1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 ) TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2!4%$G0G03%26)#%3G0G0$)4!,.%47/2+G0G0 )3$.G9/6%2!,G0.%47/2+G0G0!30%#43!.$G0G0 ora2) If the conditions in 1.3.1 are satisfied, the NSAP address can be inferred from the E.164 number.Note - For circuit mode
2、 calls, the semantic content of the AFI may be contained in the numbering andaddressing plan identification in the Q.931 or S.S. No. 7 calling/called address protocol elements. Forpacket mode calls, similar information may be found in the X.25/X.75 protocol. Until such time as aprotocol mechanism fo
3、r identifying the numbering plan and the type of number is implemented inX.25/X.75, analogous to that which exists in Q.931/S.S. No. 7, such information may be derivable fromthe X.25/X.75 address fields which may include a numbering plan escape code. It may also be possible forthe semantic content o
4、f the AFI to be implied by network arrangements.2 Fascicle III.8 - Rec. I.334b) The OSI NSAP address is comprised of an AFI + IDI + DSP, in which the IDI is semantically identical tothe ISDN number. In this case, the entire NSAP address is carried in the subaddress/address extensionfield.c) The OSI
5、NSAP address is comprised of an AFI + IDI + DSP, in which the IDI is not related to the ISDNnumber. The entire NSAP address is conveyed in the subaddress/address extension field.1.3 Encoding of NSAP addresses1.3.1 Use of the AF (Address Field)Under certain conditions, the NSAP address, as defined in
6、 ISO 8348 AD2, may be conveyed entirely in the AF.These conditions are:a) the NSAP address consists solely of the IDP (i.e. the DSP is null);b) the AFI can be deduced from the contents of the AF (e.g. with knowledge of the subnetwork to which theDTE is attached); andc) the IDI is the same as the SNP
7、A (Subnetwork Point of Attachment) address.When all the above conditions are satisfied, the AF may be used to convey the semantics of the entire NSAPaddress (the AFI is implied and the contents of the AF are equivalent to the IDI). In these cases, the AEF (AddressExtension Field) may also be used (s
8、ee 1.3.2).Fascicle III.8 - Rec. I.334 31.3.2 Use of the AEF (Address Extension Field)When the conditions in 1.3.1 are not satisfied, the AEF shall be used. The NSAP address, complete with AFI,is placed in the AEF (type of subaddress is X.213/ISO 8348 AD2). In this case, the contents of the AF are no
9、t defined bythis Recommendation.1.4 Decoding of NSAP addresses1.4.1 Absent AEF caseIf the AEF is not present, then local knowledge is required by the receiving NL (Network Layer) entity todetermine whether an OSI NSAP address is to be deduced from the content of the AF. If this local knowledge indic
10、atesthat an NSAP address is present, its abstract syntax is as follows:a) the AFI is deduced from knowledge of the subnetwork from which the packet was received;b) the IDI is the same as the contents of the AF; andc) the DSP is absent.1.4.2 AEF caseIf the AEF is present and the type of subaddress is
11、 X.213/ISO 8348 AD2, then the NSAP address is containedentirely within the AEF. The abstract syntax is as follows:a) the AFI is contained within the first two digits of the AEF;b) the IDI is the remainder of the Initial Domain Part (IDP) after any leading and trailing padding digits arediscarded; an
12、dc) the DSP, if present, constitutes the remainder of the AEF content after any trailing padding digits arediscarded.2 Means to specify the type of subaddressConsidering the three cases in which the NSAP address may be related to the ISDN address/subaddress, amechanism which permits determination of
13、 the type of subaddress present may be useful in making distinctions. Themethod of distinction is dependent upon the protocol being used.In the case of Q.931/I.451, 3 bits within octet 3 of each subaddress information element (i.e., calling and calledparty subaddress)1)establish the “type of subaddr
14、ess“. Two existing assignments, subject to change by responsibleauthorities are “user-specified“ and “X.213/ISO 8348 AD2“. All other values are reserved.The actual subaddress information is coded beginning in octet 4 with the possibility of continuing up to octet23, i.e., the subaddress information
15、element has the capacity to carry a maximum of 20 octets of subaddress information.- Under the X.213/ISO 8348 AD2 encoding of type of subaddress, the initial two digits of the subaddressrepresent the AFI which permits further distinction in subaddress encoding schemes as specified inFigure 2/I.334.-
16、 Under the user-specified encoding of type of subaddress, the subaddress field is encoded according to userspecifications subject to a maximum length of 20 octets.In the case of packet mode calls using X.25/ISO 8208, bits within the first octet of the calling/called addressextension facility paramet
17、er field indicate the “type of address extension“ in a similar manner._1)Octets 1 and 2 of the subaddress information elements serve as information element and length identifiers,respectively.4 Fascicle III.8 - Rec. I.3343 The OSI NSAP address formatFor reference purposes, a description of terms use
18、d in connection with NSAP addresses is provided below:The format of the NSAP address is:Fascicle III.8 - Rec. I.334 5IDP - Initial Domain Part. This is the part which contains all the internationally standardized parts of the NSAPaddress, i.e. those addresses and numbers which are controlled either
19、by ISO or CCITT.AFI - Authority and Format Identifier. This 2-digit code indicates the authority responsible for the number followingthe AFI, such as X.121 or E.164 and the format of the DSP. This is always two digits and is allocated as perX.213/ISO 8348 AD2.IDI - Initial Domain Identifier. This ma
20、y contain, for example, an E.164 or X.121 number. The networks which usethese numbering schemes are termed subnetwork by ISO. The overall length of the field is determined by themaximum length of the number format being used.DSP - Domain Specific Part. In the context of an E.164 IDI, this part conta
21、ins an address which is relevant only tothe domain which has been accessed beyond the domain specified within the IDI, such as a PBX extension, aLAN terminal and so on. This is a variable length field, and is constrained by the length of the IDP, as theoverall maximum length of the OSI NSAP address is 20 octets.
