1、CCITT FASCICLE 1V.L * M LIBb257I 0505422 I Mi Recommendation M.565 ACCESS POINTS FOR INTERNATIONAL TELEPHONE CIRCUITS 1 General This Recommendation specifies the access points required for testing and measuring purposes on international telephone circuits. (Access points for other types of circuit a
2、re dealt with in Recommenda- tion M.110.) 2 Types of access point and their uses 2.1 provided and used in accordance with the following principles: Three basic types of access points are required for international telephone circuits. These should be 2.1.1 The international circuit for public telepho
3、ny includes the international line (as defined in Recommenda- tion M.700). Points serving to distinguish the ends of the international line should be provided, where possible, in the form of 4-wire access points called line access points as defined below. line access points (points daccs la ligne -
4、puntos de acceso a la lnea) Points used by the CCITT to define the limits of an international line, and from which measurements are made. Only one “line access point” exists at each end of an international line. The precise location of each such point depends on the Administration concerned 11, Wher
5、e a digital international exchange is interfaced with the transmission network by primary (or higher order) digital paths, a line access point at “circuit” level cannot generally be provided. In this case, any necessary testing of circuits normally carried out at the line access point may be perform
6、ed at the digital path access point 2, nearest to the international exchange. Circuit tests which involve the sending of test signals should be carried out from the circuit access point as defined in 2.1.2 below. 2.1.2 should be provided. At the international switching centres, at the terminals of a
7、 circuit, “circuit access points” as defined below circuit access points (points daccs au circuit - puntos de acceso al circuito) Four-wire access points so located that as much as possible of the international circuit is included between corresponding pairs of these access points at the two centres
8、 concerned. These points, and their relative level (with reference to the transmission reference point) are determined in each case by the Administration concerned. They are taken as the basic practical reference points of known relative level to which other transmission measurements will be related
9、. In other words, for measurement and lining-up purposes, the level at the appropriate circuit access point is the relative level with respect to which other levels are adjusted. The requirement to provide a circuit access point within a digital international exchange will be met by any suitable met
10、hod of obtaining access to the digital bit sequence (time slot) corresponding to an individual telephone circuit. Where a circuit uses channel associated signalling it should be possible to identify and measure at the circuit access points, the signal-transmission parameters, e,g., type of signal, s
11、equence, timing, duration, level and frequency. 2.1.3 Where an international telephone circuit appears at its basic frequencies or basic bit rate within a transit country, an intermediate access point for testing and measuring purposes should be provided at that location in the transit country. ) A
12、compandor, if fitted, should be connected on the line side of the line access points and not between the line access point and the circuit access point. In this way the relationship between the nominal transmission levels at these two points on a circuit with a compandor is the same as for other cir
13、cuits. *) The access points required for digital leased circuits have yet to be specified. This matter is for further study by Study Group IV, in association with Study Group XV. 7: - 344 Fascicle IV.1 - Rec. M.565 CCITT FASCICLE IV.3 * = 4862573 0505423 3 2.2 The line access points and circuit acce
14、ss points (and any intermediate access points in transit countries) will be used by the appropriate testing points3) in all tests and measurements for the line-up and maintenance of international telephone circuits. 2.3 At the discretion of Administrations, means of giving access to the circuit acce
15、ss points and/or the line access points from remote loctions may be provided - such remote locations being within or outside the international centre. Such arrangements avoid the need for staff to enter equipment areas for circuit testing purposes, and increase the flexibility and efficiency of the
16、maintenance organization where large numbers of circuits must be maintained. 2.4 In order to line-up and maintain circuits routed on a mixture of analogue and digital systems it is necessary to carry out measurements of analogue circuit parameters at digital international exchanges. If this requirem
17、ent cannot be met by suitable digital test equipment, it may be necessary to provide a “test coder/ decoder” to convert digital access points (operated at 64 kbit/s for example) to analogue access points (at voice frequency), thus enabling analogue measuring equipment to be used. 2.5 Figure 1/M.565
18、shows typical access and test equipment arrangements for analogue and digital interna- tional exchanges. Subject to meeting the requirements in $3 2.1 to 2.4 above, the actual arrangements at a particular international centre are left to the discretion of the Administration concerned. Note - Remote
19、access arrangements, as described in Q 2.3, are only a physical extension of the access points to a more convenient location(s). Thus, in the Series M Recommendations, the terms “line access points” and “circuit access points” are used (without qualification) irrespective of the manner in which the
20、required access is obtained. 3 Transmission characteristics and choice of levels at analogue access points 3.1 The impedance at analogue access points should have a return loss against the nominal impedance of the measuring apparatus of the station (for example 600 ohms, nonreactive) of not less tha
