1、CCITT FASCICLE IV.3 * m 4862593 0505437 A m Recommendation M.562 TYPES OF CIRCUIT AND CIRCUIT SECTION 1 General 1.1 The purpose of this Recommendation is to describe the terms “circuit section” and “circuit”, as they are used in the Series M Recommendations, taking into consideration analogue, digit
2、al, and mixed analogue/digital constitutions. 1.2 The circuit types described in this Recommendation differ somewhat from those in Recommenda- tion G.lO1 i, in order to emphasize distinctions that are useful in setting forth maintenance procedures. 2 Circuit sections Each of the first three section
3、types listed below corresponds to one of the three channel types defined in Recommendation M.300. Each of the last two circuit section types corresponds to one of the two terminal types also defined in Recommendation M.300. 2.1 Analogue circuit section An analogue circuit section comprises two analo
4、gue channels, one for each direction of transmission. 2.2 Mixed circuit section A mixed circuit section comprises two mixed channels, one for each direction of transmission. 2.3 Digital circuit section A digital circuit section comprises two digital channels, one for each direction of transmission.
5、2.4 Mixed analogue/digital terminal circuit section A mixed analogue/digital terminal circuit section comprises the two directions of transmission, for one equivalent voice-frequency signal, through a PCM multiplex equipment. rn the analogue to digital direction, the mixed analogue/digital terminal
6、circuit section extends from the audio input of the PCM multiplex equipment, to the associated 64 kbit/s time slot appearance at the digital output. In the digital to analogue direction, the mixed analogue/digital terminal circuit section extends from the 64 kbit/s time slot appearance of a particul
7、ar channel at the digital input to the PCM multiplex equipment, to the associated audio output. 2.5 Digital terminal circuit section A digital terminal circuit section comprises the two directions of transmission, for one equivalent voice-frequency signal, through a digital terminal. For each direct
8、ion of transmission, the digital terminal circuit section extends from a particular 64 kbit/s time slot appearance, in the input bit sequence to the digital terminal, to the corresponding 64 kbit/s time slot appearance in the output bit sequence of the digital terminal. 3 Circuits International circ
9、uits comprise various combinations of national and international circuit sections, together with ancillary equipment as required. The following circuit types are defined in terms of their constituent circuit sections, as a basis for recommending appropriate maintenance procedures. 7 ! Fascicle IV.l
10、- Rec. M.562 339 CCITT FASCICLE IV-L * = 48b2593 0505438 T 3.1 Analogue circuit An analogue circuit comprises one or more analogue circuit sections. These circuits terminate at both ends in analogue switching machines. A schematic drawing is shown in Figure 1/M.562. 1 r- I Symbols and nomenclature u
11、sed in Figures l/M.562 to 3/M.562 ACS - Analogue circuit section MCS - Mixed anaiogue/digital circuit section DCS - Digital circuit section MTCS - Mixed analogue/digital terminal circuit section DTCS - Digital terminal circuit section -id- *. Digital switching !-.1 I 1 ! +- Analogue switching L.-.-.
