This article is marketed "AS IS". The author hereby disclaims all warranties relating to this article, expressed or implied, including but not limited to damage to hardware, software and/or data from use of this article. This is an updated version of my article, which was written on Jan. 26, 1997. Permission is granted to distribute unmodified copies of this article. This article is being marketing as freeware.
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Introduction

What's a modem?

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Now it’s time to turn our attention to the technical issues of a modem. Broadly speaking, a modem consists of a housing, a circuit, and a small speaker called audio monitor.

Modem types

There are two types of modem, internals & externals. An internal modem is connected through an expansion slot on the mainboard. Phone lines can be connected to the modem through expansion apertures at the back of the system. The external modem is a separate device using the RS-232 cable to connect to the computer's serial port. Phone lines should be connected to the external unit.

Modem housing

The modem housing is commonly made of plastic or steel. It contains a modem circuit, a power supply, and a small speaker. It allows the user to monitor the progress of a call. At the front of the modem housing, the indicator lights may be seen (this depends on the type of the modem used). Connectors at the back link the modem to phone lines, computer and power cable.

 

Display panel

The display panel is an array of small Light-Emitting Diodes (LED) placed along the front of the modem housing. These lights show the activity and the state of a modem. Each activity is accompanied with abbreviated labels. The labels are:

Label	State	Remarks
AA	Auto Answer	Modem is ready to answer
RD	Receive Data	Modem is about to receiving data
OH	Off Hook	Modem is connecting to another modem
TR	Terminal Ready	Modem is ready to transfer data
CS	Clear to Send	Modem is ready to receive data from computer and send them
RI	Ring Indicator	Modem is Recognizes the telephone rings
MR	Modem Ready	Modem is ready
CD	Carrier Detect	Modem is able to recognize signals which are sent by the another modem

Table A

Since an internal modem is inside the case and can’t be seen, some software developers displays those labels on the screen at the time of connection. In this way, showing the activity of internal modems is possible on CRT and it increases the compatibility with the user. WinCim or Compuserve Information Manager is one of the communication software that provides such facility for users. It displays two of the important above-mentioned labels on the screen: carrier detect and receive data.

 

Audio monitor

A speaker that is included as part of a modem is called an audio monitor. It allows you to listen to whatever sound is on the phone line. Since the CD LED leads you to recognize the signals that are sent by the other modem, the audio speaker is often used to let you hear busy signals or make sure that the other modem picks up the carrier too.

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As we said at the beginning of this article, most of the modems use a combination of modulation techniques to transmit multiple bits per baud. In other words, all the existing modems use a combination of these techniques in order to transmit data through the lines. Following, we will discuss three modulation techniques which most modems use a combination of: modulation protocol, error control protocol and the data compression protocol.

 

Modulation protocols

The specific technique used to encode the digital bits into analog is called "modulation protocol". Since there are too many modem manufacturers, there are too many various modulation protocols to define the exact method of encoding data. It also defines the speed of transmitted data. V.21, V.22, V.22bis, V.23, V.32, V.32bis and V.fast (also known as V.FC) are some sort of CCITT modulation protocols each with different advantages. CCITT V.21 is the standard for sending information by 300bps baud rates. This standard is seldom used in US. A 2400-bps Hayes compatible modem typically supports it. However they can theoretically yield a 9600-bps throughput, they are not high-speed modems.

In fact, high-speed modems are modems that feature modulation protocols at 9600bps or higher. V.22 is the another standard which supports information transmission at 1200bps and it is used usually outside the US. For more information on CCITT V modulation protocols, refer to Table B.

 

Error control protocols

There are two standards for error control protocols: MNP4 and V.42. MNP2-MNP4 are error correction protocols. V.42 is established by CCITT. It actually incorporates two error control schemes: LAP-M as the primary scheme and MNP4 as the alternate one. Therefore a V.42 modem will be able to establish an error-controlled connection with a modem that only supports MNP4. A modem may also use a non-standard error control protocol. For example, LAP-B is the name of the error control protocol, which is built in Hayes V-series smartmodem 9600.

 

Data compression protocols

The last modulation technique is called data compression protocol. Like error control protocols, there are two standards for these protocol one of which is called MNP5. Keep in mind that a modem can not support data compression without an error control protocol. A MNP5 modem requires MNP4 error control protocol.

