Modulation Standards

Modulation Standards

Modulation defines the method used to encode the data stream between DCE devices. There are
many modulation standards, including several proprietary methods. Modems will negotiate the
modulation standard to be used during the connection. Modern modems will alter this negotiation
during the connection, should line conditions permit. This can provide improved performance
or prevent a connection from terminating, should the line condition degrade. Table 23.2
notes the common modem modulation standards.

Modem Modulation Standards

Modulation DCE to DCE Bandwidth Status
V.22 1,200bps ITU standard
V.22bis 2,400bps ITU standard
V.32 9,600bps ITU standard
V.32bis 14.4Kbps ITU standard
V.32 terbo 19.2Kbps Proprietary
V.34 28.8Kbps ITU standard
V.fast 28.8Kbps Proprietary
V.FC 28.8Kbps Proprietary
V.34 annex 12 33.6Kbps ITU standard
K56Flex 56Kbps Proprietary
X2 56Kbps Proprietary
V.90 56Kbps ITU standard
V.92 56Kbps ITU standard—adds faster call connection
capabilities
V.61 or V.34Q 56Kbps ITU standard—adds simultaneous
voice and data capabilities


Most modems support all lower bandwidth ITU standards for backward compatibility,
and many V.90 and V.92 modems also support either X2 or K56Flex. Modems that shipped
with X2-only or K56Flex-only support—before the V.90 standard was ratified—can usually
be upgraded in the field, frequently with software only. This might not be the case when
upgrading from V.90 to V.92. You will have to consult your modem vendor for upgrade capabilities
and cost.
The V.92 standard represents three significant modem enhancements:

Quick Connect, which reduces the amount of time required for the modems to negotiate a
connection. In some instances, a user might see a 50 percent reduction in connection time.

The V.92 standard includes Modem on Hold, which enables a user to accept an incoming
phone call without terminating their existing connection. For users with only one phone
line in their home or office, Modem on Hold enables them to handle both data and voice
calls over the same telephone line.

V.92 supports PCM Upstream, which allows for faster uploading and sending of large e-mail
messages, photos, and documents. With PCM Upstream, users gain faster upstream communication
with speeds reaching up to 48,000 bits per second, as compared to 33,600 bits per second
with V.90 technology.
The modulation standards also incorporate data compression and error correction specifications,
which are detailed next.