1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 ) TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2!4%$G0G03%26)#%3G0G0$)4!,.%47/2+G0G0 )3$.G9)3$.G0G053%2G13.%47/2+G0G0).4%2 amended at Melbourne, 1988)1 GeneralThis Recommendation defines limited sets of both channel types and interface structures for
2、ISDN user-network physical interfaces.2 Definitions2.1 A channel represents a specified portion of the information-carrying capacity of an interface.2.2 Channels are classified by channel types, which have common characteristics. Channel types are specified in 3.2.3 The channels are combined into in
3、terface structures, specified in 4. An interface structure defines themaximum digital information-carrying capacity across a physical interface.2.4 In an actual access arrangement some of the channels available across an ISDN user-networks physicalinterface, as defined in the applicable interface st
4、ructure, may not be supported by the network. Some ISDN serviceswill not require the full capacity of a B-channel; in those cases in which users require only such services, the accesscapability might be further reduced. The capability provided by those channels that are actually available forcommuni
5、cation purposes, is referred to as the access capability provided through the interface.3 Channel types and their use3.1 B-channel3.1.1 The B-channel is a 64 kbit/s channel accompanied by timing.Note - The method for providing this timing is a subject of the individual interface Recommendations.A B-
6、channel is intended to carry a wide variety of user information streams. A distinguishing characteristic isthat a B-channel does not carry signalling information for circuit switching by the ISDN. Signalling information usedfor circuit switching by the ISDN is carried over other types of channels, e
7、.g. a D-channel.3.1.2 User information streams may be carried on a B-channel on a dedicated, alternate (within one call or asseparate calls), or simultaneous basis, consistent with the B-channel bit rate. The following are samples of userinformation streams:i) voice encoded at 64 kbit/s according to
8、 Recommendation G.711;ii) data information corresponding to circuit or packet-switching user classes of service at bit rates less thanor equal to 64 kbit/s, according to Recommendation X.1;iii) wideband voice encoded at 64 kbit/s according to Recommendation G.722;iv) voice encoded at bit rates lower
9、 than 64 kbit/s alone, or combined with other digital information streams.It is recognized that a B-channel may also be used to carry user information streams not covered by CCITTRecommendations.2 Fascicle III.8 - Rec. I.4123.1.3 B-channels may be used to provide access to a variety of communication
10、 modes within the ISDN. Examplesof these modes are:i) circuit switching;ii) packet switching, supporting packet mode terminals; andiii) semi-permanent connections.In case i), the ISDN can provide either a transparent end-to-end 64 kbit/s connection or a connectionspecifically suited to a particular
11、service, such as telephony, in which case a transparent 64 kbit/s connection may notbe provided.In case ii), the B-channel will carry protocols at layers 2 and 3 according to Recommendation X.25 whichhave to be handled by the network. The application of D-channel protocols for this case is for furth
12、er study.In case iii), the semi-permanent connection can be provided, for example by using circuit or packet switchingmodes.3.1.4 Single information streams at bit rates less than 64 kbit/s should be rate adapted to be carried on theB-channel as described in Recommendation I.460.3.1.5 Multiple infor
13、mation streams from a given user may be multiplexed together in the same B-channel, but forcircuit switching, an entire B-channel will be switched to a single user-network interface. This multiplexing should bein accordance with Recommendation I.460.Note - Independent routing of subrate channels cir
14、cuit switched to different destinations is for further study.3.2 D-channel3.2.1 The D-channel may have different bit rates as specified in 4.A D-channel is primarily intended to carry signalling information for circuit switching by the ISDN.A D-channel uses a layered protocol according to Recommenda
15、tions I.440, I.441, I.450 and I.451. Inparticular the link access procedure is frame oriented (Note).Note - The use of Signalling System No. 7 at a user-network interface is for further study.3.2.2 In addition to signalling information for circuit switching, a D-channel may also be used to carry tel
16、eactioninformation and packet-switched data.In certain cases where such signalling is not being utilized, the D-channel may support only teleactioninformation or packet-switched data.3.3 H-channels3.3.1 H-channels have the following bit rates, accompanied by timing:H0channel: 384 kbit/sH1channels: 1
17、536 (H11) and 1920 (H12) kbit/s.Note - The method for providing this timing is a subject of the individual interface Recommendation.Higher rate H-channels are for further study.An H-channel is intended to carry a variety of user information streams. A distinguishing characteristic is thatan H-channe
18、l does not carry signalling information for circuit switching by the ISDN.Fascicle III.8 - Rec. I.412 33.3.2 User information streams may be carried on an H-channel on a dedicated, alternate (within one call or asseparate calls), or simultaneous basis, consistent with the H-channel bit rates. The fo
19、llowing are examples of userinformation streams:i) fast facsimile;ii) video: e.g. for teleconferencing;iii) high speed data;iv) high quality audio or sound programme material;v) information streams, each at rates lower than the respective H-channel bit rate (e.g. voice at 64 kbit/s),which have been
20、rate adapted or multiplexed together;vi) packet-switched information.3.4 Other channelsFor further study.4 Interface structuresISDN user-network physical interfaces at ISDN reference points S and T shall comply with one of theinterface structures defined below.4.1 B-channel interface structures4.1.1
21、 Basic interface structure4.1.1.1 The basic interface structure is composed of two B-channels and one D-channel, 2 B + D. The bit rate of theD-channel in this interface structure is 16 kbit/s.4.1.1.2 The B-channels may be used independently; i.e. in different connections at the same time.4.1.1.3 Wit
