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1、INTERNATIONAL STANDARD ISOJIEC 10588 First edition 1993-05-01 Information technology - Use of X.25 Packet Layer Protocol in conjunction with X.21/X.21 bis to provide the OSI connection-mode Network Service Technologies de /information- Utilisation du protocole X.25 de couche de paquet conjointement
2、avec X.21/X.21 bis pour fournir le service de r b) Network Layer; c) Network Service. 3.2 Service conventions definitions This Recommendation I International Standard makes use of the following terms defined in CCITT Recommen- dation X.210 I ISO/TR 8509: a) Network Service provider; b) Network Servi
3、ce user. 3.3 Network Service definitions This Recommendation I International Standard makes use of the following terms defined in CCITT Recommen- dation X.213 I ISO/IEC 8348: a) N-CONNECT request; b) N-DISCONNECT indication. 2 CCITT Rec. X.613 (1992 E) Copyright International Organization for Standa
4、rdization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:11:35 MDTNo reproduction or networking permitted without license from IHS -,-,- Iso/IJx 10588 : 1993(E) 3.4 X.25 definitions This Recommendation I International Standard make
5、s use of the following terms defined in CCIlT Recommen- dation X.25 and ISO/lEC 8208: a) Data circuit-terminating equipment; b) Data terminal equipment; c) Registration packet; d) Virtual call. 3.5 X.21 definitions This Recommendation I International Standard makes use of the following term defined
6、in CCITT Recommen- dation X.21: Call progress signal. 3.6 ISDN definitions This Recommendation I International Standard makes use of the following term defined in CCITT Recommen- dation X.3 1: Terminal adaptor. 4 Abbreviations 4.1 Reference model abbreviations NL Network Layer NS Network Service OS1
7、 Open Systems Interconnection 4.2 Network service abbreviations CONS Connection-mode Network Service NSAP Network service access point QOS Quality of service SNPA Subnetwork point of attachment 4.3 X.25 abbreviations DCE Data circuit-terminating equipment DTE Data terminal equipment MC Highest incom
8、ing channel HOC Highest outgoing channel HTC Highest two-way channel LAPB Link access procedure -balanced LIC Lowest incoming channel LOC Lowest outgoing channel LTC Lowest two-way channel PLP Packet layer protocol CCITT Rec. X.613 (1992 E) Copyright International Organization for Standardization Pr
9、ovided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:11:35 MDTNo reproduction or networking permitted without license from IHS -,-,- 1s0/JEc 10588 : 1993(E) 4.4 ISDN abbreviations ISDN TA TB Integrated services digital network Interworking
10、 function Terminal adaptor Terminal equipment 4.5 Other abbreviations CSDN PSDN Circuit switched dam network (public or private) Packet switched data network (public or private) 5 Overview There are two basic configurations where the X.25 packet layer protocol (PLP), as standardized in ISOAEC 8208,
11、may be operated over an X.21 connection to provide the OS1 Connection-mode Network Service (CONS) defined in CCITT Rec. X.213 I ISO/EC 8348. The first configuration is shown in Figure la) where two X.25 DTEs are directly connected by one or more concatenated CSDNs, or by one or more ISDNs and one or
12、 two X.30 TAs (see Annex B). In this case each DTE operates the DTWDTE mode of ISO/IEC 8208 (X.25 PLP) and IS0 7776 (X.25 LAPB). The connection may be permanent or switched. The second configuration is shown in Figure lb) where an X.25 DTE access a PSDN. In this case the DTE operates the DTE/DCE mod
13、e of ISOAEC 8208 (X.25 PLP) and IS0 7776 (X.25 LAPB). When the connection is permanent this configuration is a regular X.25 scenario, while it relates to X.32 and X.3 1 when the connection is switched. DTE DTE X.21 bis a) ConJ+uration for DTE-TO-D7E operation DTE X.21 bis TIS00590-92 b) Configuratio
14、n for DTE-TO-PSDN operation Figure 1 - Configurations CCITT Rec. X.613 (1992 E) Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:11:35 MDTNo reproduction or networking permitte
15、d without license from IHS -,-,- ISO/IEC 10588 : 1993(E) CCIIT Rec. X.223 I IS0 8878 specifies the method for providing the OS1 Connection-mode Network Service (CONS) through the use of the X.25 PLP directly connected to another DTE or directly connected to a packet switched data network. When a cir
16、cuit switched (semi-permanent, hot line, or full circuit switched) connection is introduced into the path between the DTEs or between the DTE and a PSDN, a few provisions additional to those contained in CCIIT Rec. X.223 I IS0 8878 are necessary. These additional provisions for a DTE connected to a
