1、 Rec. ITU-R BT.1549 1 RECOMMENDATION ITU-R BT.1549*Data link protocol for interaction channel (Question ITU-R 16/6) (2001) The ITU Radiocommunication Assembly, considering a) the progress in information processing and communication technologies; b) the rapid progress towards enhanced and digital tel
2、evision delivery systems; c) the need within such systems for interactivity for a variety of purposes; d) the development of communication methods, over various delivery media, suitable for use in receiving, from viewers, return communications related to the programme material (vision, sound and dat
3、a); e) the availability of mass storage media permitting interaction without the requirement of a return channel; f) the large number of domestic receivers likely to be impacted by the adoption of interactive services; g) the large number of domestic multimedia systems likely to be impacted by the a
4、doption of interactive services; h) that return information from viewers can be also provided in a non-automatic way (viewers have to store and then take care of the delivery of the information); j) the existence of Recommendation ITU-R BT.1369 Basic principles for a worldwide common family of syste
5、ms for the provision of interactive television services; k) the need for simpler protocols than Recommendation ITU-R BT.1434, which supports Internet protocol (IP)-based protocols, such that the introduction of interactive services using a return channel might be more easily achieved; _ *Radiocommun
6、ication Study Group 6 made editorial amendments to this Recommendation in 2002 in accordance with Resolution ITU-R 44. 2 Rec. ITU-R BT.1549 l) that public switched telephone network (PSTN)/integrated services digital network (ISDN) is one of most generic interaction channels in many countries; m) th
7、e requirement that the simpler protocols should also support a simple level of the security, recommends 1 that the data link layer protocol for the interaction channel, specified in Annex 1, should be applied when using a low data rate modem and simple data transfer protocols. ANNEX 1 Data link prot
8、ocol for interaction channel 1 Scope This Recommandation specifies the data link layer protocol which can be applied when using a low data rate modem and a simple data transfer protocol for the interaction channel (i.e. non IP-based protocols such as high level data link control (HDLC) (ISO 3309, 43
9、35, 7809), basic mode control procedure for data communication systems (ISO 1745), etc.). 2 Normative references ITU-T Recommendation X.28 DTE/DCE interface for a start-stop mode Data Terminal Equipment accessing the Packet Assembly/Disassembly facility (PAD) in a public data network situated in the
10、 same country. 3 Reference models 3.1 Assumed connection model for interactive systems An assumed connection model for interactive systems to which this link layer protocol can be applied is shown below. NOTE 1 The model is compliant with the system model of ITU-T Recommendation J.110, and matches t
11、he logical model of ITU-T Recommendation J.111 with the exception that the S2 channel for data downloading is not used. Rec. ITU-R BT.1549 3 1549-01Host AReceiverHost BHost serverPSTN, etc.Shared access point networkHost No. AHost No. BFIGURE 1Symmetric bidirectional connection model(Direct connecti
12、on model: using a shared access point network, receiverscan easily select connection to any host server (host) directly)3.2 Data transfer phases Protocols using PSTN for bidirectional interactive services consist of the following five phases: Phase 1: Line connection phase Phase 2: Data link establi
13、shment phase Phase 3: Data transmission phase Phase 4: Data link termination phase Phase 5: Line disconnection phase. 4 Data link protocol for interaction channel 4.1 Conditions 4.1.1 Telecommunication conditions Telecommunication conditions for the modem of the receiver are shown in Table 1. 4 Rec.
