1、ITU-T RECMN*I.365 -1 93 4862593 0592355 226 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU INTEGRATED SERVICES DIGITAL NETWORK (ISDN) OVERALL NETWORK ASPECTS AND FUNCTIONS 1.365.1 (1 1 /93) FRAME RELAYING SERVICE SPECIFIC CONVERGENCE SUBLAYER (FR-SSCS) IT
2、U-T Recommendation 1.365.1 (Previously “CCITT Recommendation”) FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommen-
3、dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics.
4、 The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. i (Helsinki, March 1-12, 1993). ITU-T Recommendation 1.365.1 was prepared by IT-T Study Group 13 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 26t
5、h of November 1993. NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O ITU 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by
6、any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*Im3b5 .i 93 4Bb259L 0592357 OT9 CONTENTS 1 Introduction 2 Service provided by FR-SSCS . 2.1 Primitives 2.2 Description of connections . Interaction with the management a
7、nd control plane . Functions. structure and coding of FR-SSCS . 4.1 Functions in FR-SSCS 4.2 Procedures for the FR-SSCS sublayer . 5.1 State variables . 5.2 Procedures at the sender side 5.3 Procedures at the receiver side Annex A - List of acronyms 3 4 FR-SSCS-PDU structure and coding 5 Recommendat
8、ion 1.365.1 (11/93) L- Page 1 1 2 3 3 3 3 4 5 5 5 6 8 i ITU-T RECMN*I.365 .I 93 4862593 0572358 T35 Recommendation 1.365.1 AAL 5 CAR ATM-DATA requesi ATMDATA indicafon ATMSAP FRAME RELAYING SERVICE SPECIFIC CONVERGENCE SUBLAYER (FR-SSCS) (Geneva, 1993) 1 Introduction This Recommendation specifies th
9、e Frame Relaying Service Specific Convergence Sublayer (FR-SSCS). The FR-SSCS is located in the upper part of the ATM Adaptation Layer on top of the Common Part Convergence Sublayer (CPCS) of AAL type 5, as specified in clause 6A.363 and shown in Figures 1 and 2. The FR-SSCS is used at the B-ISDN TE
10、 to emulate the Frame Relaying Bearer Service (FRBS) in B-ISDN. It is also used for interworking between a B-ISDN and a Frame Relaying Network. 2 Service provided by FR-SSCS At the DL-CORE-SAP the FR-SSCS provides the Core service as described in AnnexCn.233.1. The Core service primitives, exchanged
11、 between the Core service user and the Core service provider (FR-SSCS) are summarized in Table 1 according to the table contained in A.4.2.2/Q.922. The primitives and their parameters are described in detail in Annex Cn.233.1 (see Figure 1). - DL-CORE-SAP HGURE 1n.365.1 Location of the FR-SSCS in th
12、e B-ISDN protocol reference model for a B-ISDN TE The exchange of information at the FR interworking point (FR-IWP) is described by IWF-DATA indication and IWF-DATA request primitives (see Figure 2). The parameters of those primitives are contained in Table 2. Recommendation 1.365.1 (11/93) 1 Q.922
13、core Parameter (Note) DL-CORE user data Discard Eligibility Congestion encountered backward Congestion encountered forward DLCORE-Service-User protocol control information IWFDATA lndcation Primitive DL-CORE DATA DLCORE DATA FR-SSCS-PDU field request indication X X Information field X DE X BECN X FE
14、CN X X C/R bit sai AAL 5 CPCS I CPCS-UNilATA invoke AAL 5 CAR ATM-DATA request ATM-DATA indication ATM-SAP TI 3J2910-WdO2 I FIGURE uI.35.1 Location of the FR-SSCS in the B-ISDN protocol reference model for the IWF between a frame relaying network and a B-ISDN The FR-SSCS sublayer preserves the FR-SS
15、CS-SDU sequence integrity. It uses the CPCS message mode service without the corrupted data delivery option. 2.1 Primitives The primitives exchanged across the DL-CORE-SAP are described in Table 1. TABLE 1fl.365.1 Core service primitives and parameters at the DL-CORE-SAP X Parameter is present. NOTE
16、 - Whether the DLCORE control parameter and the DLCI or DLCORE control indicator parameter are included in the DLCORE DATA primitives is for further study. 2 Recommendation 1.365.1 (lvP3) - The primitives exchanged across the FR Interworking Point (FR-IWP) are described in Table 2. Parameter DL-CORE
17、 user data Discard Eligibility TABLE 2A.365.1 Primitive IWF-DATA I WF-DATA FR-SSCS-PDU field request indication X X Information field X X DE Congestion encountered backward Congestion encountered forward X X BECN X X FECN DL-CORE-Service-User protocol control information DLCI or DL-CORE control indi
18、cator (Note 1) X X CIR bit X X D/C The primitives exchanged across the boundary between the FR-SSCS and the CPCS of AAL type 5 are described in 6.1.2.2.1fl.363. DL-CORE control (Note 2) 2.2 Description of connections Multiple FR-SSCS connections may be associated with a single CPCS connection (and w
