Winter 2002 1

CMPE 155

Week 9

Winter 2002 2

Project 8

Setting up wireless ad hoc network. Designing/implementing application-

level neighbor discovery protocol.

Winter 2002 3

Wireless and Mobile Networking

Winter 2002 4

Overview

Wireless access and mobility– Force us to rethink many of our

assumptions…

Winter 2002 5

The Physical Layer

Many possible realizations.– Radio transmission range defined by cell.

• “Nanocell”: 6 meters in diameter.

– Receiver within range can hear transmission. – Interaction of multiple transmitters at receiver:

• Collisions: receiver is “in range” of more than one transmitter.

• Capture: receiver in transmission range of 2 transmitters but able to receive cleanly from closer one.

• Interference: receiver is in range of one transmitter and slightly out of range of another transmitter; unable to receive cleanly from closer transmitter due to the other.

Winter 2002 6

MAC approaches

Contention.– More robust.– Good performance under light load.– Example: CSMA

Round-robin.– Token-based.

Reservation.

Winter 2002 7

Carrier Sense Does Not Work

Station avoids collisions by sensing carrier before transmitting.

Relevant contention at the receiver, not sender.– Collision happens when

multiple signals interfere at receiver.

– CS cannot avoid collisions at receiver.

A

B

C

Winter 2002 8

Hidden terminals

B can hear both A and C.

But A and C cannot hear each other.

If A is transmitting to B, and if C starts to transmit, hidden terminal scenario at B.

A

B

C

Winter 2002 9

Exposed terminals

B is sending to A. C is ready to

transmit but detects carrier and defers transmission.

But A is out of C’s range.

A

B

C

Winter 2002 10

Medium Access Collision Avoidance (MACA) Uses 2 short, fixed size signaling

packets: RTS and CTS.– RTS: request-to-send.– CTS: clear-to-send.

Winter 2002 11

MACA

Before sending data, send RTS.

Target responds with CTS.

RTS-generator can then transmit.

Others who hear RTS defer transmission until after CTS.

Stations overhearing CTS defer transmission. – Data packet length in RTS

and CTS messages

If station hears RTS but not CTS, can transmit.

CTS as indication of being in-range of receiver.

If CTS is not heard, or RTS collides:– Retransmit RTS after

binary exponential backoff.– Backoff doubled after each

retransmission.– Goes back to minimum

after success.

Winter 2002 12

Backoff

Can lead to unfairness– Even starvation.

Need to share congestion information.– Backoff copy.

Avoid oscillations in backoff counter.– Increase multiplicatively– Decrease additively– Improves throughput

(less contention)

A

B C

Winter 2002 13

Adding Reliability

Transmission errors:– Noise, collisions, etc.

Add an ACK after DATA transmission– If ACK not received,

sender restarts RTS/CTS again.

– If ACK was lost, receiver sends ACK instead of CTS.

A

B

C

Winter 2002 14

Fairness with RTS/CTS

An exposed terminal may not be able to compete effectively.– Can send RTS but may

not hear CTS.– Doesn’t know if RTS/CTS

was successful. Fix:

– Carrier sense.– …or a DS packet

• Short 30-byte data-sending packet.

A

B

C

Winter 2002 15

IEEE 802.11

Standard for wireless communication.

MAC-layer uses many of the ideas discussed.– RTS/CTS/ACK.– Careful backoff.

Allows two modes– Ad-hoc.– Wired/wireless.

Winter 2002 16

Mobile IP

Internet standard for “last-hop” mobility support in IP networks (RFC 2290).

How do we deliver IP packets when the endpoints move?– Mobile host must be able to communicate

after changing its link-layer point-of-attachment to the Internet.

– Mobile host must be able to communicate using its permanent (home) IP address.

Winter 2002 17

Mobile IP

Issues:– Impact on IP addressing.– Impact on routing.

Key design considerations:– Scale.

• Routing updates (size and frequency).

– Simplicity.– Incremental deployment.

Winter 2002 18

Mobile IP Terminology

Home Agent (HA)

Mobile Host (MH)

Foreign Agent (FA)

Winter 2002 19

Discovering Agents

Agents periodically beaconadvertisements

Winter 2002 20

Registration

Winter 2002 21

Tunneling

Tunneled Data Packet

HA keeps bindingbetween MH and FA

Src

Winter 2002 22

Other Mobile IP Issues

Route optimality– resulting paths can be sub-optimal– can be improved with route optimization

• unsolicited binding cache update to sender

Authentication– registration messages– binding cache updates

Winter 2002 23

What’s a MANET?

Winter 2002 24

MANET Multihop, (wireless), (mobile) ad hoc

network. What does that really mean?

– No fixed network infrastructure.– No distinction between hosts and routers.– All nodes can be traffic sources and

forwarders.

Winter 2002 25

Applications Military applications.

– Battlefield, tanks, boats.

Disaster relief scenarios. Personal area network.

