11. Recommendation G.763 DIGITAL CIRCUIT MULTIPLICATION EQUIPMENT USING A 32KBIT/S ADPLM AND DIGITAL SPEECH INTERPOLA- TION (Melbourne, 1988) 1. General 1.1 Scope This Recommendation is intended as an introduction to digital circuit multiplication equipment and systems, and as a base document for the specification of Digital Circuit Multiplication Equipment (DCME) and Digital Circuit Multiplication Systems (DCMS). Essential facilities, interface conditions and overall performance require- ments are given. Requirements for full compatibility and interoperability are under study (see Supplement No. 31). 1.2 Attributes Digital Circuit Multiplication equipment (DCME) is utilized as a means of augmenting the capacity of digital transmission systems operating between several International Switching Centers (ISC). DCME has all of the following attributes: - digital speech interpolation (DSI); - low rate encoding (LRE); - dynamic load control (DLC) arrangement in association with inter- facing; - capability to accomodate the following types of bearer service requirements: speech 3.1 kHz audio (data and speech) 64 kbit/s unrestricted (transparent) alternate speech/64 kbit/s unrestricted. The link between two DCMEs is generally one where a highly efficient traffic carrying capability is required, e.g., a long-distance link. 1.3 Application This Recommendation is applicable to the design of digital circuit multi- plication equipment intended for, but not limited to, use in an interna- tional digital circuit. Freedom is permitted in design details which are not covered in this Recommendation (Note). Note - Several additional items yet to be fully considered include the evaluation of: - instantaneous 2 bit speech encoding on overloads (to avoid clip- ping), and voice band data rate discrimination (to permit rates less than or equal to 4800 bit/s to be coded at 32kbit/s only). - silence elimination techniques (to permit saving of bearer channel capacity during the inactive periods of half-duplex voice band data calls). 2. Definitions relating to digital circuit multiplication equipment (DCME) 2.1 Digital circuit multiplication equipment (DCME) A general class of equipment which permits concentration of a number of 64 kbit/s PCM encoded input trunk channels on a reduced number of transmission channels (see 2.7). 2.2 Digital circuit multiplication system (DCMS) A telecommunications network comprised of two or more DCME termi- nals where each DCME terminal contains a transmit unit and a receive unit. 2.3 Low rate encoding (LRE) A voice band signal encoding method, e.g., adaptive differential pulse code modulation (ADPCM), which results in a bit rate less than 64 kbit/s, e.g., 40 kbit/s, 32 kbit/s, or 24 kbit/s. Note 1 - Conversion between speech signals encoded in PCM at 64 kbit/s and those encoded in ADPCM must be carried out by means of transcod- ing processes given in RecommendationsG.721 and G.723. 2.4 Variable bit rate (VBR) The capability of the encoding algorithm to dynamically switch between 32 and 24 kbit/s for speech traffic under control of the DCME. 2.5 Digital speech interpolation (DSI) A process which, when used in the transmit unit of a DCME, causes a trunk channel (see 2.9) to be connected to a bearer channel (see 2.8) only when activity is actually present on the trunk channel. This, by exploiting the probability of the Speech Activity Factor (see 2.14) of trunk channels being less than 1.0, enables the traffic from a number of trunk channels to be concentrated and carried by a lesser number of time shared bearer channels. The signals carried by a bearer channel therefore represent interleaved bursts of speech signals derived from a number of different trunk channels. Note - A process complementary to DSI is required in the receive unit of a DCME, i.e., assignment of the interleaved bursts to their appropriate trunk channels. 2.6 DCME frame A time interval, the beginning of which is identified by a "unique word" in the control channel. The DCME frame need not coincide with the multi-frames defined in CCITT Recommendation G.704. The format specification of the DCME frame includes channel boundaries and bit position significance. 2.7 Transmission channel A 64 kbit/s time slot within a DCME frame. 2.8 Bearer channel (BC) A bearer channel is a unidirectional, digital, transmission path from the transmit unit of one DCME to the receive unit of a second associated DCME used to carry concentrated traffic between the two DCMEs. Note 1 - A number of bearer channels in each direction of transmission form the both-way link required between two DCMEs. This link may be, for example, a 2048kbit/s system. Note 2 - A bearer channel may have any of the following instantaneous bit rates: 24, 32, 40 and 64 kbit/s. 2.9 Trunk channel (TC) A unidirectional, digital transmission path (generally short distance) used for carrying traffic and which connects a DCME to other equipment e.g., an International Switching Centre (ISC). Two such trunk channels (trans- mit and receive) are needed by 4 wire telephone circuits and constitute a trunk circuit. Note 1 - Signals carried by a trunk channel will be transmitted at a bit rate of 64 kbit/s. Note 2 - A number of trunk channels in each direction of transmission are required between a DCME and, for instance, an ISC. These trunk chan- nels may be carried by a number of 1544 or 2048 kbit/s systems. 