Modems in Detail

Modems in Detail

Serial Port Modem Connections

If you have an older Mac OS machine without USB, an external modem must be connected to a serial port, usually the Modem port, since this is often given a higher priority. Suitable modems come with a RS-232 interface, often provided via a 25-way D socket (DB25) and with connections configured for data circuit-terminating equipment, also known as data communication equipment (DCE). You may need to construct or obtain an adaptor cable wired as follows:-

The cable shown above includes wiring for hardware handshaking, which is essential for running a modem at 9.6 kbit/s or higher. Any cable that links pin 2 of the mini-DIN connector to pin 6 on the DB25 connector (DSR), instead of pin 5, is unsuitable for high-speed operation. If you use an even simpler cable that lacks any of these hardware handshake connections your equipment will resort to XON/XOFF software handshaking, which simply isn’t adequate for the job.

Those of a technical disposition may be interested in the following descriptions:-

• Clear to send (CTS): indicates whether the modem can transmit data
• Request to send (RTS): indicates whether the computer can accept data
• Data terminal ready (DTR): resets or puts modem into command mode or ends call

CTS handshaking must always be used. The RTS and DTR circuits can’t be employed at the same time, although both can be wired, as shown above. The modem is usually configured via an initialisation string (see below) to use either RTS or DTR.

For high-speed operation, the RTS handshaking circuit is best avoided, whilst the DTR circuit can be released for a function determined by the initialisation string. Without RTS handshaking, the computer must always accept data and its serial port must run at a rate equal to or higher than the modem’s maximum speed. If the DTR handshaking circuit isn’t used, its function must be replaced by messages generated by the computer’s software and sent over the TXD- data circuit.

• The cable shown above doesn’t provide support for DSR handshaking or DCD handshaking, as required by some terminal emulation software for checking the readiness of a modem or the existence of a carrier signal.

Speed

A fast modem isn’t just convenient, it reduces your phone bill. The speed or bit rate for sending data is measured in kilobits per second (kbps or kbit/s), also erroneously known as kilobaud (kbaud). Common values include 1.2, 2.4, 9.6, 14.4, 28.8, 33.6 and 56 kbit/s. To use the Internet you’ll need a modem that runs at 28.8 kbit/s or more, preferably at 56 kbit/s.

The bit rate shouldn’t be confused with the data rate measured in kilobytes per second (KB/s), which is around a tenth of the speed in kbit/s, allowing for the start and stop bits at the beginning and end of each byte. In practice, some bytes are retransmitted if errors are encountered, reducing the actual rate even further. Here are the best possible data rates:-

Speed (kbit/s)	Rate (KB/s)	Rate (KB/min)
33.6	3.36	201.6
56	5.6	336

If your modem is connected via a serial port then the maximum data transfer rate (DTR) of the port can restrict the speed of your modem. For example, an elderly 680x0-based Mac OS computer with AppleTalk may be limited to 9.6 kbit/s, although this increases to 57.6 kbit/s with AppleTalkturned off. A more recent machine with a GeoPort interface gives 115.2 kbit/s with AppleTalk and 230.4 kbit/s without it. Some of these maximum DTRs can be a problem, especially if your modem is using compression, which increases the speed at the serial port to four times the speed on the phone line.

You can determine the average DTR of your system by using the following equation:-

DTR in kbit/s = (File size in KB / download time in seconds) × 9

Ideally, both modems on a link should run at their highest possible speed. In practice, split rates are often used. For example, a V.90 modem (see below), although capable of receiving data at 56 kbit/s, only sends it at 33.6 kbit/s. Fortunately, this usually isn’t a problem, since most people receive more data then they send.

• Modern computers can accommodate varying data rates over the link between a modem and computer. You may find that your modem can be programmed to provide bit rate conversion if there are any problems with such a link.
• The UK’s original Prestel system received enquiries at 75 bit/s and responded at 1.2 kbit/s.

