Links to other Reference Manual pages:
This chapter discusses the following topics:
Modem
Communication Standards
Modem
Configurations
Connection Rates
Error Correction
and Data Compression
Connection Types
Modem Communication Standards
The International Telecommunications Union (ITU) is an
international organization which establishes world wide standards
(also referred to as protocols) for communications devices.
Protocols define the basic communication link between two modems
-- that is, the frequency of tones they use and how they modulate
the data.
Your modem supports the following communications protocols: Bell 103, Bell 212A, ITU V.21, V.22, V.22bis, V.23, V.32, V.32bis, V.34, V.80 and K56flex. The protocols you use are determined by the type of modem you are connecting to.
A unique set of protocols is supported for the U.S. and Canada: Bell 103 (300 bps) and Bell 212A (1200 bps), named for the Bell Labs that developed them.
Almost everyone uses the ITU standard protocols: V.21 (300 bps) and V.22 (1200 bps). For connections of 2400 bps and faster, ITU standards are universal (given the necessary line conditions and appropriate equipment support): V.22 bis (for 2400 bps), V.32 (for 4800 and 9600 bps), V.32bis (for 4800, 7200, 9600, 12,000, 14,400 bps), and V.34 (for 2400, 4800, 7200, 9600, 12,000, 14,400, 16,800, 19,200, 24,000, 26,400, 28,800, 31,200 and 33,600 bps). K56flex is used for 32,000, 34,000, 36,000, 38,000, 40,000, 42,000, 44,000, 46,000, 48,000, 50,000, 52,000, 54,000 and 56,000 bps rates.
You will probably use only one set of protocols, unless you make international calls.
Modem Configurations
Factory Default, Active, and Stored
There are three types of modem configurations: factory default,
active, and stored.
Factory Default
Before your modem left the factory, it was set up to act a
certain way. For example, it returns full-word result codes and
waits for a dial tone before dialing. Factory configurations
differ, depending on what kind of computer you are using and
whether you need error correction and data compression. These
settings comprise the "factory default configuration."
The factory default configuration is stored in the modem's permanent memory (called firmware ROM). Whenever you turn on your modem, it loads the factory default configuration into the active configuration area.
Active Configuration
The active configuration is the one you use when you have your
modem turned on. It is stored in the modem's temporary memory
(called temporary RAM).
Changes made to the active configuration are lost when you turn the modem off. If you want the changes to be in effect the next time you turn on the modem, you can add any changes from the factory defaults to your software initialization string. However, many applications issue AT&F as part of their initialization, so even if you save your active profile, the application's initialization will reset the factory defaults.
Stored Configuration:
You can save and store the active configuration in the modem's
memory.
Loading the Factory Configuration
To load the factory default configuration, issue AT&F
for IBM or compatible computers to direct your modem to use
AutoReliable mode, hardware handshaking, and CONNECT XXXX
messages that reflect your computer's rate. The factory default
configuration works for most situations.
If you need to set up your modem to make a direct connection, issue AT\N1 &K &QØ &D2.
Changing Your Active Configuration
Your modem's AT commands let you change most of the active
configuration settings by issuing single commands. The following
AT commands are particularly useful:
The command ATSr? displays the value in S Register r. For example, ATSØ? reads S Register Ø and displays three digits -- 004 if the value is 4.
The command ATSr=n writes the value n into S Register r. Occasionally, you may need to change a setting by issuing the ATSr=n command. The n must be a decimal integer from Ø to 255. For example, the command ATSØ=4 [CR] sets Register SØ to 4, which causes the modem to automatically answer all incoming calls after the fourth ring, unless the range is limited due to PTT regulations. The modem replies with OK.
However, if you have a choice, use an AT command to change S Registers indirectly rather than using the ATSr=n command.
The Using AT Commands section addresses general use of AT Commands and Result Codes. The AT Commands and S Registers are also available online.
Connection Rates
How the Modem Determines Connection Speed and
Protocol
The modem first tries to connect to the remote modem at the
highest rate it supports (56,000 bps). If the remote modem can't
connect at your modem's highest rate, your modem tries to connect
at the next highest rate (54,000 bps). If that doesn't work, it
continues trying the remaining supported speeds, one at a time in
descending order. This process is called fall back.
