1. ODMRP Enhancement

2. Multicasting in ad hoc nets
ODMRP: On Demand Multicast Routing Protocol
Most widely accepted ad hoc multicast routing protocol
Simple yet high-performing
Why multicast in ad hoc nets?
Group (1-to-many) communication
Wireless “broadcast” medium
Spectral efficiency

3. ODMRP: Initialization Phase
Join Query
Join Reply
Forwarding Node
Link
Multicast Route
On-demand approach: A source initiates Join Query only when it has data to send
The sender periodically floods Join Query control messages
All intermediate nodes set up route to sender (backward pointer)
Members send Join Reply message
Routes from sources to receivers build a mesh of nodes called “forwarding group”.
Forwarding Group S1 R S R S2 R R

4. ODMRP: operation No explicit join/leave messages
Source broadcasts data packet to neighbors
The Forwarding Group neighbors forward the M-cast packet via “restricted” flooding on the forwarding mesh
Soft state
No explicit join/leave messages
Forwarding nodes clear state upon timeout
Extremely robust to mobility
Forwarding Group R S R S2 R R

5. ODMRP Performance - PDR

6. Simulation result – O/H

7. ODMRP Soft-state Pros/Cons
Advantage
Simple
Frequent refresh helps overcoming node displacements and channel fading
Keep shortest path
Disadvantage
Too much overhead when low mobility

8. Enhancing ODMRP Adaptive route refresh rate Semi soft state:
Explicit join + timeout
Local route recovery

9. Enhancing ODMRP Explicit join
A node that wants to join multicast group starts expanded ring search to find a member or forwarder
The node connects with any node in multicast group with 3 step hand-shaking
A node does not need to wait next route refresh from the source
We can change refresh interval

10. Enhancing ODMRP Cont. Expanding Ring Search
A node broadcasts packet with increasing TTL
If TTL is too large, it may generate control packet overhead so we need maximum TTL when ring search
Maximum TTL in ring search is 3~5 since the node is still near the ODMRP mesh when the route is broken with mobility.
Reach maximum TTL, the node broadcasts Join Query Request packet. When the source receives this packet, it sends Join Query for route refresh

11. Enhancing ODMRP Cont. Local Route Recovery
Each node stores the longest data packet arrival interval that the source node sends with Join Query packet.
If nodes do not receive any data during 2*packet arrival interval, they start route recovery
When nodes recognize route breakage, they start route recovery
Process is the same to explicit join

12. Enhancing ODMRP Cont. Route Refresh Interval
The source sends Join Query every 30 second (Odmrp is 3 second)
The source sends Join Query when it receives Join Query Request packet from any member node.

13. Simulation result - Packet Delivery Rate
1 sources, 20 members, and total 50 nodes with various mobility
ODMRP and Enhanced ODMRP similar performance

14. Simulation result - Control Packet Overhead
1 sources, 20 members, and total 50 nodes with various speed

15. Unidirectional Link: Characteristics and Problems
link that connects A to B, but not B to A
Sources of asymmetry
External interference
Transmission power control
Directional antennas
Unidirectional link problem in on-demand routing
Reverse Path technique does not work. A B

16. ODMRP-ASYM
FACT: A loop must exist if the network is connected (ie, there are two unidirectional paths between a pair of nodes).
Our strategy: Discover the loop and use it!
Path from S to D S A B
Loop!
Unidirectional links E C D
Path from D to S

17. ODMRP with unidirectional links
Source S floods Join Query toward D
Reverse path to relay Join Reply from D to S is blocked at B due to link asymmetry
Join Query S A B F E C D
Join Reply

18. ODMRP-ASYM: Loop Discovery
On detecting asymmetric link, node B initiates Loop Discovery Packet (LDP) flooding
LDP goes around the loop while collecting node IDs in the loop and returns to B
LDP
LDP S A B
CEA E C D

19. Simulation
QualNet
Two-power-level model: two different tx power levels are assigned to nodes randomly
50 nodes, 1 source, 1 group with 10 members
Traffic: CBR, 4 packets/sec, 512 bytes/packet
Varying density by changing field size: 1500 x 1500 m2 ~ 3500 x 3500 m2
Mobility model: Random waypoint

20. Simulation results: Packet Delivery Ratio
Figure 1: Packet Delivery Ratios (mobile nodes with max speed = 20m/s and pause time = 15s)
Figure 2: Packet Delivery Ratios (static nodes)