US Robotics COURIER HST User'smanual page 14

USR-HST
Trellis Coded
Modulation
Asymmetric
Modulation
Enhanced Error
Control
Shorter
Response Time
THE COURIER
PRODUCT
FAMILY
MNP Error
Control at
2400/1200 BPS
COURIER HST
CHAPTER 1
INTRODUCTION
The USRobotics Courier HST modem represents powerful,
advanced electronic design that gives you optimal speed and
accuracy. USRobotics' High Speed Technology (USR-HST)
offers these advantages:
Trellis coded modulation is a convolutional coding technique
that makes data transmission less vulnerable to errors caused
by phone network impairments. It can tolerate twice the tel-
ephone channel noise power as conventional modulation
(quadrature amplitude modulation, or QAM), so there are
fewer error-control retransmissions. Trellis coded modula-
tion is also less susceptible to impulse-type noise.
At top speed, data flows in one direction at 9600 bits per
second and at 300 bits per second in the other. The modems
automatically switch the high-speed channel on demand,
i.e., depending on which transmitter has the most data to
transmit. In practice, the modems seldom need to reverse
channels, since the asymmetric design reflects typical com-
munications sessions- brief messages typed at one end of
the link, files sent from the other. The asymmetric approach
provides the most efficient and economical strategy for using
ordinary phone channels at high speeds.
The HST error control protocol uses sophisticated error
detection methods to ensure data integrity. Design effi-
ciencies have reduced the overhead (extra control infor-
mation) experienced with error control protocols at lower
speeds. On local and long-distance connections the result
is accuracy. greater speed and higher throughput-
approximately 1100 characters per second.
9600-bps modems are generally most efficient for file trans-
fers. You'll find that the Courier HST also offers faster
response times during interactive sessions.
In addition to the Courier HST's new features, the modem
incorporates the same popular capabilities of other products
in the Courier line:
The Courier HST implements the Microcom Networking
Protocol (MNP), Service Classes l, 2, and 3, at 2400 and
l 200 bits per second. This ensures compatibility with the
Courier 2400e and other MNP-compatible modems at
those speeds.
1-1