1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU v.75 Appendix II (02/98) SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK Simultaneous transmission of data and other signals DSVD terminal control procedures Appendix II: Session establishment using V
2、.75/H .245 procedures ITU-T Recommendation V.75 - Appendix II (Previously CCITT Recommendation) STDeITU-T RECMN V.75-ENGL I1778 4862591 0650705 668 ITU-T V-SERIES RECOMMENDATIONS DATA COMMUNICATION OVER THE TELEPHONE NETWORK General Interfaces and voiceband modems Wideband modems Error control Trans
3、mission quality and maintenance v.1-v.9 V. 10-V.34 v.35-v.39 v.40-v.49 V.50-V.59 “- V.60-V.99. - 9. -1 Simultaneos transmission of data and other signals Interworking with other networks Interface layer specifications for data communication Control procedures v. 100-v.199 V.200-V.249 V.250-V.299 For
4、 firther details, please refer to ITU-T List of Recommendations. ITU-T RECOMMENDATION V.75 DSVD TERMINAL CONTROL PROCEDURES APPENDIX II Session establishment using V.75M.245 procedures Source Appendix II to ITU-T Recommendation V.75, was prepared by ITU-T Study Group 16 (1997-2000) on the 6th of Feb
5、ruary 1998. STDeITU-T RECMN V.75-ENGL 1998 m 48b2591 Ob50907 ll30 m FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is
6、 responsible for studying technical, operating and tariff questions and issuing Recommendations 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
7、by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1. In some areas of information technology which fall within ITU-Ts purview, the necessary
8、 standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the p
9、ossibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside
10、of the Recommendation development process. As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the la
11、test information and are therefore strongly urged to consult the TSB patent database. O ITU 1998 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing f
12、rom the ITU. 11 Recommendation V.75 - Appendix II (02/98) CONTENTS Page Appendix II . Session establishment using V.75m.245 procedures . II . 1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.1 1 11.12 II . 13 Introduction General overview Opening channels . Capability exchange . H.245 Terminalcapa
13、bility Set structure Channel establishment Channel closure Sample session Out-of-band control channel . Establishment of suspendresume channels with the out-of-band control channel Sample suspendresume negotiation and activation procedure Establishment of suspendresume option in V.76 using V.8 bis P
14、ractical considerations TerminalCapabilitySet Tables for V.70 2 4 4 5 6 7 . Recommendation V.75 . Appendix II (02/98) 111 STD-ITU-T RECMN V-75-ENGL 3998 m 4862593 Ob50909 203 m Recommendation V.75 DSVD TERMINAL CONTROL PROCEDURES APPENDIX II Session establishment using V.75M.245 procedures (Geneva,
15、1998) 11.1 Introduction The purpose of this Appendix is to describe the establishment of a V.70 session using H.245 messages according to Recommendation V.75. Due to the number of system variables: number of channels, channel parameters, options, user input, etc., the usage of V.75 procedures to est
16、ablish a V.70 session can be accomplished in a number of ways. This Appendix cannot be therefore a complete review of all V.70 session establishment procedures, but rather a general description of the methods to establish a V.70 session. This Appendix will describe those control procedures necessary
17、 in V.70 session establishment from the point in which a V.34 startup has been completed. It is assumed that the modems have connected at a sufficient bit rate to support DSVD mode and that both ends of the connection have been made aware through the use of V.8bis to initiate a V.70 session. 11.2 Ge
18、neral overview V.75 channel establishment procedures follow generally the procedures for establishing a channel in V.42. In other words, an XID exchange on a DLC proceeds a SABMELJA exchange. One significant difference between V.42 and V.70 is that H.245 messages are contained within the XID/SABME/A
19、/DM/DISC HDLC frames. In V.76, the multiplex for V.70, the SABME/UA/DM/DISC frames have been enhanced to include an information field. In a V.70 session establishment, these information fields contain a single H.245 message. What H.245 message goes into what HDLC frame and the allowed H.245 paramete
20、rs is essentially what V.75 describes. See Figure II. 1 I Flag I Control octets I FI I H.245 message I Flag I Figure II.lN.75 - V.70 XID/SABME/A/DM/DISC frame format XID frames in V.70 are a special case. XID frames already included an information field as specified in 12.2N.42. In V.42 XID frames,
21、the first octet of the I-field is a FI (Format Identifier). This identifier indicates the encoding of the remainder of the I-field which is described in 12.2N.42. To identifj the new I-field encoding of XID frames for V.70 (H.245 messages), a new FI (133,) has been assigned. For consistency througho
