Cable Modem Theory

by: Rolf V. Østergaard

What Are The Cable Modem Standards - Page 13

Three major standards exist for Cable Modems! Guess how many we need?

First generation Cable Modems uses various proprietary protocols etc. making it impossible for the CATV network operators to use multiple vendors Cable Modems on the same system.

Around 1997 three standards emerged. DAVIC/DVB were first with a European standard, closely followed by MCSN with a US standard (DOCSIS). IEEE came last with 802.14, and clearly lost the 1st round. IEEE is now trying to leap-frog the two other standards by focusing on the next generation standards.

DVB/DAVIC

This standard is also known as DVB-RCC and as ETS 300 800. Initially run by DAVIC, but now the work has moved to DVB. Very few vendors develop for this standard, but enough that it does play a role. This standard is fighting the EuroDOCSIS standard for the European market (see below).

This standard is based on fixed cell size (ATM) and includes all the standard ways of doing quality of service (QoS) that ATM is known for. In that way, the standard is very well suited to both data i.e. TCP/IP (using AAL5) and telephony as pure ATM. VoIP to the cable modem may not be the best solution, although technically feasible.

Initially the standard lacked security (encryption), but that was added as an option in version 1.4.

Open to both internal and external implementations, and also covering Set-Top Box implementations with an additional out-of-band receive data channel.

MCNS/DOCSIS

The dominant US standard - even though it has not gone through any formal/independent standards body yet. This standard is very much driven by the wish of the large cable operators to have cable modems sold through the retail channel. Initially the chip manufacturer Broadcom played an important role, by pushing the standard and the level of chip integration at a very fast pace. As a result, the complexity of the standard is generally agreed to be much higher than what is strictly required, and is even growing.

Initially the standard did not support QoS which is required for telephony applications (VoIP) and other applications as well, but this has been added in version 1.1.

Initially open to only external box solutions with Ethernet interface, but now also allows internal modems and USB modems. Host based processing solutions is still debated (start 1999).

While originally targeted at the US domestic market, an off-spring named EuroDOCSIS is being pushed as the solution to the DVB centric European market. EuroDOCSIS is essentially the same as DOCSIS apart from the physical layer, which is DVB compliant in EuroDOCSIS.

IEEE

Lost the 1st round of the Cable Modem standards battle. What happens down the road remains to be seen. It looks like part of the IEEE group is working with Broadcom and Terayon on the next generation physical layer with increased (30 Mbps) upstream bitrate. This has also been termed DOCSIS 1.2, even though that does not seem to be official, and is certainly not approved by the DOCSIS vendor community yet.

The DVB-RCC vs. EuroDOCSIS fight

The battle for the US domestic market is clearly won by the DOCSIS standard, but the battle for the European market is still going on. It does not seem like at very fair match, but nevertheless quite interesting.