Nevertheless, it is instructive to quantify the average access delay D, defined as the
time elapsed between the time instant when the frame is put into service - i.e., it becomes
head-of-line (HOL) - and the instant of time the frame terminates a successful delivery.
Under the assumption of no retry limits, i.e., that all the HOL frames are ultimately
delivered, this computation is straightforward. In fact, we may rely on the well-known
Little??™s Result, which states that for any queueing system, the average number of customers
in the system is equal to the average experienced delay multiplied by the average customer
departure rate. The application of Little??™s result to our case yields:
10 The Ack_Timeout is specified in the Annex C (For a formal description of MAC operation, see details of the
Trsp timer setting on page 346) as:
Ack_Timeout = CTS_Timeout = aSIFS + Duration(Ack) + PLCPHeader + PLCPPreamble + aSlotTime.
According to this value, a station involved in a collision will be able to access the channel only a DIFS after the
Ack_Timeout, and thus (in the assumption of 1 Mbps control rate) only a slot-time after the end of an EIFS for
monitoring stations.
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