1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$)4!,G0G03%#4)/.3G0G0!.$G0G0$)4!,G0G0,).%G0G03934%-3$)4!,G0G03%#4)/.3G0G0“!3%$G0G0/.4(%G0G0 G0KBIT SG0G0()%2!2#(9)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.921 was
2、published in Fascicle III.5 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see
3、 below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, el
4、ectronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.5 - Rec. G.921 1Recommendation G.921Fascicle III.5 - Rec. G.921DIGITAL SECTIONS BASED ON THE 2048 kbit/s HIERARCHY(Malaga-Torremolinos, 1984; amended at Melbourne, 1988)1 Characterist
5、ics of digital sections1.1 General features1.1.1 Bit rateThe digital sections based on the 2048 kbit/s hierarchy should be able to transmit signals at the nominal bit rateswith their corresponding tolerances as indicated in Table 1/G.921.TABLE 1/G.921Tolerances on transmitted signalsNominal bit rate
6、(kbit/s)2048 8448 34 368 139 264Tolerance (ppm) 50 30 20 15Note - The 2048 kbit/s digital sections may be operating synchronously or plesiochronously withinthe same environment.1.1.2 Special propertiesThe digital sections based on the 2048 kbit/s hierarchy should be bit sequence independent.1.2 Char
7、acteristics of interfacesThe digital interfaces should comply with Recommendation G.703.1.3 Performance standardsThe performance requirements (e.g. errors, jitter and availability) are specified in terms of a HypotheticalReference Digital Section (HRDS). Such a model is defined in Recommendation G.8
8、01.1.3.1 Error performanceDepending on the various applications in the different parts of a connection as specified inRecommendation G.821, different section quality classes have been defined in Table 2/G.921.1.3.2 JitterTo ensure that the maximum network limit of jitter (see 2 of Recommendation G.8
9、23) is not exceeded withina digital network it is necessary to control the jitter contributed by transmission systems.2 Fascicle III.5 - Rec. G.9211.3.2.1 IntroductionThe jitter specifications relate to hypothetical reference digital sections (HRDS) defined in Table 2/G.921.The limits given below ha
10、ve been derived on the basis that only a few digital sections will be connected incascade and, moreover, no account has been taken of jitter originating from asynchronous multiplexing equipment.However, in certain real network configurations some Administrations may find it necessary to have more se
11、ctions incascade along with many asynchronous digital multiplex. For effective jitter control in these situations it might benecessary to satisfy more demanding limits and/or to use other means of jitter minimization.All the limits given below for digital sections are to be satisfied for all section
12、s regardless of length and thenumber of repeaters.It is important to note that the limits must be met regardless of the transmitted signal. In such circumstances thechoice for a test sequence is left to the discretion of national Administrations. The measurement guidelines given in 4of Recommendatio
13、n G.823 should be taken into account.1.3.2.2 Lower limit of tolerable input jitterThe requirements given in Figure 2/G.823 and Table 2/G.823 should be met.Note - It is recognized that for 2048 kbit/s line sections and under practical conditions of interference thepermissible maximum input jitter may
14、 have to be reduced in the frequency range f3to f4(but retaining the existing20 dB/decade slope below the frequency f3which would result in a slightly lower value for frequency f2). Consideringthat these sections are used in the lowest levels of the network and that actual 2048 kbit/s sources have v
15、ery low outputjitter in the high frequency range (cf. Recommendations G.732, G.742 and Q.551), the resulting performance will beentirely satisfactory.1.3.2.3 Jitter transfer characteristicsThe maximum gain of the jitter transfer function should not exceed the value of 1 dB.Note 1 - The low frequency
16、 limit should be as low as possible taking into account the limitations of measuringequipment. A value in the order of 5 Hz is considered acceptable.Note 2 - For line sections at 2048 kbit/s complying with the alternative national interface option (Note 2 toTable 2/G.823), a jitter gain of 3 dB is p
17、ermitted.TABLE 2/G.921Digital section quality classifications for error performanceSection qualityclassificationHRDS length (km)(see Figure 4/G.801) (Note 2)Allocation(Notes 3, 4)To be used in circuit classification(see Figure 1/G.821)(Notes 5 and 6)1 280 0.45 % High grade2 280 2 % Medium grade3 50
18、2 % Medium grade4 50 5 % Medium gradeNote 1 - There is no intention to confine any quality classification to any specific bit rate. The possibility ofintroducing additional options (for instance concerning length) requires further study.Fascicle III.5 - Rec. G.921 3Note 2 - The indicated values of l
19、ength are those identified in Recommendation G.801. They should be understoodto correspond to maximum lengths of real digital sections. If a real digital section is shorter, there will be noreduction of the bit error allocation (i.e. percentage value in the third column). This takes into account tha
20、t:- in many line systems (especially on copper wire pairs) most bit errors occur at the ends of the system;- in the interest of economy, short-haul systems may be designed with greater per-kilometre error ratio than long-haul systems.If a real digital section is longer (e.g. 450 km), its overall all
21、ocation should correspond to that of an integer numberof HRDSs (of the same quality classification) the combined lengths of which are at least as long as the real sectionlength (e.g. 2 x 280 km).Note 3 - The values in this column are percentages of the overall degradation (at 64 kbit/s) specified in
22、Recommendation G.821; i.e. of the 8% errored seconds, of the 10% degraded minutes and of the 0.1% severelyerrored seconds which are allocated according to the same rules as the two other parameters.Note 4 - To obtain 64 kbit/s error performance data from error measurement at primary bit rates and ab
