This preview shows page 1. Sign up to view the full content.
Unformatted text preview: RD FOR COMMUNICATIONS OPERATION PROCEDURE-1 5.1.10 TRANSMISSION COUNT VARIABLES
When a Type-AD or Type-BC Transfer Frame is lost, the normal recovery procedure is to
retransmit it. If, however, there is a serious problem on the underlying communications link,
no amount of retransmissions will permit an acknowledgement for the Transfer Frame to
appear in the CLCW for the Virtual Channel.
If nothing were done, there would be no way for COP-1 to detect the error. Therefore, all
FDUs containing user data passed from the Higher Procedures at the sending end (as well as
all directives from the Higher Procedures) have associated with them, at least implicitly, a
limit to the number of times the corresponding Transfer Frame is to be transmitted.
In order to keep from declaring that the link has failed when it is in fact getting Transfer
Frames into the receiving end, the transmission count limit shall apply only to the first
Transfer Frame on the Sent_Queue. Once that Transfer Frame is acknowledged, the count
is reset, even though the remaining Transfer Frames on the Sent_Queue have already been
transmitted, possibly more than once. The effect is that the transmission count can be
considered to be associated with the Sent_Queue, rather than with each Transfer Frame;
therefore, each Transfer Frame will be transmitted ‘at least’ the number of times
corresponding to the value given by Transmission_Limit, unless this value is set to 1. In this
last case, no retransmission will be tried and each Transfer Frame will be transmitted only
Three FOP-1 variables are used for controlling the retransmissions:
b) Timeout_Type (TT);
The Transmission_Limit holds a value which represents the maximum number of times the
first Transfer Frame on the Sent_Queue may be transmitted. This includes the first
‘transmission’ and any subsequent ‘retransmissions’ of the Transfer Frame. A Transfer
Frame in the Sent_Queue that moves from...
View Full Document
- Spring '14