1、NTERNATIONAL TELECOM M U N CATION UNION V.42 bis CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE DATA COMM UNICATION OVER THE TELEPHONE NETWORK DATA COMPRESSION PROCEDURES FOR DATA CIRCUIT TERMINATING EQUIPMENT (DCE) USING ERROR CORRECTION PROCEDURES Recommendation V.42 bis Ge
2、neva 1990 CCITT RECflNxV.42 BIS 90 = 48b259L 0573261, 73T m INTERNATIONAL TELECOMMUNICATION UNION CCITT V.42 bis THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE DATA COMMUNICATION OVER THE TELEPHONE NETWORK DATA COMPRESSION PROCEDURES FOR DATA CI RCUIT TERM I NATI NG EQUIPMENT (DCE)
3、 USING ERROR CORRECTION PROCEDURES Recommendation V.42 bis Geneva 1990 CCITT RECMNtV.42 BIS 90 = 4862591 05732b2 b7b FOREWORD The CCITT (the International Telegraph and Telephone Consultative Committee) is the permanent organ of the International Telecommunication Union (ITU). CCITT is responsible f
4、or studying technical, operating and tariff questions and issuing recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its St
5、udy Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988). Recommendation V.42 bis was prepared by Study Group XVII and was approved under the Resolution No. 2 procedure on the 31st
6、 of January 1990. O IT 1990 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 from the ITU. CCITT RECMN*V=42 BIS 90 D 48b259L 0573263 502 I The CCIT
7、T, considering (a) that the use of V-Series DCEs for transmission of asynchronous data on the general switched telephone network (GSTN) is widespread; Recommendation V.42 bis I 1 scope DATA COMPRESSION PROCEDURES FOR DATA CIRCUIT TERMINATING EQUIPMENT CE) USING ERROR CORRECTING PROCEDURES terminal e
8、quipment (DTE) b) a decoding procedure which recovers the strings of characters from received codewords; c) an automatic transparent mode of operation when uncompressible data is detected. An exploration of the parameters used in this Recommendation is given in 5 10. I I 1.2 Requirements for error c
9、orrecting procedures For correct operation of the data compression function it is necessary that an emr correcting procedure be implemented between the two entities using this Recommendation. In the case of V-series Recommendations, this requires that the LAPM (link access procedure for modems) erro
10、r correcting procedures defined in Recommendation V.42 or the error correcting procedures in Recommendation V.120 2 be implemented. Note - Undetected bit errors will cause misoperation of the data compression function. Use of a 32-bit frame check sequence (FCS) as defined in IS0 3309 3 substantially
11、 reduces the possibility of such errors. It may therefore be desirable to use this 32-bit FCS (which is an option in V.42 LAPM) in environments with severe impairments. 1.3 The elements of an error correcting V-series DCE are specified in Recommendation V.42. A DCE employing data compression The dat
12、a compression function may be used with an error-correcting DCE, as shown in Figure lN.42 bis. Recommendation V.42 bis 1 _- CCITT RECMN*V*42 BIS 70 4862573 05732b4 449 Data Error compression control function function Signal GSTN converter Asynchronous DTE Interchange 4 - circuits Control function 17
13、ooa69 FIGURE 1W.42 bis DCE employing data compression and error control 2 Definitions 2.1 character Single data element, encoded using a predefined number of bits (N3 = 8). 2.2 start-stop or asynchronous format Start-stop or asynchronous format is defined in Recommendations V.7 4 and V.14 5. 2.3 ord
14、inal value example, the character “A“ when encoded as 0100OOO1 would have an ordinal value of 65,. The ordinal value of a character is the numerical equivalent of the binary encoding of the character. For 2.4 alphabet Set of all possible characters which ma be sent or received across the DTEDCE inte
15、rface. It is assumed in this Recommendation that the ordinal values of ti e alphabet are contiguous from O to N, - 1, where N4 is the number of characters. 2.5 codeword A codeword, within the context of this Recommendation, is a binary number in the range O to N, - 1 which represents a string of cha
16、racters in compressed form. A codeword is encoded using a number of bits, C, where C, is initially 9 (Le. N, + 1) and increases to a maximum of N, bits (see 8 7). 2.6 control codeword compression function whilst in the compressed mode of operation (see 9). A control codeword is reserved for use in D
17、CE-to-DCE signalling of control information related to the 2.7 command code Octet which is used for DCE-to-DCE signalling of control information related to the compression function whilst in the transparent mode of operation. Command codes are distinguished from normal characters by a preceding esca
18、pe character (see 8 2.13). 1 I 2 Recommendation V.42 bis CCITT RECMN*V=42 BIS 90 D 4862591 05732b5 385 D 2.8 tree structure initiai character (see Figure 2N.42 6is and 8 6.1). Abstract data structure which is used in this Recommendation to represent a set of strings with the same 2.9 leaf node (see
19、8 6.1). Point on a tree which. represents, within the context of this Recommendation, the last character in a string 2.10 root node A root node is a point on a tree which represents, within the context of this Recommendation, the firnt character in a string (see Figure 2N.42 bis and 8 6.1). 2.1 1 co
