Links to other Reference Manual pages:
This section addresses some of the issues that you should be
aware of as you work with modems that communicate at speeds of
33,600 bps and above. The following topics are discussed:
56K Modem
Technology
33,600 bps
Connections
Asymmetric Data
Rates
Selecting a
Modulation to Control High Speed Connections
Hardware Flow
Control
56K Modem Technology
Communicating at 56,000 bps is the new lightning fast modem
technology which offers the most rapid transmission speeds
available over ordinary phone lines. With 56K technology, you can
surf the Internet and explore web sites at up to twice the speed
of 28,8000 bps modems.
Current analog modems allow for a maximum data connection of 33,600 bps between two modems or a single modem and an Internet Service Provider(ISP). The maximum theoretical transmission rate across the Public Switched Telelphone Network (PSTN) is roughly 35,000 bps.
This normal data transmission rate is described by Shannons Law which calculates the fastest speeds at which data can flow across the twisted pair of copper wires between two analog sites. Your ISP is connected to the phone company Central Office, which converts the signals from analog to digital upon receipt. This digital signal is transmitted through the PSTN to the Central Office located near you. Then it is converted once again from digital to analog and delivered to your home or office.
Recently, phone companies have begun to deliver a digital line (in the form of a T-1, equivalent to 48 voice lines) from their Central Office to the ISP. By upgrading ISP dial-up modems with a digital connection to a T-1 line, one less analog to digital conversion is made between the ISP and their Central office. This reduces the amount of error introduced into the transmission, and provides for data rates up to 56,000 bps delivered to homes and offices from an ISP over standard copper twisted-pair lines.
56 Kbps Asymmetric Data Rates
Your 56K modem uses asymmetric data rates; the send and receive
speeds differ. While your modem is capable of receiving at
a rate of 56,000 bps, the send rate is 33,600 bps or lower.
56 Kbps Requirements
For you to achieve 56 Kbps performance from your modem, your ISP
must have these items in place:
33,600 bps Connections
Your modem supports 33,600 bps connections with modems from many
different manufacturers. However, it may be unealistic to expect
that you will always be able to achieve full 33,600 bps data
speeds every time you connect with another 33.6 Kbps modem. Line
conditions, serial port speed, and other factors impact actual
connection speeds.
Unlike earlier modems, 33,600 bps modems are designed to modify their actual transmission speeds during a data connection; so a minor difference in the phone line can have a significant impact on the connection. For example, during one attempt to make a connection, you might connect at only 24,000 bps while a later call on the same lines and to the same modem may result in a 33,600 bps connection.
One reason for such variable results is that speeds of 33,600 bps are higher than normal analog phone lines were designed to handle. Data communications of 33,600 bps are best achieved when the phone lines between the two modems are of high quality and are free of limiting factors such as line noise.
Another factor to keep in mind when attempting high speed communication is the modem on the other end of your connection. While all 33.6 modems use the same core technology, not all of them may be as capable as yours. If you are unable to achieve the top data speeds, it may be that the modem to which you are connecting is adjusting the speed down because it is unable to communicate at top speed.
The slowdown may also be because the computer at the other end of the connection is not capable of the highest speeds.
Problem issues such as serial port speed, slow downloads, multitasking, and line noise are discussed the the High Speed Connections section of the Solving Problems chapter.
Asymmetric Data Rates
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.
Selecting a
Modulation to Control High Speed Connections
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:(0,1,2,3,9,10,11,56,64,69), (Ø, 1), (300-56000),
(300-56000), (Ø, 1), (Ø, 1), (300-33600)
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. You use a variation
on the +MS command to report selected options and
supported options.
The command format for AT+MS is:
AT+MS = <mod>,<automode>,<minimum
rate>,<maximum rate> [CR]
For example, to set your modem to operate as a V.32bis modem, enter:
AT+MS = 10,1,9600,14400[CR]
You can leave a parameter at its current value by leaving that parameter's place blank and entering a comma to separate the parameters or by entering a carriage return ([CR]) if it is the last parameter. For example, to change the selected modulation from the setting shown above (AT+MS=10,1,9600,14400) to a Default mode of full V34, V32bis, Bell 212, and Bell 103, enter:
AT+MS=11,,300,[CR]
Displaying Selected Values
You can use AT+MS? to display the values you selected with
the AT+MS command. The modem displays the selected values
in the following order:
<mod>, <automode>, <min. rate>, <max. rate>, <x-Law>, <rb-signal>, <max-up rate>
For example, if you enter AT+MS?[CR], your modem might display
+MS: 56,1,300,56000,1,Ø,33600.
This display means that the values currently selected on your modem are K56flex with automode enabled. The minimum rate is set at 300 bps, the maximum download rate is set at 33,600 bps, A-law codec type is selected, robbed bit detection has been disabled, and the maximum upload rate is set at 33,600 bps.
The <mod> Parameter
To specify a preferred modulation, enter the "Numeric"
modulation setting from the <mod> column of the following
table.
