains 8 binary digits, and so the gross digit rate is 8000 X 8 X 32=2048 kbit/s.
The 30 speech channels are interleaved sequentially, sample by sample in each 125 | s frame, but the TS0 frame-alignment word alternates between two values, and the TS16 slot, which has to carry signaling information for all 30 channels, is further submultiplexed over a period of 16 frames (i.e., 2ms), this group of 16 frames being called a multiframe.
The frame-alignment signal in TS0 is 0011011 transmitted in binary digit spaces 2-8 in alternate frames. Digit space 1 of TS0 is reserved for international use, but is normally set to a "1". In the intermediary frames only binary digit space 2 is fixed as a "1"; the other spaces are not allocated except that binary space 3 indicates the state of the multiplex alarms. Likewise binary space 6 of TS16 in frame 0 of a multiframe indicates the state of alarms associated with channel signaling.
The Figure 7.4-1 shows the 30-channel frame format. The actual signals sent to line are not unipolar since such a pulse train would contain a large DC component (e.g., a succession of all "1"s) together with high amplitude low-frequency components which are undesirable. The 2048 kbit/s binary digit stream is instead encoded to HDB3.
The 30-channel 2048 kbit/s stream is the basic building module of the digital network in Europe and China. It is the bit rate at which switching takes place in stored program control (SPC) switches. The multiplex is widely adopted as the access link from digital PABXs and LANs to SPC switches, and to interconnect PABXs or LANs on separated sites.
The CEPT began its deliberations leading to a PCM standard in the early 1970s, it was clear that digital switching was a practical possibility. For all these reasons the CEPT chose a 32-channel primary multiplex: 30 speech channels and one channel reserved for frame alignment and system information and a second channel reserved for signaling (see Figure 7.4-1).
For channel associated signaling a 16-frame multi-frame is defined by the frame-alignment signal contained in channel 0 of even frames and the multi- frame alignment signal contained in channel 16 of frame 0. Channel 0s of odd frames are reserved for PCM link status indications and the channel 16s of frames 1 to 15 each carry two signaling quartets relating to two specific voice channels. Thus the signaling capacity for channel associated signaling is 2 kbit/s
For common channel signaling, channel 16 can be readily used as a 64 kbit/s common channel and the multi-frame can be reduced to two frames. A further advantage of the system is that, with 8 bits per frame for frame alignment and 8 bits per 16 frames for multi-frame alignment, alignment can be regained much more quickly than with the 24-channel systems.
The coding law adopted was the A-law, retaining some measure of compatibility with the existing European PCM systems but finally removing all hope of compatibility with the North American systems. When the ITU-U at last produced recommendations for PCM primary multiplexes, this divide was recognized internationally by defining two systems , Recommendation G.733, and the CEPT system, Recommendation G.732. Both coding laws are defined in Recommendation G.712. Recommendation G.711 requires that all international links be coded to requirements of the A-law and G.732 in situations where dissimilar systems exist in the countries so linked.
Despite the increased transmission problems of a PCM system with a 2.048 Mbit/s bit rate, CEPT system has been received with favor internationally and is the predominant system worldwide.
The basic PCM30/32 multi-frame and single frame have the formats as shown in Figure 7.4-1.
As shown in the above diagram, in the 125 |is sampling cycle, each channel alternatively sends the 8 bit voice code groups once, i.e., each channel seizes one time slot. 30 channels plus the synchronization and signaling time slots form a single frame. TS0 is used to transmit frame synchronization codes. TS16 is used to transmit the signaling codes in each channel (such as seizure, called off-hooking, caller on-hooking, and disconnection).
It should be noted that the PCM 30/32 frame format (E1) is the unique synchronously multiplexing frame format in plesiochronous digital hierarchy. The rest frame formats, E2, E3 and E4, are all asynchronous multiplexing frame format.
In a single frame, the characteristic data of the 30-channel PCM system is as follows:
Voice frequency band: 300-3400 Hz
Sampling frequency: 8000 Hz
Frame cycle: 125 | s
Coded bits for each sample value: 8 bit
Rate per channel: 64 kbit/s
Time slot number in each frame: 32 Bits in each frame: 256 Channels in each frame: 30 Duration of each time slot: 3.9 |is Bit duration: 0.488 |is Total data rate 2.048 Mb/s Compressing law: Law A, and A=87.6 Signaling capacity:
channel associated signaling: 2 kbit/s common channel signaling: 64 kbit/s In a multi-frame, the 30-channel PCM system shall have the following characteristic data: Multi-frame frequency: 500 Hz Multi-frame cycle: 2 ms Time slots in each multi-frame: 32x16=512 Bits in each multi-frame: 256x16=4096
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