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The September 13, 2005 Issue Provided by System Dynamics Inc.
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Your Voice -- Readers Comments

We heard from readers commenting on cable bandwidth as well as seeking answers on mixed mode Wi-Fi, HomePlug problems and Dynamic DNS for VPNs.

Cable Bandwidth

We heard from Dave Large and Ron Shani, whose recent NCTA paper we credited in last month's article.

Dave wrote "I think you have inadvertently overstated the bandwidth gain possible with 1024QAM. Most operators are already using 256 QAM, which transmits 8 "bits" of information in each symbol period. Upgrading to 1024 QAM increases that only to 10 bits per symbol, a maximum potential increase of 25% ... as a practical matter, the increase is not that large, as more aggressive error correction would probably be used to partially compensate for the increased sensitivity to noise.

Similarly, channel bonding, while it provides higher peak speed capability for data services doesn't do anything to solve the bandwidth crunch or increase system throughput. All channel bonding does is to effectively combine the capabilities of several data channels so that selected individual customers can have speeds greater than possible through a single channel."

And Ron wrote "I believe that DOCSIS 3.0 will be a very significant addition to the MSOs toolbox when they come to compete with FTTx. Iím not sure that having better spectral efficiency is what the industry currently needs (e.g. 1024 QAM), but am more sure that having virtual wider channels (the channel bonding) will provide higher data rates and better statistical usage, leading to more competitive service offerings."

Does b Degrade g?

Joe asked "I've heard that having a 802.11b client on a network will cap the throughput of 802.11g clients. So, if I have a Tivo hooked up using a 'b' adaptor, all my other systems will have a max throughput of the Tivo?"

We replied "That's a very good question. I'm afraid the best answer is 'it depends on exactly what 802.11g equipment you have'. All 11g equipment is degraded somewhat by 11b, and 11b is degraded by 11g.

It's not that the 11g equipment will have 'the max throughput of the TiVo', but the 11g performance is reduced in mixed mode. How much depends on which manufacturers chips are used in all the devices and which version of firmware and client software is running.

It's getting even worse now with many 'super-G' and MIMO products coming to market. They all seem to have mixed-mode problems with each other."

Problems with HomePlug

Gregg Zupcsics wrote "I recently installed a whole house standby electric generator. When we did that, half of our Homeplug/Powerline network stopped working. I suspect the problem is that half of the network is now isolated from the signals on the other half of the network where the WAN connection resides. Do you know whether an X10 signal bridge would also serve to bridge Homeplug signals between two parts of an electrical network?"

We pointed him to test tools from Corinex. He later reported "A 'phase coupler' (aka 'phase bridge') ... intended for use with X10 networks works fine, and gives just as good throughput as before."

Getting Dynamic DNS Working for VPN

Chris had a question about our website discussion of VPN between our Florida condo and our house: "I successfully setup a VPN between two FVS318 v2.4 firmware routers. Personally I ran into the same problem you did with dynamic DNS. The router would not update the Dynamic DNS when the IP changed and the router was not retrieving the proper IP address from the dyndns addresses. I personally think it is a bug in the firmware that is yet to be resolved. If you find a way to get the dynamic DNS working please let me know.

We replied "I did get dynamic DNS working on the FVS318 -- after a lot of digging and experimenting. The correct approach is described on the Netgear support forum. The posting says Netgear routers don't support Custom DNS, but only regular subdomains. The trick is to register for a free subdomain at Dyndns, then make a "cname record" under "Advanced" to point to it: (eg --> This does seem to work fine."