The second data compression protocol is V.42bis, which is established by CCITT. V.42bis modem requires V.42 error control protocol. The maximum compression ratio that a MNP5 modem can achieve is 2:1. So a 14,400bps MNP5 modem can transfer data up to 28,800bps.

For V.42bis modems, the maximum compression ratio is 4:1. That’s why all those V.32 modem manufacturers claim that their modems provide throughput up to 38,400bps. Keep in mind that they are not very useful when you are downloading files from online services. The reason lies behind the fact that such modems emulate actual 38,400bps modems. By the way, you should only expect to see an actual throughput between 950 to 1100 cps, even if the modem supports V.32/V.42bis throughput up to 38,400bps.

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The purpose of buying a modem is to have the ability of transporting a raw bit stream of data from one computer or terminal to the another. Various physical media can be used for actual transmissions. Choosing the best physical media depends on too many parameters including but not limited to bandwidth, delay, cost and ease of installation and maintenance. Following, we’ll discuss about two most commonly-used physical media in detail, and will have a quick look to the other media in general. From then on, we’ll focus on some problems caused by unwanted energies called noise. Through this section we’ll turn our attention to some cables used in communication industry such as RS-232-C and its technical issues.

 

Lines

There are too many ways to connect to a remote modem using different lines. Each has some advantages and disadvantage. Moreover, you can connect to a remote system using wireless media, such as satellites. We are not going to focus on transmission media in detail. Instead, we’ll have a precise look to two commonly-used physical media: phone lines and leased lines.

 

Phone lines

The first mentioned transmission media is phone line, which is well-known. Phone lines usually have some restrictions. They often use "twisted pairs" and however they are twisted by each other in order to reduce electrical interference from similar pairs close by, they can be affected by the energies from outside of the cable. Therefore, it may cause a bad impression on the receiver’s mind, when these unwanted energies create a garbled message. On the other hand, the maximum speed of transmitted data can’t be beyond 115,000 bps. However the maximum bandwidth (bandpass) of phone lines depends on the error-correction protocols, and the clarity of line, but it is also commonly used because of its adequate performance and low cost. Figure A show a normal view of twisted pairs.



Figure A

 

Leased lines

The second media is called a leased line. A leased line is a specific telephone line that directly connects two computers. It is usually rented from the Telephone Company. A leased line doesn’t have many of the electronic restrictions that a dialup line has, so data can be sent faster. For example, data can be sent over these lines with a speed ranged 9600bps to 64Kbps. Carrier companies often use "Coaxial cable" as physical media in such transmissions. A coaxial cable consists of too many parts, one of which is a thick core at the middle of the cable. It is made of copper, insulating material surrounds the mentioned core. The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh. A protective plastic sheath covers the entire cable. Following is a cutaway view of a coaxial cable.



Figure B

 

Problem with line noise

Sometimes a whole line of text may be jumbled or even lost. This, however, is not a serious problem, since you can still read the text. But have you ever seen strayed characters (which become part of your message), when you are transmitting the text of a mail? These garbled messages may create a bad impression.

If transmission media were perfect, the receiver would receive exactly the same signal that the transmitter sent. Unfortunately, media are not perfect, so the received signal is not the same as the transmitted signal. For digital data, this difference can lead to errors. Transmission lines suffer from 3 major problems one of which is line noise. Noise, is unwanted energy from sources other than the transmitter. Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable. In general, the higher the bps rate of your modem, the more line noise will appear. However, error control protocols strive to eliminate line noise.

 

Error correction, error detection code

According to the last section, network designers have developed two basic strategies for dealing with errors. One way is to include enough redundant information along with each block of data sent to enable the receiver to deduce what the transmitted character must have been. The other way is to include only enough redundancy to allow the receiver to deduce that an error occurred, but not which error, and request a retransmission. The former strategy uses error-correcting codes and the latter uses error-detecting codes.

 

MNP modem

Newly developed modems are able to eliminate the problem of line noise. They include a built-in error correction protocol called MNP. MNP stands for 'Microcom Networking Protocol'. Never forget that MNP modems work with any communication software, but not with all networks or on-line services. So you can get the benefits of the MNP modem protocol with your favorite communication program. There are different MNP levels. Level 1-4 are error control protocols and level 5 is a data compression protocol that can compress data to about 50% of its original size. A modem with MNP-5 also has MNP-4. MNP1-4 is also included in the CCITT V.42 error-correction system. In this section, we’re going to introduce different MNP levels or more commonly MNP Classes.

