[ Pobierz całość w formacie PDF ]
.Thus, it is important to achieve a suitable trade-off betweenthe goals of maintaining an efficient route and limiting disruption to continuous mediatraffic while maintaining low handoff latency at the same time.In order to not over-load the switch, this must be done while keeping the switch updates due to connectionrerouting, low.Connection rerouting procedures for ATM-based wireless networks include handoffschemes, which are Switched Virtual Circuit (SVC) based and PVC-based schemes.Connection rerouting involves the location of the COS.A COS is defined to be thefarthest switch from the fixed endpoint that is also the point of divergence between thenew and old routes connecting the mobile and fixed endpoint.Four general approaches toward connection rerouting are proposed:" Optimistic handoff approach" Ordered handoff approach" Predictive handoff approach" Chaining handoff approach.The goal of an optimistic handoff scheme is to perform simple and fast handoffswith the optimistic view that disruption to user traffic will be minimal.The COS simplyreroutes data traffic through a different path to the new BS with the connection from thesource to the COS remaining unmodified.The goal of an ordered approach is to provide ordered lossless data delivery duringhandoffs.The incremental and multicast-based rerouting schemes fall into this category.188 HANDOFF IN MOBILE AND WIRELESS NETWORKSHowever, complex protocols with resynchronization mechanisms and buffering at the BSare necessary to ensure lossless connection rerouting.In predictive approaches to connection rerouting, the key idea is to predict the next BSof the mobile endpoint and perform advance multicasting of data to the BS.This approachrequires the maintenance of multiple connection paths to many or all the neighbors of thecurrent BS of the mobile endpoint.10.4 OPTIMIZED COS DISCOVERY THROUGHCONNECTION GROUPINGAmong a large number of MMUP connections, groups of connections going to the sameexternal host can occur naturally.Within each such group, all connections will probablyshare the same path between the MMUP and their common external host owing to thelimited number of border switches within the MMUP network.A single common COScan help reroute all connections within such a group at the time of a handoff.Dependingon the size of such groups, a single COS discovery per group (instead of per connection)can cut down the total time required for handoffs significantly.Since connection hand-offs are performed within the confines of the cellular network, connections to differentexternal hosts that share a common subpath within the cellular network can be groupedtogether as well.Since hop-by-hop path information, which is necessary to perform such grouping ofconnections, is not accumulated during connection setup, switches within the cellularnetwork are updated to run a modified Private Network to Network Interface (PNNI)protocol that accumulates and forwards hop-by-hop path information to the MMUP duringconnection setup.For connections originating at a host external to the MMUP, a pathlist is created at the first gateway switch (in the cellular network) encountered duringconnection setup and is eventually passed onto a border switch within the MMUP.Eachborder switch in the MMUP network maintains a group database wherein connectionssharing a common subpath are placed together.Each intermediate switch that receives apath list simply appends its identifier and forwards it to the next hop on the path.Forconnections originating within the MMUP, the path list is created at the access pointon the path and forwarded up to a gateway switch, which returns the list to the MMUPalong the partial path established using a special signaling message.Intermediate nodesare required to simply forward any such incoming messages onto the next hop alongthe path.When presented with subpath information at the time of a connection setup,a border switch within the MMUP groups connections according to the commonality oftheir subpaths within the cellular network.At the time of a handoff, the border switchpasses on the group information to the access point, which is responsible for initiatingCOS discovery.A list of VCs that belong to a common group accompanies the group-COS discovery request so that path state can be setup and resources procured individuallyfor each individual connection.HANDOFF IN LOW EARTH ORBIT (LEO) SATELLITE NETWORKS 18910.5 SCHEDULE-ASSISTED HANDOFFSPreplanned travel schedules can be used to improve smoothness of handoffs in high-speedMMUP application scenarios.A schedule provides the MMUP with information aboutthe upcoming cell in advance of its intercell moves.Consequently, an MMUP can triggerCOS discoveries for existing connections a short time before it establishes contact with thenext AP.This time period should be determined individually for each application scenarioon the basis of the specific system characteristics and trial observations and should beset to the necessary minimum.Advance COS discoveries are not needed if cell overlapregions are large enough.When the MMUP is close enough to the next AP for the mechanism to be triggered,it initiates a COS discovery for some or all existing connections (or groups) through thecurrent AP.The COS-discovery process results in establishment of new subpaths from theCOS to the next AP.These connections are maintained if a call proceeding sends signalingto the next AP until the MMUP establishes contact with it (or the timer expires), uponwhich all pending connection requests are forwarded onto the MMUP.After the MMUPconfirms successful reestablishment of the connections, the COS begins switching dataalong the new paths and initiates tear down of the old subpaths.Since connection resources are held along the new subpaths until the MMUP makes themove to the next cell, it is important to keep the advance trigger threshold to the necessaryminimum.Nevertheless, some connections that are handed off early may terminate beforethe actual move is made.They are rejected by the MMUP at the next AP, and thecorresponding new subpaths are torn down.10.6 HANDOFF IN LOW EARTH ORBIT (LEO)SATELLITE NETWORKSA handover rerouting algorithm, referred to as Footprint Handover Rerouting Protocol(FHRP), has been proposed to handle the intersatellite handover problem.The protocoladdresses the trade-off between the simplicity of the partial connection rerouting and theoptimality of the complete rerouting.The FHRP is a hybrid algorithm that consists ofthe augmentation and the Footprint Rerouting (FR) phases.In the augmentation phase,a direct link from the new end satellite to the existing connection routes is found.Thisway the route can be updated with minimum signaling delay and at a low signaling cost.In case there is no such link with the required capacity, a new route is found, usingthe optimum routing algorithm.In the FR phase, connection route is migrated to a routethat has the same optimality feature with the original route.The goal of the rerouting isto establish an optimum route without applying the optimum routing algorithm after anumber of handovers.This property is significant because, in the ideal case, the routingalgorithm computes a single route for each connection.The optimality of the originalroute is maintained after the FR phase [ Pobierz całość w formacie PDF ]