US Robotics COURIER HST User'smanual page 68

OVERVIEW
Throughput
MNP Service
Classes
Service Class 1
Service Class 2
COURIER HST
APPENDIX A
ERROR/FLOW CONTROL CONCEPTS
At 9600 bps. the Courier HST uses the USR-HST error
control protocol, a proprietary protocol developed by
USRobotics. At 2400/1200 bps, the modem uses the MNP
error control protocol originally developed by Microcom.
Inc. and now in the public domain. In USRobotics imple-
mentations, we use the general term ARQ (automatic repeat
request) to denote error control at all three speeds.
Error-free data transmission is ensured through two features:
• error-detection and retransmission techniques:
• data buffering and flow control.
NOTE: As with all protocols, error control only works when
the Courier HST connects with another modem that imple-
ments the HST protocol at 9600 bps or the MNP protocol
at 2400/1200 bps.
Chapter 3 offers recommended modem settings to gain max-
imum throughput at 9600 bps. Optimal throughput is about
1100 characters per second when the link speed is 9600 bps
and the terminal transmits to the modem at 19.2k bps. The
design of the HST protocol builds on and enhances previous
protocols by integrating much of the additional error control
information (overhead) within the data blocks instead of
transmitting separate ARQ data.
For optimal throughput at 2400/ ! 200 bps under MNP, see
Service Class 3, below.
At 2400/ ! 200 the ARQ protocol implements one of three
MNP
service classes. as follows:
This service class, often called Block Mode, supports half-
duplex (one way at a time) transmission. The sending
modem transmits a block of data and waits for an acknowl-
edgment from the receiving modem before sending the next.
Service Class 1 is the slowest and is included because
some computer equipment is restricted to either sending or
receiving at any one time. This mode of operation is usually
observed in communications between terminals rather than
in terminal-to-computer or computer-to-computer links.
This class, often called Stream Mode, supports full-duplex
transmission in which data flows in both directions at the
A-1