If error correction and data compression are enabled (they are when shipped), the modem next tries to "negotiate" error correction and data compression protocols with the other modem. Your modem tries the protocols in the following order: first, error correction; then data compression, and finally, neither error correction or data compression. The protocols used are the most powerful protocols that both modems support.
Because your modem supports so many rates and protocols, the negotiation process can take several seconds to complete. If it seems to take an extremely long time, you may be having line noise problems; refer to "Problems with High Speed Connections" in Chapter 10, Solving Problems.
Once connected, your modem sends your computer a Result Code that tells it the connection rate. Depending on how your software handles the code, you see either the Result Code on your screen, or you see your software's interpretation of it. Also depending upon how the modem is set, the Result Code will show the connection rate between the two modems (DCE), or the rate at which the computer is communicating with the modem (DTE).
Error Correction and Data
Compression
Your modem features error correction and data compression
protocols that allow you to transmit data fast and error-free.
Error Correction
For error correction, the modem breaks your transmission down
into blocks of data (frames) and calculates a checksum (the sum
of all data bytes in the block). The block and its checksum are
transmitted. The receiving modem calculates a checksum on the
received data block and compares it with the received checksum.
If these numbers are different, the data block was not
transmitted properly, and the receiving modem instructs the
transmitting modem to resend the data.
Error correction is necessary for high speed communications. However, error correction will not perform miracles on extremely noisy lines. If you have lots of errors, you may need to have your telephone circuits checked or change long distance carriers.
Data Compression
To accomplish data compression, the sending modem analyzes the
outgoing data for often repeated patterns (such as the letters
"A" and "E" in English text). It then assigns
a shorter data code that transmits faster than the original data.
The more often the same data codes occurs in the transmission,
the better compression the modem can achieve.
Certain file types such as text files and graphics screens are good candidates for optimal compression. Files that have already been compressed with a program like Stuffit, PKZIP, or ARC may see little or no performance improvement, because the files are already compressed.
Error Correction and Data Compression
Standards
The error correction and data compression standards your modem
supports include MNP 2-4 and V.42 for error correction, and MNP 5
and V.42bis for data compression. MNP 5 offers a compression rate
of up to 2:1. MNP 5 connections also use MNP error correction.
V.42 is the ITU error correction protocol, also known as LAPM. V.42bis adds BTLZ data compression to the V.42 protocol. It offers compression of up to 4:1. V.42bis connections require V.42 error correction.
Both your modem and the remote modem must support the same data compression/error correction standards if they are to make a data compressing/error correcting connection.
Using ZModem, XModem, or Pre-Compressed
Files
Error Correction
It is usually advantageous to use MNP 2-4 or V.42 error
correction for ANY file transfer, regardless of compression or
software-based error correction standards such as Zmodem, Xmodem,
etc. The error correction standard strips start and stop bits
from each character (to improve throughput) and performs error
correction on smaller chunks of data than software-based error
correction standards. Thus if an error does occur, less data has
to be retransmitted, which takes less time.
V.42bis Data Compression (Mode Default)
V.42bis recognizes that there is no benefit to compressing
software-compressed data further, so it does not attempt to use
V.42bis compression if no additional compression is possible. Use
V.42bis rather than MNP5 whenever possible to receive these
benefits.
For better performance, disable MNP 5 data compression (%C) when using an MNP Reliable connection and transferring files that are already compressed by compression software such as ARC, .ZIP, or StuffIt. When files compressed by these programs are transferred using MNP 5, the files may take longer to transfer than files transferred using a Reliable connection and no data compression.
The default settings in your modem require the modem first attempt to establish a V.42/V.42bis connection, and only attempt an MNP connection if the V.42/V.42bis connection is not possible.
Connection Types
All Normal and Reliable connections use flow control, which
allows your computer and modem rates to be different. With Direct
connections, the rates must be the same.
You can only use data compression if you have made an error-correcting connection (MNP or LAPM) with another modem that supports data compression. MNP 5 is only used with MNP error correction, and BTLZ is only used with LAPM error correction. If a data compression connection cannot be made, or if data compression has been disabled, only error correction is attempted.
The commands for controlling connections (not to be used during an active connection) are listed with the AT Commands under AT\N and AT&Qn.
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