Basic Characteristics of High-Rate Multiplexing Systems with Time Compression of Channels According to ITU-T Recommendations

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1 Telecommunications and Radio Engineering, 64(3): (2005) Basic Characteristics of High-Rate Multiplexing Systems with Time Compression of Channels According to ITU-T Recommendations Part 1. Basic Characteristics of Multiplexing Systems of the First Level of Plesiochronous Digital Hierarchy S. G. Emelianov, O. I. Atakishchev, I. S. Zakharov, E. A. Titenko, E. Yu. Musakin, and A. A. Chizhov $%675$&7 +LHUDUFK\ RI KLJKUDWH PXOWLSOH[LQJ V\VWHPV ZLWK WLPH FRPSUHVVLRQ RI FKDQQHOV SULQFLSOHV RI FRQVWUXFWLRQ DQG EDVLF FKDUDFWHULVWLFV RI ILUVWOHYHO KLHUDU FK\ V\VWHPV DFFRUGLQJ WR WKH UHFRPPHQGDWLRQV E\,787 * * KDYH EHHQ GLVFXVVHG At present, digital communications is a serious and dynamically expanding market of telecommunication services that experiences a rapid increase in all trends and parameters. A substantial number of methods for exchanging different types of information have appeared at the user level. However, along with the newly generated data transmission methods, a considerable traffic share in the current data exchange systems belongs to PDH (plesiosynchronous digital hierarchy) communication utilizing aggregate signal multiplexing techniques described in this article. An intensive development of time channel compression systems began in early 70 s of the previous century. During first years, the development remained within the frames of national communication systems. Multiplexers for time compression of channels produced during that period typically differed from each other by their parameters [1]. Time compression as the most effective method for multiplexing of channels was approved by the Consultative Committee for International Telephone and Telegraphy (CCITT) and was accepted for wide use in the world communication network. 195 ISSN Begell House, Inc.

2 S. G. Emelianov et al. First recommendations by CCITT for high-rate multiplexing systems of the first level were accepted in The VI Plenary Assembly held in 1976 accepted recommendations for all classes of multiplexing systems, namely: - low-rate systems (aggregate transmission rate of up to 1200 bps) designed for telegraphy Recommendation R. 44 [2]; - medium-rate systems (aggregate transmission rate from 1200 to 9600 bps) designed for telegraphy and data transmission Recommendation R. 111 (Part II) [2]; - high-rate systems of zero level (aggregate transmission rate of 64 kbps) designed for telegraphy and data transmission Recommendation R. 111 (Part I) [2], Recommendations X. 50 and X. 51 [2]; - high-rate systems of first to fourth level (aggregate transmission rate over 64 kbps) designed for digital systems Recommendations G. 732 G. 734, G. 742 G. 746, G. 751, G. 752 [3]. The high-rate multiplexing systems with time compression of channels are intended to transmit digital signals and are therefore referred to in literature as digital transmission systems [4 6]. These systems also serve to transmit data and telegraph messages via 64 kbit time channel intervals. The prime tasks of this class of systems include conversion of an analog signal to a digital form through a pulse-code modulation (PCM) method, formation at the transmitting terminal of a digital sequence (aggregate signal), transmission of this sequence through digital linear paths to a predetermined distance and reconversion of the signal at the receiving terminal. Digital transmission systems (DTS) provide signal transmission in a unified digital form independently of the type of data being transmitted and thereby possess the following primary advantages over analog systems [4]: - insignificant noise accumulation along the trunk length; - high stability of channel parameters under the conditions of line attenuation fluctuations and influence of other destabilizing factors; - high data transmission capacity; - capability of simultaneously transmitting signals of different data practically without their interference; - capability of total signal regeneration at regeneration terminals; - substantially smaller dimensions of hardware due to preferential utilization of integrated circuits, specifically highly integrated chips; - easier and less expensive operation resulting from identity of many hardware units. 196