Speed Arbitration

You should always use your modem at the highest possible speed. However, when connecting with another modem the two devices automatically agree over the choice of speed, together with other options for compression and error-control. So if one modem works at 33.6 kbit/s and the other is limited to 28.8 kbit/s the communications session runs at 28.8 kbit/s.

A poor circuit can cause the modems to agree on a very slow rate. When this happens you could try making another call to see if things improve. Just to confuse you, the FCC in the USA have limited the maximum speed of a 56 kbit/s modem to 53 kbit/s. In practice, the actual rate of a 56 kbit/s modem varies somewhere between 46 and 50 kbit/s.

The final speed of a link will match one of the following values in bits/s, irrespective of your modem’s maximum speed:-

2400	4800	7200	9600	12000	14400	16800	19200	21600	24000	26400	28800	29333	30667
31200	32000	33333	33600	34000	34667	36000	37333	38000	38667	40000	41333	42000	42667
44000	45333	46000	46667	48000	49333	50000	50667	52000	53333	54000	54667	56000

• With some modems and older software you have to press certain keys to get the modems to agree terms. For example, in a communications application, you may have to press Return for a CR (carriage return) or press Ctrl-C for an ETX (End of Text) message. However, most modern applications don’t require such primitive methods.

Standard Protocols

The International Telegraph and Telephone Committee (CCIT), now known as the International Telecommunications Union (ITU-T) has defined numerous standards for the transmission of data over telephone lines. Fortunately modern modems can be used with most, if not all, of the following standards:-

CCIT	Speeds (kbit/s)	Notes
V.21	0.3	Incompatible with Bell 103 standard (USA)
V.22	1.2	Incompatible with Bell 212A standard (USA)
V.22bis	2.4
V.26ter	2.4	For noisy telephone lines (rarely used)
V.29	9.6	Group III fax protocol (half-duplex)
V.32	4.8, 9.6	Full-duplex
V.32bis	7.2, 12, 14.4	Full-duplex
V.17	14.4	Group III fax protocol (half-duplex)
V.34	28.8
V.34bis	33.6
V.42bis	38.4
V.90	56	Replaces x2 and k56flex standards

bis = second version of specification

ter = third version of specification

Prior to acceptance of the V.90 standard by the ITU-T there were two competing systems at 56 kbit/s; x2 technology from US Robotics and k56flexfrom Rockwell. Fortunately, most (but not all) modems can be updated using a flash upgrade. This usually involves downloading data from the Internet, which is then loaded into the modem.

To continue using x2 or k56flex you must use an Internet service provider (ISP) that recognises these standards. It’s worth noting that an ISP sometimes issue special phone numbers for each kind of modem. Although some ISPs accommodate both x2 and k56flex customers, x2 providers are less common. Remember, if you use a modem with the wrong ISP or phone number it won’t work at 56 kbit/s. Ideally, you should get your modem upgraded to V.90.

Error Correction

To avoid file corruption, every block or packet of data must be sent correctly over the link. Following the initial connection, both modems agree on the type of error detection, if any, to be used. Several different techniques have been developed, mainly based on the LAP-B and LAP-M systems. The modern V.42 standard incorporates basic LAP-M error correction, as well as Microcom Networking Protocol (MNP) correction at levels 2 to 4, commonly known as MNP2 to MNP4. The correction mechanism is in the modem’s hardware or in the form of software. Although older modems and software may not fully support V.42, this standard can be used with all ITU-T standards, including the older V.22, V.22bis, V.26ter, V.32 and V.32bis systems. Alternative error correction, such as MNP10, is used for unreliable links via a cellular telephone.

When using these protocols you must set the modem to 8 data bits, no parity and one stop bit. A modem with MNP hardware should confirm that it exists whenever a /REL string is sent to it (see below for more about the Hayes protocol for modems).

Other error correction protocols, including Claris Kermit Tool, XMODEM, YMODEM and ZMODEM, give varying levels of performance. Some of these are supplied as communications tools in the Classic Mac OS and can be selected from within any communications application that supports Apple’s Communications Toolbox (CTB).

