There are 2 basic physical types of modems: Internal & External modems. External modems sit next to the computer and connect to the serial port using a straight-through serial cable.
An internal modems is a plug-in circuit board that sits inside the computer. It incorporates the serial port on-board. They are less expensive than external modems because they do not require a case, power supply and serial cable. They appear to the communication programs as if they were an external modem for all practical purposes.
Modem Types
There are many types of modems, the most common of which are:
Optical Modem - Uses optical fiber cable instead of wire. The modem converts the digital signal to pulses of light to be transmitted over optical lines (more commonly called a media adapter or transceiver).
Short Haul Modem - A modem used to transmit data over 20 miles or less. Modems we use at home or to connect computers together among different offices in the same building are short haul modems.
Acoustic Modem - a modem that couples to the telephone handset with what looks like suction cups that contain a speaker and microphone. Used by traveling salespeople to connect to hotel phones.
Smart Modem - A modem with a CPU (microprocessor) on board that uses the Hayes AT command set. This allows auto-answer & dial capability rather than manually dialing & answering.
Digital Modem - Converts the RS-232 digital signals to digital signals more suitable for transmission. (also called a media adapter or transceiver)
V.32 Modem - a milestone modem that uses a 2400 baud modem with 4 bit encoding. This results in a 9600 bps (bits per second) transfer rate. It brought the price of high speed modems below $5,000.
Baud is the speed at which the analog data is changing on the voice channel and bps is the speed at which the decoded digital data is being transferred.
Features of Modems
Speed - The speed at which the modem can send data in bps (bits per second). Typical modem speeds are: 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K, 28.8K bps.
Auto Dial/Re dial - Smart modems can dial the phone number and auto re dial if a busy signal is received.
Auto Answer - Most modems have Ring Detect capability and can automatically answer the telephone when an incoming call comes in.
Self Testing - Newer modems have self-testing features. They can test the digital connection to the terminal/computer and the analog connection to a remote modem. They can also check the modem's internal electronics.
Voice Over Data - Voice Over Data modems allow a voice conversation to take place while data is being transmitted. This requires both the source and destination modems to have this feature.
Synchronous or Asynchronous Transmission - Newer modems allow a choice of synchronous or asynchronous transmission of data. Normally, modem transmission is asynchronous (we send individual characters with just start and stop bits). Synchronous transmission or packet transmission is used in specific applications.
Modem Speeds/Standards
| Bell 103 | 300 bps FSK - Half duplex |
| Bell 113 | 300 bps FSK - Full duplex |
| Bell 202 | 1200 baud half duplex |
| Bell 212A | 1200 bps DPSK (Dibit Phase Shift Keying) - V.22 compatible |
| MNP1-3 | Microcon Networking Protocol - Basic error detection and control of errors |
| MNP4 | Error correction + adapts to line conditions |
| MNP5 | Error correction + adapts to line conditions; adds compression technique to double the data transfer rate |
| RS-232D | Cable and connector standard |
| V.22 | 1200 bps DPSK (Dibit Phase Shift Keying) - Bell 212A compatible |
| V.22bis | 2400 bps - International Standard |
| V.24 | European Mechanical specifications for RS-232D |
| V.26. | Synchronous 2400 bps modem |
| V.27 | Synchronous 4800 bps DPSK modem |
| V.28 | European Electrical specifications for RS-232D |
| V.29 | Synchronous 9600 bps QAM |
| V.32 | 9600 bps QAM |
| V.32bis | 14.4 Kbs QAM1 |
| V.33 | 14.4 Kbps Trellis Coded Modulation for noise immunity |
| V.34 | 28.8 Kbps modem standard |
| V.34bis | 33.6 Kbps modem standard |
| V.42bis | Compression technique to roughly double the data transfer rate. Uses Automatic Repeat Request ARQ and CRC (Cyclic Redundancy Checking). |
| WE201 | Synchronous Western Electric 2400 bps DPSK |
| WE208 | Synchronous 4800 bps DPSK |
| WE209 | Synchronous 9600 bps |
Transfer Rate versus PC Bus Speed
The lowliest XT PC can out-perform the fastest modem transfer rate. For example, an XT has an 8 bit parallel expansion bus operating at 4.77 MHz. This equates to a data transfer rate of:
8 bits x 4.77 MHz = 38.16 Mbps
Compare this to the fastest modem transfer rates of 57.6 kbps!
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