23、ove, themethod described in Recommendation G.821, Annex D, should be used.Note 5 - May also be used within a lower grade portion of the connection as defined per Figure 1/G.821.Note 6 - To take account of adverse propagation conditions on radio systems as detailed inRecommendation G.821, an addition
24、al percentage of 0.05% of severely errored seconds has been allocated to a2500 km radio-relay HRDP for systems operating in the high and medium grade quality part of the HRX. Thiscorresponds for a 280 km section to a value of 0.0055% to be added to section quality classification 1 and 2allocation wh
25、en applied to severely errored seconds.This would result in an additional allowance of 0.025% of severely errored seconds available for the medium gradepart of the connection if it is realized entirely with class 1 radio sections. Where the medium grade portion of thenetwork is realized with a mixtu
26、re of different classifications, part of this additional allowance may be allocated toclasses 3 and 4 at the discretion of Administrations.To be consistent with the statistical assumptions made in G.821 3.3 b) regarding the number of radio sections inthe HRX, and the occurrence of worst month effect
27、s it may be necessary to take into account the probability ofworst month effects occurring simultaneously for all radio sections in a connection. A statistical model to be usedfor network planning and performance evaluation to assess the consistency of a given connection to the overallobjective of G
28、.821 is under study.1.3.2.4 Output jitter in the absence of input jitterThe maximum peak-to-peak jitter in the absence of input jitter, for any valid signal condition, should not exceedthe limit given in Table 3/G.921.4 Fascicle III.5 - Rec. G.921TABLE 3/G.921The maximum output jitter in the absence
29、 of input jitter for a digital section(Measurements are made in accordance with the method shown in Figure 1/G.823)Maximum output jitter for a digitalsectionMeasurement filter bandwidthBitrate(kbit/s)HRDSlength(km)Lowfrequencylimit(f1-f4)Highfrequencylimit(f3-f4)Band-pass filter having a lower cut-o
30、fffrequency f1or f3and an upper cut-offfrequency f4unit intervalpeak-to-peakunit intervalpeak-to-peakf1f3f42 048 50 0.75 0.2 20 Hz 18 Hz(700 Hz)100 kHz8 448 50 0.75 0.2 20 Hz 3 kHz(80 kHz)400 kHz34 368 50 0.75 0.15 100 Hz 10 kHz 800 kHz34 368 280 0.75 0.15 100 Hz 10 kHz 800 kHz139 264 280 0.75 0.075
31、 200 Hz 10 kHz 3 500 kHzNote - For interfaces within national networks the frequency values (f2and f3) shown in parenthesis may be used.1.3.3 AvailabilityUnder study.This performance requirement will be defined taking into account Recommendations G.821, E.800 and CCIRRecommendation 557.1.4 Fault con
32、ditions and consequent actions1.4.1 Fault conditionsThe digital sections should detect the following fault conditions.1.4.1.1 Internal power failure of the line terminal equipmentNote - Line refers to both cable and radio-relay equipments.Fascicle III.5 - Rec. G.921 51.4.1.2 Error ratio 1 10-3The co
33、nsequent actions should be taken when the bit error ratio is considered to exceed 1 10-3. Some form ofpersistence check should be employed to establish with appropriate confidence that a fault condition does exist. In anycase, the alarm indication should be given with 500 ms of the start of the faul
34、t condition; this period includes thedetection and any persistence check time.The alarm indication should be removed once it has been established, with appropriate confidence, that the faultcondition has disappeared.1.4.1.3 Loss of the signal at the receiving terminalNote - The detection of this fau
35、lt condition is required only when it does not result in an indication“error ratio 1 l0-3“.1.4.1.4 Loss of alignment when alphabetic line codes or additional frames are usedNote - The detection of this fault condition is required only when it does not result in an indication“error ratio 1 10-3“.1.4.
36、1.5 Loss of incoming interface signal1.4.2 Consequent actionFurther to the detection of a fault condition, appropriate actions should be taken as specified in Table 4/G.921.TABLE 4/G.921Fault conditions and consequent actions for digital sections based on the 2048 kbit/s hierarchyMaintenance alarms
37、AIS toEquipment Fault conditions prompt deferred(see Note)Line side Interface sideLine terminalequipmentInternal power failure yes if practicable if practicableError ratio 1 10-3yes yesLoss of the signal at thereceiving terminalyes yesLine side only Loss of alignment whenalphabetic line codes oraddi
38、tional frames are usedyes yesInterface side only Loss of incoming signal yes yesNote - As far as network performance objectives are concerned, criteria to activate deferred maintenance alarm are needed. Theyshould be provided by the systems if possible.6 Fascicle III.5 - Rec. G.9211.4.2.1 Prompt mai
39、ntenance alarm indication generated to signify that performance is below acceptable standards andmaintenance attention is required locally.Note - The location and provision of any visual and/or audible alarm activated by the alarm indications given in 1.4.2.1 above, is left to the discretion of each
40、 Administration.1.4.2.2 AIS applied to the line side (see Notes 1 and 2).1.4.2.3 AIS applied to the interface side.Note 1 - The equivalent binary content of the alarm indication signal (AIS) is a continuous stream of ones.Note 2 - The bit rate of this AIS should be within the tolerance limits define
41、d in Table 1/G.921.Note 3 - In the case of power failure apply AIS only if practicable.1.4.3 Time requirements for application of AISIn general, the AIS should be transmitted coincident with the detection of the fault conditions given inTable 4/G.921, except for AIS under the fault condition “error
42、ratio 1 10-3“; in this latter case the time requirementsgiven in 1.4.1.2 should be respected.Note - For wholly national digital sections and, with the agreement of the countries involved digital sectionswhich cross international boundaries, an option to delay the transmission of an AIS of up to a fe
43、w seconds may beneeded when the application of an AIS is controlled by means of a G.921 threshold monitoring process based on theseverely errored second G.821 parameter. Short downstream alarms in international digital links which are routed viawholly national digital sections may appear during these few seconds.