 

Class 1:

In this Class, information is sent in one direction at a time and it is about 70% faster than those modems which use no error-correction protocol. Class 1, is called block mode too.

 

Class 2:

This is about 84% faster than those that don’t use error-correction protocol. It also sends information in two directions. It’s called stream mode too.

 

Class 3:

In this Class, sending and receiving information is under more rules, one of which is partitioning data into small packets. When the sending computer wants to send information, it strips start bit and stop bit from the first and the end of the packet. Then sending modem, sends the information to the other end. The receiving modem will receive these packets and it adds those omitted bits in order to send them to the receiving computer. It’s about 8% faster than transmissions that don’t use error-correction protocol. Following diagram shows the state of this operation:



Figure C

 

Class 4:

Modems that support such class are about 20% faster in transmission as those that use no error-correction protocol. This class has got an advantage which others do not have. As we said before, data divides into small packets in order to be sent to the other end. The size of these packets in this Class depends on the quality of line. What does it mean? It means that if the modem (which supports class 4) detects a noisy line, it will divide data into small packets. Otherwise, data will be transmitted in larger blocks. This improves the speed of the transmission. How? Let us say line is noisy. In such lines, data can be easily re-sent if an error occurs. Since the data is in small packets, the modem doesn’t have to send the entire file. It will send the bad packet repeatedly until it receives the acknowledgement.

 

Class 5:

Class 5 supports error-correction protocol, but also compression protocol. It means that if you are going to send a file to the other end, the Class 5 modems compresses it automatically and the data will be decompressed on the other end before sending it to the receiving computer. It is about 100% faster than those which don’t support error-correction protocol. However, keep in mind that if the file is compressed using any compression program, the speed of the transferring data will be drop down than if you were using no data compression.

 

Modem Cables

The back of the external modem has two connectors for phone hookup. One connects to the telephone and the other connects directly into the wall phone jack. A separate cable connects the modem to a serial port on the computer. Keep in mind that most modems are wired as DCE nowadays and most computers are wired as DTE. If you are connecting a DTE serial port on your computer to a DCE serial port on another computer, use a standard cable. Otherwise, you should use a reverse cable, if you want to connect either DCE to DCE or DTE to DTE. In the standards, the terminal or computer is officially called a DTE (Data Terminal Equipment) and the modem is officially called a DCE (Data Circuit-Terminating Equipment).

The mechanical specification for the modem cable which is called RS-232-C is for a 25-pin connector 47.04� .13mm wide, with all the other dimensions equally well specified. The top row has pins numbered 1 to 13 (left to right); the bottom row has pins numbered 14 to 25 (also left to right).

Lets look to the figure below. It shows 9 pins that are nearly always implemented. The remaining ones are frequently omitted. When the terminal or computer is powered up, it asserts DTR (pin 20). When the modem is powered up, it asserts DSR (pin 6). When the modem detects a carrier on the telephone line, it asserts CD (pin 8). RTS (pin 4) indicates that the terminal wants to send data. CTS (pin 5) mean that the modem is prepared to accept data. Data is transmitted on the Terminal circuit (pin 2) and received on the Receive circuit (pin 3).



Figure D

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Communication softwares

Networks and communications softwares enable users to connect their computers to other systems. This is done either via a modem or network link. This allows information to be accessed or transferred between them. Typical functions include electronic messaging commonly referred to as 'E-mail', transferring other data files, and using remote facilities like printers or databases. There is too much communications software each with different advantages such as CrossTalk, Qmodem, Smartcom, and so on. There are other types of applications like ‘PC Any Where’, which are designed for the benefit of the users. With such applications, you can directly connect to another PC (using the remote program). The other system uses host program to answer the remote one.

 

Operating modes

Now it is time to turn our attention to the modes that every modem has working in them: command mode and online mode.

 

Command mode

When you turn the computer on, the modem is in the command mode. In this mode, every character that is sent to the modem is interpreted as a command. Hence, the modem tries to perform each command and it displays a result code (numerical or verbal) on CRT indicating the results of that command. This mode is sometimes called local or terminal mode too.

