Quote:
Originally Posted by Sephiroth
Then to address qasi's comment; TDMA is not confined to DOCSIS. So I'd still like to know what it is about DOCSIS that causes jitter.
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I don't think my point was addressed at you - but in any case, it's the CSMA nature of DOCSIS TDMA that makes it jitter. A modem wanting to transmit upstream data must request a timeslot. To do so it first checks if any other modem is transmitting on the channel, and if so, it aborts the request and then waits
a random amount of time before trying again. Modems also don't get allocated fixed timeslots but timeslots are allocated on-demand and in a seemingly random manner, so in a loaded network the time from one slot to the next can be vastly different.
xDSL and 3G both don't use TDMA let alone CSMA hence jitter is lower in both cases. In both these cases a modem wanting to transmit simply transmits immediately, and can do so at any time. It doesn't have to wait, request a timeslot, then wait again until that timeslot is granted. GSM (2G) telephony also sees next to zero jitter as each call is given a fixed, dedicated timeslot. Only the initial call setup uses CSMA, whereas on VM cable (I say that as it's not an absolute requirement of DOCSIS - only that VM haven't implemented the alternative) CSMA is used for just about single upload request.
GPON (i.e. BT Infinity FTTP) I don't know enough about but I believe it operates on a similar timeslotting arrangement to cable, hence why I've said in the past it brings several of the same disadvantages as cable. I've not seen enough actual numbers from PON tech to make any kind of judgement on it though.
That's jitter. Latency on the other hand is a different matter. Excessive latency is simply a result of excessively long buffers, which aren't required as part of DOCSIS. VM simply chooses to stick excessively long buffers in place. It helps the performance of only those using excessively high amounts of bandwidth and degrades the performance of everyone else. It's completely unneccessary.