14、 ITU-R BT.1549 TABLE 1 Telecommunication conditions for the modem of the receiver 4.1.2 Modem The modem shall support the specifications, listed above, equal to ITU-T Recommendation V.22 bis or higher (e.g. ITU-T Recommendations V.22 bis, V.32 and V.32 bis) and with the error correction scheme equal
15、 to MNP4 or higher (e.g. MNP4, MNP5, ITU-T Recommendations V.42 and V.42 bis). 4.1.3 Line connection and disconnection phase During the line connection and disconnection phase, the receiver connects to and disconnects from the host using PSTN, etc. The line connection and disconnection are controlle
16、d by AT (attention) commands for modems, terminal adapter (TA) and data communication adapters (ADP) of mobile phones. 4.2 Protocol for data link establishment and termination phase In this phase, which starts after the line connection has been effected, a data transmission is established between th
17、e receiver and a host. In addition, after data transmission has been completed, the link between the receiver and the host is terminated. These phases can be applied to data transfer protocols, which do not specify a destination address for each data packet. Therefore, this phase is applicable to va
18、rious types of protocols for interaction channels. Parameters Note Data length (character length) 8 bits Parity None Stop bit 1 bit Transmission code system Specified by each application Local echo back None (remote echo back may be used) CR/LF control From receiver to host: only CR is sent. From ho
19、st to receiver: CR and LF are sent Code for transmission delimiter CR (0D H) Code for new line LF (0A H) Code for input correction BS (08 H) Communication conditions for line connection Data transfer sequence Specified by each application Transmission mode Asynchronous full duplex transmission Trans
20、mission speed Above ITU-T Recommendation V.22 bis (2 400 bit/s) Flow control RS/CS MNP class Class 4 or above Communication conditions for modem CR: transmission mode LF: code for new line MNP: Microcom Networking Protocol Rec. ITU-R BT.1549 5 When basic modems are used, error detection and correcti
21、on protocols are either performed at the physical layer (MNP4), the data link layer or the network layer, whichever is specified by the operational rule. Table 2 shows the protocol stack for this phase. TABLE 2 Protocol stack for data link establishment and termination phase 4.3 Host number When rec
22、eivers connect to hosts through telephone networks and so on, they should connect to the shared access point at first and send the host number command to identify host servers. 4.4 Sequences 4.4.1 Connection sequence 1549-02AccesspointPSTN, etc.IRDCallIndication ofcall receptionRequest forconnection
23、ResponseHandshakeNotificationof connectionNotificationof connectionHost number commandRemote echoService signal COMHostFIGURE 2Normal sequenceModem/ADPADP: adapterIRD: integrated receiver/decoderProtocol stack Application layer Selected according to service Data link layer Protocol conforming to a p
24、art of ITU-T Recommendation X.28 (requires a facility for specifying the number of host servers) Physical layer Receiver Host Basic modem Conforming with ITU-T Recommendation V.22 bis and later 6 Rec. ITU-R BT.1549 1549-03FIGURE 3Error sequence(error caused by host number command)Request forconnecti
25、onCallResponseHandshakeHost number commandRemote echoIndication ofcall receptionAfter ERR INV has been sent, it waits for host number command. Then it watches whether any communication is performed or not.(Time out duration: T1)Note 1 The value of T1 is given by each application.Notificationof conne
26、ctionService signal ERR INVNotificationof connectionNotificationof connection( means space)1549-04AccesspointPSTN, etc.IRDCallIndication ofcall receptionRequest forconnectionResponseHandshakeNotificationof connectionNotificationof connectionNo communicationDisconnectHostFIGURE 4Error sequence(error
27、caused by time out process of host server after waiting for host number command)Modem/ADPAfter specified time for the timer which watches the communication has been passed, the line connection is broken.(Time out duration: T1)Note 1 The value of T1 is given by each application.T1Rec. ITU-R BT.1549 7
28、 1549-05FIGURE 5Error sequence(rejection of call reception caused by host)Request forconnectionCallResponseHandshakeHost number commandRemote echoIndication ofcall receptionNotificationof connectionDisconnectNotificationof connection1549-06AccesspointPSTN, etc.IRDCallIndication ofcall receptionReque
29、st forconnectionResponseHandshakeNotificationof connectionHost number commandRemote echo with the different host number from the transmitted oneDisconnectHostFIGURE 6Error sequence(error of remote echo)Modem/ADPNotificationof connection8 Rec. ITU-R BT.1549 1549-07FIGURE 7Error sequence(time out occu
30、rred at receiver after waiting for remote echo)Request forconnectionCallResponseHandshakeHost number commandNo communicationIndication ofcall receptionNotificationof connectionDisconnectNotificationof connectionAfter sending host number command, it watches the communication, and after specified time
31、 T1 is passed without communication, it breaks the line connection. (Time out duration: T1)Note 1 The value of T1 is given by each application.T11549-08AccesspointPSTN, etc.IRDCallIndication ofcall receptionRequest forconnectionResponseHandshakeNotificationof connectionHost number commandService sig
