PDH Networks PDH Networks: Principles of Digital Transmission 137 in Software

Printer Code 128B in Software PDH Networks PDH Networks: Principles of Digital Transmission 137

Figure 73 PDH transmission systems are standardized only at the ITU-T G703 hierarchical interfaces at the input and output of the transmission terminal On the line side, every manufacturer may use a different media code and management overhead structure, so standardized testing can be done only at the G703 interface In the media transmitter, a clock is recovered from the incoming coded PDH hierarchical signal, and the G703 interface code (eg, HDB3 or CMI) is removed and binary data and clock streams derived The terminal then adds its own overhead for network management, error control, framing, and order wire, before the composite signal is encoded for media transmission over microwave radio or fiber The receiver reverses this process to regenerate the PDH signal

Figure 74 The 32-timeslot frame at the 2048 Mbps (E1) primary rate, which begins with the frame alignment word The frame, which repeats every 125 s, is a fully synchronous frame because there is no provision for additional justification bits for synchronization

721 European 2048 Mbps primary rate PDH hierarchy Primary rate frame structure The fundamental building block for the European or

international PDH hierarchy is the 2048 Mbps E1 frame structure, comprising the 32-timeslot synchronous frame defined by ITU-T Recommendations G704 and G732 (shown in Figure 74)

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Strictly speaking, no frame structure is required; a feature of the 2 Mbps hierarchy, defined in ITU-T Recommendation G703, is that transmission can be bit sequence independent In other words, 2 Mbps and 64 kbps facilities are clear channel and do not require any particular signal structure to pass through the network Although this transparency can be useful for transmission of wideband signals, sending an unstructured signal into the network can have drawbacks An apparently random signal cannot be monitored in-service by the service provider for transmission errors, and it is impossible to provide bandwidth grooming or switching of channels It is likely that the network operator will not be able to guarantee network performance with unstructured 2 Mbps traffic In view of this, most private and public networks operate with the standard 2 Mbps frame structure defined in ITU-T Recommendation G704 Each 2 Mbps frame contains 256 bits (32 timeslots, each of 8 bits) at a repetition rate of exactly 8 kbps The first timeslot (timeslot zero, TS0) is reserved for framing, error-checking, and alarm signals; the remaining 31 can be used for traffic The individual channels can be used for 64 kbps PCM, subdivided further for low-rate data or voice compression such as ADPCM (Adaptive Differential PCM), or aggregated for wideband signals such as videoconferencing or LAN interconnection Sometimes a timeslot (such as TS16) is reserved for signaling (ISDN primary rate D channel signaling such as Q931, for example, or channel-associated ABCD signaling) The start of the 32-timeslot frame is signified by the frame alignment word (0011011) in the TS0 of alternate frames, as shown in Figure 75 In the other frame,

Figure 75 The 7-bit frame word is transmitted in alternate frames as shown The frame alignment cri-

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Figure 76 Timeslot 16 (TS16) contains alternately the multiframe alignment signal or pairs of signaling bits for channel-associated signaling (CAS) The multiframe provides a reference so that the receiving equipment can decode the 4-bit signaling word for each of the 30 PCM channels When common-channel signaling (CCS) is used, TS16 can be used for traffic or to carry the CCS signal

bit 2 is set to 1 and bit 3 contains the A-bit for sending an alarm to the far end The S-bits are all intended for international and national use and, when unused, are set to logical 1 Once the demultiplexer has achieved frame alignment, it can separate the individual 64 kbps channels in the frame If three out of four frame alignment words are received in error, the terminal declares loss of frame alignment and initiates a resynchronization process The recovery criterion is one correct frame alignment word, one nonframe word bit 2 (logical 1), followed by one correct frame alignment word When the 2 Mbps frame was used exclusively for PCM voice transmission, the frame alignment criterion was very reliable With data transmission, however, the traffic can simulate the frame alignment and nonframe alignment words, meaning false framing is possible A new, more robust standard has been developed, building on the earlier framing standard; this will be discussed shortly Once the multiplexer has gained frame alignment, it searches in TS16 for the multiframe alignment signal (0000) in bits 1 4 This marks frame 0 of the group of 16 frames, called the multiframe (shown in Figure 76) The multiframe is necessary only when channel-associated signaling (CAS) is used Timeslot 16 then contains pairs of 4-bit ABCD signaling words Over a complete multiframe, all 30 channels are serviced If common-channel signaling (CCS) is used, then multiframe alignment is unnecessary; TS16 is used simply as a 64 kbps data channel for CCS messages, or it can be turned over to revenue-earning traffic (giving a total of 31 channels for the payload) The 2 Mbps frame structure just described is in widespread use It has some limitations, however, particularly with increased data transmission and demand for online performance monitoring

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