1、ITU-T RECMN*V-/4 (REV* revised Helsinki, 1993) 1 Scope 1.1 This Recommendation describes a method of conveying start-stop characters over synchronous bearer channels using an async-to-sync converter in the data signalling rate range of up to 19 200 bids. Start-stop characters at signalling rates bel
2、ow or equal to 300 bids can be conveyed over synchronous bearer channels by oversampling at a signaiiing rate of at least 1200 bids. NOTE - The conversion method provided here replaces the conversion methcd applied earlier to Recommendations V.22, V.22 bis. V.26 ter and V.32. 1.2 This converter may
3、be an intermediate device inserted into the data lines of both circuit 103 in the transmitter and circuit 104 in the receiver inside a synchronous DCE (see Figure A.l), or a stand-alone unit in certain applications. 1.3 Operation of DTE-DCE interface after faiiure to establish error-correcting (Reco
4、mmenda- tion V.42) operation DCEs may be designed to employ primarily the asynchronous-to-synchronous conversion associated with the error- correcting function specified in Recommendation V.42. Such DCEs are normally also capable of interworking, in a fallback mode, with DCEs employing the asynchron
5、ous-to-synchronous conversion in conformance with this Recom- mendation, but retaining the buffering of data and the flow control across the DTE-DCE interface. Further details may be found in 7.9N.42. 2 Data signalling rates The conversion method shall be limited to signalling rates of up to 19 200
6、bids preferring the standard signalling rates of Recommendation V.5. The nominal signalling rates for both the start-stop characters and the synchronous DCE shall be the same. The tolerance of the signalling rate of the synchronous transmission shall be i 0.01%. 3 The conversion method is capable of
7、 tolerating the signalling rates of the DTE in two ranges: Signalling rate ranges of the start-stop characters at the converter input a) b) basic range: +1% to -2.5%; extended range: +2.3% to -2.5%. The use of the basic signalling rate range is preferred since it results in lower distortion. The c-o
8、ice of range s-all be made at the time of installation, and shall be the same for both transmitter and receiver. It is not intended to be under customer control 4 Start-stop character format It shall be possible to condition the converter to accept the following formats; viz: a) b) a one-unit start
9、element, followed by seven data units, and a stop element of the unit in length (9-bit characters); a one-unit start element, followed by eight data units, and a stop element of one unit in length (10-bit characters); Recommendation V.14 (03/93) 1 - - _ _ ITU-T RECMN*V-34 (REV*%) 93 4862591 0582983
10、9D5 c) a one-unit start element, followed by nine data units, and a stop element of one unit in length (11-bit characters); the converter may also accept characters consisting of: a one-unit start element, followed by six data units, and a stop element of one unit in length (8-bit characters). d) No
11、te that character formats c) and d) do not conform to International Alphabet No. 5. The character format selected shall be the same for both transmitter and receiver. The characters shall be in accordance with Recommendation V.4 regardless of whether they conform to International Alphabet No. 5. It
12、shall be possible to transmit characters continuously or with any additional continuous stop element of arbitrary length between characters. NOTE - In each of the four formats, data units can be replaced by additional stop units. For example, format c) will allow 11-bit characters consisting of a on
13、e-unit start element, followed by eight data units and a stop element of two units to be handled. 5 The effective net margin of the converter for transmitting of start-stop characters applied to the input of the converter shall be at least 40%. This figure is a subject for further study. Margin of t
14、he converter input 6 Selection for synchronous or asynchronous modes of operation shall be provided by switch (or similar means) enabling the user to perform normal transmission and testing in each mode of operation, respectively. In synchronous mode of operation the converter is totally bypassed in
15、 both directions. Selection of synchronous or asynchronous modes of operation 7 Async-to-sync conversion method The general method to handle the speed differences between the intracharacter signalling rate of the start-stop characters and the data signalling rate of the synchronous bearer channel wi
16、ll be the insertioddeletion of stop elements at the transmitter and reinsertion of deleted stop elements at the receiver. Means are provided to transfer continuous start polarity (break signals) as well. 7.1 Transmitter In the transmit direction the start-stop characters shall be adapted to the sign
17、alling rate of the synchronous bearer channel by: - - deleting stop elements in case of overspeed of the start-stop characters; insertion of additional stop elements in case of underspeed of the start-stop characters. 7.1.1 Basic signalling rate range No more than one stop element shall be deleted f
18、or any eight consecutive characters. 7.1.2 Extended signalling rate range No more than one stop element shall be deleted for any four consecutive characters. 7.2 Receiver The intracharacter signalling rate provided by the converter shall be in the range of the nominal data rate to the limit of the s
19、pecified overspeed tolerance, i.e. +1% in the basic and +2.3% in the extended data signalling range. The length of the stop element shall not be reduced by more than 12.5% for the basic signalling rate range (or 25% for the optional extended signalling rate range) to allow for overspeed in the trans
20、mitting terminal. The nominal length of the start and data elements for all characters shall be the same. 2 Recommendation V.14 (03/93) ITU-T RECMN*V.L4 (REV*L) 93 m 4862591 0582982 841 m NOTE - Equipments exists in the field which delete stop elements more fiequently than specified in 7.1.1 and 7.1
21、.2. However, in these equipments there will always be at least one additional inserted stop element between deleted stop elements. 7.3 Breaksignai 7.3.1 Transmitter If the converter detects M to 2M + 3 bits all of “start” polarity, where M is the number of bits per character in the selected format,
22、the converter shall transmit 2M + 3 bits of “stop” polarity. If the converter detects more than 2M + 3 bits all of “start” polarity the converter shall transmit all these bits as “start” polarity. NOTE - The converter must receive at least 2M bits of “stop” polarity after the “start” polarity break
23、signal in order to ensure that it regains the character synchronism. 7.3.2 Receiver The 2M + 3 or more bits of “start” polarity received from the transmitting modem shall be transferred to the output of the converter, and the character synchronism shall be regained from the following “stop” to “star
24、t” transition. NOTE - In some earlier implementations an uninitiated NUL character may precede the break signal at the output of the converter when no measures have been taken to prevent this. 7.4 Tandem operation Tandem operation between two ends comprising async-to-sync conversions can be establis
25、hed only by using cascaded synchrotous bearer channels. 7.5 Testing facilities AU the tests recommended in the relevant Recommendations can be performed in asynchronous operation as well, where this converter is used, with the exception of self test end-to-end. Recommendation V.14 (03/93) 3 Annex A
26、Inclusion of an async-to-sync converter into a synchronous DCE (This annex forms an integral part of this Recommendation) I signal te-sync converter 104 DTE - ITU-T RECMN*V. data input to the DCE TD Transmitted data; the synchrms output of the converter following the async-tesync conversion of start
27、-stop characters to be transmitted TDC Transmitter signai element timing; intemal timing information for the generation of synchronous transmitted data RDC Receiver signal element timing; internai timing information associated with synchro- nous received data RD 104 Received data; the input of the c
28、onverter for the restoration of start-stop characters Received data; data output from the DCE SY Synchronous mode; selection of the required mode of operation (asynchronous or synchronous) FIGURE A. IN. I4 NOTE - Other interchange circuits which are provided are not involved in the operation of the async-to-sync converter but must comply with the requirements of the relevant DCE Recommendations including the conditions of the hing circuits &e. 113,114 and 115) during both the asynchronous modes of operation. 4 Recommendation V.14 (03/93)
