V.92 Features
The three mayor features of V92 modem are Modem-on-Hold (MOH), PCM upstream (PCMU) and Quick Connect (QC). Each
of these features can be used independently.
PCM Upstream
V.92 specification allows a PCM upstream rate from 24,000bps to 48,000bps. The rate increments
in steps of 1.33 kbps as in V.90. Upstream rates could be 24k, 25.3k, 26.6k, 28k, etc. This is a 30%
increase over the 33.6K bps attainable by V.90 modems. This higher upstream speed offers numerous
benefits including a reduction in the upstream data transfer time for large e-mails with attachments and for ftp
site uploads, as well as improved operation for interactive applications. Due to the real conditions of the phone lines
the 48 kbps upstream is very hard to obtain.
Modem On Hold
Allows modems to stay connected "on-hold" while you take an incoming call waiting call,
or initiate an outgoing voice call (works only if V.92 server modem is configured to allow this feature). The amount of time
you can place your Internet connection on hold while taking another call will be up to your Internet Service
Provider (ISP).The V.92 recommendation permits ISPs to set their modems to wait for 0 to 16 minutes, or,
to wait indefinitely. V.92 client modems may include software that will warn the user when the ISP determined timeout approaches, as well as to display
call waiting caller ID info (requires telco call waiting/caller ID service). ISPs have an incentive to limit
the use of this feature: when a call is placed on hold, the ISP modem remains unavailable to take other calls.
The ISP may need to add more modems to maintain quality of service if this feature is enabled. The ISPs also have the
posibility of do not accept modem-on-hold if the total time of modem-on-hold in the same session is greater
that a determined value.
Quick Connect
In the ITU V.92 recommendation the time to establish a connection may be reduced with
faster handshaking, using a short phase 1 and phase 2, reducing the connection time to about 20 seconds. It is
also possible to reduce more this connedction time to around 10 seconds using short phase 3 and phase 4 (this
feature is not in the ITU V.92 recommendatin) by forcing V.92 modems to remember
line conditions and connect rates on past connections by storing data about those connections in a buffer. The modem
will compare line quality variables as it dials in to a server modem with those in the buffer, and if a match is
found, the handshake starts at the rate previously negotiated. Quick Connect will also be used with Modem On
Hold: when a call is switched back to the modem after using the MOH feature, a retrain is required. When
the V92 modem uses short phase 3 and phase 4 and the line conditions are different, the V.92 modem have to fall-back
to a full retrain.
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