 

Online mode

When a connection with a remote system is established, your modem switches to the online mode (which is called data mode too). In this mode, the modem interprets all the information it receives as data (except the escape sequence). Now, the question is "is it possible to switch between these modes? If so, how?"

After you establish a connection with a remote modem you can switch modes in two ways: If you send the escape sequence (+++) to the modem in data mode, you'll switch to the command mode. In this mode, you can get back to the online mode by sending the "ATO" command.

 

Measurement units

Modems are generally categorized with the speed they transmit data. The two units of measurement used are the "baud rates" and "bits per second" (BPS). Though they are used interchangeably, they are not same. Baud refers to the number of packets of data sent each second. If each packet contains a single bit, then the baud and the BPS will be the same. Newer modems may include several data bits in each packet. For example a 2400-baud modem transmitting 4 bits per packet transmits data at 9600 bits per second.

 

Communication Standards

CCITT is an int'l organization. It sets global standards for modem types, computer networks, and facsimiles. It comes from French and in English stands for 'International Telegraph & Telephone Consultative Committee'. There are too many types of these standards, one of which, namely "V Protocols series', is tabulated as follows:

Protocol	Remarks
CCITT V.21	A standard for sending data by 300bps baud rates. This is seldom used in US.
CCITT V.22	A standard for sending information by 1200bps baud rates. This is seldom used in US too.
CCITT V.22 bis	A standard for sending information by 2400bps baud rates. As a matter of fact, this is created by repeating V.22 twice a time, which made the modem to send data in 2400 rates.
CCITT V.23	A standard for sending information by 1200/75 bps. It’s usually used in Europe.
CCITT V.32	A standard for sending data in 4800bps or 9600bps baud rates.
CCITT V.32 bis	A standard for sending data in 14400bps baud rates. This is compatible with V.32 standard modems. (i.e. this type of modems can reduce the baud rates to 9600bps)
CCITT V.42	This is a standard that is used for error correction. This cause data to be filtered & educe the correct data.
CCITT V.42 bis	A standard for increasing data rates. This compresses data and on the receiving end, they will decompress.

Table B

 

Hayes

Hayes Microcomputer Products, Inc. is the producer of Hayes modems which is well-known by its users. They introduce a set of commands, which are fully compatible with ITU’s tariffs. Hence, you may hear "this modem is Hayes compatible". This means that the mentioned modem is fully compatible with ITU’s tariffs and can process standard AT commands.

Today, most of the modems are compatible with Hayes and understand the standard AT commands. Following is a table which provide you Hayes Command Set (BASIC ones only) and Register Formats:

Hayes Command Set	Description
+++	Standard escape sequence
AT,	Delay processing of next character
AT;	Return to command state after dialing
AT!	Time break recall
AT@	Instructs the modem to listen for five seconds silence before continuing. The value help in S7 determines the number of seconds the modem waits for silence.
AT	Standard attention sequence
ATA	Force to answer in the originate mode
ATC0	Transmitter off
ATC1	Transmitter on
ATD <digit/characters>	Dial number (Digits/characters 0-9 A B C D # * are numbers and characters the modem can dial. The characters A B C D # * represent specific tone pairs and therefore can be used only when tone dialing is selected.
ATDP <digit/characters>	Pulse dial (Digits/characters are same as above)
ATDT <digit/characters>	Tone dial (Digits/characters are same as above)
ATE0	Disable local character echo
ATE1	Enable local character echo
ATF0	Half Duplex (echoes characters)
ATF1	Full Duplex (doesn’t echo characters)
ATH0	Force line on hook
ATH1	Force line off hook
ATH2	Force line special off hook
ATI0	Request product code, formatted PPR, PP=Prod., R= Rev.
ATI1	Request ROM checksum
ATI2	Perform ROM checksum
ATI4	Identifying product feature
ATL1	Speaker volume low
ATL2	Speaker volume medium
ATL3	Speaker volume high
ATM0	Speaker always off
ATM1	Speaker on except while carrier present
ATM2	Speaker always on
ATM3	Speaker on except while dialing & carrier present
ATO	Force modem into online state
ATP	Sets modem to default pulse dial
ATQ0	Modem returns result codes
ATQ1	Modem doesn’t return result codes
ATR	Sets modem to answer mode after dialing out
ATSn	Sn is S register number n
ATT	Sets modem to default tone dial
ATV0	Send numeric codes
ATV1	Send word result codes
ATX0	Basic result code set "connect", no dial tone detect
ATX1	Extended result code set "connect 1200", no dial tone detect
ATX2	Wait for dial tone, extended result codes
ATX3	Detect busy signal, extended result codes
ATX4	Wait for dial tone, detect busy, extended result codes
ATW	Wait for dial tone
ATZ	Reset to power up condition

Table C

All modems have certain settings. Those settings available will vary from modem to modem, as well as how you set them. Modems work by responding to commands. These commands are known as dial modifiers or AT commands, which stands for Attention.

