Routing in Ad Hoc Wireless Networks

Routing in Ad Hoc Wireless Networks

PROF. MICHAEL TSAI

2011/12/15

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

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

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)

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

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

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

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

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

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

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.

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

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

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

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).

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

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

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

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 ?

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

Gossiping vs. Flooding

• Gossip-Based Routing

• Node forward packets with some probability p_G < 1

• How good is it?

• 35% less overhead than flooding

• What determine P

?

S

Gossip: Probabilistic Flooding

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

Network Connectivity

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

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.

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

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

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.

AODV – Route Maintenance

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

• Failing to receive allowed_hello_loss packets consecutively

• 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

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

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

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

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

 pkts received / pkts sent

• Forward delivery ratios obtained from neighbors (piggybacked

on probes)

Route ETX

Route ETX 1

2 2 3

Route ETX = Sum of link ETXs

5

Throughput 100%

50%

50%

33%

20%

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

= 𝜶 × 𝑺𝑵𝑹 + 𝟏 − 𝜶 × 𝑺𝑺𝑵𝑹

• SNR: SNR value of a newly received packet

• 𝑆𝑆𝑁𝑅_𝑖−1: old SSNR value before receiving the packet

• 𝑆𝑆𝑁𝑅_𝑖: 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.

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.