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Comcast Xfinity service is now based on advanced video transmission and high-speed Internet service based on the DOCSIS 3.x system.

Installation: April, 2013

Initial benchmarking shows performance roughly 3 times what was available from AT&T U-Verse.

First speed tests

Comcast's speed test software:

Download: 54.3 Mb/s (megabits);
Upload: 10.04 Mb/s

"Real-life" transfers to server at Yale University:

Download: 5 - 7 MB/s or 40 - 56 Mb/s;
Upload: 1.4 MB/s or 11.2 Mb/s
(1 MB (megabyte) is roughly 8 Mb (megabits).)

Note: These are spot checks that do not allow for possible congestion / loading slow-down during heaving network usages periods.

Unfortunately, there appears to be no PC-based performance / error monitoring software available for Xfinity that is comparable to UV Realtime for U-Verse.

Modem / Router

We were supplied with an SMCD3GNV modem/router, running version firmware. The modem's web status page offers the following summary of channels and modulation:

Comcast Xfinity DOCSIS 3 modulation scheme

Compare this data with the more generic information in the Wikipedia article. The Xfinity advertised download rate for "Blast!" service is "up to" 50 Mb/s. (A 105 Mb/s service is also available.) The DOCSIS spec indicates that downloads of at least 171 Mb/s are supported by the scheme, while the SMC modem datasheet claims to support up to 300 Mb/s down and 120 Mb/s upstream. In any case, there is a potential for much higher Internet performance in the future.

The download channels are all above 600 MHz, with relatively little chance of Amateur Radio interference. The upload channels at 23.6 and 30.0 MHz might have problems with nearby amateur HF bands, however, as upload (transmit) channels, they operate at high signal levels. The upstream receivers for these channels are presumably somewhere in the neighborhood, but not very close by.

The Xfinity modem only decodes the Internet bitstream, which would include VOIP service (a minor bandwidth item). This is unlike AT&T's U-Verse modem which decodes all incoming traffic, including digital TV signals and Internet from a single bitstream. Xfinity's video service operates outside the DOCSIS 3 channels, using QAM signalling ranging from 55 MHz to 1 GHz. (See channel lists.)