|
Ensuring end-to-end bounded delay and fair allocation of bandwidth to a backlogged session are no more the only criterias for declaring a queue service scheme good. With the evolution of packet-switched networks, more and more distributed and multimedia applications are being developed. These applications demand that service offered to them should be homogeneously distributed at all instants contrarily to back-to-back packet's serving in WFQ scheme. There are two reasons for this demand of homogeneous service: (1) In feedback based congestion control algorithms, sources constantly sample the network state using the feedback from the receiver. The source modifies its emission rate in accordance to the feedback message. A reliable feedback message is only possible if the packet service is homogeneous. (2) In multicast applications, where packet replication is performed at switches, replicated packets are probable to be served at different rates if service to them, at different output ports, is not homogeneous. This is not desirable for such applications as the phenomena of packet replication to different multicast branches, at a switch, has to be carried out at a homogeneous speed for the following two important reasons: (1) heterogeneous service rates of replicated multicast packets result in different feedback informations, from different destinations (of same multicast session), and thus lead to unstable and less efficient network control. (2) in a switch architecture, the buffer requirement can be reduced if replication and serving of multicast packets are done at a homogeneous rate. Thus, there is a need of a service discipline which not only serve the applications at no less than their guaranteed rates but also assures a homogeneous service to packets. The homogeneous service to an application may precisely be translated in terms of maintaining a good inter-packets spacing. EWFQ scheme is identical to WFQ scheme expect that a packet is stamped with delayed value of service start time of packet in corresponding GPS scheme, This delay is meant to consider the packet slots which might be occupied by a packet of precedently served session. Then EWFQ scheme serves the packets in the increasing order of their stamp values. It provides an end-to-end bounded delay service to applications. For multicast sessions, this scheme ensures a homogeneous service rate to all the replicated packet thus permits the replicator to work at a rather constant speed. Session's packets get distributed more accurately with low cost, moreover EWFQ scheme is highly probable to perform lesser number of operations than other schemes (e.g. WF2Q) while ensuring good inter-packets spacing.
|