– Communications between personal devices PDA’s, Laptops, Watches, Play Stations.

SoHo (Small office Home office). Business Indoor Applications.

– Exhibitions, Symposiums.– Demos, Meetings.

Winter 2002 26

Issues Mobility

– Widely varying mobility patterns.– Routes can change rapidly.– Does it make sense to use stateful routing

protocols? Power, bandwidth, and processing

capabilities.– May be limited.– Multihop.– Power-aware algorithms.

Winter 2002 27

Winter 2002 28

Ad-Hoc Routing Requirements Distribution paths

– Multi hop paths.– Loop free.– Minimal transmission data overhead.

Self starting and adaptive to dynamic topology.

Low consumption of memory, bandwidth, power.– Scalable with number of nodes.– Localized effects of link failure.

Winter 2002 29

Unicast routing classification

Table Driven - Proactive

Routing Protocols

Source Initiated - On Demand Hybrid

DSDV WRPAODV DSR TORA LAR ZRP DDR

Winter 2002 30

Problems using DV or LS

DV protocols: – May form loops: wasteful in wireless

environment• Bandwidth and power.

– Loop avoidance may be complex LS protocols:

– Higher storage and communication overhead.

Winter 2002 31

Flooding ?

Flooding may deliver packets to too many nodes and in worst case all nodes reachable from sender may receive the packet.

S

R

Winter 2002 32

Alternatives to flooding

Flood only control packets – Use flooding to set up routes and use

established routes for data.– Need to limit flooding as much as

possible.

Winter 2002 33

Proactive protocols Table Driven.

– Maintain consistent, up-to-date routing information from each node to every other node in the network.

– Each node has to maintain one or more tables to store routing information.

– Periodically propagate updates throughout network to account for link changes.

Winter 2002 34

DSDV Destination Sequenced Distance Vector:

– Based on Bellman-Ford algorithm. – Guarantees loop freedom.

Each node maintains routing table.– Next hop.– Cost metrics.– Destination sequence number.– Each node periodically sends its local routing table

with an incremented sequence number.

Winter 2002 35

On-demand protocols AODV

– Primary Objectives• Provide, unicast, broadcast and multicast capability.• Minimize broadcast of control packets.• Disseminate information about link breakages to

neighboring nodes using the link.

– Characteristics• On-demand route creation.

Winter 2002 36

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

Winter 2002 37

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

Winter 2002 38

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents links on Reverse Path

Z

Y

M

N

L

Winter 2002 39

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

Winter 2002 40

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Winter 2002 41

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

• Node D does not forward RREQ, because node D is the intended target of the RREQ

M

N

L

Winter 2002 42

Route Reply in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents links on path taken by RREP

M

N

L

Winter 2002 43

Forward Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Forward links are setup when RREP travels alongthe reverse path

Represents a link on the forward path

Winter 2002 44

Data Delivery in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Routing table entries used to forward data packet.

Route is not included in packet header.

DATA

Winter 2002 45

Timeouts A routing table entry maintaining a reverse path is

purged after a timeout interval– timeout should be long enough to allow RREP

to come back

A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval– if no is data being sent using a particular

routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)

Winter 2002 46

Link Failure Reporting

A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval

When the next hop link in a routing table entry breaks, all active neighbors are informed

Link failures are propagated by means of Route Error messages, which also update destination sequence numbers

Winter 2002 47

Link Failure Detection

Hello messages: Neighboring nodes periodically exchange hello message

Absence of hello message is used as an indication of link failure

Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure

Winter 2002 48

Dynamic Source Routing (DSR) [Johnson96] When node S wants to send a packet to

node D, but does not know a route to D, node S initiates a route discovery

Source node S floods Route Request (RREQ)

Each node appends own identifier when forwarding RREQ

Winter 2002 49

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

Winter 2002 50

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ

Winter 2002 51

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

• Node H receives packet RREQ from two neighbors: potential for collision

Z

Y

M

N

L

[S,E]

[S,C]

Winter 2002 52

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

[S,C,G]

[S,E,F]

Winter 2002 53

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

• Nodes J and K both broadcast RREQ to node D• Since nodes J and K are hidden from each other, their transmissions may collide

N

L

[S,C,G,K]

[S,E,F,J]

Winter 2002 54

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

• Node D does not forward RREQ, because node D is the intended target of the route discovery

M

N

L

[S,E,F,J,M]

Winter 2002 55

Route Reply in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

RREP [S,E,F,J,D]

Represents RREP control message

Winter 2002 56

Data Delivery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

DATA [S,E,F,J,D]

Packet header size grows with route length

Winter 2002 57

Use of Route Caching

B

A

S E

F

H

J

D

C

G

IK

[P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format)

M

N

L

[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

Z

Winter 2002 58

Route Error (RERR)

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

RERR [J-D]

J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails

Nodes hearing RERR update their route cache to remove link J-D