22、ut V.70, V.75 specifies that this new FI be present as the first octet in not just XID I-fields but in all HDLC control frames. 11.3 Opening channels Opening a channel consists of an optional capability exchange using XID frames followed by a channel establishment phase using SABMELJA/DM. This proce
23、dure is repeated for each channel to be opened. V.70 mandates the support of one data channel and one audio channel. No specific order Recommendation V.75 - Appendix lI (02/98) 1 is required, but it is recommended that the data channel be opened first so that existing data applications are assigned
24、a data channel as soon as possible. 1 2 3 11.4 Capability exchange Channel capabilities are exchanged in V.70 just like V.42 with XID frames. V.70 terminal that wishes to establish a channel for audio or data should fust complete an XID command-response frame exchange to signal to the remote termina
25、l the local capabilities. Here V.70 diverges somewhat from V.42. In V.42, one XID command-response exchanges capabilities of both the remote and local terminal. As described in V.75, a V.70 XID frame exchange contains the H.245 Terminalcapability Set and TerminalCapabilitySetAck message which only t
26、ransfers capabilities in one direction. In order that both the remote and local terminals have the complete capability set, two XID command-response exchanges are necessary - one initiated from each side. V.70 specifies that the reception of an XID command frame on a particular DLCI should trigger a
27、 V.70 terminal to initiate the corresponding XID command-response exchange in the reverse direction in order that both terminals capabilities on that DLCI are known by both V.70 terminals. To allow for faster channel establishment, V.70 does allow you to skip the capability exchange completely and g
28、o straight to opening a channel. When skipping the capability exchange, the V.70 user must open the channel with a guess of what the remote terminal can support, and therefore risks a rejection from the remote terminal. The procedure described above is referred in the Recommendations as an in-band c
29、apability exchange as it is transmitted on a particular DLCI contained in the control octets in Figure 11.1. The capabilities described in the exchange pertain only to that DLCI in which the exchange occurred. For that reason, the H.245 TerminalCapabilitySet message must only contain a single “Alter
30、nativeCapabilitySet“ within a “simultaneousCapabilitySet“. (If it contained more than one AlternativeCapabilitySet, the H.245 message would be describing capabilities on more than one channel. Refer to the description of these structures in Recommendation H.245.) G.729 Annex A O ,231 1 * / G.729 Ann
31、ex A w. Annex B I 129334 Data (v76Compression) /I 2 11.5 H.245 TerminalCapabilitySet structure The H.245 Terminalcapability Set structure uses two separate structures to define the terminal capabilities. The structures are the “CapabilityTable“ and the “CapabilityDescriptor“. The CapabilityTable is
32、an array of two elements; the first element is a number (CapabilityTableEntryNumber) from 1 to 65535 and the second element (Capability) is the capability type. The second structure, the CapabilityDescriptor expresses what combination of capabilities from the CapabilityTable the terminal can support
33、 simultaneously. See Figure 11.2. 4 , 65535 Capability Table simultaneousCapability 1 Capability DeScfiPtors I Data (dsvdContro1 = OOB) Il 1 II /I Il Capability i CapabilityTableEnyNumber ! alternativeCapabilitySet ! capabilityDescriptorNumber Figure II.ZN.75 - Example of a TerminalCapabilitySet str
34、ucture 2 Recommendation V.75 - Appendix II (02/98) Referring to Figure 11.2, the CapabilityTable indicates that the V.70 terminal can support 4 separate capabilities: G.729 AnnexA with and without silence suppression, a data channel with compression and an out-of-band control channel. The Capability
35、Descriptor then describes which of the capabilities in the CapabilityTable can be supported simultaneously in the V.70 terminal. This is done using the H.245 structures alternativecapabilityset and simultaneousCapabilities. In the example above the CapabilityDescriptor number 1 indicates two V.70 co
36、nfigurations. The first descriptor says the V.70 can support two channels simultaneously: one with either G.729 Annex A or G.729 Annex A with silence suppression, and the other data with compression. The second descriptor indicates that the V.70 terminal can support three channels: one with either G
37、.729 Annex A or G.729 Annex A with silence suppression, the second just data with compression and a third out-of-band control channel. Out-of-band capability exchange in which simultaneous capabilities on multiple channels can be described is discussed in Out-of-Band Control Channel subclause below.