32、nal with errorDisconnectHostFIGURE 8Error sequence(error of service signal)Modem/ADPRemote echoNotificationof connectionRec. ITU-R BT.1549 9 1549-09T1After receiving remote echo, it watches the communication, and when specified time T1 is passed without communication except remote echo after the tra
33、nsmission of host number, it breaks the line connection.(Time out duration: T1)Note 1 The value of T1 is given by each application.FIGURE 9Error sequence(time out occurred at receiver after waiting for service signal)Request forconnectionCallResponseHandshakeIndication ofcall receptionNotificationof
34、 connectionNotificationof connectionHost number commandNo communicationDisconnectRemote echo4.4.2 Break sequence 1549-10AccesspointPSTN, etc.IRDDisconnection of the lineIndication ofdisconnectionDisconnectioncommandHostFIGURE 10Disconnection initiated by receiversModem/ADP1549-11Disconnection of the
35、 lineFIGURE 11Disconnection initiated by hostDisconnectioncommandIndication ofdisconnection10 Rec. ITU-R BT.1549 An example format of the host number command and service signal is shown in Table 3. TABLE 3 Example format of the host number command and service signal 4.4.3 Receiver behaviour after tr
36、ansmitting host number command 4.4.3.1 Waiting for remote echo of transmitted host number After sending the host number, receivers move into the waiting status in order to receive the remote echo. The behaviour of the receivers in this status is shown in Table 4. TABLE 4 Behaviour of the receivers w
37、aiting for the remote echo Format Note Host number command N1N2N3N4N5N6N7N8 CR Characters which are echoed back N1N2N3N4N5N6N7N8 CR LF 8 alphanumeric characters are echoed back Notification of connection CR LF COM CR LF CR LF Service signal Command error CR LF ERRINV CR LF means space Received signa
38、l Behaviour after receiving the signal Same remote echo as the transmitted host number Reception of N1N2N3N4N5N6N7N8 CR LF (8 characters prior to the CRLF are compared and the other characters are neglected) Moves into the waiting status for service signal Different remote echo from the transmitted
39、host number Reception of CR LF ( means code strings with any length larger than 0 byte other than N1N2N3N4N5N6N7N8) Disconnects the line immediately After sending host number or re-sending it, it does not receive CR LF within the specified time (time out duration: T1)(1)Disconnects the line immediat
40、ely (1)The timer of the receiver which watches the communication starts from the transmission of host number command or its retransmission. (The value of T1 is specified by each application.) Rec. ITU-R BT.1549 11 4.4.3.2 Waiting for service signal After receiving the same remote echo as the transmi
41、tted host number, N1N2N3N4N5N6N7N8 CR LF, receivers move into the waiting status for service signal. The behaviour of the receivers in this status is shown in Table 5. TABLE 5 Behaviour of the receivers waiting for the service signal 4.4.4 Remote echo When a receiver sends the host number command, t
42、he host side echoes back to the receiver. Thus the local echo back in the receiver is not necessary. The host side echoes back after receiving the host number command, and sends a service signal in succession. 4.4.5 Start timing of the timer at the server side The timer at the host side which watche
43、s the communication starts count-up from the completion of the line connection (end of the modem negotiation). The value of time out T1 is specified according to this start time. The timer resets after sending CR LF ERRINV CR LF. Received signal Behaviour after receiving the signal Correct service s
44、ignal (notification of connection)(1)Reception of CR LF COM CR LF Moves into data transfer sequence Correct service signal (command error)(1)Reception of CR LF ERRINV CR LF ( means space) Retransmits host number immediately. Number of retransmission: 3 times. (It disconnects when CR LF ERRINV CR LF
45、is received four times) Service signal with errors(1)Reception of CR LF COM, CR LF ERR, or CR LFCR LF ( means codes other than CR. means codes other than space. means strings with any length larger than 0 byte other than COM and ERRINV) Disconnects the line immediately After sending host number or r
46、esending it, it does not receive correct service signal within the specified time (time out duration: T1)(2)Disconnects the line immediately (1)The data which is received from the transition to the waiting status of service signal to the time when the first CR LF is received are discarded. (2)The ti
47、mer of the receiver which watches the communication starts from the transmission of host number command or its retransmission. (The value of T1 is specified by each application.) 12 Rec. ITU-R BT.1549 APPENDIX 1 TO ANNEX 1 Example protocols used for direct connections (data transmission phase) toget
48、her with the specified data link protocol (data link establishment and termination phase) In the following protocols, error detection and correction protocols are either performed at the physical layer (MNP4), the data link layer or the network layer, whichever is specified by the operational rule.
49、NOTE 1 Protocols at the physical layer in this Recommendation mean those of the physical layer and transport layer in ITU-T Recommendation J.111 (or Recommendation ITU-R BT.1434) and ITU-T Recommendation J.113 (or Recommendation ITU-R BT.1435), and, Protocols at the data link and higher layers mean the network independent protocols in ITU-T Recommendation J.111. These differences come from the IP protocol layer. In this specification IP is stated at the network layer, which is stated at the higher medium layer in I