<mod> Numeric | <mod> Verbose | Protocol | Possible Rates (bps)¹ | ||
---|---|---|---|---|---|
0 | V21 | V.21 | 300 | ||
1 | V22 | V.22 | 1200 | ||
2 | V22B | V.22bis | 2400 or 1200 | ||
3 | V23C | V.23 | 1200² | ||
9 | V32 | V.32 | 9600 or 4800 | ||
10 | V32B | V.32bis | 14400, 12000, 9600, 7200, or 4800 | ||
11 | V34 | V.34 | 33600, 31200, 28800, 26400, 24000, 21600, 19200, 16800, 14400, 12000, 9600, 7200, 4800, or 2400 | ||
56 | K56 | K56flex | 56000, 54000, 52000, 50000, 48000, 46000, 44000, 42000, 40000, 38000, 36000, 34000, 32000 (Default) | ||
64 | B103 | Bell 103 | 300 | ||
69 | B212 | Bell 212 | 1200 | ||
74 | RVFC | V.FC | 28800, 26400, 24000, 21600, 19200, 16800, or 14400 | ||
1 See optional <automode>,
<minimum rate>, and <maximum rate>
parameters. 2 For V.23, originating modes transmit at 75 bps and receive at 1200 bps; answering modes transmit at 1200 bps and receive at 75 bps. The rate is always specified as 1200 bps. |
The <automode> Parameter
The <automode> (automatic modulation negotiation) parameter
is an optional numeric value that enables or disables automatic
modulation negotiation. There are modulations for which automatic
negotiation is not available, such as Bell 212.
The default value, 1, enables automode. This allows the modem to "fall back" to other protocols within the specified <minimum rate> and <maximum rate> range for which standard automode procedures exist. For example, failed V.34 connection attempts can automode to other protocols, but a Bell 103 connection cannot automode to V.23.
The following table displays the available automode options:
<automode> | Option Selected | Notes |
---|---|---|
0 | Automode disabled | |
1 | Automode enabled using V.8 or V.32bis Annex A | Default |
Setting <automode>
If you set <automode> to 1, and you are using the default
settings, your modem attempts to connect at the highest possible
rate and protocol using either V.8 or V.32bis Annex A. If your
modem cannot make a connection at that speed, it automatically
negotiates the rate downward until a connection is made. For
example, if you issue the command AT+MS=10,1,1200,14400[CR]
the modem starts negotiating the rate downward from V.32bis 14400
bps if a connection is not made.
If you set <automode> to zero, and if a V.32bis connection cannot be made, the modem will not try a V.22bis connection. For example, if you give the command AT+MS=9,0,1200,4800[CR] the modem will try all speeds of V.32, but will not try the next lower protocol (i.e., V.22bis 2400 bps.)
The <minimum rate> Parameter
The <minimum rate> is an optional number that specifies the
lowest rate at which the modem may establish a connection. The
default is 300 bps.
The <maximum rate> Parameter
The <maximum rate> is an optional number that specifies the
highest rate at which the modem may establish a connection. The
default is 56,000 bps.
Note: Minimum and maximum rates are based on the <mod> parameter. If you issue +MS=9,1,300,33600 the maximum rate at which you can actually connect is 9600, as shown in the "Possible Rates" column in the previous <:mod>Parameter table.
The <x_Law> Parameter
The <x-Law> is an optional number which specifies the codec
type. The options are:
Note: ATZ will reset the
The <rb_signaling> Parameter
The <rb_signaling> is an optional number which enables or
disables "robbed bit" signaling generation in a server
modem or enables or disables robbed bit signaling detection in a
client modem. The options are:
Note: ATZ will reset the <rb_signaling> selection to Ø (disabled). (Note: u-Law is mainly used in USA, Canada and Japan; all other countries use A-Law.)
Factors that Affect the AT+MS Stored
Settings
The AT+MS command has one active setting and one stored
setting. It is possible to inadvertently change the stored
setting of AT+MS. If the AT+MS command's maximum
speed is changed, it remains at the lower rate until you reset
it.
If you drop your DTE rate to a slow rate, as long as you do not issue an AT&W at the slower setting, you need only to issue an ATZ after increasing the DTE rate again to reset the maximum speed.
Requesting a Retrain (%E)
Be cautious when requesting a Retrain. It is disabled by default.
The %E command is useful only if you want your modem to
Retrain without Rate Renegotiation (%GØ) when line
conditions are bad.
%EØ = Disabled. (Default)
%E1 = Enable Auto Retain based on line quality
Hardware Flow Control
To communicate at high speeds using external modems, it is
CRITICAL that you use a cable with "RTS/CTS Hardware
Handshaking" (such as the one supplied with your modem).
With error correction and data compression, the modem can send
data to the computer faster than the computer can accept it. When
this happens, the computer tells the modem to slow down.
Similarly, if data is not sent correctly to the modem and needs
to be resent, the modem needs to be able to tell the computer to
wait. This is called "flow control" or
"handshaking" and helps to prevent data from being
lost.
How to Get a Hardware Handshaking Cable
If your modem didn't come with a cable or you've lost your cable,
your local dealer should be able to provide you with one that
supports hardware handshaking. When you ask for a new cable, be
careful to specify that it needs to support hardware handshaking.
If your modem came with a cable, it supports hardware
handshaking.
Using Hardware or Software Flow Control
Enable hardware (RTS/CTS) flow control rather than software
(XON/XOFF) flow control because the results are generally faster
and more reliable.
To enable hardware flow control:
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