The basic mechanism by which
this is achieved is via a reservation-based, time-division multiple access control system.
When an Ethernet frame arrives at the 10/100/1000Base-T or USB port on a CM, the
CM determines the size of the packet and then sends a request message to the CMTS
to reserve an appropriately sized timeslot in which to send the packet. The request message
itself is sent in contention using a slotted Aloha protocol with binary exponential
backoff in the event of a collision. The CMTS prioritizes the request and then grants
Hybrid Fiber-Coax 153
the CM the requested timeslot. The CMTS communicates the scheduling of upstream
transmissions by periodically broadcasting a map message, which identifies contention
request timeslots as well as timeslots that are granted to a particular CM.
Since the cabling distance, and hence propagation delay, between the CMTS and
each individual CM may vary widely, all CMs are synchronized to a common time base
using a periodic ranging mechanism. When triggered by an upstream ranging timeslot
granted to it by the CMTS, the CM sends a ranging request message. The CMTS then
calculates and sends a ranging response message that includes a timing adjustment as
well as transmit power and frequency adjustments, if necessary.
Pages:
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395