User manual

Computer Gateway User Manual 5-4 5/96
5.3.2
5.3.2 Dual-Link Bisynch Sequences
Although a single-link connection to the host computer is recommended, there is an option
for a dual Bisynch-link connection. The advantage of dual links is improved throughput
rates. The disadvantage is the need for more complex host-computer software necessary to
react to the CG on startup and in failure recovery conditions and the maintenance of send
and receive maintenance counters.
In a dual-link configuration, the CG checks the Message Counter of each message. It must
be between 1 and 30,000 and be different from the last transmission of any data message
(duplicates are ignored). The host processor should perform the same checks.
For the dual-link configuration, the line-bid sequence occurs only at startup. The lines are
then kept open using TTD (Temporary Text Delay). Line two is used for host-processor
transmissions and line one for CG transmissions. Note that these are half-duplex
transmissions with the protocol messages (ACK, NAK) returned over the same line they
were received on. The resulting throughput is close to that of a full-duplex implementation
because there are no line bidding delays.
Examples of Dual Link sequences:
Line Bid (at startup only)
SENDER ENQ
RECEIVER ACK0
Line Idle Mode
SENDER TTD
RECEIVER NAK
Message Transfer (single block)
SENDER DLE STX "transparent text" DLE ETX BCC
RECEIVER ACK*
Multiblock Message Transfer (first and intermediate blocks)
SENDER DLE STX "transparent text" DLE ETB BCC
RECEIVER ACK*
Multiblock Message Transfer (final block)
SENDER DLE STX "transparent text" DLE ETX BCC
RECEIVER ACK*
NOTES: 1) ACK* means the correct ACK0 or ACK1, which alternate.
2) Idle Mode exchanges (TTD-NAK) can take place between blocks of a multi-
block transfer.
3) NAK received in place of ACK* requires retransmission of that block.