APerformance Comparison of Multi-Hop Wireless Ad Hoc NeWork

A Performance Comparison of

Multi-Hop Wireless Ad Hoc NeWork Routing

Josh Broth

David A. Maltz

David B. Johnson

Yih-Chun Hu

Computer Science Department

Carnegie Mellon University

Pittsbur~, PA 15213

http: //w.monarch. cs .cmu. edu/

Abstract

An ad hoc networkis a collwtion of wirelessmobilenodesdynamically

formingatemporarynetworkwithouttheuseofanyexistingnetworkirrfras-

tructureorcentralizedadministration.Duetothelimitedtransmissionrange

of ~vlrelessnenvorkinterfaces,multiplenetwork“hops”maybe neededfor

onenodeto exchangedataivithanotheracrox thenetwork.Inrecentyears,

a ttiery of nelvroutingprotocols~geted specificallyat this environment

havebeen developed.but little pcrfomrartwinformationon mch protocol

and no ralistic performancecomparisonbehvwrrthem ISavailable. ~Is

paperpresentsthe resultsof a derailedpacket-levelsimulationcomparing

fourmulti-hopwirelessadhocnetworkroutingprotocolsthatcovera range

of designchoices: DSDV,TORA,DSR and AODV. \Vehave extended

the/~r-2networksimulatortoaccuratelymodeltheMACandphysical-layer

behaviorof the IEEE 802.1I wirelessLANstandard,includinga realistic

wtrelesstransmissionchannel

model, and present the resultsof simulations

ofnet(vorksof 50mobilenodes.

1 Introduction

In areas in which there is little or no communication infrastructure

or the existing infrastructure is expensive or inconvenient to use,

wireless mobile users may still be able to communicate through the

formation of an

adhocrte~ork. Insuch anehvork, each mobile node

operates not only as a host but also as a router, forwarding packets

for other mobile nodes in the network that may not be within direct

wireless tmnsrnission range of each other. Each node participates in

an ad hoc routing protocol that allows it to discover “muki-hop” paths

through thenehvorkto anyothernode. Theideaofadhocnehvorking

is sometimes also called

infras[mcturelms neworfing [13], since the

mobile nodes in the network dynamically establish routing among

themselves to form their own nehvork “on the fly? Some examples of

the possible uses of ad hoc nehvorting include students using laptop

computers to participate in an interactive lecture, business associates

sharing information during a meeting, soldiers relaying information

for situational awareness on the battlefield [12, 21], and emergency

disaster relief personnel coordinating efforts after a hurricane or

earthquake.

This work was supported in part by Ihe National Science Foundation @S~ under

CAREER AwardNCR-9S027M, by the Air Force hfateriet Command (AFLIC) under

DARP,\ contract number F19628-9W~61, and by the AT&T Foundationunder a

Special Purpose Grant in Science and Enginwnn& Datid Nfaltzwas atso supported

undrr mr IBhl Coopcmtive Fellowship, and Mh-Chun Hu was also supported by an

NSF Graduate Fellowship. The views and conclusionscontained here arc those of the

authorsand shouldnot be interpretedas neces~ly representingthe officialpolicies or

endomements,eitherexpressorimplie& ofNSF,AFhfC, DARPA,theAT&TFoundation,

~hf, Carnegie hlellon Unirersi~, or the U.S. GovernsnenL

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Protocols

Jo~eta Jetcheva

Many different protocols have been proposed to solve the multi-

hop routing problem in ad hoc nehvorks, each based on different

assumptions and intuitions. However, little is known about the actual

performance of these protocols, and no attempt has previously been

made to directly compare them in a realistic manner.

This paper is the first to provide a realistic, quantitative analysis

comparing the performance of a variety of multi-hop wireless ad hoc

network routing protocols. Wepresent results of detailed simulations

showing the relative performance of four recently proposed ad hoc

routing protocols DSDV [18], TORA [14, 15], DSR [9, 10, 2]. and

AODV [17]. To enable these simulations, we extended the ns-2

nehvork simulator [6] to include

●

Node mobility.

A realistic physical layer including a radio propagation model

suppotiing propagation delay, capture effects, and carrier

sense [20].

Radio netiork inte~aces with properties such as transmission

power, antenna gain, and receiver sensitivity.

The

IEEE 802.11 Medium Accas Control (MAC) protocol using

the Distributed Coordination Function (DCF) [8].

Ourrestdts inthispaperarebasedon simulations ofan ad hoc network

of 50 wireless mobile nodes moving about and communicating with

each other. Weanalyze the performance of each protocol and explain

the design choices that account for their performance.