22、h the basic interface structure, two B-channels and one D-channel are always present at the ISDN user-network physical interface. One or both B-channels, however, may not be supported by the network. See Appendix I.4.1.2 Primary rate B-channel interface structuresThese structures correspond to the p
23、rimary rates of 1544 kbit/s and 2048 kbit/s.4.1.2.1 The primary rate B-channel interface structures are composed of B-channels and one D-channel. The bit rateof this D-channel is 64 kbit/s.4.1.2.2 At the 1544 kbit/s primary rate the interface structure is 23 B + D.4.1.2.3 At the 2048 kbit/s primary
24、rate the interface structure is 30 B + D.4.1.2.4 With the primary rate B-channel interface structures, the designated number of B-channels is always presentat the ISDN user-network physical interface. One or more of the B-channels may not be supported by the network.4.1.2.5 In the case of a user-net
25、work access arrangement containing multiple interfaces, it is possible for theD-channel in one structure to carry the signalling for B-channels in another primary rate structure without an activatedD-channel. When a D-channel is not activated, the designated time slot may or may not be used to provi
26、de anadditional B-channel, depending on the situation; e.g., 24 B for a 1544 kbit/s interface.4.2 H-channel interface structure4.2.1 Primary rate interface H0-channel structures4.2.1.1 The primary rate interface H0-channel structures are composed of H0-channels with or without a D-channel,as indicat
27、ed below. When present in the same interface structure the bit rate of the D-channel is 64 kbit/s. Additionalprimary rate interface H0-channel are for further study.4 Fascicle III.8 - Rec. I.4124.2.1.2 At the 1544 kbit/s primary rate interface the H0-channel structures are 4 H0and 3 H0 + D. The use
28、of theadditional capacity across the interface is for further study. When the D-channel is not provided, signalling for theH0-channels is provided by the D-channel in another interface4.2.1.3 At the 2048 kbit/s primary rate interface the H0-channel structure is 5 H0+ D. In the case of a user-network
29、access arrangement containing multiple interfaces, it is possible for the D-channel in one structure to carry thesignalling for H0-channels in another primary rate interface without a D-channel in use.4.2.1.4 With the primary rate interface H0-channel structures, the designated number of H0-channels
30、 is alwayspresent at the user-network physical interface. One or more of the H0-channels may not be supported by the network.4.2.1.5 In the case of a user-network access arrangement containing multiple interfaces it is possible for theD-channel of one structure to carry the signalling for H0-channel
31、s in another primary rate interface structure without anactivated D-channel. When a D-channel is not required in a 1544 kbit/s interface, the 4 H0-channel structure may beused.4.2.2. Primary rate interface H1-channel structures4.2.2.1 1536 kbit/s H11-channel StructureThe 1536 kbit/s H11-channel stru
32、cture is composed of one 1536 kbit/s H11-channel. Signalling for theH11-channel, if required, is carried in a D-channel on another interface structure within the same user-network accessarrangement.4.2.2.2 1920 kbit/s H12-channel StructureThe 1920 kbit/s H12-channel structure is composed of one 1920
33、 kbit/s H12-channel and a D-channel. The bitrate of the D-channel is 64 kbit/s. Signalling for the H12-channel, if required, is carried in this D-channel or theD-channel of another interface structure within the same user-network access arrangement.4.3 Primary rate interface structures for mixtures
34、of B- and H0-channelsA primary rate interface may have a structure consisting of a single D-channel and any mixture of B- andH0-channels. The bit rate of the D-channel is 64 kbit/s. In the case of a user-network access arrangement containingmultiple interfaces, a D-channel in one interface structure
35、 may also carry signalling for channels in another interfacestructure. When a D-channel is not activated, its 64 kbit/s capacity may or may not be used for the mixture of B- andH0-channels, depending on the situation, e.g. 3 H0+ 6 B for a 1544 kbit/s interface.4.4 Other interface structure(s)For fur
36、ther study.5 Examples of application of interface structures5.1 Access arrangement for PABX, terminal controller, local area network, etc.Figure 1/I.412 illustrates a typical PABX, or LAN access arrangement. For this particular configuration it isnot necessary to apply the same interface structure a
37、t both S and T reference points. For example, basic interfacestructures may be used for interfaces located at reference point S. Either basic or primary rate or other interfacestructures may be used at interfaces located at reference point T.Fascicle III.8 - Rec. I.412 5APPENDIX I(to Recommendation
38、I.412)Access capabilitiesI.1 As stated in 2.4, not all of the channels present in an ISDN user-network physical interface are necessarilysupported by the network. The resulting capability provided in an ISDN user-network access arrangement is defined asthe access capability.To assist in guiding the
39、implementations of ISDN equipment and services around the world, several preferredaccess capabilities are identified here. While these preferred arrangements do not preclude the implementation of otheraccess capabilities, they are intended to assist in the worldwide commonality which is a key object
40、ive of ISDN.I.2 Preferred access capabilitiesa) Preferred basic access capabilities- 2 B + D- B + D-Db) Primary rate - B-channel access capabilities- n B + Dn 23 for 1544 kbit/s primary rate, unless signalling is provided in another physical interface(see 4.1.2.5); then n = 24 may be allowed.n 30 fo
41、r 2048 kbit/s primary rate, unless signalling is provided in another physical interface(see 4.1.2.5) then n = 31 may be allowed.c) Primary rate - H0-channel access capabilities- n H0+ Dn 3 for 1544 kbit/s primary raten 5 for 2048 kbit/s primary rate- n H0n 4 for 1544 kbit/s primary rated) Other channel structure access capabilitiesFor further study.
