Norman Contreras posted an update 3 days, 16 hours ago
3. Dissemination protocols: outlineIn the following, we identify for each aforementioned class a representative protocol; these protocols are used in the next section to investigate the main advantages and disadvantages of the different dissemination approaches, under high data throughput conditions.3.1. Timer based packet by packet forwardingUnder such a technique, upon receipt of a packet, a vehicle determines whether it is qualified to forward the packet across the VANET by using relevant state conditions such as: Inhibition Rule (IR) and packet sequence number. If qualified, a vehicle determines whether it should elect itself as the forwarder for this packet, or not, by initiating a timer mechanism. The Targeted Timeout Level (TTL) for the expiration of the timer depends on a calculation of a forwarding qualification functionwhich can depend on a state vector sv that is associated with the state of this vehicle, of the packet, of the system and of the local environment.In this class we consider two representative protocols:? the Distance Based Forwarding (DBF) algorithm;? the Timer-based Backbone Network dissemination protocol (TBN).Both protocols are representative of beacon-less logic. They are implemented in a distributed manner and are quite simple to implement. The first is the well known DDT , where a hop count field is added in each packet. We consider this protocol since its main behavior is at the basis of several other protocols proposed in the literature, also considered in the GeoNetworking ETSI standard. The second is an enhancement of the DBF, where, instead of having the relay selection at the furthest distance, this is achieved at some suggested positions in the road span. TBN takes into account also some Physical and MAC layer issues to enhance the data dissemination .Under a Distance Based Forwarding (DBF) algorithm, TTL is set to a value that is a decreasing function of the distance between the link\’s source purchase SQ 22536 and this node. While, under a TBN algorithm, TTL is proportional to the distance between the vehicle and the closest nominal relay position; these nominal RN locations are separated spatially by a distance range (indicated as D in the following) that is chosen so as to cause each receiving relay node to record an SINR level that is sufficiently high for supporting the intended packet transmission data rate.To describe the basic principles of a timer-based dissemination logic, we introduce the following notations:designates a packet of a data flow transmitted in the VANET, identified by sequence number k;is the set of vehicles that receive a packet transmitted by node X.The basic logic in case of a DBF protocol is outlined in the following, referring to the example depicted in Fig. 1. RSU initially sends packetin the VANET. Vehicles in the radio transmission range of RSU (say, A and B in the left side) receive the packet and compute their distance from RSU. Each one of these nodes sets a timer whose duration (TTL) is a decreasing function of the distance from RSU. Once the timer elapses, the packet is passed to the MAC layer and scheduled for transmission. While the timer for is running in a node that has received this packet, the reception of another packet with the same sequence number k triggers the Inhibition Rule (); i.e., prompting node to dropand to cancel the timer.Considered scenarios: single RSU and single data flow Download high-res image (49KB)Download full-size imageFig. 1. Considered scenarios: single RSU and single data flow.Under an ideal dissemination protocol, only a single vehicle residing in the neighborhoodof sender S will eventually forward the packet. In doing so, it will inhibit all other nodes that are members of from forwarding this packet. Fig. 1 illustrates the desired behavior of the forwarding scheme. In this way,hops from one forwarding vehicle to another. It is thus advantageous to design a scheme under which the set of relay vehicles is selected with a twofold goal: i) inhibit some vehicles from re-broadcasting packets, aiming to reduce channel overloading, and hence reduce access contentions and packet collisions; ii) prefer to take long hops along the road, serving to speed up the dissemination process.