12、_I - Analogue transmission -1- Analogue attenuator (see Note) =- Digital transmission +- Transmultipiexer +- Mixed terminai -.i- Digital terminal j Station boundaries Note - The use of this symbol in the figures indicates the location of any analogue attenuation required to meet the CCIT transmissio
13、n plan. FIGURE 1/M.562 Analogue circuit 3.2 Mixed analogue/digital circuit A mixed analogue/digital circuit comprises any combination of circuit sections that includes one or more analogue to digital, or digital to analogue, conversion processes. Mixed analogue/digital circuits may terminate at eith
14、er end in analogue or digital switching machines. Combinations of various types of circuit sections that are acceptable in making up mixed analogue/digital circuits are constrained by the need to avoid excessive transmission impairments. These constraints are discussed in 0 5 below. Examples of perm
15、itted mixed analogue/ digital circuit configurations are shown schematically in Figure 2/M.562. 340 -Fascicle IV.l - Rec. M.562 7 -1 CCITT FASCICLE IV-L * 4862593 0505419 .- L I- - - - 1 a) Analogue switching at each end; digital transmission; mixed terminals r-i r-i I b) Analogue switching at each
16、end; both analogue and digital transmission, with transmultiplexer at transmission interface ; mixed terminal r-1 r-l c) Digital switching at one end, with analogue switching at the other end; analogue transmission; transmultiplexer d) Digital switching at one end, with analogue switching at the oth
17、er end; both digital and analogue transmission, with transm uitipexer a F transmission in teriace FIGURE 2/M.562 Mixed analogue/digital circuits Fascicle IV.l - Rec. M.562 341 CCITT FASCICLE IV.3 * W 4862593 0505420 8 J .-.-.i e) Digital switching at one end, with analogue switching at the other end
18、: digital transmission: mixed terminal !-1 r-i f) Digital switching; analogue transmission: transmultiplexer -T-“-T-T- j ! L J L _I i - I L J L J CClTl 11110 g) Digital switching; both digital and analogue transmission; transmulfiplexers at the transmission interfaces Note - Explanation of symbols a
19、nd nomenclature is given in Figure UM.562. FIGURE 2/M.562 (con&) Mixed analogue/digital circuits 3.3 Digital circuit A digital circuit comprises one or more digital circuit sections. In addition to the digital circuit section), a digital circuit may include one or more digital terminal circuit secti
20、ons. These circuits terminate at both ends in digital switching machines. A schematic drawing is shown in Figure 3/M.562. 4 Ailocation of losses in mixed analogue/digital circuits In Figure 21M.562, the attenuators needed to control any variability in the analogue portions of the circuits, arising f
21、rom loss variations with time, or attenuation distortion, are shown symmetrically for both directions of transmission. However, in practice, such arrangements may require nonstandard levels at the boundaries between circuit sections. Administrations are advised that should they prefer to adopt an as
22、ymmetric arrangement, e.g., by putting all the loss into the receive direction at only one end of a circuit or circuit section, then, provided that the loss is small, e.g., a total of not more than 1 dB, the small amount of asymmetry that results in the international portion of the connection will b
23、e acceptable, bearing in mind the small number of international circuits encountered in most actual connections. 342 Fascicle IV.l - Rec. M.562 CCITT FASCICLE IV-1 * 4862571 0505L121 T m I a) Digital circuit I r b) Digital circuit with digital terminal circuit section Note - Explanation of symbols a
24、nd nomenclafure is given in Figure 1/M.562. FIGURE 3/M.562 Digital circuits 5 Number of unintegrated PCM digital processes 5.1 General principle It is recognized that in the mixed analogue/digital period, there could be a considerable presence of unintegrated digital processes in the worldwide telep
25、hone network. Consequently, it is important that the incorporation of these processes should take place in such a way that when integration of functions can occur, unnecessary items of equipment will not remain in the all-digital network. 5.2 Restrictions due to transmission impairments In the mixed
26、 analogueldigital period, it may be necessary to include a substantial number of unintegrated digital processes in international telephone connections. To ensure that the resulting transmission impairments (quantization, attenuation and group-delay distortion) introduced by such processes do not acc
27、umulate to the point where overall transmission quality can be appreciably impaired, it is recommended that the planning rule given in Recommendation G.113, 0 3 2, be complied with. The effect of this rule is to limit the number of unintegrated digital processes in both the national and internationa
28、l parts of telephone connections. In the case of all-digital connections, transmission impairments can also accumulate due to the incorpora- tion of digital processes (e.g., digital pads). The matter of accumulating such impairments under all-digital conditions is also dealt with in Recommendation G.113, 0 3 2. References CCIT Recommendation The transmission plan, Vol. III, Rec. G.101. CCIT Recommendation Transmission Impairments, Vol. III, Rec. G.113. i 2 Fascicle IV.l - Rec. M.562