20、mpressed operation of the data received from the DTE (see 8 7.1). Compressed operation has two modes. Transitions between the modes may be automatic based on the content 2.11.1 compressed mode A mode of operation in which data from the DTE is transmitted in codewords. 2.11.2 transparent mode form. T
21、ransparent mode command code sequences may be inserted into the data stream. A mode of operation in which compression has been selected but data is being transmitted in uncompressed 2.12 uncompressed operation A mode of operation in which compression has not been selected. The data compression funct
22、ion is inactive. 2.13 escape character Within the context of this Recommendation, the escape character is a character which, in transparent mode, indicates the beginning of a command code sequence. This has an initiai vaiue of zero, and is adjusted on each appearance of the escape character in the d
23、ata stream from the DTE, whether in transparent mode or compressed mode (see 8 9.2). 3 Abbreviations The abbreviations introduced in this Recommendation are: EID Escape in Data, a command code defined in 8 9. ETM Enter Transparent Mode, a control codeword defined in 8 9. ECM Enter Compressed Mode, a
24、 command code defined in 8 9. 4 Overview of the operation of a DCE incorporating a data compression function 4.1 General A XE employing data compression, as depicted in Figure lN.42 bis, contains the following components: a) DTEDCE interchange circuits; b) a signai converter; c) a control function;
25、Recommendation V.42 bis CCITT RECMNxV.42 BIS 90 W 4862573 05732hb 213 d) an error control function; and e) a data compression function. The control function shall have additional capabilities beyond those needed for an error correcting DCE as described in Recommendation V.42. The additional capabili
26、ties of the control function are described in 5, and the operation of the data compression function is described in $5 6 to 9. The remainder of this section provides an overview of the control function and data compression function. 4.2 the following aspects of operation; Overview of the conhol func
27、tion The control function shall perform, in addition to the functions defined in 0 6.2 of Recommendation V.42, negotiation of the presence of the data compression function in the remote XE, and of parameters associated with the operation of the data compression function; initialization or re-initial
28、ization of the daia compression function; coordination of the establishment of an error controlled connection for use by the peer data compression functions; coordination of the delivery of data between the DTE/DCE interface and the data compression function, in accordance with the procedures define
29、d in Recommendation V.42, $0 6.2 and 8.4, including the provision of the flow control procedures defined therein; coordination of the delivery of data between the data compression function and the error control function; action on detection of an exception condition. 4.3 Overview of the data compres
30、sion function The data compression function shall impiement the procedures defined in this Recommendation, which result in the efficient encoding of data prior to transmission over the error controlled connection, and shall have the following capabilities: a) initialization of the data compression f
31、unction: b) data compression encoding and decoding; c) a mechanism for switching between compressed and transparent modes of operation. 4.4 Communication between the controlfunction and the data compression function Communication between the control function and the data compression function is mode
32、lled as a set of abstract primitives of the form X-NAME-TYPE which represent the logical exchange of information and control to accomplish a task or service. In the context of this Recommendation the control function is viewed as the “service user“ while the data compression function is viewed as th
33、e “service provider“. The types of primitive are request, indication, response and confirm. The services expected by the control function are shown in Table 1N.42 bis. 5 Operation of the control function 5.1 Negotiation of the data compression function The use of the data compression function and th
34、e associated parameters shall be negotiated at link establishment via a protocol (for example, using the XID procedure defined in Recommendation V.42), following which they remain unchanged for the duration of the error corrected connection. 4 Recommendation VA2 bis CCITT RECMN*V*i.12 BIS 90 m 48625
35、93 05732b7 158 m TABLE 1n.42 bis Services expected by the control function Service Initialize the data compression function Indicate an error to the control function Transfer uncompressed data to/frm the data compression function Transfer compressed data to/from the data compression function Flush r
36、emaining untransmitted data from the encoder 9 I Primitive I 5.2,5.6 C-ERROR C-FLUSH s Parameter Po specifies whether or not compression is to be used. This parameter also specifies the directions (transmit only, receive only, or both directions). The default value of Po is O, indicating no Compress
37、ion in either direction. If compression is proposed for only one direction, then the only valid response is for the proposed direction or no compression. If compression is proposed for both directions, then valid responses are for both directions, for either single direction, or for no compression.