21、n 20 dB over the range 600-3400 Hz and not less than 15 dB over the range 300-600 Hz. 3.2 It should be recognized that the analogue link access point shown in Figure lb/M.565 is suitable only as a maintenance access point. It is not intended for the setting-up or lining-up of individual circuits, be
22、cause the circuits levels at these points are not specifically defined. This occurs because the shape of the group and supergroup filters have not been compensated for at this point by the channel translating equipment adjustments. 3.3 It is not possible to recommend a value for the nominal transmis
23、sion loss between the circuit access points of a switched public telephony circuit, because of the freedom accorded to Administrations in choosing the transmission levels at these points. However, bearing in mind that the attenuation between the circuit access points and the virtual analogue switchi
24、ng points will have a fixed and known value and that it is possible to build out the wiring to circuit access points to a known loss, the send and receive level at the circuit access point should be chosen such that the circuit level diagram is respected. 3.4 It is advantageous to adopt the same val
25、ue of relative level at the send line access points for every circuit connected to the exchange. Similarly, all the receive line access points could also be at a particular common nominal value of relative level. When relative levels are made uniform in this way, line-up and maintenance activity is
26、greatly simplified. Also, lines can be readily cross-connected at the line access points, which is useful in the immediate replacement of faulty lines in an emergency. 3.5 If the nominal relative level at the receive line access point is chosen to be higher than that at the send line access point of
27、 the same exchange, this leve difference can be used to offset the inherent transmission loss in the signalling and switching equipment, and the requirements of the CCITT transmission plan can be met without the obligation to install supplementary audio-frequency amplifiers. Note - It is preferred t
28、o make transmission measurements between 4-wire access points but, as a permissible alternative, a terminating unit may be provided together with an associated 2-wire access point for measurement purposes. The transmission levels and losses must be chosen so that the nominal loss between virtual ana
29、logue switching points is 0.5 dB (or 0.0 dB for wholly digital circuits), and the circuit level diagram is respected. 3, For example, those defined in Recommendations M.717 and M.718. I Fascicle IV.l - Rec. M.565 345 CCITT FASCICLE IV-L * m 4862591 0505424 5 I, c O a, vj 3 .“ E6 $ me il I a, o a o o
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31、 2 0- a, 346 Fascicle IV.l - Rec. M.565 . CCITT FASCICLE 1V.L tt = 4862591 0505425 7 4 Interface requirements at digital access points 4.1 interface requirements of Q 1.2.3 of Recommendation G.703 i. 4.2 meet the interface requirements of Recommendation G.703 i. 4.3 are under study by Study Group IV
32、. Digital access points at 64 kbit/s should be operated in the contradirectional mode and should meet the Digital path access points, operated at 1544 kbit/s or 2048 kbit/s (or higher hierarchical bit rates) should Interface requirements for digital access points on circuits using an encoding techni
33、que other than PCM Reference i CCIT Recommendation, Physical electrical characteristics of hierarchical digital interfaces, Vol. III, Rec. G.703. Recommendation M.570 CONSTITUTION OF THE CIRCUIT; PRELIMINARY EXCHANGE OF INFORMATION 1 As soon as it is decided to bring a new circuit into operation, th
34、e technical services of the terminal countries should agree upon the circuit control station, and the technical service of each transit country should advise the other technical services concerned of the name of the sub-control station chosen for its territory. If the circuit is routed in a direct g
35、roup or block crossing a transit country without demodulation or demultiplexing, no sub-control station need be provided for the transit country. When a circuit is subjected to analogue-to-digital conversion using, for example, a transmultiplexer, the location of the transmultiplexer should be desig
36、nated as circuit sub-control station. Also the technical services of all the countries concerned should send to the technical service responsible for the circuit control station information which will be required for the preparation of the circuit routing form (see the Appendix I to this Recommendat
37、ion) using the letter and number code on the form. The information for a circuit without audio sections will consist of the numbers of the groups or blocks used and the number of the channel in each group or block. In the case of a both-way circuit, the circuit order of selection should be stated, i
38、ncluding the method by which the International Switching Centre selects circuits (i.e., sequentially by circuit, fully randomly or sequen- tially by block of circuits but randomly within the block, etc.). When the circuit is assigned its designation (according to Recommendation M.140, 0 i), the Admi
39、nistra- tion with control station responsibility will assemble the necessary technical and operational information. This should be entered into the list of Related Information (as defined in Recommendation M.140, 0 2) which consists of the items shown in Annex A. 2 Exchange of information The inform
40、ation should preferably be sent by telex and the examples below show typical telex messages concerning the provision of Bucuresti-London 1 when the circuit is analogue, digital or mixed analogue/digital. This method using the telex services enables agreement on routing details to be obtained quickly and also enables circuit routing forms to be completed by the technical services responsible for the circuit control stations as soon as a circuit is put into service or rearranged. - r _I Fascicle IV.l - Rec. M.570 347