There are two command sets, basic and extended. Basic commands are typically a character or two and are preceded by the AT characters, for example:

ATDT - Sets the dial to a tone

ATDP - Sets the dial to a pulse

ATM0 - Turns the speaker off

These commands can be also issued on one line. If so, the AT should be given only once, for example:

ATDTM0

The extended commands start with an ampersand. Again, the AT characters precede them. For example:

AT&K0 - Turns off data compression.

Extended commands can be issued in conjunction with basic commands on one line. Again, the AT should be issued only once, for example:

ATDTM0&K0

While most settings are available on all modems, some modems have a few settings that others do not. For example, some modems accept &K4 and &K5 and some modems don't. The available settings for your specific modem will be found in your modem manual. You should know that we can control the way our modem works through registers too. Modems like all of the central processing units have got some registers.

Unfortunately, not all modems support all of these registers and some of them have got registers other than those listed here. Also note that some of them are just read only. However these registers can be used by all of us, but those are useful for programmers; because they prefer to control any device directly.

AT Sr = ## - to set a register

AT Sr? - to read a register

Where ‘r’ is the register number.

Register #	Values	Register Function
0	0-255	Ring to answer on (0 means don’t answer)
1	0-255	Ring count (clear after 8 second) (read only)
2	0-255	Escape char, normally "+", 128-255 disable escape
3	0-127	End of line character
4	0-127	Line feed character
5	0-32, 127	Backspace character
6	2-255	Pause before dialing in seconds
7	1-255	Wait for carrier in seconds
8	0-255	Pause for comma in dial string in seconds
9	1-255	Carrier detect response time in 1/10 second
10	1-255	Carrier loss delay in 1/10 second, 255 = ignore CD
11	50-255	Touch tone dial speed, in milliseconds
12	20-255	Escape guard time, in 1/50 second, 0 = no delay
13	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7	Power up asynchronous data format unused result code, 0 = basic; 1 = extended parity, 0 = disabled; 1 = enabled parity, 0 = odd; 1 = even data bits, 0 = 7 bits, 1 = 8 bits undefined buffer overflow flag, 0 = disabled; 1 = enabled 8th bit, 0 = space, 1 = mark (8 bit only)
14	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7	Option register status data format unused local echo, 0 = disabled; 1 = enabled result codes, 0 = enabled; 1 = disabled result codes, 0 = numeric; 1 = word command recognition, 0 = enable; 1 = disable dial method, 0 = touch tone; 1 = pulse unused 0 = answer; 1 = originate
15		Flag register
16	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7	Test status Analog loop, 0 = inactive; 1 = active unused digital loop, 0 = inactive; 1 = active remote request digital loop, 0 = inactive; 1 = active remote digital loop, 0 = inactive; 1 = active self test RDL, 0 = inactive, 1 = active self test analog loop, 0 = inactive; 1 = active unused
18	0-255	Remote test timer in seconds
21	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7 bit 43 00 01 10 11	Option status jack type, 0 = RJ11, RJ41, RJ45; 1 = RJ12, RJ13 unused CTS state, 0 = follows RTS, 1 = forced on DTR modem behavior (see below) DTR modem behavior (see below) DCD, 0 = forced on; 1 = follows carrier DSR, 0 = forced on; 1 = behaves normally long space disconnect, 0 = disabled; 1 = enabled DTR modem behavior ignore DTR assume command state when DTR drops assume command state & disable auto answer when DTR drops reset when DTR drops
22	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7 bit 10 01 10 11 bit 32 00 01 10 11 bit 654 100 101 110 111	Option status speaker volume (see below) speaker volume (see below) speaker setting (see below) speaker setting (see below) X command in effect (see below) X command in effect (see below) X command in effect (see below) make/break ratio, 0 = 39/61; 1 = 33/67   low med high   speaker always off speaker on until carrier detected speaker always on speaker off during dialing, then on until CD   X1 command in effect X2 command in effect X3 command in effect X4 command in effect
23	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7 bit21 00 01 10 11 bit 54 00 01 10 11 bit 76 00 01 10	Option status remote digital loop request, 0 = denied; 1 = OK baud rate (see below) baud rate (see below) unused parity (see below) parity (see below) guard tone setting (see below) guard tone setting (see below)   0-300bps 600 bps 1200 bps 2400 bps   even parity parity bit always 0 odd parity parity bit always 1   no guard tones 550 Hz guard tone 1800 Hz guard tone
25	0-255	DTR detect delay in seconds
26	0-255	RTS to CTS delay in milliseconds
27	7/6/5/4/3/2/1/0 0 1 2 3 4 5 6 7 bit 10 00 01 10 11 bit 45 00 01 10	Option status synch/async mode (see below) synch/async mode (see below) phone line, 0 = public; 1 = leased Unused clock type (see below) clock type (see below) protocol, 0 = CCITT V.22, 1 = BELL 212A unused   asynchronous mode async/sync mode sync auto-dial sync manual dial   internal clock used external clock used slaved clock used

