Some modems offer various forms of hardware error detection and correction. Such features usually require matching firmware in the modems at both ends.
Data compression is a means of shrinking files into smaller packets, which results in faster connections and requires less space on the host and client machine for storage. Some modems can perform "on-the-fly" data compression; both modems must be able to understand the compression for it to work. On-the-fly compression will significantly enhance the amount of data sent between modems during a given time period. There are now a variety of industry standards for data compression, based in part on the work done by various Internet-related committees.
There were no standards during the early days of modem communication. The only way to ensure data transmission was to place identical modems at the sending and receiving ends of the transmission. Compatibility was a great concern, and proprietary modems were the norm.
Today, modems comply with several standards. There are two sources for these standards:
- Manufacturers have placed specifications of their modem functions in the public domain. These specifications can now be copied and used by any manufacturer. If enough manufacturers use a specification, it becomes a standard on its own merits.
- Standards committees are formed when there is enough interest expressed by users, vendors, or regulatory committees to develop a set of rules for a class of data or modems.
Early Bell Standards
Bell Telephone produced the first generally accepted modem standards (103 and 212A); they developed out of the market-dominant position of the telephone company in telecommunications. To compete, other vendors offered products that would recognize the Bell command set. This scenario occurred more than once.
The CCITT modem standards are commonly known as Vdot standards (because each is named using the letter "V" followed by a decimal point and a number). The Vdot standards set out detailed requirements for the use of various modem speeds, incorporation of data-compression schemes, and error correction.
The MNP standards (Micron Network Protocols-named for Micron, the company that developed them) set forth a series of error-correction methods.
The following table compares the standards set by the aforementioned organizations.
|Bell 103||300||300||U.S. and Canadian standard, now largely obsolete.|
|Bell 212A||600||1200||U.S. and Canadian standard.|
|V.21||300||300||Similar to B 103. However, B 103 modems are not compatible with V.21.|
|V.22||600||1200||Incompatible with 212A. This standard is primarily used outside the U.S.|
|V.22bis||600||2400||Offers world-wide compatibility.|
|V.23||1200/75||Provides split data transmission.|
|V.29||9600||Used for Class 3 fax machines.|
|V.32||2400||4800/9600||Provides error correction. Full-duplex.|
|V.32bis||2400||14,400||An improved version of V.32.|
|V.32fast||2400||28,800||An extension of V.32 and V.32 bis.|
|V.34 or V.fast||2400||28,800||Allows optional higher speeds of 31.2 Kbps and 33.6 Kbps.|
Error-Detection and Data-Compression Protocols
In addition to speed standards, some CCITT standards include error-detection and data-compression protocols. The following table shows the standards that include error detection.
|V.42||2400||2400 and up||Error correction|
|MNP 1-4||2400||2400 and up||Error correction|
|V.42 bis||2400||9600/38.4K||Data compression||V.42 must be present.|
Telephone lines are capable of carrying 56 Kbps of data; however, conversion from analog to digital signals and back comes with a price. That price is a speed limit of 33.6 Kbps. Because many telephone systems are now digital, it is possible to transmit, in some instances, at a full 56 Kbps in one direction. The return data, however, is still limited to 33.6 Kbps. For these reasons, it is unlikely that you can achieve a full data transfer rate with a 56-K modem (56-K refers to a modem that transmits at 56 Kbps). In order to achieve the best performance, the following conditions must be met:
- Digital to analog conversion should be limited so that it takes place only once within the network. Each conversion slows the communication process.
- The host must be connected digitally.
- Both modems must support the 56-K technology.
There are currently three 56-K modem standards: K56flex, x2, and V.90. Unfortunately, these standards are not compatible at 56 K, so in order to achieve the highest possible speed, both modems must use the same standard. Several companies developed the K56flex standard, and U.S. Robotics developed the x2 standard. Currently, the V.90 standard is replacing both the K56flex and the x2, and most 56-K modems can be upgraded to this standard. If you have a 56-K modem and want to upgrade to V.90, check the manufacturer's Web site for instructions and download the appropriate software.