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• The request/data IE: Provides an interval of mini-slots, in which either requests for bandwidth or short data PDUs can be transmitted on a contention basis. Since transmissions might collide, the CMTS must acknowledge any data PDUs that it received correctly. (Note that this IE is different from the request IE.)
• The initial maintenance IE: The IE provides an interval of mini-slots, during which new CMs can join the network. The transmission during these mini-slots is contention-based.
• The station maintenance IE: Defines an interval of mini-slots, during which CMs are expected to perform some aspect of routine maintenance, such as ranging or power adjustment. The transmission during these mini-slots is contention-based. Alternatively, the CMTS can specify a particular CM to perform a network maintenance task.
• The short and long data grants IEs: Define an interval of mini-slots, during which CMs can transmit short data PDUs and long data PDUs, respectively. These IEs are issued either in response to bandwidth requests, or to an administrative policy to provide bandwidth to some CMs. A data grant cannot be larger than 255 mini-slots.
• Data acknowledge IE: Used by the CMTS to acknowledge that a data PDU has been received. This IE is used only in conjunction with contention transmission. It was not designed to provide a reliable transport between CMs and CMTS.
A CM must request a number of mini-slots in order to transmit an entire frame. This frame might be: a single MAC frame; a MAC frame that has been formed by concatenating multiple MAC frames; or a MAC frame containing a fragment of a long data PDU. The request must be large enough in order to accommodate the MAC frame and the necessary physical layer overheads. A CM cannot request more than the required number of minislots necessary to transmit one MAC frame, and it can only have one request outstanding at a time.
In Figure 11.17, we give an example of the upstream transmission scheme between the CMTS and a CM. The following events take place:
Slots mapped by first MAP
Figure 11.17 An example of the upstream transmission scheme.
THE CABLE-BASED ACCESS NETWORK
• At time tb the CMTS transmits a MAP message to a CM that starts at time t3. That is, the CMTS expects to receive the first mini-slot of this mapped group at time t3. The difference between ti and t3 is needed to allow for all of the delays involved from the time that the CMTS transmits the MAP to the time that a CM can respond. These delays include: the worst-case round-trip propagation delay, queueing delays within the CMTS, processing delays within the CM, and other delays introduced by the PMD layer. Within this MAP, there is a request IE which will start at t5.
• At t2, the CM receives the MAP message and scans it for request opportunities. In order to minimize request collisions, it calculates t6 as a random offset.
• At time t4, the CM transmits a request for as many mini-slots as are needed to transmit the data PDU. Time t4 is chosen based on the ranging offset so that the request will arrive at the CMTS at t6. The ranging offset is used to place all of the CMs at the same virtual distance from the CMTS. This is necessitated by the fact that the CMs are at different distances from the CMTS. Because of this, a MAC frame transmitted by a CM might not arrive at the mini-slot at which it is expected. A ranging protocol is used to calculate a ranging offset by which a CM delays its transmissions. The other purpose of the ranging process is to make all CMs transmit at a power level that makes all of their transmissions arrive at the CMTS at the same level of power.
• At t6, the CMTS receives the request and schedules it for service in the next MAP message.
• At time t7, the CMTS transmits a new MAP message, whose effective starting time is t9. Within this MAP, there is a data grant for the CM that starts at t11.
• At t8 the CM receives the MAP message and scans it for its data grant.
• At t10, the CM transmits its data PDU so that it will arrive at the CMTS at t11. Time t10 is calculated from the ranging offset.
As seen above, for a given interval, the CMs can use contention transmission. In this case, a CM selects a random number within its back-off window, which is determined by the CMTS. This random number indicates the number of transmit opportunities that the CM must skip before it starts its transmission. (For simplicity, assume that each mini-slot is a transmit opportunity, although this is not generally the case.) That is, if the CM has a back-off window from 0 to i0, and it randomly selects 8, then the CM must skip eight slots before it starts transmitting. For instance, let us assume that it has a bandwidth request to transmit to the CMTS, and let us assume that the request IE in the first MAP message defines an interval of six slots and in the second MAP message it defines an interval of four. Then, the CM will skip the mini-slots in the first interval, and it will transmit in the third mini-slot of the next interval.