1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 4 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%2-).!,G0G0%15)0-%.4G0G0!.$G0G002/4/#/,3G0G0 amended at Melbourne, 1988)The CCITT,considering(a) that the Teletex service will be introduced in different types of networks, i.e. circuit switched public datan
2、etworks (CSPDN), packet switched public data networks (PSPDN) and public switched telephone networks(PSTN);(b) that depending on the service provided by the physical level, the link level procedures may have to cater fora half-duplex transmission facility;(c) that some Administrations are considerin
3、g the provision of a Teletex service with a half-duplextransmission facility on the PSTN;(d) that modems according to Recommendation V.26 bis are suitable for half-duplex transmission at 2400 bit/son PSNTs,unanimously declaresthat this Recommendation defines the link level procedure using LAPB exten
4、ded for half-duplex physical levelservice.1 Introduction1.1 General1.1.1 Figure 1/T.71 shows the half-duplex transmission module (HDTM) for extending the use of LAPB foroperation of Teletex terminals connected to the PSTN where use of half-duplex 2400 bit/s modems is planned. This isreferred to in R
5、ecommendation T.70 as LAPX.2 Fascicle VII.5 - Rec. T.711.1.2 Before the HDTM begins operation the physical circuit must be established by the appropriate PSTN callcontrol procedures. The operation of the HDTM is such that the calling DTE will initially have the right to transmit.For the link address
6、ing conventions refer to Recommendation T.70.1.2 Architecture1.2.1 Level relationshipsIt is an objective to avoid modification of the definition of LAPB in order to adapt it for half-duplex operation.However, there is a functional requirement that the HDTM inhibit LAPB from sending frames during cer
7、tain phases ofthe half-duplex procedure. The means of accomplishing this functional requirement is not defined.The logical relationships between LAPB, the HDTM and the physical level are as shown in Figure 2/T.71.1.2.2 Control (C) and status (S) functionsThe following logical functions are defined t
8、o describe the interactions between LAPB and the HDTM:Control Revert to the HDTM idle state since LAPB has entered the disconnected phase (equivalent to ADM ofHDLC).Status LAPB is enabled to send frames.Status LAPB is inhibited from sending frames.Fascicle VII.5 - Rec. T.71 32 State diagram and desc
9、riptions2.1 State diagramThe state diagram shown in Figure 3/T.71 describes the procedure for controlling the right to transmit. Thenumber in each ellipse is the state reference number.2.2 State definitions2.2.1 Idle state (state 0)The DTE is in an inactive state. This is the initial state prior to
10、call establishment and the final state after calltermination.4 Fascicle VII.5 - Rec. T.712.2.2 Half-duplex sending state (state 1)The DTE is in a half-duplex sending state, so that all signals generated by LAPB are passed to the physicallevel.2.2.3 Wait for receiving state (state 2)The DTE is awaiti
11、ng indication that the remote DTE has entered the half-duplex sending state. No signalsgenerated by LAPB are passed to the physical level.2.2.4 Half-duplex receiving state (state 3)The DTE is in a half-duplex receiving state, so that no signals generated by LAPB are passed to the physicallevel. The
12、remote DTE is considered to be in the half-duplex sending state.2.2.5 Wait for sending state (state 4)The DTE is awaiting indication of the availability of the physical level for transmission of frames to the remoteDTE. All signals generated by LAPB are passed to the physical level, but LAPB is inhi
13、bited from sending frames.2.3 Table of transitions between statesTable 1/T.71 shows the events that cause transitions from one state to another, along with any resulting actions.This shows a generalized description of the operation of the HDTM.2.4 State definitions expressed in terms applicable to a
14、 modem interfaceThe following definitions apply to the use of the HDTM with the V.26 bis modem interface, as an example.2.4.1 Idle state (state 0)Circuit 107 is OFF.2.4.2 Half-duplex sending state (state 1)Circuit 105, circuit 106 and circuit 107 are ON. LAPB is connected to circuit 103 and enabled
15、to send frames.2.4.3 Waitfor receiving state (state 2)Circuit 107 is ON, circuit 105 is OFF. LAPB is inhibited from sending frames and disconnected from circuit103, which is held in the binary 1 condition. Timer T is running.2.4.4 Half-duplex receiving state (state 3)Circuit 107 is ON, circuit 105 i
16、s OFF. LAPB is inhibited from sending frames and disconnected from circuit103, which is held in the binary 1 condition.2.4.5 Wait for sending state (state 4)Circuit 105 and circuit 107 are ON, and circuit 106 is OFF. LAPB is connected to circuit 103 but is inhibitedfrom sending frames.2.5 Table of t