19、ith the corresponding ATM connection), allowing multiplexing at the FR-SSCS. Within a CPCS connection the FR-SSCS connections are uniquely identified by the Data Link Connection Identifiers (DLCIs). X X DL-CORE control 3 For further study. Interaction with the management and control plane 4 Function
20、s, structure and coding of FR-SSCS 4.1 Functions in FR-SSCS The functions provided by the FR-SSCS include: a) Multiplexing/demultipiexing This function provides the capability to multiplex multiple FR-SSCS connections into a single CPCS connection. Inspection of the FR-SSCS-PDU length These function
21、s inspect the FR-SSCS-PDU to ensure that it consists of an integral number of octets and to ensure that it is neither too long nor too short as specified in 1.233.1. b) Recommendation 1.365.1 (1Yg3) 3 ITU-T RECMN*I*365 *II 93 4Bb259L 05923bL 52T c) Congestion control These functions provide the mean
22、s for the network to notify the end user, in forward and backward direction, that congestion avoidance procedures should be initiated, where applicable (congestion control forward and congestion control backward). In addition, the functions provide the means for the end user and/or the network to in
23、dicate what frames should be discarded in preference to other frames in a congestion situation (congestion control discard eligibility). Congestion management procedures are described in Recommendation 1.370. 4.2 FR-SSCS-PDU structure and coding The peer-to-peer communication between FR-SSCS entitie
24、s uses FR-SSCS-PDUs. The format of the FR-SSCS-PDU structure is shown in Figure 3. NOTE - The structure is exactly the same as the Q.922 frame relay frame without the flags, zero bit insertion and FCS. The details of the formats and the coding of the fields are given in A.3/Q.922. As every octet of
25、an ATM-PDU is defined to be transmitted from the Most Significant Bit (MSB) to the Least Significant Bit (LSB), the FR-SSCS-PDU in the ATM-PDU is also to be transmitted from MSB to LSB. Though this bit transmission order is different from that of Recommendation 4.922, the user of the FR-SSCS should
26、regard the transmission order of the FR-SSCS to be the same as that of Recommendation Q.922, as the original bit order is restored after receipt. Default Header Format 3 Octet Header Format 4 Octet Header Format (upper) DLCI information field (integer number of octets: 1 . 262 . 1600 . octets) FE BE
27、 CN CN O DE EA DLCI DLCi FE BE EA DE CN CN O (lower) DLCI or DLCI I I DLCi C/R Command/Response-bit EA Address Extension Bit FECNIBECN ForWarcBackward Congestion Notification DE Discard Eligibility FCS Frame Check Sequence DIC Data Link Connection identifier (1 Oll 6/17/23 DLCI or DL-CORE control in
28、dicator II I lits FIGURE 3D.365.1 Structure of FR-SSCS-PDU with 2,3 and 4 octet header formats 4 Recommendation 1.365.1 (11/93) ITU-T RECMN*I.365 .I 93 W 4862593 0592362 466 FR-SSCS-PDU field DLCI UR The support of the default (2 octet) header format is mandatory. The support of 3 and 4 octet header
29、 formats is optional. In B-TE In IWF Is set to the value which is associated to the FR-SSCS connection during the connection Set-up or at the subscription Is set to the value of the parameter DL-CORE- Service-User protocol control information (As in B-TE) (As in B-TE) 5 Procedures for the FR-SSCS su
30、blayer DE 5.1 State variables Most-recent-CI-received Is set to the value of the parameter Discard Eligibility Most-recent-CI-received: This variable is initialized to O. It is used if the FR-SSCS connection is bidirectional. This variable records the value of the CPCS-CI parameter of the most recen
31、t CPCS-UNITDATA signal primitive received for this connection in the reverse direction. DL-CORE control 5.2 When FR-SSCS receives: Procedures at the sender side For further study (As in B-TE) - - a DL-CORE DATA request from the core service user in a B-TE, or an IWF-DATA request from the Q.922 core
32、in an IWF, it shall construct the FR-SSCS-PDU as specified in Table 3, according to the format in Figure 3. EA Information field TABLE 3A.365.1 Values for the FR-SSCS-PDU at the sender side Is set to O at the first octet(s) of the header and to 1 at the last octet of the header Is set to the value o
33、f the parameter DL-CORE user data (As in B-TE) (As in B-TE) Value set by the FR-SSCS I I FECN Is set to O I Is set to the value of the parameter Congestion encountered forward I BECN Is set to O or optionally to the value of the variable Is set to logical OR of the values of the variable Most-recent