38、 DISC XID 11.6 Channel establishment After the capability exchange, audio or data channels may be established. Following procedures in V.42, channels are established using a SABME-UA (or SABME-DM for rejection) frame exchange. In V.70, both the SABME and UADM include Melds that contain an H.245 mess
39、age. Which H.245 message is contained in which HDLC frame is specified by V.75. Table 11.1 below shows the mapping between HDLC frames and H.245 messages. CloseLogicalC hannel TerminalCapability Set or TerminalCapabilitySetAck or TerminalCapabilitySetReject Table II.lN.75 - Mapping of HDLC frame Mel
40、ds and H.245 messages I HDLC Frame I H.245 message in I-field I I SABME I OpenLogicalChannel I UA OpenLogicalC hannelAck or CloseLogicalC hannelAck I DM I OpenLogicalChannelNack I The OpenLogicalChannel message in the SABME frame contains all of the information needed to establish the correct mode o
41、f operation on the DLCI specified in the control octets of the SABME frame. The DLCI value of the SABME frame and the parameters in the OpenLogicalChannel message in the I-field should match capabilities exchanged on the DLCI in the previously completed XID command-response exchange. For V.70 termin
42、als, the value of n401 contained in both the OpenLogicalChannel and TerminalCapabilitySet message shall be negotiated in octets. 11.7 Channel closure At the end of a V.70 session, or if a terminal wishes to close a channel in the middle of a session, either terminal may initiate a channel close proc
43、edure. Closing a channel in a V.70 session is Recommendation V.75 - Appendix II (02/98) 3 STD-ITU-T RECMN V.75-ENGL 1998 m 48b2591 Ob50912 8T8 m accomplished by transmitting an H.245 CloseLogicalChannel message in the V.76 I-field of a DISC frame. A V.70 terminal receiving such a frame shall then re
44、spond with a UA frame containing a CloseLogicalChannelAck message. 11.8 Sample session Figure 11.3 is an example of a basic V.70 session depicting the HDLC frames exchanged between V.70 terminals to establish data channel on DLCI O and an audio channel on DLCI 1. Initiator Responder After V.34 start
45、up completion XID, DLCI O (Daia) ACK. DLCI O XID, DLCI O (Data) Y ACK, DLCI O b SABME, DLCI O (Open Data logical channel) r UA, DLCI O XID, DLCI 1 (Audio) 4 ACK, DLCI 1 4 XID, DLCI 1 (Audio) ACK, DLCI 1 4 r SABME, DLCI 1 (Open Audio logical channel) F UA, DLCI 1 T 1600840-97 Figure 11.3N.75 - Sample
46、 of a basic V.70 session, without Suspend/Resume 11.9 Out-of-band control channel An out-of-band channel is a third type of channel that a V.70 terminal can open in addition to data and audio. The out-of-band control channel is an option in V.70 terminals. Only H.245 messages are transmitted. Table
47、6N.75 lists those H.245 messages that are currently defined as valid in a V.70 out-of-band control channel. In general, an out-of-band control channel transmits control messages that affect the entire V.70 session and not just a single channel or DLCI. An out-of-band control channel must be opened j
48、ust like an audio or data channel before it can be used for H.245 message transfer. H.245 messages transmitted in an out-of-band control channel are transferred reliably in I-frames. 11.10 Establishment of suspend/resume channels with the out-of-band control channel Suspendresume is a V.76 option in
49、 V.70 that allows frames from channels that are real-time in nature to “suspend“ transmission of other frames before the HDLC end-flag to lower real-time channel latency. In addition, the suspendresume option reduces overhead by reducing the number of control octets. The suspendresume feature is accomplished by redefining the ABORT flag as a “suspend“ flag, and is therefore a feature of the multiplex layer in V.70. As the multiplex layer is 4 Recommendation V.75 - Appendix II (02/98) STD-ITU-T RECMN V.75-ENGL 3998 M 4862593 0650933 734 responsible for transmission of all
