Signal Stability-based Adaptive Routing Protocol

SSA

SSA tries to discover stronger routes based on signal strength and location stability of the nodes. The location stability defines paths which have existed for a longer period of time. SSA is beacon-based like ABR which means, that the signal strength of the beacons is measured for determining the link stability between the nodes. With the help of the link stability and the location stability the links are classified as stable or unstable.
The signal strengths of the neighbor beacons are stored in a table named SST (signal stability table). Each node is constantly updating his SST with the beacon messages. The informations from the SST are used to forward the RouteRequest messages only over stable links to the destination. SSA uses next-hop routing, which means that each node keeps a routing table where the next hop to the target is stored.

When we compare SSA to ABR, one difference is that nodes only forward the RouteRequest messages if they receive them over stable links. In the other case the packets are dropped. Another difference is that SSA only minds on one metric which is the signal strength between the nodes.

The main objective of SSA is to only use routes that have stronger connectivity.

Route Establishment

Before the Route Establishment protocol is started the initiator checks its route cache for available routes to the target. Only when no route is known, the Route Establishment protocol is started. SSA is partly based on DSR.

1. First the network is flooded with the RouteRequest messages.
2. Now each node that receives the RouteRequest packet, checks if the packets were send to it over a stable link. All packets received over unstable (weak) links and all duplicate packets are dropped. He RouteRequest packet stores the address of each intermediate node of the route.
3. When the first packet arrives at the target over the stable links, the target immediately initiates a RouteReply packet to notify the selected route to the source. The RouteReply packet is returned over the samepath.

RouteRequest packets are only forwarded over stable links to the destination:
Path: 1-2-4-8-13-15

Route Maintenance

1. Immediately when a link breaks, both end nodes send a notification to the source and destination node.
2. The source node then floods the network again with RouteRequest messages to find another stable route to the destination.
3. Older routes are only deleted when the data packet with the route information fails to reach the next node.
4. If no way through stable links can be found, only then also weak links are used!

Broken link: 2-5
New strong path: 1-5-4-8-13-15

Advantages

The main advantage of SSA is that this protocol finds more stable routes to a destination then DSR. The shortest path isn’t necessary the best! With the beacons between the nodes, SSA classifies the link as stable or unstable to find the strongest path.

Disadvantages

Broken links are locally detected but not repaired and the multiple flooding of RouteRequest messages restricts the bandwidth. Another disadvantages is, that the RouteRequest packets received over weak links are not considered but dropped.

References

[Dube1996] Original Paper
[Murthy2004] All Pictures
[Dube1997]

"Signal Stability-based Adaptive Routing Protocol" is mentioned on: Ad-Hoc Protocols (History) | Ad-Hoc Workshop Winter 04/05 (Termine)