Fall 2008

For more details, and talks in past semesters, consult the full schedule of talks.

Tentative talks:

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Past topics can (and should) be repeated occasionally. In addition, here are some topics people might like to hear about:

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A Crash Course in Modem Jargon

by Anu Rao

Table of Contents

Concepts

There are two types of communication rates:
Connection Rate
the rate at which your modem and the remote modem connect, such as 9600 baud (baud=bps=bits per second). This rate is determined by the modems when they connect. (also called Line Speed, Modem-to-Modem Speed, DCE Speed)
Computer Rate
the rate at which your modem and computer communicate. This rate is modified via your modem software. (also called Serial Port Speed, Modem-to-Computer Speed, DTE Speed)
We will concentrate on the Connection Rate.

Modems come with a base baud rate, which can be improved using:

Error Correction
the modem breaks the transmission into blocks of data, calculates the checksum (the sum of all data bytes in a block); the data block and checksum are transmitted to the receiving modem, which calculates the checksum for the block it received and compares it to the checksum it received for the data block; if the received and calculated checksums are different then the block was transferred incorrectly and the modem resends it.
Data Compression
the modem looks for often-repeated patterns and replaces them with a shoter data code that will transmit faster
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Protocols and Buzzwords

We will discuss the cryptic terms encountered when trying to figure out which modem is best for you. There are three types of protocols:
  1. Modulation Protocols

    The modem converts digital signals generated by your computer into analog signals which can be transmitted over a phone line, and transforms the incoming analog signals back into their digital equivalents. Here are tables of the modulation protocols and their corresponding baud rates:

    2400 bps Modems
    Protocol Base Speed
    Bell 103 300 bps (USA)
    Bell 212A 1200 bps (USA)
    V.22 1200 bps (outside USA)
    V.22 bis 2400 bps (CCITT)
    High Speed Modems
    Protocol Base Speed
    V.32 9600 bps (CCITT)
    V.32 bis 14,400 bps (CCITT)
    V.FC 28,800 bps
  2. Error Control Protocols

    There are two standards:

    • MNP 2-4 (Microcom Networking Protocol)
    • V.42 -- uses LAPM (Link Access Procedure for Modems), uses MNP 2-4 as an alternate scheme
    These protocols filter out line noise and automatically retransmit corrupted data. When two modems establish an error-controlled connection, this is called a "reliable" link. These protocols can also improve throughput (effective transfer rate) by stripping start and stop bits (go from 10 to 8 bits -- a 20% reduction without any compression!).
  3. Data Compression Protocols

    There are two standards:

    • MNP 5 -- gives a maximum of 2:1 compression
    • V.42 bis -- adds BTLZ data compression to the V.42 error protocol (BTLZ offers up to 4:1 compression and requires LAPM error control)
    An MNP 5 data compression protocol requires MNP 4 error protocol, and a V.42 bis modem requires V.42 error protocol.

    Although V.42 includes MNP 4, V.42 bis does not include MNP 5. But almost all high speed modems that support V.42 bis also incorporate MNP 5.

    Note that when dealing with compressed data, the V.42 vin/LAPM protocol detects which data cannot be compressed further and sends it as is, whereas the MNP protocol tries anyway and slows things down.

Note that local and remote modems must both support the same error correction and data compression protocols in order for them to take effect.
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Improving Throughput for High Speed Modems

Now that we are familiar with the different protocols, let's see how the various types interact to affect our baud rates:
V.32: base speed=9600 bps
using we can get up to
MNP 5 (2:1) 19,200 bps
V.42,LAPM,BTLZ (4:1) 38,400 bps
V.32 bis: base speed=14,400 bps
using we can get up to
MNP 5 (2:1) 28,800 bps
V.42,LAPM,BTLZ (4:1) 57,600 bps
V.FC: base speed=28,800 bps
using we can get up to
V.42,BTLZ 115,200 bps
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Changing Your Modem Configuration

A modem has three types of memory:
  1. factory default (ROM)
  2. active (RAM)
  3. stored (active settings are stored here to be used again after repowering modem; also known as non-volatile RAM)
You need the factory handbook that came with your modem to change the settings; many commands are specific to the manufacturer. All sorts of options can be changed using the AT commands. For example, here are some sample commands from my modem book:
ATD number
dials number
AT L0
set to low volume
ATA
answer manually
AT&F
restore modem to factory settings
AT&W
store changes made to non-volatile RAM
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Flow Control

There are two types of flow control: Both types tell the remote modem to stop sending when the local modem buffer is full, and to start sending again when it is clear. Hardware control is faster; software control (XON,XOFF) can cause data errors with binary files containing CRTL-S or CRTL-Q.
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Types of Modems (Hardware-Wise)

There are internal modems (plug in as a card) and external modems (external devices that must be connected via an external cable). In general, external modems are more expensive than internal modems.
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Some Examples of PC Communication Programs

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References

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http://math.arizona.edu/~swig/documentation/modems/index.php
Last modified: Fri, 14 Dec 2007 15:50:52 -0700
E-mail: swig@math.arizona.edu
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