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Modem Interface V-Series Standards

The V Series recommendations from the International Telecommunication Union (ITU) are listed below. They include the most commonly used modem standards and other telephone network standards. Prior to the ITU-T standards, the American Telephone and Telegraph Company and the Bell System offered its own standards (Bell 103 and Bell 212A) at very low transfer rates. Another set of standards, the Microcom Networking Protocol (MNP) offered error correction. Some modems offer both MNP and ITU Standards.

In general, when modems handshake, they agree on the highest standard transfer rate that both modems can achieve. Beginning with V.22bis, ITU-T transfer rates increase in 2400 bps multiples. (Bis refers to a "second version" of the standard and Terbo refers to a "third version" of the standard.)


Modem Interface V-Series Standards


Provides 1200 bits per second at 600 baud (state changes per second).


The first true world standard, it allows 2400 bits per second at 600 baud.


Provides 4800 and 9600 bits per second at 2400 baud.


Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second.


Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard.


Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits per second and backwards compatibility with V.32 and V.32bis.


Provides up to 33,600 bits per second or fallback to 31,200 or V.34 transfer rates.


The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters.


Same transfer rate as V.32, V.32bis, and other standards but with better error correction and therefore more reliable.


Provides up to 56,000 bits per second downstream (but in practice somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell's K56flex technology.

x2 is a technology from US Robotics (now 3Com) for the downstream transmission of data over ordinary phone lines at 56 Kbps (thousands of bits per second). The 56 Kbps speed is achieved in the downstream direction only (to your home or business). Upstream speed is at the regular maximum speed of 33.6 Kbps. (The actual achieved downstream speed is reported by users to be about 53 Kbps.) x2 provided input to and has been replaced by the V.90 ITU standard.

K56flex: Rockwell Corporation's K56flex modem chipset gave users the capability to receive data on ordinary phone lines at 56 Kbps (thousand bits per second). K56flex was similar in capability to US Robotics' x2 modem and transmission technology. K56flex and X2 contributed to and were replaced by the ITU V.90 standard. Modems built with K56flex can be upgraded to the V.90 standard by getting a software update from the modem manufacturer. (Note that Rockwell does not build modems itself, but only the chipsets for them.) Conexant, a Rockwell subsidiary company, now builds V.90 chipsets


V.92 is the new dial-up modem specification from the ITU (International Telecommunications Union) that introduces three new features that will add convenience and performance for the modem user. The three features are Quick Connect, Modem-On-Hold and PCM Upstream.

Quick Connect: Quick Connect will shorten the time it takes to make a connection by remembering ("training") the phone line characteristics and storing them for later usage. Typically, the modem handshake (all that noise you hear) takes from 25 to 27 seconds. Surveys indicate that people are quite irritated at this length of time. Quick Connect will cut the modem handshake time in half for most calls, a significant improvement.

Modem-On-Hold: Many households use the same phone line for both voice calls and data (Internet), so when the user is browsing the Internet, an incoming call cannot get through. Modem-on-Hold allows you to receive an incoming call and stay connected to the Internet (Call-Waiting service from your phone company is all that is required). It also works in reverse; you can initiate a voice call while connected and keep the modem connection.

PCM Upstream: PCM Upstream boosts the upstream data rates between the user and ISP to reduce upload times for large files and email attachments. A maximum of 48 Kbps upstream rate is supported. PCM Upstream will work particularly well with new equipment such as Internet-connected digital cameras, which primarily upload rather than download information.


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