Modem and Port Information

General information

General Information:

A modem is what most of us use to connect. The computer transfers data through a serial port to the modem ... The modem transfers the data across phone lines to a modem at the provider which again transfers the data through a serial port to the computer.

When you dial and connect you hear an obnoxious set of sounds which may go on for awhile . What is happening during this period is that your modem and the other modem negotiates connection speed and protocols . They talk to each other ask and tell what type of capabilities they have and agree on a set both can support.

Serial port : The major item of importance is the UART <Universal Asynchronous receiver transmitter> . It gets the bits <not bytes> from the computer one by one and send them to serial port to the modem for transmittal . The  modem receives them from the communication link and sends it through the serial port to the computer one at the time. The older UARTs had no buffers thus every bit being received and transmitted needed the attention of the processor and got it by issuing interrupts. The newer UARTs <16550 types> have a 16 bit buffer thus they can store bits and issue interrupts less frequently and needs less direct attention. When Installing a modem the port of choice is Com2 and use Com1 for the mouse. Using Com3 or Com4 may result in conflicts. Com1 and Com3 shares interrupts thus a mouse on com1 and a modem on com3 will not work. Com4 has address conflicts with some S3 video chips . And Com2 does have higher priority than Com1

Modem :   Modems comes in all types. With the graphics content around on the net a 28800 or better modem is needed. Internal modems have there own UART and all the new 28800 or better modems will have a 16500 or equivalent while external uses the I/O  port in the computer . Here you will find the older as well as the newer depending on the age of the computer.
When buying modems the old rule often applies "you get what you pay for". While all 28800 <or 33600 modems will support v.34 some may not support all the optional portions such as split <asymmetrical> receive/transmit. Some will use RIP where error control and compression is handled in software rather than in hardware thus puts a higher demand on the processor. Thus often compression may result in connect problems since it puts additional demands on the data flow through the port and also on the processor if software rather than hardware compression is employed.

Modem speed:  There are several speeds and they are often confused. The first DTE <data terminal equipment>  is the speed at which the computer may communicate with the modem. This is also referred to as the port speed and is the setting in modem setup. The speed at which the two modems communicate is the DCE <data communication equipment> which is negotiated between the two modems during the initial connect phase. This rate is subject to change during a connect period and may change as a result of line conditions and errors during a connect period . This is the speed at which data is being transferred

Modem Compression: The common ones are MNP5 and V.42 .  These compress compressible files to give an apparent increase in transfer speed . A  file maybe compressed prior to transmission to a much smaller size then transmitted at the DCE speed and uncompressed at the other side. Thus it appears as if a 28800 modem really is transmitting data at 57600 with a 2:1 compression. This would then also require that the computers on both sides were able to transmit data through the ports to and from the modems at 57600. (There is some additional overhead  which is ignored> .
What amount of compression can reasonably be achieved ??? Most files you download will not compress any significant amount since they are already zipped or in an archive format ..... Actually, for these type files MNP5 compression might slow down the transfer . Text files like word document etc a typical compression might be about 2:1 while sparse documents like spread sheets and data base files may compress up to 4:1 .

X2 Modems: There is presently no agreed to standard for these modems. They work like any other modem but take advantage of the potential that your service provider has the proper modem and also uses a digital connection <rather than analog > to the net. This means a analog to digital conversion process can be eliminated and will provide for a potential increase in download speed . You will not see this speed increase the other way.

The following  are from a paper entitled Of Line noise, The Phone Company, and Your Modem by Patrick Moore, Supra Corp.
" -  Repeatable results with V.FC/V.34 modems on real-world phone lines are sometimes difficult to obtain.  Unlike V32.bis modems,  V.FC/V.34 modems will actually modify their data coding (symbol  rate) and transmit levels during the connect phase.  Very minor differences in the phone line can lead to quite different results.
 -  A 28.8K connection on some real-world lines may not occur very often, if it occurs at all.  Preliminary testing in the lab indicates that "normal" phone line attenuation makes a 28.8K  connection difficult even in the absence of line noise.  This is  not surprising considering that the V.FC & V.34 coding schemes are    relying on the bandwidth of the phone system being greater than what is actually available in some areas.  Very high-speed connections assume that there is a substantial amount of digital technology in the system (thereby normally increasing the usable bandwidth).  Some users are going to find that their local phone system effectively limits them to 16.8K or 19.2K.
 -  Both the originating and the answering modem are adjusting to their line conditions during the connect phase.  The fact that  a connection to a given modem at a given location occurs at a lower rate (19.2k for example) may have nothing to do with the modem the call is placed with, or the location it is placed from, as the modem on the other end may be adjusting the baud rate down."

 What Can we do to maximize our connect speed and make it work as good as possible !!!!

1. Make sure you have a 16550 type UART if using an external modem
2. Let win95 install the modem for you if it detects it. If not install it manually. Again the preference is Com2
3. Set the port speed to 57600 for a 28800 modem and 38400 for a 14400 modem
4. Enable error control and compression -
5. Make sure you do not check connect only at this speed in the modem setup
6. Get the latest drivers from your Modem manufacture - normally available on their web site and if you have a modem which can be flashed get the latest firmware

Still having speed problems !!!

1. Check your house wiring and installed equipment. It is often source of trouble. To many phones - old phones poor wiring!!! An easy way to check is to get a twisted pair then run it directly from the phone companies box were it enters the house to your modem <disconnecting the others > and then try a few times and see what happens !!!
2. Phone companies lines_ If you have noise on the line or maybe sometimes can hear other conversations in the background you may try to complain to the phone company about line quality. However , saying I can't connect at 28800 is not likely to get much action. They never guaranteed that in the first place!

Having Connect Problems !!!!

1. Try lowering the port speed down towards the modem speed and turn compression off. This is required for setups using old UARTs. They will not handle anything above 19200 and may not do well above 9600. Also, may help with an older processor. Try to run less tasks concurrent.

2. Try adjusting the FIFO buffer settings if you have a 16550 type UART

See Navas FAQ for good information

Copyrighted All rights reserved
Last revised: May 29, 2005