38、Parameter P, represents a proposed value of N2 the total number of codewords. P, shall have a default value of 512, which is its minimum value: a maximum value is not specified within this Recommendation. Any attempt to specify less than the minimum value shall be considered a procedural error and r
39、esult in disconnection. When values of Pl are exchanged during the negotiation procedure in one or both directions of transmission, the lower value shall be selected and assigned to N, in both DCEs. Note - See Appendix II for guidance on the choice of value of N, and its effect on performance. Param
40、eter P2 is the proposed value for N7, the maximum string length. The default value of P2 is 6, and the permitted range is from 6 to 250. The values outside this range are invalid; and attempt to specify such values shall be regarded as a procedural error and result in disconnection. When values of P
41、2 are exchanged during the negotiation procedure, the lower value shall be selected and assigned to N7 in both DCEs. 5.2 Initialization of the data compression function Following successful negotiation of data compression parameters, the control function shall issue the C-INIT request primitive to t
42、he data compression function. The primitive shall indicate the values of the negotiated parameters. 5.3 Connection establishment indicate to the DTE that data transfer may commence. On receipt of the C-INIT confirm primitive from the data compression function, the control function shall 5.4 Coordina
43、tion of the transfer of data between the DTEIDCE interface and data compression function On completion of connection establishment, the control function shall request encoding of the data received on the DTE/DCE interface. Recommendation V.42 bis 5 1 CCITT RECMN*V-42 BIS 90 m 4862591 0573268 094 m T
44、o encode data, the control function shall issue a C-DATA request primitive to the data compression function. This primitive shall indicate the data to be encoded. On receipt of a C-DATA indication primitive from the data compression function, the control function shall deliver the decoded data to th
45、e DTE/DCE interface. Flow control procedures will be necessary in order to avoid potential loss of data due to buffer overflow. When the procedures defined in this Recommendation are used in conjunction with those defined in Recommendation V.42, the flow control procedures defined in Recommendation
46、V.42,$ 7.3.1 and 8.4.2, shall be applied. 5.5 shail issue an L-DATA request primitive to the error control function. issue a C-TRANSFER request primitive to the data compression function. Coordination of the transfer of data between the data compression function and error control function On receipt
47、 of a C-TRANSFER indication primitive from the data compression function, the control function On receipt of an L-DATA indication primitive from the error control function, the control function shall 5.6 conditions: Reinitialization of the data compression function The control function shall issue a
48、 C-INIT request to the data compression function on the following a) L-ESTABLISH indication or confirm; b) L- SIGNAL indication or confirm, where the primitive indicates a destructive form. It is the responsibility of the control functions to ensure that C-INIT request primitives are issued oniy whe
49、n no data is in transit between the data compression functions (e.g. in the error control functions) to ensure synchronization between the encoders and decoders. 5.7 Eapedited b) receipt of a codeword, at any time, equal to Cl; c) receipt of a codeword representing an empty dictionary entry; d) receipt of a reserved command code. 6 Procedures for dictionary use and maintenance 6.1 General The data compression function employs an algorithm in which a siring of characters read from the DTE is encoded as a fixed length codeword. The process employs dictionaries, in w
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