Quote:
Originally Posted by Chrysalis
Well previously I assumed when there was jitter this was due to bandwidth saturation on the ubr port.
But cable modem technology means they need a timeslot allocated to them and this can vary in situations of multiple users requesting at once, so I guess would explain a situation where one can upload at full speed (available bandwidth on UBR port) but still see's jitter (not available timeslot)?
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No - the timeslots provided are when the modems actually transmit data so no timeslot allocation no transmission. The only transmissions that are done outside of provided timeslots are when modems come online and when they request timeslots to transmit.
A CMTS periodically (every 1-10ms) transmits (downstream) an upstream MAP for each upstream that services that downstream. This MAP indicates how the mini-slots are allocated for that period until the next MAP.
There are 3 mini-slot types - contention slots, where modems request data slots, these are unallocated unless DOCSIS 1.1 or higher QoS is being used so are
CSMA. The CMTS tells the modems when the contention slots are coming up and they fight amongst themselves for them. This is what I mentioned as being that other source of congestion however with a typical upstream MAP frequency being between 1 and 4ms it's not a huge source of jitter.
Next is data slots, these are allocated to individual modems, so the MAP for these is a schedule of which modem gets to transmit, and how many mini-slots they get to transmit for.
Last are maintenance slots, there are two types of these. One is initial maintenance which are CSMA, for modems that are just coming online so the CMTS can't allocate them slots, it's not heard from them before, and there are station maintenance slots which are reserved for modems to respond to. You may notice that the T4 disconnect message refers to not seeing a unicast maintenance opportunity and this is what it refers to, that modem doesn't get a station maintenance slot in its' MAP for 35 seconds so considers itself disconnected.
A MAP is broadcast on the downstream for every upstream, so on the overlay network even though you can only acquire 1 upstream on your node you will see either 4 or 8 MAPs with different channel IDs depending on how many nodes your bonded downstream group covers and how many upstream ports are active on the card.
Congestion on the cable network is when all the data slots, and therefore usable upstream bandwidth, is being consumed. The bandwidth will never be 100% consumed as there will never be a case where every single burst fits exactly into every mini-slot however if all the data slots are consumed there is no usable bandwidth for modems to use to transmit their data.