Wireless Ad Hoc and Sensor Networks

8 Wireless Ad Hoc and Sensor NetworksThe packets are initially sent to the first-hop node to which the sourceend system attaches. As each packet arrives at this node, it stores the packetbriefly in the input buffer of the corresponding node, determines the nexthop of the route by searching the routing table (created through a routingprotocol) with the routing control information in the packet header, andthen moves the packet to the appropriate output buffer associated withthat outgoing link. When the link is available, each packet is transmittedto the next node en route as rapidly as possible; this is, in effect, statisticaltime-division multiplexing. All of the packets eventually work their waythrough the network and are delivered to the intended destination.Routing is essential to the operations of a packet switching network.Some sort of adaptive or dynamic routing technique is usually necessary.The routing decisions that are made change as conditions on the networkchange. For example, when a node or trunk fails, it can no longer be usedas part of a route; when a portion of the network is heavily congested, itis desirable to route packets around the area of congestion rather thanthrough it.To maximize network resource (e.g., bandwidth and buffer) utilizationwhile satisfying the individual user’s QoS requirements, special QoScontrol mechanisms should be provided to prioritize access to resourcesat network nodes. For example, real-time queuing systems are the core ofany implementation of QoS-controlled network services. The provision of asingle class of QoS-controlled service requires the coordinated use ofadmission control, traffic access control, packet scheduling, and buffermanagement. Other techniques include flow and congestion control andQoS routing, as briefly explained in the following subsections. Each ofthem will be further explained with detailed references later in the booksome for computer networks and other wireless for wireless networks.1.2.1 Admission ControlAdmission control limits the traffic on the queuing system by determining ifan incoming request for a new user can be met without disrupting the serviceguarantees to established data flows. Basically, when a new request is received,the admission control (AC) or call admission control (CAC) in the case of anATM network is executed to decide whether to accept or reject the request.The user provides a source traffic descriptor — the set of traffic parametersof the request or ATM source (e.g., PCR, SCR, MBS, and MCR), QoS requirements(such as delay, delay variation, and cell loss rate), and conformancedefinition (e.g., generic cell rate algorithm (GCRA) or dynamic GCRA(DGCRA) for ABR). The network then tries to see whether there are sufficientnetwork resources (buffer and bandwidth) to meet the QoS requirement.Given that most real-time queuing systems cannot provide QoScontrolledservices at arbitrarily high loads, admission control determines