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Comcast Digital Voice (CDV) now known as Xfinity Voice is a telephone product offered by Comcast Corporation. Comcast offers CDV in both residential and business class and is packaged with Xfinity.
Xfinity Voice is similar to VoIP, but differs in that it operates on a dedicated server and assigns priority to the information sent over the Xfinity Voice network, and also means that the information is not transmitted over the public Internet. On the HFC (cable) network, calls are placed into individual Unsolicited Grant Service (UGS) flows, based on DOCSIS 1.1 QoS standards.
It utilizes E911, rather than the standard 911 service utilized by other large VoIP carriers. This allows the 911 service to immediately know where a caller is located, regardless of whether or not they are able to vocally relate their location to the operator.
The term "digital voice" is misleading. While voice calls are delivered as a digital stream over the Comcast network, the voice lines are converted to plain old telephone service (POTS) lines in the customer cable modem and output on standard RJ-11 jacks. The only true digital lines available today in the U.S. are either ISDN BRI and PRI lines or T1.
SMC corporation is a major provider of cable modems for Comcast. They have introduced the SMCV4702 cable modem that provides (digital) ISDN PRI output but it is not clear if or when Comcast will start deploying this modem to business Comcast voice customers.