2 Simulation Environment

m is a discrete event simulator developed by the University of

California at Berkeley and the VINT project [6]. While it provides

substantial support for simulating TCP and other protocols over con-

ventional nehvorks, it provides no support for accurately simulating

the physical aspects ofmulti-hop wireless nehvorks or the MAC pro-

tocols needed in such environments. Berkeley has recently released

ns code that provides some support for modeling wireless LANs, but

this code cannot be used for studying multi-hop ad hoc networks as

it does not support the notion of node position; there is no spatial

diversity (all nodes are in the same collision domain), and it can only

model directly connected nodes.

In this section, we describe some of the modifications we made to

ns to allow accurate simulation of mobile wireless nehvorks.

2.1 Physical and Data Link Layer Model

To accurately model the attenuation of radio waves behveen anten-

nas close to the ground, radio engineers typically use a model that

attenuates the power of a signal as 1/rz at short distances (T is the

distance between the antennas), and as l/r4 at longer distances.

The crossover point is called the

reference distance, and is typically

around 100 meters for outdoor low-gain antennas 1.5m above the

ground plane operating in the 1–2GHz band [20]. Following this

practice, our signal propagation model combines both a free space

propagation model and a two-ray ground reflection model. When a

transmitter is within the reference distance of the receiver, we use

——

the free space model where the signal attenuates as l/T2. Outside of

this distance, we use the ground reflection model where the sigud

falls off as l/T4.

Each mobile node has a position and a velocity and moves around

on a topography that is specified using either a digitd elevation map

or a flat grid. The position of a mobile node can be crdculated as

a function of time, and is used by the radio propagation model to

cdcdate the propagation delay from one node to another and to

determine the power level of a received signal at each mobile node.

Each mobile node has one or more wireless network interfams,

with dl interfaces of the same type (on dl mobile nodes) linked

together by a single physical chmel. When a network interface

transmits a packet, it passes the packet to the appropriate physical

channel object. This object then computes the propagation delay

from the sender to every other interfam on the channel and schedties

a “packet reception” event for each. This event notifies the receiving

interface that the first bit of a new packet has arrived. At this time,

the power level at which the packet was received is compared to two

different values: the carrier sense threshold and the receivethreshold.

If the power level falls klow the carrier sense threshold the packet

is discarded as noise. If the received power level is above the carrier

sense threshold but below the receive threshold the packet is marked

as apacket in error before beiig passed to tie MAC layer. Otherwise,

the packet is simply handed up to the MAC layer.

Once the MAC layer receives a packe~ it checks to insure that its

receive state is presentiy “ide? If the receiver is not ide, one of two

things cm happen. If the power level of the packet aheady being

received is at least 10dB greater tha the received power level of the

new packet, we assume capture, discard the new packet, and allow

the receiving interface to continue with its current receive operation.

Otherwise, a collision occurs and both packets are droppe~

If the MAC layer is ide when an incoming packet is passed up

from the network interface, it simply computes the transmission time

of the packet and scheddes a “packet reception complete” event for

itse~. When this event occurs, the MAC layer verifies that the packet

is error-free, performs destination address filtering, and passes the

packet up the protocol stack.

2.2 Medium ACCMControl

The link Iayerofoursimtiatorimplements the complete EEE 802.11

standard [8] Medium Access Control WC) protocol Distributed

Coordination Function ~~ in order to accurately model the

contention of nodes for the wireless medium. DCF is similar to

MACA [11] and MACAW [1] and is designed to use both physi-

cal carrier sense and virtwl cam.er sense mechanisms to reduce the

probability of collisions due tohidden termin~s. The transmission of

each unicast packet is preceded by a Request-to-SenWClear-to-Send

@TSICTS) exchange that reserves the wireless chael for trans-

mission of a data packet. Each correcfly received unicast packet is

followed by an Acknowledgment (AcK)to the sender, which retrsms-

mits the packet a limited number of times until this

ACKis received.

Broadcast packets are sent ordy when virtual and physical carrier

sense indicate that the medium is clear, but they are not preceded by

an RTS/CTS and are not acknowledged by their recipients.

23 Addrws Resolution

Since the routing protocols dl operate at the nehvork layer using fP

addresses, an implementation of ARP [19], modeled after the BSD

Unix implementation [23], was included in the simulation and used

to resolve F addresses to link layer addresses. The broadcmt nature

of an ARP ~Q= packet (Section 6.3) and the interaction of ARP

with on-demmd protocols (Section 6.4) make ARP an important

deti of the simdation.

