1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 ) TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2!4%$G0G03%26)#%3G0G0$)4!,.%47/2+G0G0 )3$.G9).4%2.%47/2+G0G0).4%2 however, at least one network provider should contain an ISDN.The internetwork termination (IT) is a functional grouping associated with t
2、he proper physical andelectromagnetic termination of the network as well as section, link and/or circuit termination of the network. Note thatspecific functions of IT may be performed with one or more pieces of equipment.Reference point Q should be one of the equipment interfaces listed in Recs. G.7
3、02 and G.707. The specificationof Q can be used as the common description of the layer 1 specification for the different logical interfaces K, M and N.The digital link of each network should be terminated at Q.2.2 Physical realizations of reference configurationFigure 2/I.511 gives examples of confi
4、gurations made up of combinations of physical interfaces at referencepoint Q; Figure 2a/I.511 shows an interface without transmission section (line or radio); and Figures 2b/I.511 and2c/I.511 show interfaces with transmission sections.In every case, reference point Q should appear as the equipment i
5、nterface.The mandatory functions of IT described in 3 are the same in each application, while the optional functionsmay be different according to the following cases, if interworking:- with or without transmission sections,- with or without master-slave relation, such as master-slave synchronization
6、 and remote maintenancebetween two network providers.2 Fascicle III.9 - Rec. I.5113 Layer 1 interworking functionsLayer 1 interworking functions at Q, which may be performed by the IT, should be classified into mandatoryand optional functions.Fascicle III.9 - Rec. I.511 3 4 Fascicle III.9 - Rec. I.5
7、113.1 Mandatory functionsEvery item related to mandatory functions should always be carried out in order be classified into mandatoryand optional functions.3.1.1 Providing standardized equipment interfacesReference point Q should be applied to one of the equipment interfaces standardized in the G.70
8、0-G.900 SeriesRecommendations for digital networks, transmission systems and multiplexing equipment.Items to be standardized are as follows:1) Interface bit rateInterface bit rate at Q should be selected from among the hierarchical bit rates defined in Recs. G.702 andG.707.It should be noted that th
9、e interworking hierarchy be applied to the international interworking as definedin Rec. G.802 when interconnection based on an asynchronous hierarchy is adopted.2) Physical/electrical characteristicsPhysical/electrical characteristics at Q should conform to the relevant part of G.700-G.900 SeriesRec
10、ommendations.3) Functional characteristicsFunctional characteristics at Q should conform to the relevant part of G.700-G.900 SeriesRecommendations.4) Time slot assignmentThere are two methods for assigning time slots in the frame structure to various channels: the fixed formatmethod and the variable
11、 format method. A set of examples of both methods is described in Figure 3/I.511.Fixed format - Time slots for interworking information channels are preassigned in a fixed manner in theinterworking frame structure by bilateral negotiation.Variable format - A flexible time slot is allocated to any in
12、formation channel on a demand assignmentbasis.5) Time slot sequence integrityTime slot sequence integrity should be performed. Furthermore, it is preferable to perform time slotsequence integrity with 8 kHz integrity.Fascicle III.9 - Rec. I.511 53.1.2 Providing layer 1 maintenance capabilityReferenc
13、e point Q should meet the maintenance requirements defined in the relevant part of the M-Series andN-Series Recommendations.Items to be standardized are as follows:1) Termination of the digital linkTermination of the digital link should conform to the relevant part of the M-Series Recommendations.2)
14、 Termination of the digital circuitTermination of the digital circuit should conform to the relevant part of the M-Series Recommendationsand is deferred for further study.3.2 Optional functionsNot all of the items of the optional functions can be achieved at reference point Q. Only some of them ares
15、elected according to the features of each connection type or differences in the relationship between network providers.3.2.1 Providing interworking between different connection types in layer 1In some applications, connection types which are different in layer 1 items can be successfully interconnec
16、tedover reference point Q by using the optional capabilities listed below.Items to be standardized are as follows:1) Coding rule conversioni) -A law coding rule conversion should be performed according to Rec. G.802 in the case of speechand 3.1 kHz audio services;ii) Unrestricted 64 kbit/s digital s
17、ervice shall not be subject to network provided conversion.2) Interconnection among connection types with different layer 1 attributesRate adaption should be performed according to Recs. I.460, I.461, I.462, I.463 and I.464.3.2.2 Providing network synchronization clockIf network synchronization is performed over reference point Q by other methods than the plesiochronousmethod, the clock should meet the requirement defined in Rec. G.812.
