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Technology Stocks : COM21 (CMTO) -- Ignore unavailable to you. Want to Upgrade?

To: pat mudge who wrote (558)	6/14/1999 2:34:00 PM
From: Mark Laubach	Read Replies (2) \| Respond to of 2347

Hi Pat, I just wanted to point out one technical item from your nice comparison of the two companies. DOCSIS 1.2 is an extension of DOCSIS 1.1; i.e. it is DOCSIS 1.1 with the new "hi performance" upstream physical layer. CableLabs is requiring that all iterations of DOCSIS 1.x are fully backwards compatible to DOCSIS 1.0. That means regardless of where you are, if you purchase a DOCSIS certified cable modem, it will always be able to operate with a DOCSIS 1.0 only CMTS. This has a couple effects: 1) in order to be certified for DOCSIS 1.1, a cable modem must first be certified for DOCSIS 1.0. Similarly, a DOCSIS 1.2 cable modem, must be certified at DOCSIS 1.0 and at DOCSIS 1.1, 2) if a cable operator wants to enable the features of 1.x+1 for operation in a particular plant, they may be force to allocate "new" upstream channels for the 1.x+1 modems. For example, DOCSIS 1.1 adds packet fragmentation services in the upstream channel. This was done to allow the CMTS to better allocate upstream resources to control delay and jitter for packet voice services. DOCSIS 1.0 only transmits whole Ethernet packets (64 bytes to 1500 bytes). In DOCSIS 1.1, the CMTS has the capability to command a cable modem to send less than a whole packet, e.g. less than 1500 bytes. This is especially useful if one or more cable modems were holding time sensitive packets, while other cable modems were holding maximum length packets. In DOCSIS 1.0, there is no fine grain control over jitter. In DOCSIS 1.1 there is. The kicker is that in order to have all cable modems use fragmentation on an upstream channel, the channel must only have DOCSIS 1.1 modems allocated to it, requiring that DOCSIS 1.0 modems be allocated to one or more other upstream channels. It is technically possible to operate both DOCSIS 1.1 and DOCSIS 1.0 modems in the same upstream channel, even with the DOCSIS 1.1 modems using fragmentation. If the delay/jitter requirements of the traffic mix require more control than a mixed channel provides, then the cable operator must allocate additional upstream channels and allocate the DOCSIS 1.0 modems out of the way of the DOCSIS 1.1 modems requiring the tighter bandwidth allocation management. DOCSIS 1.2 gets a little more interesting. The TDMA upgrades in DOCSIS 1.2 are a proper superset of the DOCSIS 1.0/1.1 upstream PHY. Any DOCSIS 1.2 compliant cable modem therefore has everything it needs to operate with DOCSIS 1.0/1.1 upstream PHY modes. If the cable operator wishes to use features that are in DOCSIS 1.2, such as a higher TDMA modulation order or a higher symbol rate, then one or more additional channels must be provisioned in the upstream for this operation. The S-CDMA mode of DOCSIS 1.2 requires that an additional 6-MHz upstream channel be allocated. The last I looked at the draft specification, TDMA and S-CDMA will not be mixed in the same upstream channel. If a cable plant has a mixture of "legacy" [<g>] DOCSIS 1.0 or DOCSIS 1.1 modems with DOCSIS 1.2 modems, additional channels will be required for operation of DOCSIS 1.2 features. If a cable plant only has DOCSIS 1.2 modems provisioned, that is all subscribers have DOCSIS 1.2 modems, than these restrictions don't apply, and the upstream channels can all be run with DOCSIS 1.2 extensions (e.g. enhanced TDMA or S-CDMA). If any subscriber has a DOCSIS 1.0/1.1 cable modem, than either the cable operator must support one or more DOCSIS 1.0/1.1 channels or they must find a way to upgrade those modems (replacement, incentives or whatever) to DOCSIS 1.2. Last November, the IEEE 802.14 working group voted the enhanced TDMA as mandatory and S-CDMA as optional in the cable modem. I'm not sure whether the "optional" will survive when the draft gets approved or in the manner that DOCSIS implements it. I wasn't at the last two 802.14 HiPHY meetings and I've lost track of exactly what's happened on this item. Hope this helps clarify some items about DOCSIS 1.2 and 802.14. Mark Laubach