1、I I 2326414 000b165 b W Page 1 TJN 34-02 E Recommendation T/N 34-02 E (Edinburgh 1988) MESSAGE CHNWECL SPECIFICATION, LAYERS 1-6 FOR USE IN AUDIOVISUAL COMMUNICATION Recommendation proposed by Working Group T/WG 14 “Network Aspects” (NA) Text of the Recommendation adopted by “Telecommunicaions” Comm
2、ission: “The European Conference of Postal and Telecommunications Administrations, considering that there is an increasing demand for audiovisual services, and especially national and international multipoint confer- encing ; such services consisting of basic audiovisual facilities and a variety of
3、optional enhancements, and recogn ising the need for flexible controls and indications (C 2326414 000bLb 3 = T/N 34-02 E Page 5 Byte 3: The first bit is an extension bit, set to O if no extension. Five bytes maximum can be added for future use. The second bit is a “protocol discsiminator” (PD). When
4、 using layer 6 as defined in chapter 6, PD = O and the third byte has the following structure: O O X X X X X X Other codes may be used in future for fiirther extension of layer 3, if needed. Note. If no layer 6 content (e.g. Address Allocation Procedure as in 5.3.), PD=O. where X is “dont care” 5.3.
5、 Address Allocation Procedure The MCU number is pre-allocated by prior agreement, e.g. in the reservation phase. The goal of this address allocation procedure is to let each Terminal know its own address, and to let each MCU know whether its ports are connected to a Terminal, or another MCU. A) from
6、 the MCU side, the procedure takes place in three steps: Step 1 : Sent to each port a layer 3 message, i.e. 3-byte containing its own MCU number, the port number, the extension byte (no information field, at this stage). Step 2: Three possible messages may then be received: i) OO/OO/OO: message sent
7、 by a Terminal which has not yet received the message sent in step 1, ii) MCU/00/00: message sent by a Terminal which received the message sent in step 1, and which acknowledges it. iii) (Other MCU)/(other port)/00: message sent by another MCU as described in step 1. In this case, step 3 must be act
8、ivated. Step 3: The MCU re-sends a layer 3 message with the content MCU/OO/OO, acknowledging the other MCU. (MCU =other MCU number.) B) from the Terminal side, the procedure takes place in three steps: Step 1 : The Terminal continuously sends a layer 3 message consisting of OO/OO/OO. Step 2: Two pos
9、sible messages may then be received: i) OO/OO/OO: message sent by another Terminal, implying a point-to-point configuration. ii) MCU/port/OO: message sent by a MCU. In this case, step 3 is activated. Step 3: The Terminal replies with a message with MCU/OO/OO. Figure 3 (T/N 34-02) illustrates the pro
10、cedure as described. Edition of October 31, 1989 - E 2326434 O006370 T E T/N 34-02 E Page 6 MCU MCU/port/OO Terminal MCU MCU Layer 6 Activated Terminal Terminal oo/oo/oo MCU/port/OO MCU MCU/OO/OO MCI port/00 OO/OO Figure 3 (T/N 34-02). Address Allocation Procedure. 6, LAYER 6 6,l. Coding of messages
11、 At layer 6, the messages are encoded, according to CCITT specification X.409, in the ILC (Identifier, Length,Content) form. An X.409 Identifier byte is of the form: Primitive/Constructor 0/1 msb lsb Tag Class Tag In the above diagram the bits are allocated as follows: Tag class: defines the type of
12、 identifier O1 Application wide, e.g. unique to audio/video conferencing message repertoire. 10 Context specific, e.g. unique within a part of a message. Primitive: single ILC. Edition of October 31, I 232b414 0006171 1-0 i O0 T/N 34-02 E Page 7 O0 Point to point Specific TML O0 Specific MCU Constru
13、ctor: nested ILCs e.g, IL(ILC)(ILC). Tag : Note. Maximum of 2 levels of nesting are recommended. uniquely defines the identifier (according to its class). Note. The 5 bits available for tagging allow 30 unique values. Values higher than 30 require the use of the “extended form” of tagging. All messa
14、ges contain the following: Function Code, Destination Address, Source Address. Those messages which include data will also have an Information field. 6.2. Fiinction Code The X.409 facility of “tagging” an Identifier allows the Function Code of each message to be placed in the tag of the overall mess
15、age Identifier, Le. in the first byte, as shown in the following example: Constructor (1) I Tag Class= Tag = Application Wide (01) Function Code (e.g. 21) Note Note. For Function codes greater than 30, the Tag above is set to 11 11 1 which indicates that an extension byte) follows. An extension byte
16、(s) caters for Tags greater than 30. The last extension byte must have msb bit 8 set to zero. 6.3. Address Fields The Contents of the Layer 6 Destination and Source fields are 3 octets long and are encoded as shown below: Destination: identical to the layer 3, octets 1 and 2. Note. Destination has n
17、o routing function but is retained in layer 6 for possible future use. Source: 1st MCU Byte I 2$g I Source Byte 3: The third byte allows sub-addressing, e.g. Participant number or Microphone number. Set to zero when not used. The Identifier bytes for Destination and Source can be dened uniquely sinc
18、e, from 6.1., both are of class: Context Specific, and both are always of type: Primitive. X.409 Tags are allocated as shown, giving Identifier bytes for each field as follows: Destination (tag O) Identifier has value 80H Source (tag 1) Identifier has value 81H Edition of October 31, 1989 y, 7 F -7 - - _- i- , -
