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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.

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Computer Expressions 40CG Manual de usuario Pagina 176