Appendix: Modem Standards
Tables 7.4 and 7.5 list Bell and ITU modem-speed standards. Standards have changed in two ways: Initially, they were set by the Bell system (AT&T). Now the International Telecommunications Union headquartered in Geneva, Switzerland, sets them. Standard modem speeds have increased from 300 bps to 56,000 bps.
| Standard | Modem Type | Speed (bps) | Fall-Back Speed | Comments |
|---|---|---|---|---|
| Bell 103 | Dialup | 300 | ||
| Bell 212 | Dialup | 1200 | ||
| Bell 208A | Leased | 4800 | ||
| Bell 208B | Dialup | 4800 | ||
| V.21 | Dialup | 300 | 300 | Used by Group 3 fax in half duplex mode for negotiation and control. |
| V.22 | Dialup, leased | 1200 | 300 | |
| V.22bis | Dialup | 2400 | 1200 | Compatible with V.22. |
| V.23 | Dialup | 1200 | 600 | Specifies 75-bps reverse channel. |
| V.27ter | Dialup | 2400 4.8Kbps | 2400 | Used by Group 3 fax for image transfer. |
| V.29 | Four-wire leased, two-wire leased, dialup | 9.6Kbps | 4.8Kbps, 7.2Kbps | Used by Group 3 fax for image transfer at 9.6Kbps and 7.2Kbps. |
| V.32 | Two-wire leased, dialup | 9.6Kbps | 4.8Kbps | With V.42, provides 38.4Kbps throughput. |
| V.32bis | Two-wire leased, dialup | 14.4Kbps | 4.8Kbps, 7.2Kbps, 9.6Kbps | Has rapid rate negotiation feature for fast speed changes. |
| V.32ter | Two-wire leased, dialup | 19.2Kbps | 18.8Kbps, V.32 V.32 bits | An AT&T specification that is compatible with V.32 and V.32bis. |
| V.33 | Four-wire leased | 14.4Kbps | 12Kbps | |
| V.34 | Two- and four-wire leased, dialup | 33.6Kbps | 28.8, 19.2Kbps V.32bis | Initial speed was set at 28.8Kbps, known as V.fast prior to standard being set. |
| V.90 | Dialup | 33.6Kbps upstream 56Kbps downstream | V.34 | Downstream from network to customer is digital. |
| V.92 | Dialup | 48 Kbps upstream 56Kbps downstream | V.34 | Same as V.90 with faster upstream, quicker connects and capability to put data call on hold and take a voice call. |
| V.10 | Cable modem standard for North and South America | Upstream frequencies between 5 and 42 MHz; downstream frequencies between 50 and 750 MHz | Actual speed depends on available capacity and traffic levels | DOCSIS, data over cable system interface specifications. DOCSIS 1.0 modems certified and available. DOCSIS 1.1 is being certified in 2001. Availability predicted by 2002. Has improved security, billing and quality of service (QoS) for voice over IP. |
| DAVIC 1.0-1.4 | Cable modem, fixed wireless and set-top box standards for Europe | Sets parameters for fixed rate as well as variable speeds | Standards for Internet access and near video on demand. Adopted by ISO/IEC. Set by Digital Audio Visual Council (DAVIC). | |
| bis = ITU term designating a second-generation standard.
ter = ITU term designating a third-generation standard. V series standards, except for V.32, are promulgated by ITU. Fall-back speed is the speed that a modem can drop down to with noisy lines. | ||||
Table 7.5 lists various error control standards and techniques available in modems. It also lists compression methods used. Compression removes white spaces and commonly repeated characters to improve throughput (efficiency) on data communicated. The repeated characters are replaced by abbreviated versions of the characters. A text file sent using a 4:1 compression scheme might only need to send 25% as many bits as a noncompressed file.
| Standard/Technique | Description |
|---|---|
| V.44 | A new data compression method used with V.92 modems. It improves V.42 compression 20% to 60% and makes modem downloads faster. |
| V.42 | Specifies both Microcom Networking Protocol (MNP) 2-4 and Link Access Procedure for modems (LAPM) M error correction for full duplex modems. Compatible with V.34, V.22, V.22bis, V.26ter, V.32, V.32bis and the proprietary versions of V.32ter. |
| V.42bis | A data compression protocol that theoretically allows up to 4:1 file compression. |
| V.54 | A standard for local and remote loop test devices in modems. |
| LAP M | The preferred error control protocol specified by the V.42 and V.42bis standard for error control in modems. |
| MNP 1 | MNP 1 has been superseded by later versions. |
| MNP 2-4 | Part of the V.42 standard for error control. |
| MNP 5 | A data compression protocol that provides a 2:1 compression ratio. |
| MNP 10 | Optimizes modem performance over adverse line conditions and cellular links. |
| EC2 | Developed by Motorola. Modifies cellular transmission levels to avoid clipping, which is common at high frequencies. |
| TX-Cel | An error-correcting algorithm that reduces cellular amplitude distortion. |
| ETC | Enhanced Throughput Cellular (ETC), newer than MNP 10, works with V.42bis, the ITU-specified standard error control that works with V.42bis. Enhances throughput on cellular modems. Owned and licensed by Paradyne (formerly part of AT&T). |
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