17、CSDN, or to an ISDN via a TA, are specified by this Recommendation I International Standard. The applicable protocol stack is given in Figure 2. The X.25 PLP is used to convey all elements of all three phases of the OS1 CONS. CCIIT Rec. X.223 I IS0 8878 applies with regard to the mapping of the CONS
18、 primitives and parameters to the elements of the X.25 PLP. However, in certain cases discussed in the clauses below, additional provisions apply before these mappings can take place (e.g., to cover establishment of a circuit switched connection) or in addition to these mappings (e.g., to cover an u
19、nexpected disconnection of the circuit switched connection). 4 This Recommendation 1 International Standard in conjunction with Recommendation X.223 1 IS0 8878 ISO/IEC 8208 (X.25 PLP) - 3 - 2 - X.21 orX.21 bis 1 - TISOO590-92 . 4 +d Signalling for Packet switched signailing OSI circuit switched acce
20、ss and information transfer layers (see Note) NOTE -These procedures are not used in the leased circuit case. Figure 2 - Protocol stack At the physical layer (and for signalling for a circuit switched connection), either X.21 or X.21 bis may be used. At the data link layer IS0 7776, as constrained b
21、y clause 7 of this Recommendation I International Standard, is used. At the network layer ISO/lEC 8208, as constrained by clause 8 of this Recommendation I International Standard, is used. The mapping of the elements of the CONS to the protocol and procedures of ISO/IEC 8208 shall be as required by
22、CCITI Rec. X.223 I IS0 8878. The remaining clauses of this Recommendation I International Standard specify the provisions required in addition to these mappings. 6 Control of underlying connections The following clauses cover the two types of underlying connections that can be operated : leased circ
23、uit connection, and circuit switched connection including direct call. CClTT Rec. X.613 (1992 E) 5 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:11:35 MDTNo reproduction or
24、networking permitted without license from IHS -,-,- ISO/IEC 10588 : 1993(E) 6.1 Leased circuit connection Both the X.21 and X.21 his interfaces provide for a leased circuit connection. No additional procedures are required. 6.2 Circuit switched connection (including direct call) Both the X.21 and X.
25、21 bis interfaces provide for a circuit switched connection. 6.2.1 Circuit switched connection originated by DTE The following additional procedure shall be used if a circuit switched connection is not already established or if an additional circuit switched connection is needed to support the addit
26、ional traffic. The receipt by layer 3 of an N-CONNECT request primitive shall first cause the X.21 or X.21 bis procedures to be used to establish the connection. Following the successful establishment of this connection and the X.21 or X.21 bis procedures for entering the data transfer phase at laye
27、r 1, the IS0 7776 and ISO/IEC 8208 procedures shall apply as constrained by clauses 7 and 8 of this Recommendation I International Standard. Failure to establish the connection is indicated to the NS user by means of a N-DISCONNECT indication primitive with the originator parameter indicating “NS pr
28、ovider” and the reason parameter as given in Table 1. Table 1 - Mapping of the X.21 call progress signals associated with clearing to the CONS Reasons NOTES X.21 code CONS Reason parameter 20 Connection rejection-NSAP unreachable/transient 21 Connection rejection-reason unspecified/transient 22 Conn
29、ection rejection-reason unspecified/transient 23 Connection rejection-reason unspecified/transient 41 Connection rejection-reason unspecified/permanent 42 Connection rejection-reason unspecified/permanent 43 Connection rejection-N b) an exchange is always needed between the two DTEs; or c) the need,
30、 or lack of need, of an exchange depends on other factors known to the two DTEs. For the mode of operation where a circuit switched connection is used to connect a DTE and a DCE, the need for an identification exchange is governed by the PSDN requirements and the PSDN services the DTE desires to use
31、 (services are defined in Recommendation X.32). When an exchange of identification information is needed, the method(s) of exchange will be selected from the methods given in Recommendation X.32 (see Note 1). For DTE-to-DTE operation, the method is agreed between the two DTEs on an a priori basis (s
32、ee Note 2). For DTE-to-DCE operation, the requirements of the PSDN will govern which method is to be used. CCIIT Rec. X.613 (1992 E) 7 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/
33、2007 10:11:35 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO/IEC 10588 : 1993(E) NOTES 1 Although Recommendation X.32 specifies the identification procedures as DTE-to-DCE, they are applied to the DTE-to-DTE case by this Recommendation I International Standard. 2 One o
