29
Problem issues such as serial port speed, slow downloads, multitasking, and line noise are discussed
in Solving Problems.
A
SYMMETRIC
D
ATA
R
ATES
A connection where the carrier rates can be different in each direction is referred to as
“Asymmetric.” Many K56flex and V.34 modems have the ability to use the “best match” carrier rate
in each direction. Thus the modem can use the highest possible stable carrier rate in each direction,
rather than using the lower stable rate for both directions. Each modem is responsible for
negotiating for the fastest speed at which it can receive data, given current line conditions.
Your modem provides for the use of asymmetric data rates, so the send and receive speeds may be
different. If the conditions in one direction do not match those in the other direction, your modem
will adjust its operation to achieve the best rate in each direction. The modem will attempt to
establish the fastest stable carrier rate(s) possible for the current phone line conditions. Without
asymmetric rates, problems with the modem or the line quality on either side will affect the
performance in
both
directions.
S
ELECTING
A
M
ODULATION
TO
C
ONTROL
H
IGH
S
PEED
C
ONNECTIONS
If your modem uses the V.34 or K56flex protocol, you can display and control the modulation your
modem uses to negotiate a connection by entering the
AT+MS
command.
Displaying Supported Values
A variation of the
AT+MS
command allows you to display supported values and selected values.
Issue
AT+MS=?[CR]
to display the values supported by your modem. The modem displays a list
of values in the following order:
[supported <mod> values], [supported <automode> values],
[supported <minimum rate> values], [supported <maximum rate> values]
The modem response might look like this:
+MS: (V21, V22, V22B, V23C, V32, V32B, V34, K56, B103, B212), (Ø, 1), (300 - 56000), (300 -
56000) (Ø, 1)
Setting Values with the AT+MS Command
The
AT+MS
command allows you to select a specific modulation, enable or disable automode, and
specify the highest and lowest connection rates for your modem.
The command format for
AT+MS
is:
AT+MS = <mod>,<automode>,<min rate>,<max rate> [CR]
