18-01-2013, 12:40 PM
Ad-hoc Networks
1Ad-hoc Networks.pdf (Size: 27.5 KB / Downloads: 29)
Why Ad-hoc?
Ad-hoc networking is often justfied by scenarios where you do not
want or where you cannot deploy & manage an infrastructure.
Spontaneous meetings (at work, airport):
exchange files, play games
Special circumstances:
disaster relief
Or simply cabeling costs, management overhead:
old building, “wearable LAN”, wireless headset
Ad-hoc Network = multi-hop Network
No default router available
Potentially every node becomes a router:
must be able to forward traffic on behalf of others
Higher fan-out, multiple routes:
a wireless network has potentially more route alternatives,
how to find out the “best” path in a communication mesh?
Pro-active Routing
Current Internet routing protocols are pro-active.
Also called table-driven routing protocols
Imagine that each node knows the full topology
! connectivity matrix
Simplification: each node maintains a vector
– indexed by potential destination names
– entry contains forwarding information for this dest.
RIP (routing information protocol), distance-vector
OSPF (open shortest path first), link-state
Problems of Distance Vector (in Ad-hoc)
Topology changes are slowly propagated
Count-to-infinity problem (! poisened reverse)
Moving nodes create confusion:
they carry connectivity data which, at the new place, are wrong!
! fatal routing loops
Table exchange eats bandwidth.
Scaling Problems
Common assumption of current ad-hoc research:
small networks with less than 100 nodes.
How to scale this to 1000 and more nodes? ! routing hierarchy
Create logical cells (partitioning into clusters)
– use one of AODV, DSDV etc inside a cell
– designate a “cluster-head” or gateway
Two-level routing:
– direct routing inside a cluster
– inter-cluster routing via cluster-heads
– add more levels if needed
Associating IP-Subnet with Ad-hoc Network
How to include ad-hoc networks into IP?
Install a gateway with wireless interface,
– gateway participates in ad-hoc routing
– can forward data to the Internet
Assign an IP network number to this ad-hoc network.
Each ad-hoc node has its IP host number inside this subnet.