34、f the identification methods allowed by Recommendation X.32 is use of the capabilities of the switched network for identification. When using such an identification method, the specific protocol mechanism must also be agreed among the two DTEs and the switched network. 7 Data link layer 7.1 Mode of
35、protocol The basic mode (modulo 8) single link procedures shall be implemented. The extended mode (modulo 128) single link procedure may also be implemented. 7.2 Address assignment For DTE-to-DTE operation over a switched connection, and for DTE access to a PSDN via a switched connection, the A and
36、B addresses of IS0 7776 shall be determined as follows: the calling end is assigned address A and the called end is assigned address B unless the assignment is lmown a priori. For access to a PSDN via a leased circuit, the A and B addresses shall be as specified in Recommendation X.25. For DTE-to-DT
37、E operation over a leased circuit, the A and B addresses shall be known a priori. 7.3 Parameter values It is recommended that the Tl timer be capable of being set to a value of 5 s in order to cope with multiple satellite links. It is recommended that 103 1 octet frames be supported in order to acco
38、mmodate satellite connections. NOTE - In addition to these recommendations, the use of large windows, requiring the use of extended mode (modulo 128) operation may be advisable, particularly if multiple satellite hops are likely. 8 Packet layer 8.1 General In general the features defined in CCITT Re
39、c. X.223 I IS0 8878 apply with regards to the QOS parameters and addressing. However, the procedures defined in Recommendation X.21 imply the operation of some specific mechanisms as defined in Recommendations X.25, X.32 and X.31 when a PSDN is involved. 8.2 Addressing 8.2.1 Direct DTE-to-DTE config
40、uration The X.21 addressing elements are used to establish the X.21 connection between the two concerned SNPAs. The X.25 Address Field is not used (i.e., it is empty) in this case, and the Called and CaIIing NSAP addresses are entirely conveyed in the Called and Calling Address Extension facilities.
41、 If packets are received which contain addressing information in the Address Field, this information shall be ignored. 8 CCITT Rec. X.613 (1992 E) Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Re
42、sale, 04/21/2007 10:11:35 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO/IEC 10588 : 1993(E) 8.2.2 DTE-to-DCE configuration The X.21 addressing elements are used to establish the X.21 connection between the two concerned SNPAs (i.e., the DTE and the PSDN). The X.25 Add
43、ress Field, and the Called and Calling Address Extension facilities are used as defined in CCIlT Rec. X.223 I IS0 8878. 8.3 QOS parameters 8.3.1 Throughput QOS parameters The calling NL entity should have an a priori knowledge of the data signalling rate of the X.21 connection. This a priori knowled
44、ge is generally obtained by means of subscription information. This knowledge is used to determine the value of the throughput and to determine if a new connection is to be established to meet the NS user requirements. 8.3.2 Transit delay QOS parameter Due to the limited signalling capabilities of X
45、.21, the calling NL entity should have an a priori knowledge of the transit delay of the X.21 connection. This a priori knowledge may be obtained via knowledge of the network topology or via monitoring of the connection characteristic. The value of the X.21 connection transit delay is used as part o
46、f the calculation in determining the cumulative transit delay carried in the End-to-End Transit Delay Negotiation facility (see CCITT Rec. X.223 I IS0 8878). 8.4 Window size and packet size parameters If only one virtual circuit is to be used, an appropriate window size should be supported. It is re
47、commended that 1024 octet data packets be supported in order to accommodate satellite connections. 8.5 Logical channel ranges The logical channel ranges (UC, I-UC, LTC, HTC, LOC and HOC in ISO/IEC 8208) to be used are determined by local knowledge if available. If local knowledge is not available, t
48、hen only a single two-way logical channel is available and LTC and HTC shah be set to 1. If additional logical channels are required, they may be negotiated by using the Registration packet as described in ISO/IEC 8208. In order to avoid collision of Registration packets, the circuit initiator is re
49、sponsible for initiating the negotiation. 8.6 Role selection In the absence of a priori knowledge, the Restart procedure shall be used for role selection as described in ISO/-IEC 8208. CCITT Rec. X.613 (1992 E) 9 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Techn