3 High-Rate Multiplexing Systems. Part 1 A specifically important advantage of the digital transmission method is the possibility to construct transmission and switching systems according to a common principle and on a common elemental base, which has allowed implementation of an integrated digital communication network for the transmission of signals of different data types. The DTS are used in order to multiplex digital flows being transmitted in cable, radio relay, and satellite links. 1. Hierarchy of High-Rate Multiplexing Systems with Time Compression of Channels According to aggregate transmission rates, the high-rate multiplexing systems with time compression of channels (TCC) form a so-called hierarchic structure (Table 1 where an asterisk marks the rates recommended by ITU-T). The creation of hierarchic structure systems rests on a general principle of uniting several systems of a preceding hierarchy step at each subsequent step. The number of preceding steps of hierarchy united in a subsequent step is referred to as a multiplexing factor. Table 1 demonstrates that the ratio of the rate of a subsequent step to the rate of a preceding step exceeds the multiplexing factor. This results from a need of introducing service information into an aggregate signal. The transmission rate of 2048 kbps in Europe and 1544 kbps in North America and Japan corresponds to the first level of hierarchy. The lack of standard type equipment for the first-level systems has led to a similar situation with systems of subsequent steps. Therefore, three systems currently exist in the hierarchic structure: North-American, Japanese, and European. The hierarchic structure for Europe has been accepted by CEPT (Conference of European Postal and Telecommunication Administrations). The hierarchic structure levels marked in Table 1 by asterisks were accepted by the VI Plenary Assembly of CCITT in Basic characteristics of systems of these levels are analyzed below. Table 1. Hierarchic structure of high-rate multiplexing systems with time compression Type of system System level depending on aggregate transmission rate, Mbps zero first second third fourth fifth North-American * 1,554 * 6,312 * 44,736 * 274,176 Japanese * 1,554 * 6,312 * 32,064* 97, ,200 European * 2,048 * 8,448 * 34,368 * 139,264 * 564,

4 S. G. Emelianov et al. 2. Basic Characteristics of High-Rate Multiplexing Systems of First Level Basic characteristics of a PCM system of first level with aggregate transmission rate of 2048 kbps (Recommendation ITU-T G. 732) Time compression method: synchronous [2]. Trunking method: character-by-character [1, 2]. Aggregate transmission rate: 2048 kbps. Channel rate: 64 kbps (after conversion of an analog signal to a discrete one). Number of channels: 32, of which 30 are intended for voice signal transmission and 2 for service signal transmission. Channel code: eight-element. The voice signal coding corresponds to A-law analyzed in ITU-T Guide G. 711 (see Section 6). Input channel type: code-dependent [1]. Aggregate transmission channel: digital path rated 2048 kbps. System cycle structure (shown in Fig. 1). Supercycle structure. Channel interval 16 is intended for alarm information transmission. If this channel interval is not needed for alarm, it can be used in other purposes except organizing a voice-band channel by means of a PCM hardware coder. Alarm may be implemented both via a common channel and via dedicated channels. FIGURE 1. Structure of the PCM system cycle of the first level with aggregate transmission rate of 2048 kbps (G. 732). 198

5 High-Rate Multiplexing Systems. Part 1 Table 2. Supercycle structure of time channel interval 16 Cycle 0 Cycle 1 Cycle 2 Cycle XYXX abcd abcd abcd abcd abcd abcd Channel 1 Channel 16 Channel 2 Channel 17 Channel 15 Channel 30 Note: 1. X is a reserve bit corresponding to 1, when not used. 2. Y is a bit used to indicate supercycle synchronization loss. 3. a, b, c, d are bits of alarm of the i-th voice channel of the system, i = 1, If bits b, c, and d are not used, they are assigned the following values: b = 1, c = 0, d = 1. Combination of 0000 is not used for alarm purposes in channels For the purpose of alarm via dedicated channels, a supercycle structure comprising 16 sequential cycles numbered from 0 to 15 is used in channel interval 16. Under this condition, alarm for each voice-band channel is transmitted by bits a, b, c, and d (Table 2). A supercycle structure synchronization signal corresponds to 0000 and occupies synchro positions from 1st to 4th of the time channel interval 16 of cycle 0. The distribution of supercycle structure bits is given in Table 2. Basic characteristics of PCM system of first level with aggregate transmission rate of 1544 kbps (Recommendation ITU-T G. 733) Time compression method: synchronous. Trunking method: element-by-element. Aggregate transmission rate: 544 kbps. Channel rate: 64 kbps (after conversion of an analog signal to a discrete one). Number of channels: 24, all channels used for voice signal transmission. Channel code: eight-element. The voice signal coding corresponds to M-law analyzed in ITU-T Guide G. 711 (see Section 6). In order to represent a zero (analog) quantity, two code combinations are used ( ) and ( ). An all-zero ( ) code combination in some networks is suppressed in order to avoid timing data loss in a digital path. Input channel type: code-dependent. Aggregate transmission: digital path rated 1544 kbps. System cycle structure (shown in Fig. 2). A single F character is added to each cycle and is used as a cycle or a supercycle synchronous signal or for alarm purposes, and thus the number 199

