[ Pobierz całość w formacie PDF ]
.8.12.3.3 Expiry of timer T403If timer T403 expires, the error-correcting entity will act as follows (it should be noted that timer T401 is neither running nor expired):a) set the retransmission count variable to 0;b) enter the timer-recovery condition (see 8.5.3);c) transmit a supervisory command with the P bit set to 1 as follows:- if there is not an own-receiver busy condition, transmit an RR command; or- if there is an own-receiver busy condition, transmit an RNR command;d) start timer T401; ande) inform the control function after N400 retransmissions.8.13 Transfer of break8.13.1 GeneralUpon receipt of an L-SIGNAL request primitive from its control function, an error-correcting entity shall transmit a UI command frame with its P bit set to 0.The information field of the UI command frame shall be encoded to indicate a break (BRK) message and shall contain the break-handling option as indicated by the control function.If the L-SIGNAL request primitive includes a break length, it shall also be encoded in the information field of the UI frame.(See 12.3 on the encoding of UI frames for transferring a break signal.) Actions taken by the DCE (including the error-correcting entity) are specified in Table 4.On receipt of a UI command frame indicating a BRK, the error-correcting entity shall issue an L-SIGNAL indication primitive to its control function, conveying the break-handling option and, if present, the break length and follow the actions specified in Table 5.On receipt of an L-SIGNAL response primitive from its control function, an error-correcting entity shall transmit a UI response frame as soon as possible with its F bit set to the same binary value as the received UI command frame.The information field of the UI response frame shall be encoded to indicate a break acknowledgement (BRKACK) message.On receipt of a UI response frame with a BRKACK message in reply to a UI command frame conveying a BRK message, an error-correcting entity shall issue an L-SIGNAL confirmation primitive to its control function.NOTE - The exchange of UI frames, in and of itself, does not provide a confirmed service.The confirmed nature of the L-SIGNAL service provided to the control function, as described here, comes about through the association and interpretation of the contents of the information fields of the UI frames exchanged and not the association of a UI response frame with a previously-transmitted UI command frame.8.13.2 State variables and parameters8.13.2.1 Send and receive sequence numbersTo distinguish between unique and duplicate UI frames carrying break information, an error-correcting entity shall perform a sequencing operation, modulo 2, on the information field of the UI frame.Bit 8 of the first octet of the information field shall be used for this purpose.As such, bit 8 serves as a break send sequence number, N(SB), in BRK message while it serves as a break receive sequence number, N(RB), in BRKACK messages.8.13.2.2 Send state variable V(SB)The error-correcting entity shall maintain the break send state variable V(SB).V(SB) denotes the value of N(SB) in the next BRK message sent as a result of receiving an L-SIGNAL request primitive from the control function.V(SB) is complemented each time a transmitted BRK message is correctly acknowledged by a BRKACK message.Initially, when the physical connection has been established, V(SB) is set to zero.8.13.2.3 Receive state variable V(RB)The error-correcting entity shall maintain the break receive state variable V(RB).V(RB) denotes the expected value of N(SB) in the next BRK message to be received.If N(SB) in the next received BRK message is equal to V(RB), then V(RB) shall be complemented prior to sending the BRKACK message.Initially, when the physical connection has been established, V(RB) is set to zero.8.13.3 Break procedures8.13.3.1 Transmitting a BRK messageOn receipt of an L-SIGNAL request primitive, the error-correcting entity shall transmit a BRK message in a UI command frame with its P bit set to 0.The error-correcting entity shall set N(SB) to the current value of V(SB), start the acknowledgement timer T401 (see 9.2 [ Pobierz caÅ‚ość w formacie PDF ]