V.92 Modem FAQ

V.92 has four main advantages over the older V.90 standard:
1. Faster Downloads
   
2. Modem On Hold
   
3. Quick Connect
   
4. Faster Uploads
   

For V.92 modems, a new compression program has been developed. It is called V.44 and replaces the older V.42 compression routine. V.44 may improve data compression up to 6:1 compared to the 4:1 maximum with the existing V.42 compression standard. Note that software modems will be able to take advantage of this, but most serial ports limit maximum transfer rate to 115.2k. With a 48k connection and 6:1 compression, a data rate of 288kbps could be achieved. (Data compression depends upon the nature of the data transfer.) Compressed data files such as .gif, .jpeg, and .zip will not experience additional compression or speed improvement.

Modem on Hold allows end users to suspend their data connection to either initiate or receive a voice call. If the phone conversation is completed within the allotted timeframe, the user may resume the data connection without redialing. Under previous standards, analog modems were not compatible with the call waiting service offered by the telephone companies. When the phone line was engaged in a data session, call waiting was either disabled (caller gets a busy signal) or the modem disconnected when interrupted by the call waiting tone. However, V.92 modems use the call waiting beep to trigger the on-hold feature. To receive calls while online, users must subscribe to a call waiting service, and for initiating calls, the three-way calling service. When a call comes through a phone line tied up by a data connection, the call waiting beep prompts the client modem to alert the user to an incoming call. With software added to the client's PC, the alert message is displayed in a pop-up dialog box. For users who subscribe to a caller ID service, the incoming call's number is also displayed.

The Quick Connect feature of V.92 shortens the modem connection time up to 50 percent where the connection is recognized by the modem. This reduction in modem start-up time is accomplished by storing the calling line parameters in the user's modem, which enables a faster handshake between the modem and the server. On these recognized connections, V.92 may shorten the connection time from the typical 25-30 seconds to about 15 seconds. Initial implementations have improved connect times by more than 10 percent, and future modem implementations should produce further reductions.

With V.90, modems assume that each call is made on a different line to a different destination. The following sequence takes place:


The client modem calls the server modem.

The two modems perform a "handshake".

The link layer connection, including error control and data compression, is established.

PPP negotiation and authentication take place.


With V.92, the client modem learns and remembers the line characteristics of the previous call. During call setup, the client modem probes the line to compare its characteristics with those stored in memory. If there is a match, the handshake starts at the previously negotiated rate and bypasses the full training probe. If it does not recognize the line characteristics, a normal V.90 handshake begins.

PCM Upstream increases the upstream data rate from the current V.34 speed (33.6 kbps) to as high as 48 kbps.

PCM Upstream redesigns the upstream modulation process to minimize signal loss during the analog-to-digital conversion. Higher upstream data rate is accomplished by manipulating the client modem settings so that the analog signal it transmits can be reconstructed to a more precise digital signal on the central office PCM Codec. A filter is inserted into the client analog modem transmitter. The server modem determines the channel characteristics and designs coefficients for the client filter to use so that line impairments are mitigated.

The most obvious benefit to PCM Upstream is faster uploading of files, including ftp uploads or e-mails with large image file attachments. As an example, digital camera users who frequently upload photos for printing or sharing with family and friends can accomplish this task much quicker.

Note: Increasing upstream rate decreases the downstream rate to a maximum 48 kbps.

By increasing the upstream bandwidth, PCM Upstream introduces more symmetry to the dial up connection. This improves the quality of applications that require symmetric data flow, such as voice over IP calls and multiplayer online gaming. For users who pay local or long distance toll charges, faster uploads speed up their session, which saves them money.