This ensures that each
CM??™s transmissions stay within the timeslots allocated to it by the CMTS.
The DOCSIS 1.1 specifications and beyond include MAC layer extensions to support
quality of service (QoS). For example, certain packet streams (service flows) can be
given low latency by establishing an upstream scheduling type for the flow. By default,
upstream service flows are of the ???best-effort??? scheduling type and so gain access to the
upstream channel by the mechanism described previously. The alternative upstream
scheduling types include a real-time polling service and an unsolicited grant service.
Real-time polling service achieves lower latency for constant packet-rate but variable
bit-rate streams (e.g., video telephony) by eliminating the contention request and backoff
mechanism. The CM is instead given periodic, dedicated (i.e., contention-free) request
opportunities in which to request timeslots to transmit its packets. Unsolicited
grant service, on the other hand, achieves low latency and jitter for constant bit-rate
streams (e.g., voice telephony) by eliminating the request mechanism entirely. The CM
is given periodic, fixed-size timeslots (grants) in which to transmit its packets.
Pages:
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396