17、ransitions between states expressed in terms applicable to a modem interfaceTable 2/T.71 shows, in terms of the V.26 bis modem interface, the events that cause a state transition and theresulting action(s).Fascicle VII.5 - Rec. T.71 52.6 Timer TThis timer is used to recover from an apparent failure
18、of the remote DTE to take the right to transmit. To avoida contention condition during this recovery process, different values of timer T are to be used by the called and callingDTE. A calling DTE uses the value Taand a called DTE uses the value Tb.The values of Ta and Tbare system parameters and mu
19、st be studied further in relationship to interworkingrequirements and other system parameters in Recommendation T.70.TABLE 1/T.71Description of state transitionsPresent state Event Action New state0Calling DTE: Data circuit established (e.g. dataset ready, ready for data)40Called DTE: Data circuit e
20、stablished (e.g. dataset ready, ready for data)Start timer T 24Indication of availability of the physical levelfor transmissionSend indication to the remote DTE that thehalf-duplex sending state has been enteredStatus (see Note 1)11 Conclusion of transmissionSend request that remote DTE enter thehal
21、f-duplex sending state(see Note 4)Start timer TStatus (see Note 2)22Reception of indication that the remote DTEhas entered the half-duplex sending stateStop timer T 32 Expiry of timer T 43 Reception of notification that the remote DTEis requesting a change in the direction oftransmission41LAPB has e
22、ntered a disconected phase(i.e. Control , see Note 3)03LAPB has entered a disconnected phase(i.e. Control , see Note 3)0Any Physical level has no circuit to a remote DTE 0Note 1 Status indicates to LAPB that the sending of frames is enabled.Note 2 Status indicates to LAPB that the sending of frames
23、is inhibited.Note 3 Control indicates that LPAB has entered the disconnected phase (equivalent to ADM of HDLC).Note 4 HDTM uses the idle data link channel state indication (at least 15 contiguous 1s) for requesting that the remote DTE enterthe half-duplex sending state.6 Fascicle VII.5 - Rec. T.71TA
24、BLE 2/T.71Description of state transitions in terms of the V.26 bis modem interfacePresent state Event Action New state0 Calling DTE: Circuit 107 ONTurn circuit 105 ONConnect LAPB to circuit 10340 Calling DTE: Circuit 107 ON Start timer T 24 Circuit 106 ON Enable sending of LAPB frames(see Note 1)11
25、 Transmission concluded (see Note 2)Inhibit sending of LAPB framesDisconnect LAPB from circuit 103Hold circuit 103 in the binary 1 conditionTurn circuit 105 OFF (see Note 3)Start timer T22 Reception of a flag Stop timer T 32 Expiry of timer T Turn circuit 105 ONRelease circuit 103 from binary 1 cond
26、itionConnect LAPB to circuit 10343Reception of 15 contiguous 1 bits (see Notes 4and 5)Turn circuit 105 ONRelease circuit 103 from binary 1 conditionConnect LAPB to circuit 10341 LAPB has entered a disconnected phase Turn circuit 105 OFF 03 LAPB has entered a disconnected phase 0Any Circuit 107 OFF T
27、urn circuit 105 OFF 0Note 1 It is necessary to ensure that at least one full flag is transmitted after circuit 106 comes ON. This flag may be the openingflag of the first frame.Note 2 The HDTM may determine that a transmission by the LAPB module has been concluded by either of the following: countin
28、g a sequence of contiguous flags on circuit 103 while in state 1, a time-out, T, a signal from another source, e.g., from a higher level.However, if no frame is transmitted while in state 1, not less than five contiguous flags shall be sent in state 1 before entry into state2.Note 3 It is recommende
29、d that circuit 105 not be turned OFF until 15 bit times after the binary 1 condition is established on circuit103. This will assure transmission of an idle sequence to the remote DTE.Note 4 It is recognized that whether or not an idle sequence is sent by the remote DTE, the DTE will detect an idle s
30、equence aftercircuit 109 goes OFF, since according to Recommendation V.26 bis, this will hold circuit 104 in the binary 1 condition.Note 5 It is understood that circuit 109 will go OFF. Entry into state 4 may be made dependent on this OFF condition, as animplementation option.Fascicle VII.5 - Rec. T
31、.71 7ANNEX A(to Recommendation T.71)Additional rules making for greater efficiency in half-duplex transmissionA.1 General considerations Greater efficiency is obtained in recovery situations. The application of these rules is optional. The application of these rules does not imply any incompatibilit
32、y or entail any amendment of DTEs (orDCEs) which observe the procedures described in Recommendation T.71.A.2 Rules of operation1)Before the DTE (or DCE) gives the turn back, it ensures that it has acknowledged all the frames receivedand accepted before it received the turn.2)If the DTE (or DCE) receives or takes the turn, it will always first retransmit all the I-frames which havenot been acknowledged.3)The DTE (or DCE) must replace the last RR frame in each turn, if any, by an REJ frame carrying theappropriate N(R).
