Ad-Hoc Wireless Networks: Challenges and Routing Protocols - Prof. Mr, Schemes and Mind Maps of Wireless Networking

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15-441: Computer Networking

Lecture 24: Ad-Hoc Wireless Networks

Scenarios and Roadmap

• Point to point wireless networks (last lecture)
  • Example: your laptop to CMU wireless
  • Challenges: Poor and variable link quality, hidden and exposed terminals
• Ad hoc networks (no infrastructure)
  • Example: military surveillance network
  • Extra challenges: Routing and possible mobility
• Sensor networks (ad hoc++)
  • Example: network to monitor temperatures in a volcano
  • Extra challenge: serious resource constraints
• Vehicular networks (ad hoc+++)
  • Example: vehicle-2-vehicle game network
  • Extra challenge: extreme mobility

Ad Hoc Networks

• All the challenges of wireless, plus:

• No fixed infrastructure
• Mobility (on short time scales)
• Chaotically decentralized
• Multi-hop!
• Nodes are both traffic sources/sinks and forwarders, no specialized routers
• The biggest challenge: routing

Ad Hoc Routing

• Find multi-hop paths through network

• Adapt to new routes and movement / environment changes
• Deal with interference and power issues
• Scale well with # of nodes
• Localize effects of link changes

Problems Using DV or LS

• DV loops are very expensive

• Wireless bandwidth << fiber bandwidth…

• LS protocols have high overhead

• N^2 links cause very high cost

• Periodic updates waste power

• Need fast, frequent convergence

Proposed Protocols

• Destination-Sequenced Distance Vector (DSDV)
  • Addresses DV loops
• Ad Hoc On-Demand Distance Vector (AODV)
  • Forwarders store route info
• Dynamic Source Routing (DSR)
  • Route stored in the packet header
• Let’s look at DSR

DSR Components

• Route discovery

• The mechanism by which a sending node obtains a route to destination

• Route maintenance

• The mechanism by which a sending node detects that the network topology has changed and its route to destination is no longer valid

DSR Route Discovery

• Route discovery - basic idea

• Source broadcasts route-request to Destination
• Each node forwards request by adding own address and re-broadcasting
• Requests propagate outward until:
  • Target is found, or
  • A node that has a route to Destination is found

C Broadcasts Route Request to F

A Source C G H Destination F E D B Route Request

H Responds to Route Request

A Source C G H Destination F E D B G,H,F

Forwarding Route Requests

• A request is forwarded if:

• Node doesn’t know the destination
• Node not already listed in recorded source route (loop avoidance)
• Node has not seen request with same sequence number (duplicate suppression)
• IP TTL field may be used to limit scope

• Destination copies route into a Route-reply

packet and sends it back to Source

Route Cache

• All source routes learned by a node are

kept in Route Cache

• Reduces cost of route discovery

• If intermediate node receives RR for

destination and has entry for destination in

route cache, it responds to RR and does not

propagate RR further

• Nodes overhearing RR/RP may insert

routes in cache

Discussion

• Source routing is good for on demand

routes instead of a priori distribution

• Route discovery protocol used to obtain

routes on demand

• Caching used to minimize use of discovery

• Periodic messages avoided

• But need to buffer packets

• How do you decide between links?

Forwarding Packets is Expensive

• Throughput of 802.11b =~ 11Mbits/s

• In reality, you can get about 5.

• What is throughput of a chain?

• A -> B -> C?

• A -> B -> C -> D?

• Assume minimum power for radios.

• Routing metric should take this into account