2.10 Assignment message The message specifying the interconnections required between trunk channels and bearer channels. 2.11 Assignment map A record, held in a memory of a DCME, of the interconnections required between trunk channels and bearer channels. This record is dynamically updated in real time in accordance with the traffic demands made on the DCME. 2.12 Control channel A unidirectional transmission path from the transmit unit of one DCME to the receive unit of one or more associated DCMEs which is dedicated primarily to carrying channel assignment messages. In addition, the con- trol channel transmits other messages such as idle noise levels, dynamic load control, and alarm messages. Note - An alternative name for "control channel" is "assignment chan- nel". 2.13 Ensemble activity The ratio of the time active signals and their corresponding hangover time and front end delay occupy the trunk channels to the total measuring time, averaged over the total number of trunk channels included in the measurement. 2.14 Speech activity factor The ratio of the time speech signals with their corresponding hangover time and front end delay occupy a trunk channel to the total measuring time, averaged over the total number of trunk channels carrying speech. 2.15 Voice band data ratio The ratio of the number of trunk channels carrying voice band data sig- nals to the total number of trunk channels averaged over a fixed interval of time. 2.16 64 kbit/s unrestricted digital data ratio The ratio of the number of trunk channels carrying 64 kbit/s unrestricted digital data signals to the total number of trunk channels averaged over a fixed interval of time. 2.17 DCME overload (mode) The condition when the number of input trunk channels instantaneously active carrying speech exceeds the number of 32 kbit/s channels avail- able for interpolation. 2.18 Overload channels The additional bearer channel capacity which is generated using variable bit rate (VBR) encoding to minimize or eliminate DSI competitive clip- ping. 2.19 Average bits per sample The average number of encoding bits per sample computed over a given time window for the ensemble of active interpolated bearer channels within a given interpolation pool. Only bearer channels carrying speech are included in this calculation. 2.20 Transmission overload The condition when the average bits per sample goes beyond the value set in accordance with speech quality requirements. 2.21 Freeze-out The condition when a trunk channel becomes active and cannot immedi- ately be assigned to a bearer channel, due to lack of available transmis- sion capacity. 2.22 Freeze-out fraction (FOF) The ratio of the total time that the individual channels experience the freeze-out condition to the total time of the active intervals and their cor- responding hangover times and front end delays, for all trunks over a fixed interval of time, e.g., one minute. TABLE 1/G.763 +––––––––––––––––––––+–––––––––+––––––––+–––––––––––––+–– –––––––––––––––––––––––+ _ Synchronization _ Buffer _ Slip _ Location _ Figure _ _ Type _ Size _ Size _ _ No. _ _ Note 3 _ Note 1 _ Note 2 _ Note 4 _ _ +––––––––––––––––––––+–––––––––+––––––––+–––––––––––––+–– –––––––––––––––––––––––+ _1. No Buffering _ _ _ _ _ _ _ _ _ _ _ _ A Asynch _No buffer_ - _ - _I 1/G.763 _ _ _ _ _ _ _ _ B Synch _No buffer_ - _ - _I 2/G.763 I 15/G.763 _ _ _ _ _ _I 12/G.763 _ _ C Synch _ _ _ _ _ _ Analogue to _No buffer_ - _ - _I 5/G.763 _ _ Digital _ _ _ _ _ +––––––––––––––––––––+–––––––––+––––––––+–––––––––––––+–– –––––––––––––––––––––––+ _2. Plesiochronous/ _ _ _ _ _ _ Buffering _ _ _ _ _ _ _ _ _ _ _ _ A Asynch _ 0.5 ms _2 frames_Trunk side _I 3/G.763 _ _ _ _ _ _ _ _ B Synch _ 0.5 ms _2 frames_Bearer side _I 4/G.763 _ _ _ _ _ _I 13/G.763 _ _ _ _ _ _I 16/G.763 _ +––––––––––––––––––––+–––––––––+––––––––+–––––––––––––+–– –––––––––––––––––––––––+ _3. Plesiochronous/ _ _ _ _ _ _ Doppler Buffering_ _ _ _ _ _ _ _ _ _ _ _ A Synch _ 1.7 ms _2 frames_Bearer side _I 6/G.763 _ _ _ _ _ _I 11/G.763 I 17/G.763 _ _ _ _ _ _I 14/G.763 I 19/G.763 _ _ _ _ _ _ _ _ B Synch _ 2.4 & _ _Bearer side &_I 7/G.763 _ _ _ 1.7 ms _ _Trunk side _ _ _ _ _ _ _ _ _ C Asynch _ 1.7 ms _2 frames_Trunk side _I 9/G.763 _ _ _ _ _ _ _ _ D Synch _ 2.4 & _ _Trunk side _I 8/G.763 _ _ _ 1.7 ms _ _ _ _ _ _ _ _ _ _ _ E Synch _ 1.7 ms _2 frames_Trunk side _I 10/G.763 _ _ _ _ _ _I 18/G.763 _ +––––––––––––––––––––+–––––––––+––––––––+–––––––––––––+–– –––––––––––––––––––––––+