6 S. G. Emelianov et al. FIGURE 2. Structure of the PCM system cycle of the first level with aggregate transmission rate of 1544 kbps (G. 733). Table 3. Formation of cycle synchronous signal Cycle number Signal of synchronization of cycle supercycle S S Table 4. Supercycle structure Cycle number Synchronization signal of No. of the character in each channel interval for cycles supercycle(s) code combinations alarm Alarm channel A B 200

7 High-Rate Multiplexing Systems. Part 1 of N characters in the cycle is 193 (N = = 193). The F character occupies the first position in the cycle. The formation of a cycle synchronous signal and the S-bit distribution (for supercycle synchronization or alarm) are presented in Table 3. Supercycle structure. A supercycle structure comprised of 12 cycles is used for alarm purposes (see Table 4). A supercycle synchronous signal is formed from S-bits as shown in Table 4. At output from cycle synchronism, the S-bit is changed in cycle 12 from 0 to 1. Cycles 6 and 12 are alarm ones. The eighth bit of each channel interval in each alarm cycle serves to transmit signal information of a respective voice-band channel. Basic characteristics of system of first level with aggregate transmission rate of 1544 kbps for operation with digital switching stations (Recommendation ITU-T G. 734) Application of the system in digital paths of 2048 kbps capacity connected to digital switching stations. The cycle structure of this system is compatible with the cycle structure of the initial PCM system described above and may be used with digital paths linking similar PCM hardware to digital switching stations, as well as to digital paths interlinking the digital switching stations. Time compression method: synchronous. Trunking method: character-by-character. Aggregate transmission rate: 2047 kbps. Channel rate: 64 kbps. The number of channels: 32. The channels may serve to transmit both voice information and data received, for example, from the output of a zerolevel system that meets ITU-T Recommendation X. 50. Channel code: eight-element. The voice signal coding corresponds to A-law considered in ITU-T Guide G Channels allocated for other types of communication, for example, for synchronous data transmission (Recommendation X. 50), may need a coordinated method of employment. The type of the input channel: code-dependent. The aggregate path channel: digital path of 2048 kbps. The cycle structure and cycle synchronism procedures: as determined by ITU-T Recommendation G. 732 (see above). 201

8 S. G. Emelianov et al. Additional time channel intervals. If a higher capacity of an alarm channel between switching stations is needed, additional time channel intervals may be employed to provide alarm via a common channel. They are selected from a number of intervals intended within PCM hardware for data transmission. In routings between switching stations that comprise more than one digital path of 2048 bps, it might become possible to provide a respective capacity of the alarm system without the use of channel intervals of all systems in the routing. In such cases, channel intervals 16 not used for alarm may be assigned to transmit voice signals or signals of other types of communications. A zero channel interval is reserved for cycle synchronization, alarm, and network synchronization, and should not be used for alarm or voice signal transmission purposes. In general, the accomplished analysis of peculiar features of the first hierarchy level with the account for complex realization of similar digital signal processing algorithms has demonstrated the possibility of implementing all-purpose multiplexing/demultiplexing units in distributed corporate computing systems on the basis of modern all-purpose digital data processing devices. All signal structures described above may be processed by a unit occupying a single PCI (ISA) interval in a PC. REFERENCES 1. CCITT, (1976), Orange Book, Vol. III.2, Line Transmission: Recommendations of Series G, H, J, Geneva. 2. Radioelektronika in 1974, Obzor po materialam inostrannoi pechati. II. Sistemy i sredstva svyazi. Sistemy i sredstva peredachi dannykh (Radio Electronics in 1974, Review of Foreign Publications, II. Communication System and Facilities. Data Transmission Systems and Facilities) (1975), Moscow. 3. Bylyanski, P. and Ingrem, D. (1980), Tsifrovye sistemy peredachi (Digital Transmission Systems), Svyaz Press, Moscow. 202