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Chapter 7Performance optimisation for the support of TCIS p7.2.2 Continuous Reserved Request (CRR)The ‘Continuous Reserved Request (CRR)’ mechanism allows a station to request onemore slot than needed, regardless of the state of the transmission queue (see Figure 7.2).If the transmission queue of the NIU is empty, then the extra slot is not used. Thismechanism may reduce the mean access delays, because for every two MCI cycles (6ms), there is a reserved slot in which to place a request. A drawback of this mechanismis that unused extra slots cause a considerable waste of bandwidth during congestionperiods.RR: Reservation Request = 3 SlotsRR: Reservation Request = 5 SlotsCRR: Reservation Request = 3 SlotsCRR: Reservation Request = 6 SlotsFigure 7.2 – Functionality of the CRR mechanism for different packet sizes in thetransmission queue.7.2.3 Enhanced Pure Reservation Access (EPRA)A disadvantage of the pure reservation access, the RR and the CRR requestmechanisms, is that they are all based on a per-packet transmission basis rather than aper-block transmission. In other words, a station only requests the number of slotsneeded to transmit the current data message (up to 256 ATM cells). As the offered loadis increased, short messages tend to accumulate and then saturate the NIU buffer. Thus,long packet access delays render the system unstable. This effect is true even at lowerloads but does depend on of the traffic type.7-3