Table D

 

When referring to file transfers, a protocol is a method of sending and receiving a program. There are many methods available, each with different advantages and disadvantages. In this section, we are going to discuss the different protocols and both their advantages and disadvantages.

Kermit

An almost error-free file transfer protocol usually used for text transfers. It was developed at Columbia University.

Xmodem

A file transfer protocol developed by Ward Christensen around 1977. He invented this protocol for the 8-bit computer CP/M of that era. It is fairly slow in comparison to today's standards, but was the first widespread FTP. It uses blocks of 128 bytes, and after each block is sent, it sends a 1-byte checksum to check for errors. The block will be resent, if an error is encountered. Most communication programs offer this protocol.

Xmodem/CRC

The same as Xmodem, but this protocol has a 16-bit CRC instead of the checksum, which makes it more reliable. CRC stands for "Cyclic Redundancy Check".

Xmodem/1K

This is similar to the Xmodem/CRC, except that it uses blocks of 1024 bytes, rather than 128. It needs to be stopped less often to check for errors, so it is faster than Xmodem. This is sometimes wrongly called Ymodem.

Ymodem

A file transfer protocol witch can transfer more than one file at a time. It transfers both a file and some information about the file (including its length & the name of the file). It's similar to the Xmodem/CRC, except Ymodem can transfer more than one file at a time. It may use CRC-16 or a 1 byte checksum where applicable. It uses both 1024 byte blocks and 128 byte blocks.

Ymodem/g

This is Ymodem changed to provide best results with error-correcting modems. If there were any errors in the transmission, the transmission would be aborted.

Zmodem

A file transfer protocol, which is known for its speed, as well as the ability to transfer information about the files which it sends. It has crash recovery and auto download features and can use a 32-bit CRC, which makes it almost error-free.

 

Choosing a modem is as easy as pie. All you have to do is make one choice, then follow the simple guidelines. As you saw in this article, there are two types of modems: Internal and External. How do you decide what type of modem to buy? First, forget about price. To make your choice, use the following guideline. If you understand a lot about modems and you are 100% sure that you will only use the modem in one computer, think about getting an internal one. It will take up less room and be more convenient. Otherwise, get an external one. Generally speaking, both have got some advantages and disadvantages: It’s easy to move an external modem from one computer to another -even from a PC to MAC- and there are indicator lights on the front which are sometimes helpful if something goes wrong. Imagine your modem has hang up and you need to reset it. With an external modem, you can turn it off and on to perform the reset operation, but with an internal one, you will have to restart the entire computer.

Choosing between an internal and external modem is always the first step. But what’s the second one? You should make sure that you get the right speed. Remember that not to buy a modem which is slower than 28.8Kbps! Also be warned that a slower modem will cost less, but this is just about the worst place to try to save money. If you are paying by the minute for Internet access, a slow modem will force you to pay a lot more for the extra time it takes to download large files. Therefore, if you make sure you get a 28.8Kbps modem, it will have all the correct standards built into it (V.34, V.42, and V.42bis).