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Chapter 8Final conclusions pthe increase in system performance, which can be of up to ≈12% of the maximumchannel capacity.There are however modifications proposed herein, namely modifications at the INAscheduler and the NIU for the support of enhanced reservation request mechanisms. Thepiggyback mechanism and the continuous piggyback have already being integrated inthe third version of the DVB/DAVIC protocol [35] due to their remarkably performanceover the default reservation mechanism (defined in the second version). We stronglyrecommend to cable network operators to buy EuroModems supporting these piggybackmechanisms, here it was proved that the increase in system throughput could be up to25% of the maximum channel capacity for upstream channels supporting Internet andvoice traffic (see Section 7.2.7, Figure 7.8).The unsolicited grant slots (UGS) mechanism, however can achieve a better systemperformance than the piggyback mechanisms in terms of access delays for the supportof CBR traffic, here access delays below 7 ms can be obtained on high traffic loads. Theimplementation of this mechanism requires slight modifications to the INA schedulerand NIU MAC protocol. Here the ‘IA_Capabilities’ field of the Default ConfigurationMessage and the ‘IU_Capabilities’ field of the Sign-on Response Message, contain areserved field (of 5 bits for the) for future implementations, which can be used for thesupport of the UGS mechanism. This functionality requires only one bit, leaving theother four bits for future implementation. This reserved field is of one bit in the lastversion of the DVB/DAVIC standard [35], which can also be used for this functionality.With the UGS mechanism, as stated in Section 7.2.6, the number of slots needed by astation and the periodic intervals can be negotiated during the connection set-up thoughthe use of the ‘Resource Request Message’. This structure is the same as the one usedfor the establishment of fixed-rate connections, with the exception that instead ofassigning slots to a NIU in the fixed-rate region of the signalling frame, these slots areallocated in the reserved access region of the signalling frame, using the ‘ReservationGrant Message’.8-17