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Lecture 5
Mobile Ad-Hoc Networks:
Routing
Reading: •^ “Routing Protocols for Ad Hoc Wireless Networks,” in
Ad Hoc Wireless
Networks: Architectures and Protocols, Chapter 7. • D. Johnson, “Routing in Ad Hoc Networks of Mobile Hosts
,” Proceedings of the
IEEE Workshop on Mobile Computing Systems and Applications, Dec. 1994. • E. Royer and C.-K. Toh, "A Review of Current Routing Protocols for Ad-Hoc Mobile Wireless Networks,"
IEEE Personal Communications Magazine, April
1999, pp. 46-55. • J. Broch, D. Maltz, D. Johnson, Y.-C. Hu, and J. Jetcheva, "A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols,"
Proc.
Mobicom '98, Oct. 1998. • S. Singh, M. Woo, and C. Raghavendra, "Power-Aware Routing in Mobile Ad Hoc Networks,"
Proc. Mobicom '98 , Oct. 1998.
Mobile Ad-Hoc Networks(MANETs)
^ Definition^ ^
“A collection of wireless mobile hosts forming a temporary networkwithout the aid of any centralized administration or standard supportservices.” ^ Ad-hoc network topology is dynamic—nodes enter and leave thenetwork continuously ^ No centralized control or fixed infrastructure to support networkconfiguration or reconfiguration ^ Example scenarios for MANETs^ ^
Meetings Emergency or disaster relief situations Military communications Wearable computers Sensor networks
MANETs (cont.)
^ Route-finding:^ ^
Want to determine an “optimal” way to find “optimal” routes
^ Dynamic links^ ^
Broken links must be updated when a node moves out ofcommunication range with another node New links must be formed when a node moves intocommunication range with another node Based on this new information, routes must be modified
^ Frequency of route changes a function of node mobility
Conventional Routin
g
Protocols for Wired Networks
^ Distance vector routing^ ^
Each router has a table giving the distance from itself to all possibledestinations Each router periodically broadcasts its table to its neighbors^ ^ Neighbors will update their tables based on this information toensure they are using the shortest route to reach each destination To route a packet, router checks table to find next hop to destination Tradeoff in how often routing tables are exchanged^ ^ Too often
Æ^ large amount of overhead, excess BW and computational resources used Too infrequent
Æ^ sub-optimal or invalid routes (stale routes) ^ Can send routing table updates whenever information in the tablechanges due to a new link or a broken link
Problems with using ConventionalRouting in MANETs ^ Unidirectional links^ ^
A can communicate with B does not always imply B cancommunicate with A
^ Redundant links^ ^
In wired networks, only one or a small number of routersconnecting two networks In wireless networks, may be several “gateway” nodesbetween a source and a sink due to wireless channelproperties^ ^ Several redundant paths may be generated^ ^ Waste of BW, computation, and storage
Problems with using ConventionalRouting in MANETs ^ Periodic routing updates^ ^
Cost of sending routing updates (BW, energy) is much greaterin a wireless network than in a wired network Routing updates required even if nothing has changed In highly-connected network, routing updates may collide Mobiles cannot enter “sleep” state b/c need to hear all routingupdates
Æ^ wastes energy
Routing Protocols for MANETs
^ Route discovery and route maintenance ^ Route discovery^ ^
Initial discovery of valid route from source to destination Source node can send a query for a destination node Only destination node responds to query If destination located in source’s transmission range,destination responds and link established^ ^ No periodic routing updates needed^ ^ Approach must be extended to case where destinationnode not in source node’s transmission range Want an approach that is simple and efficient
Route Discovery (cont.)
^ One approach is to perform controlled flooding of the query^ ^
Nodes receiving query will append their address to the route beingrecorded in the packet header and broadcast updated packet to allneighbors^ ^ When a node receives a query, it checks to see if its address isalready in the header (indicating this packet was already floodedby this node)^ ^ If address present, node drops packet Each query labeled with unique “request ID”^ ^ Each node keeps a cache with request ID’s of packets it hasalready forwarded^ ^ Discards packets with request ID listed in node’s cache^ ^ Avoids duplicate queries sent throughout the network Node only propagates first copy of each route request packet it sees
Route Discovery (cont.)
^ Intermediate nodes can cache routes discovered^ ^
Can use these routes if want to send a packet to a node listedin route Reduces overhead in terms of number of route requestpackets required
^ Nodes can operate in “promiscuous” mode^ ^
Listen to all packets exchanged on network Cache routes listed in packets
Route Maintenance
^ Nodes can determine broken links through ACK/NACK includedwith most protocols ^ If link broken^ ^
Node that detects broken link reports this information back tosending node Or node can try to fix the broken link by sending out its ownroute request to the destination
^ If no ACK/NACK present in the link-layer protocol, nodes canlisten to channel to determine if next hop transmits packet or not^ ^
If do not hear forwarding of packet, assume link lost
^ Explicit routing acknowledgements can also be used to determinethe state of links
Proactive Routing Protocols
^ Each node maintains consistent, up-to-date routing information in theform of a table with the next-hop to reach every node in the network ^ Changes in link state transmitted throughout the network to update eachnode’s routing table ^ Proactive routing protocols^ ^
DSDV CGSR
DSDV
^ Destination-sequenced distance-vector (DSDV)^ ^
Each node maintains a table with^ ^ All possible destination nodes^ ^ Number of hops required to reach that node^ ^ Next hop along route to that node^ ^ Sequence number Sequence number used to distinguish new routes from old routes Routing table updates transmitted throughout network ^ “Full dump” routing updates^ ^
Large packet that carries all routing information Transmitted infrequently when little change in existing links ^ “Incremental” routing updates^ ^
Contains only information that has changed since last routing update
Clusterhead Gateway SwitchRouting (CGSR)
^ Clustering multihop approach ^ Cluster head controls nodes in cluster^ ^
Nodes within cluster use CDMA-type SS code to avoid inter-clusterinterference Cluster head can control channel access ^ Cluster head selection algorithm within cluster^ ^
Can perform minimum cluster changes to reduce overhead E.g., only change cluster head if cluster head moves out of cluster oranother cluster head moves into cluster ^ DSDV used as underlying routing mechanism
CGSR (cont.)
^ Nodes send data to cluster head ^ Cluster head sends data to “gateway” nodes ^ Gateway nodes route packet to new cluster head ^ Packet goes from cluster head
Æ^ gateway
Æ^ cluster head until it
reaches destination’s CH Each node keeps “cluster member table” that contains CH node for eachmobile in network^ ^ Updates sent as in DSDV