Data Compression

Data compression speeds up data transmission, allowing the speed between the modem and a computer to be increased to as much as four times the modem speed. For example, a modem operating at 14.4 kbit/s can use a serial link at 57.6 kbit/s whilst one running at 28.8 kbit/s can use 115.2 kbit/s.

Microcom Networking Protocol Level 5 (MNP5) compression is invariably built into any modem that provides MNP error correction (see above). All recent types of modem provide V.42bis compression, based on the Lempel-Zip-Welch (LZW) technique, giving a compression ratio as high as 4:1. Although V.42bis is up to 40% more efficient than MNP5, modems designed for V.42bis can also use MNP5 if required. However such modern devices often don’t accommodate older communication standards such as V.32 or V.32bis (9.6 and 14.4 kbit/s).

Transmission Modes

A modem can be used in several different modes, depending on the application and the nature of the link. Each CCIT standard employs one of the following modes:-

• Simplex

  Allows messages to be sent in only one direction, as in vintage teleprinter systems.

• Half Duplex

  Gives two-way communication, with each modem transmitting or receiving in turn, although the switching takes time. Half-duplex is rare in ITU-T standards, although it appears in the V.29 standard for Group III fax machines.

• Full Duplex

  Allows both modems to send messages at any time. Modern ITU-T standards employ echo-cancelling whilst older systems use a secondary carrier at a different frequency to the main carrier. This back channel or secondary channel, owned by the recipient, runs at a lower rate than the caller’s main channel. RS-232C ports often provide duplicate data, handshake and other circuits for a secondary channel, although they’re not needed for echo-cancelling.

Modem Settings

If you simply use your modem for working over the Internet you’ll probably never change its basic settings. However, if you want to transfer data directly to another computer by means of a communications application, you may need to set the three basic parameters shown below. The most common settings, which work in most circumstances, are shown in italics.

• Data Bits (7, 8)

  The number of bits in each burst of data. A binary file uses 8 bits, with values from 0 to 255, whilst an ASCII text file only has 7 bits, with values from 0 to 127, although the eighth bit can be used for parity error detection (see below).

• Parity (None, Odd, Even)

  Determines what kind of parity checking, if any, is used for correcting minor transmission errors. The parity flag, sent with each data burst, shows whether the burst should contain an odd or even number. If the flag doesn’t match what’s received a correction can be made. This can work well if there’s only one error, but if there are two errors the corrected result can still be wrong. Parity checking has been mainly replaced by modern software error correction systems. Parity should only be used for 7-bit data, as found in an ASCII text file.

• Stop Bits (Auto, 1, 1.5, 2)

  The width of the pulse defining the end of a data burst, usually the same width as a single bit.

Special Modems

A fax-modem is a special device that works with your computer to provide similar features to a Group III fax machine. Although faxes are normally sent at 9.6 kbit/s the speed is automatically reduced to 7.2, 4.8 or 2.4 kbit/s whenever the other device can’t cope or when there’s a poor circuit. A real Group III machine can’t work at over 9.6 kbit/s, although 14.4 kbit/s or even 33.6 kbit/s can be used for a fax-modem, assuming the device at the other end can also accommodate such a speed. Some fax-modem software may use JBIG compression to handle graphics that contain shaded material.

A Telecommunications Device for the Deaf (TDD) allows anyone with hearing disabilities to communicate via a keyboard. It operates at speeds used for a Teletype (TTY) or Telex machines, corresponding to 45.5 bit/s in the USA and 50, 110 or 300 bit/s in other countries. Older devices employBaudot code, although newer models can convey normal ASCII text.

• If you have a modem capable of running at TDD rates you can use standard communications software to communicate with a TDD that accepts ASCII text. However, if your destination only requires Baudot code you’ll have to simulate a Baudot terminal by employing a special TDD modem and a communications application that handles 5-bit Baudot code. Unfortunately, you’ll then be limited to only using uppercase characters.