2.4 Packet Buffering .

Each node has a queue for packets awaiting transmission by the

network interface that holds up to 50 packets and is managed in a

droptail fashion. Each on-demand routing protocol (i.e., TORA,

DSR, or AODV), can buffer separately an additiond 50 packets that

that are awaiting discove~ of a route through the nehvork.

3 Ad Hoc Netiork Routtig Protocok Studied

In this section, we briefly describe the key features of the DSDV,

TORA, DSR, and AODV protocols studied in our sirmdations. We

rdso describe the partictiar parameters that we chose when imple-

menting each protocol.

The protocols were care~y implemented according to their spec-

ifications published as of April 1998 and based on clfications of

some issues from the designers of each protocol and on our own

experimentation with them. b particdar, during the process of im-

plementing each protocol and analyzing the restits from early simu-

lation runs, we discovered some modifications for each protocol that

improved its performance. The key improvements to each protocol

ae higtiighted in the respective protocol descriptions below. We

dso made the following improvements to W of the protocols:

●

3.1

To prevent synchronization, periodic broadcasts and packets

sent in response to the reception of a broadcast packet were

jittered using a random delay uniforrrdy distributed between O

and 10 milliseconds.

To insure that routing information propagated through the net-

work in atimely fashion, routing packets being sent were queued

for transmission at the head of the nehvork interface transmit

queue, whereas W otier packets (ARP and data) were inserted

at tie end of the interface ~mit queue.

Each of the protocols used link breakage detection feedback

from the 802.11 MAC layer that indicated when a packet codd

not be forwarded to its next hop, with tie exception of DSDV

as explained in Section 3.1.2.

D=tination-Sequenced Distance Vector @SDm

DSDV [18] is a hopby-hop distance vector routing protocol requir-

ing each node to periodicdy broadcmt routing updates. The key

advantage of DSDV over traditionrd distance vector protocols is that

it guarantees Ioopfreedom.

3.1.1

Basic Mectiisms

Each DSDV node maintains a routing table fisting the “next hop” for

each reachable destination. DSDV tags each route with a sequence

number and considers a route

R more favorable than R’ if R has a

greater sequence number, or if the hvo routes have equal sequence

numbers but

R has a lower metic. Each node in the network ad-

vertises a monotonically increasing even sequence number for itself.

When a node B decides that its route to a destination D has broken,

it advertises the route to D with an itinite metric and a sequence

number one greater than its sequence number for the route that has

broken (making an odd sequence number). This causes any node

A routing packets through B to incorporate the itite-metric route

into its routing table until node A hears a route to D with a higher

sequence number.

3.1.2

Implementation Decisions

We did not use tink layer breakage detection from the 802.11 MAC

protocol in obtaining the DSDV data presented in this paper, because

after implementing the protocol both with and without it, we found

the performance significantly worse

with the hnk layer breakage

detection. The reason is that if a neighbor N of a node A detects

that its link to A is broken, it will broadcast a triggered route update

containing an infinite metric for A. The sequence number in this

triggered update will be one greater than the last sequence number

broadcast by A, andtherefore is the highest sequence nurnberexisting

anywhere in the network for A. Each node that hears this update will

record an ifinite metric for destination A and will propagate the

—

._—

-. -.

. ..

——

TableI Consmts usedintheDSDV-SQsimulation.

Periodicrouteupdateinteti 15s

Periodicupdatesmissedbeforefinkdeclaredbroken

Initialtiggered updateweightedsetig time

Weightedsetig timeweightingfactor 718

Routeadvertisementaggregationdme

Maximumpacketsbufferedpernodeperdestination 5

information further. This renders node A unreachable from dl nodes

in the network until A broadcasts a newer sequence number in a

periodic update. A will send this update as soon as it learns of the

irdiuite metric &lrrg propagatd for iL but large numbers of packets

can be dropped in the meantime.

Our implementation uses both Ml and increment updates as

required by the protocol’s description. However, the published de-

scription of DSDV [18] is ambiguous about specitilng when trig-

gered updates shotid be sent. One interpretation is that the receipt

of a new sequence number for a destination shotid cause a triggered

update. We crdl this approach DSDV-SQ (sequence number). The

advantage of this approach is that broken links wti be detected and

routed ~ound as new sequence numbers propagate around the broken

~i and create dtemate routes. The second interpretation, which we

crdl simply DSDV, is that ordy the receipt of a new metric shotid

cause a triggered update, and that the receipt of a new sequence rmm-

ber is not sufficiently impomt to incur the overhead of propagating

a triggered update.

