• Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint
Share this Page URL

Chapter 17. How a Modem (Really) Works > Standards and Protocols

Standards and Protocols

Today, there are a number of industry standards that allow equipment from one manufacturer to communicate with equipment from another manufacturer. Telecommunications standards include Bell and ITU-T. The Bell standards (for example, Bell 103 and Bell 212A) have been followed primarily in the United States and are, relatively speaking, obsolete. The ITU-T (International Telecommunications Union Telecommunication Standardization Sector), formerly known as CCITT, covers international standards followed worldwide. All the standards prefixed with “V” are ITU-T standards. For example, the latest modulation scheme is V.92.

Standard Definition

V.8– Covers the initial handshaking process.

V.17 fax— For making facsimile connections at 14,400 bps, 12,000 bps, 9,600 bps, and 7,200 bps.

V.21— For modems operating in asynchronous mode at speeds up to 300 bps, full duplex, on PSTNs.

V.22— For modem communications at 1,200 bps, compatible with the Bell 212A standard observed in the United States and Canada.

V.22bis— For modem communications at 2,400 bps. Includes an automatic link negotiation fallback to 1,200 bps and compatibility with Bell 212A/V.22 modems.

V.27ter— For facsimile operations, specifies modulation at 4,800 bps with fallback to 2,400 bps.

V.29— For facsimile operations, specifies modulation at 9,600 bps with fallback to 7,200 bps.

V.32— For modem communications at 9,600 bps and 4,800 bps. V.32 modems fall back to 4,800 bps when line quality is impaired.

V.32bis— Extends the V.32 connection range: 4,800, 7,200, 9,600, 12,000, and 14,400 bps. V.32bis modems fall back to the next lower speed when line quality is impaired, fall back further as necessary, and also fall forward (switch backup) when line conditions improve.

V.34— Allows data rates as high as 28,800 bps.

V.34+— An enhancement to V.34 that enables data transfer rates as high as 33,600 bps.

V.42— Defines a two-stage process of detection and negotiation for LAP-M error control.

V.42bis— An extension of ITU-T V.42 that defines specific data compression scheme for use during V.42 connections.

V.44— For modem data compression. It provides for a 6:1 compression ratio.

V.90— For 56-Kbps modem communications. It uses the digital telephone network to increase the bit rate of the receive channel by eliminating the analog-to-digital conversion commonly found in modem connections. V.90 connections require a modem with V.90 technology calling a digitally connected ISP or corporate host site compatible with V.90.

V.92— For advanced 56-Kbps modem communications. This technology offers three new features to enhance the V.90 standard: Modem on Hold, Quick Connect, and V.PCM Upstream. The V.92 technology can be utilized only if a V.92 modem is dialing into an ISP that supports and provides a digital V.92 signal.



Not a subscriber?

Start A Free Trial

  • Creative Edge
  • Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint