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(1)

Routing in Ad Hoc Wireless Networks

PROF. MICHAEL TSAI

2011/12/15

(2)

Ad Hoc Wireless Networks

• No base station or access point to relay the packets

• Relaying is necessary to send information to destinations out of our range

• Initial application: military usage

Ad hoc Network

Infrastructure-based Network

(3)

Why do we need new protocols?

• No centralized control

• No dedicated routers

• Unpredictable network topology changes

• Time-variant wireless channel

• Link breakage is common in wireless network  Connectivity problem

• Links are not always bidirectional and/or symmetric

• Power Limitation

(4)

Conventional Routing Protocols

• Not designed for highly dynamic and low bandwidth networks

• Loop formation when topology changes

• Flooding causes high control overhead

(e.g., Link State)

(5)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

A 1 A

C 1 C

Dest Cost Next Hop

A 2 B

B 1 B

(6)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

A Infinity Null

C 1 C

Dest Cost Next Hop

A 2 B

B 1 B

(7)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

A 3 C

C 1 C

Dest Cost Next Hop

A 2 B

B 1 B

Routing update

(8)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

A 3 C

C 1 C

Dest Cost Next Hop

A 4 B

B 1 B

Routing update

This continues until the cost reaches infinity (unreachable).

During the process, the packets destined for A will bounce

back and forth between B and C

(9)

S

n4

n2

D1

n8

n7

n6

D2 n3

n5

Existing Routing Protocols

S

n4

n2

n3

D

S

n4

n2

n3

D

Table-Driven:

•S and all other nodes maintain full routing information

•Require periodic table update

Hybrid Scheme

•Network is divided into multiple zones

•Use Table-Driven within the zone

•Demand-Driven across the zones through boundary nodes

Demand-Driven

Route is discovered when S wants to talk to D

A Route only needs to be maintained for as long as S and D are still talking

EX: Dynamic Source Routing (DSR)

Zone A

Zone B

(10)

Proactive vs. Reactive Routing

• Proactive

• Table driven

• Rely on periodic update to keep track of the topology change

• No latency in route discovery

• Need large storage space to keep information of the entire network

• A lot of routing

information may never be used

• Reactive

• On demand

• Route Discovery by local flood or gossiping

• Additional latency during route discovery

• Not appropriate for real-time communication

• Route maintenance

Feedback from Link Level ACK

Issue new route discovery when link breaks

(11)

Destination Sequenced Distance Vector

(DSDV)

Proactive Routing Protocols

• Each node advertises a monotonically increasing sequence number

• Each Route entry is tagged with a sequence number generated by destination to prevent loops (count-to-infinity problem)

• Sequence number indicates the “freshness” of a route

• Routes with more recent sequence numbers are preferred for packet forwarding

• If same sequence number, one having smallest metric is used

C. E. Perkins and P. Bhagwat. “Highly dynamic Destination Sequenced Distance-Vector routing (DSDV) for mobile computers”, In Proceedings of the SIGCOMM ’94

Conference on Communication Architecture, Protocols and Applications, pages 234-244, August ‘94.

(12)

Example: DSDV

• For each reachable node in the network the routing entry contains:

• Destination Address

• Next Hop

• Distance (Metric)

• Sequence Number

Destination Next Hop Distance Sequence Number

A A 0 S205_A

B B 1 S334_B

C C 1 S198_C

D D 1 S567_D

E D 2 S767_E

F D 2 S45_F

(13)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

Seq. #

A 1 A 1

C 1 C 1

Dest Cost Next Hop

Seq. #

B 1 B 1

C 1 C 1

(14)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

Seq. #

A Infinity Null 2

C 1 C 1

Dest Cost Next Hop

Seq. #

B 1 B 1

C 1 C 1

Sequence Number:

• Even numbers for link updates from neighbor nodes

• Odd numbers for link updates from the

destination itself

(15)

Count-to-infinity Problem

A

B

C

Dest Cost Next Hop

Seq. #

A Infinity Null 2

C 1 C 1

Dest Cost Next Hop

Seq. #

A 2 B 1

C 1 C 1

Routing update

C’s routing update will not change

B’s routing table since the sequence

number is smaller (older).

(16)

DSDV: Topology changes

• Assign a metric of ∞ to

A broken link

Any route through a hop with a broken link

• “∞ routes” are assigned new sequence numbers by any host and immediately broadcast via a triggered update

• If a node has an equal/later sequence number with a

finite metric for an “∞ route”, a route update is

triggered

(17)

Dynamic Source Routing [DSR]

Route Discovery

• Source node

Broadcasts the Route Request (RREQ) <id, target>

• Intermediate node

Discards if the id has been seen before, or node is in the route record (header of RREQ)

Else append address in the route record and rebroadcast

• Destination Node

• Return Route Reply (RREP)

Use previously cached route to source node

Call Route Discovery for source node, with route reply piggy backed

Use reverse sequence of Route Record, in case of bidirectional links

D. B. Johnson, D.A. Maltz, and J. Broch. “DSR: The Dynamic Source Routing Protocol for Multi-Hop Wireless Ad Hoc Networks”, Ad Hoc Networking, pages 139-172, 2001.

A

B C

D

E F

G

A

H

A,B

A,B,F

A

B C

D

E F

G

A,B,F,E

H

A,B,F,E

A,B,F,E

(18)

DSR: Route Maintenance

• Monitoring the route

• Passive Acknowledgement – overhearing the next-hop node sending packet to its next-hop

• Set a bit in packet to request explicit next hop acknowledgement

• Route Error

• Rely on data link layer to report the broken links;

• Notify source of the broken link via Route Error (RERR)

• Source truncates all routes which use nodes mentioned in RERR

• Initiate new route discovery

A

B

E F

F transmits the packet I just sent to her. That means she

received my packet correctly.

A

B

E

F RERR

RERR

(19)

DSR: Optimizations for efficiency

• Use cached entries to create RREP at intermediate node

• Promiscuous mode to add more routes

Caching overheard RREQ/RREP

• Use hop based delays to prevent RREP storms

A lot of neighbors know the route to target and attempt to send RREP in response to RREQ

Delay RREP for a period d = H x (h – 1 + r)

r : random number between 0 and 1

H : small constant delay

h : number of hops to source from that node

I heard A said B-C-D

G H

I’m 4-hop away from

D

I’m 2-hop away from D Where is D ?

(20)

Expanding Ring Search

• Route Request Hop Limit

• Use TTL in the packet header to specify the first ring

boundary

• RREQ is initially forwarded n times (n hops)

• If destination is not within n- hop

• Increase TTL to a larger value

Hi!

This is useful if destination is close to the source

(21)

Gossiping vs. Flooding

Gossip-Based Routing

• Node forward packets with some probability pG < 1

• How good is it?

• 35% less overhead than flooding

• What determine P

G

?

S

D

Gossip: Probabilistic Flooding

Z. Haas, J. Halpern and L. Li, Gossip-based ad hoc routing, in: IEEE INFOCOM (2002)

Network Connectivity

(22)

Network Connectivity

• Sub-Critical

• Low connectivity

• Mobile nodes are sparsely distributed in the network

• Performance is limited !!

• Super-Critical

• High connectivity region

• Most or all the nodes can communicate

Connectivity: Fraction of nodes that is connected to the network

(23)

Ad-Hoc On Demand Distance Vector Routing (AODV)

• Protocol overview - Pure on-demand protocol

Node does not maintain knowledge of another node unless it communicates with it

Routes discovered on as-needed basis and maintained only as long as necessary

Little or no periodic advertisement

C. E. Perkins and E. M. Royer. “Ad-Hoc On Demand Distance Vector Routing”, Proceedings of the IEEE Workshop on Mobile Computing Systems and Applications (WMCSA), pages 90-100, 1999.

(24)

AODV – Route Discovery

• Initiation

• Source node sends a Route Request (RREQ) when it has no information about destination node in its table

• RREQ contains

Source and destination’s address and sequence number

Broadcast id

Hop count

• Source address and broadcast id uniquely identify RREQ

• Reverse Path Setup

• Neighbor increments hop count and broadcasts to neighbors

• Records address of neighbor which first sends the RREQ

(25)

AODV – Route Discovery

• Forward Path Setup

Intermediate node satisfies RREQ if

Destination itself

Has route entry in table with destination sequence number ≥ that given in RREQ

Unicasts RREP to neighbor which sent RREQ

Source address

Destination address and sequence number (updated)

Hop count

Lifetime

As RREP travels backwards, each node sets pointer to sending

node and updates destination sequence number and timeout

entry for source and destination routes

(26)

AODV – Route Discovery

• Other nodes

RREQ times out : Route Request Expiration Timer

Deletes corresponding pointers

• More than one RREP received

One with greater destination number

Lesser hop count

• Source node starts transmission - updates if a better RREP is received

C. E. Perkins and E. M. Royer. “Ad-Hoc On Demand Distance Vector Routing”, Proceedings of the IEEE Workshop on Mobile Computing Systems and Applications (WMCSA), pages 90-100, 1999.

(27)

AODV – Route Maintenance

• Nodes send hello message if it has not sent a packet in

hello_interval

• Failing to receive allowed_hello_loss packets consecutively

means link is broken

• In case of broken link

unsolicited RERR sent to affected source node

Source initiates new RREQ

Sequence number updated

Hop count set =

• Route Caching Timeout after the route is considered invalid

• Optional* AODV-LL uses link layer ACK instead of hello messages

(28)

Link Quality Metrics

• The protocol chooses the route with the smallest hop count

Long hops will be included

• Long hops usually have lower SNR  high PER  retransmission!

• Original thought: lower hop count = lower bandwidth usage

• New thought: retransmission means wasted bandwidth

A

B C

D E

(A,E,D), hop count=2

(A,B,C,D), hop count=3

SNR=3

SNR=15 SNR=15

SNR=15 SNR=2

(29)

Link Quality Metrics

• Instead of using hop count only, we need to take “link quality” into account!

• What is a good metric for link quality?

• RSSI (representing SNR)

• ETX (Expected Transmission Count)

• Then we combine hop count + link quality to choose

an optimal route

(30)

Minimize total transmissions per packet (ETX, Expected Transmission Count)

Example: ETX

Link throughput  1/ Link ETX

Delivery Ratio 100%

50%

33%

Throughput 100%

50%

33%

Link ETX 1

2

3

(31)

Measuring delivery ratios

• Each node broadcasts small link probes (134 bytes), once per second

• Nodes remember probes received over past 10 seconds

• Reverse delivery ratios estimated as

r

rev

 pkts received / pkts sent

• Forward delivery ratios obtained from neighbors (piggybacked

on probes)

(32)

Route ETX

Route ETX 1

2 2 3

Route ETX = Sum of link ETXs

5

Throughput 100%

50%

50%

33%

20%

(33)

Example: SNR-based Metrics

• For each link (each of a node’s neighbor), maintain a “expected”

SNR value.

This is to eliminate the small fading effects in SNR

• Each time receiving a packet, calculate:

𝑺𝑺𝑵𝑹

𝒊

= 𝜶 × 𝑺𝑵𝑹 + 𝟏 − 𝜶 × 𝑺𝑺𝑵𝑹

𝒊−𝟏

SNR: SNR value of a newly received packet

𝑆𝑆𝑁𝑅𝑖−1: old SSNR value before receiving the packet

𝑆𝑆𝑁𝑅𝑖: new SSNR value

𝛼: a sensitivity parameter. 0 < 𝛼 ≤ 1.

• Then set a threshold to classify the links into good and bad links

• New route discovery process:

First try to discover routes consists of only good links

If no route can be found, then relax the condition to include bad links as well.

(34)

References for link quality aware routing metrics

• D. Couto, D. Aguayo, J. Bicket, and R. Morris, “A high-throughput path metric for multi-hop wireless routing,” in Proc. ACM Intern.

Conf. on Mobile Comput. and Networking (MOBICOM), San Diego, CA, USA, September 2003, pp. 134–146.

• H.-M. Tsai, N. Wisitpongphan, and O. K. Tonguz, “Link-quality aware ad hoc on-demand distance vector routing protocol,” in Proc. IEEE Wireless Pervasive Computing, January 2006, pp. 6–.

• R. Dube, C. D. Rais, K. Y. Wang, and S. K. Tripathi, “Signal stability- based adaptive routing (SSA) for ad hoc mobile networks,” IEEE Wireless Personal Communications, vol. 4, no. 1, pp. 36–45, February 1997.

• C. Toh, “Associativity-based routing for ad hoc mobile network,” J.

Wireless Personal Commun., vol. 4, no. 2, pp. 103–139, March 1997.

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