34、C1-received and the parameter Congestion encountered backward (As in B-TE) D/C Is $et to indicate whether the last header octet is negotiated during the connection Set-up or at subscription to contain DLCI bits or the DL CORE control field (As in B-TE) Recommendation 1.365.1 (1v93) 5 The FR-SSCS-PDU
35、 is then forwarded to the CPCS sublayer in the Interface Data (ID) parameter of the CPCS-UNITDATA invoke primitive. Other parameters of that primitive are set as follows: - The CPCS-Loss Priority (CPCS-LP) parameter is either: a) b) Both a) and b) above shall be supported so that network operators c
36、an decide at connection Set-up or subscription on a CPCS connection by CPCS connection basis which one is used. The method of selecting between the two cases outlined above is outside the scope of this Recommendation. The CPCS-Congestion Indication (CPCS-CI) parameter is always set to zero. The CPCS
37、-User-to-User Indication (CPCS-VU) parameter is always set to zero. set to the value of the parameter Discard Eligibility of the DL-CORE DATA request primitive or the IWF-DATA request primitive; or always set to zero or to one. - - 5.3 Procedures at the receiver side Upon reception of a CPCS-UNITDAT
38、A signal primitive, the variable Most-recent-CI-received is set to the value of the parameter CPCS-Congestion Indication (CPCS-CI) and the FR-SSCS-PDU is extracted from the parameter Interface Data (ID). The FR-SSCS-SDU, contained in the FR-SSCS-PDU information field is then passed: to the upper lay
39、er, in the case of a B-TE, using the DL-CORE DATA indication primitive, or to the Q.922 core, in the case of an IWF, using the IW-DATA indication primitive. - - The parameters of the primitives are defined in Table 4. 6 Recommendation 1.365.1 (11/93) TABLE 4fi.365.1 DLCORE user data Discard Eligibil
40、ity Values for the parameters of DL-CORE DATA indication and IWF-DATA indication primitives at the receiver side FR-SSCS-SDU (information field) (Note I) (Note 2) (As in B-TE) Parameters Congestion encountered backward Congestion encountered forward I I Is set to the value of the BECN field in the F
41、R-SSCS-PDU Is set to the logical OR of the values of the FECN field in the FR-SSCS-PDU and the parameter CPCS-CI of the CPCS-UNITDATA signal primitive (As in B-TE) (As in B-TE) 1 DL-CORE DATA indication primitive IWF-DATA indication primitive (IWF) -TE) I DL-CORE-Service-User protocol control inform
42、ation Is set to the value of the CIR field in the (As in B-TE) FR-SSCS-PDU DLCI or DLCORE control indicator (For further study) Is set to the value of the D/C field in the FR-SSCS-PDU Is set to the value of the DLCORE control field in the FR-SSCS-PDU, if present NOTES I conveyed to other entities li
43、ke management. 2 At the receiver side of the B-TE the information in the CPCS-Loss Priority (CPCS-LP) parameter at the FR-SSCS may be At the receiver side of the IWF the Discard Eligibility (DE) parameter shall be set to either: a) the value of the DE field in the FR-SSCS-PDU; or b) the logical OR o
44、f the values of the DE field in the FR-SSCS-PDU and the parameter CPCS-LP of the CPCS-UNITDATA signal primitive. Both a) and b) above shall be supported by the IWF so that the network operators can decide at connection Set-up or subscription on a CPCS connection by CPCS connection basis which one is
45、 used. The method of selecting between the two cases outlined above is outside the scope of this Recommendation. Recommendation 1.365.1 (1Y93) 7 ITU-T RECMN*I.365 .1 93 YB62591 0592365 175 AAL ATM B-ISDN B-TE BECN C/R CPCS CPCS-CI CPCS-LP CPCS-w D/C DE DLCI EA FECN FR FR-IWP FR-sscs FRBS ID IWF PDU
46、SAP SAR SDU Annex A List of acronyms (This annex forms an integral part of this Recommendation) ATM Adaptation Layer Asynchronous Transfer Mode Broadband Integrated Services Digital Network B-ISDN Terminal Backward Explicit Congestion Notification CommandResponse bit Common Part Convergence Sublayer
47、 CPCS-Congestion Indication CPCS-Loss Priority CPCS-User-to-User Indication DLCI or DL-CORE control indicator Discard Eligibility Data Link Connection Identifier Address Extension bit Forward Explicit Congestion Notification Frame Relaying Frame Relaying Interworking Point Frame Relaying Service Spe
48、cific Convergence Sublayer Frame Relaying Bearer Service Interface Data Interworking Function Protocol Data Unit Service Access Point Segmentation and Reassembly Sublayer Service Data Unit 8 Recommendation 1.365.1 (11/93) ITU-T RECMN*I*365 01 93 4862591 O592366 001 D Printed in Switzerland Geneva, 1994
