DOWNLOAD MY Ph.D Thesis - UNAM

Chapter 5Upstream channel capacity and characterisation pIn this section and also in Section 5.4.1, we have approached the highest systemperformance when transmitting maximum-size Ethernet packets in terms of offered loadand maximum number of active stations supported. In typical networks, the packet sizesvary according to application services. Hence, in the following scenarios we addresstraffic patterns consisting mainly of Internet traffic, Voice over IP traffic, and mixedtraffic configurations.5.4.3 Effects of contention access for data transmissionAs introduced in Section 3.5, the DVB/DAVIC protocol specification supports threedifferent access modes for the transmission of data messages: contention, reservationand fixed-rate access. This section evaluates the system performance when stations areallowed to transmit small packets in the contention-based access region. The analysispresented here examines to what extent the system performance can be maximised byranging the ‘Maximum Contention Access Message Length’ of a message, measured inATM cells, that may be transmitted using contention access. Any message greater thanthis parameter is transmitted using reservation access.The analysis addresses a worst case scenario of a novel and topical mixed trafficscenario (defined below), in which most of the packets are either minimum lengthEthernet packets (in case of Internet traffic) or 40 bytes for VoIP streams. This trafficscenario was considered because small packet sizes benefit the response time ofinteractive applications. On the other hand, small packets put higher stress on thenetwork. This is because more requests per volume of data have to be issued, whichincreases the protocol overhead and the probability of collisions.The traffic load was created as follows: each active station generated 32 kbps Internettraffic as defined in Section 5.3.1. Additionally, 12.4 kbps Voice over IP traffic asintroduced in Section 5.3.2, was multiplexed into the data stream. Thus, the mean datarate per active station was set to 44.4 kbps. The simulations were performed using theexponential backoff algorithm. Figures 5.5 to 5.8 present the results when the‘Maximum Contention Access Message Length’ was ranged from 0 to 6 ATM cells.5-14