Weimplemented both DSDV-SQ and DSDV and found that while

DSDV-SQis much more expensive in terms of overhead it provides

amuch better packet defiveryratio inmost cases. The second scheme

(DSDw is much more conservative in terms of routing overhea~ but

because finkbreakages are not detected as quic~y, more data packets

are dropped All of the resdts in this paper use DSDV-SQ, with the

exception of Section 6.2, which comp=es this with DSDV.

Table I Hststhe constants used in our DSDV-SQ simdation.

3.2 TemporWyOrdered Routing Algorithm (TORA)

TORA [14, 15] is a distributed routing protocol breed on a “link

reversal’ dgorithrn. It is designed to discover routes on demand,

provide mrdtiple routes to a destination, estabfish routes quicuy,

md minimize communication overhead by Iocdizing rdgorithrnic

reaction to topological changes when possible. Route optimrdity

(shortest-path routing) is considered of seconm importrmce, and

longer routes are often used to avoid tie overhead of discovering

newer routes.

The actions taken by TORA can be described in terms of water

flowing downhill towards a destination node through a nehvork of

tubes that models the routing state of the red nehvork. The tubes

represent W behveen nodes in the network, the junctions of tubes

represent the nodes, and the water in the tubes represents the packets

flowing towards the destination. Each node has a height with respect

to the destination that is computed by the routing protocol. If a tube

between nodes A and B becomes blocked such that water can no

longer flow through it, the height of A is set to a height greater than

that of any of its remaining neighbors, such that water will now flow

back out of A (and towards the other nodes that had been routing

packets to the destination via A).

3.2.1

Basic hlectinisms

At each node in the nehvork, a Iogicrdly separate copy of TORA is

run for each destination. When a node needs a route to a particuhrr

destination, it broadcmts a Q~RY packet containing the address of

the destination for which it requires a route. This packet propagates

through the network until it reaches either the destination, or an

intermediate node having a route to the destination. The recipient

of the QHY then broadcasts an UPDAmpacket listing its height

with respect to the destination. As this packet propagates through

the network, each node that receives the UPDA~ sets its height to a

value greater than the height of the neighbor from which the UPDA~

was received. This has the effect of creating a series of direetd

links from the ongind sender of the Q~Y to the node that inititiy

generated the UPDA~.

When anode discovers that a route to a destination is no longer

vdl~ it adjusts its height so that it is a Iocd maximum with respect

to its neighbors and transmits an UPDA~ packet. If the node has no

neighbors of finite height with respect to this destination, then the

node instead attempts to discover a new route as described above.

When anode detects a network partition, it generates a ~Rpacket

that resets routing snte and removes invalid routes from the network.

TORA is layered on top of MP, the htemet MA~T

Encapsdation Protocol [5], which is required to provide reliable,

in-order detivery of dl routing control messages from a node to each

of its neighbors, plus notification to the routing protocol whenever a

fink to one of its neighbors is created or broken. To reduce overhead

fMEP attempts to aggregate marry TORA and MP control mes-

sages (which ~P refers to as objec~s)together into a single packet

(as an object block) before transmission. Each block carries a se-

quence number and a response list of other nodes from which an ACK

has not yet been received, and ordy those nodes ACKthe block when

receiving i~ WP retransmits each block with some period, and con-

tinues to retransmit it if needed for some mtimurn toti period after

which time, the link to each unacknowledged node is declared down

and TORA is notified. MP cart rdsoprovide network layer address

resolution, but we did not use tis service, as we used ARP [19] with

fll four routing protocols. For link status sensing and maintaining

a list of a node’s neighbors, each ~P node periodically transmits

a BmCON (or “BWCON-eqrdvdent”) packet, which is ~wered by

each node hearing it with a ~Lo (or “~Lo-equivdent”) packet.

3.2.2

Implementation Decisions

~P must queue objects for some period of time to allow possi-

ble aggregation with other objects, but the ~P specification [5]

does not define this time period and we discovered that the ovedl

performance of the protocol was very sensitive to the choice of this

value. After significant experimentation, we chose as the best ba-

lancebetween packet overhead and routing protocol convergence, to

aggregate ~LLO and ACKpackets for a time unifotiy chosen be-

tween 150ma and 250 rns, and to not delay TORA routing messages

for aggregation. The reason for not delaying these messages is that

the TORA link reversal process creates short-lived routing loops that

exist from the time that the hnk-reversd starts until the time that dl

nodes that need to be aware of the reversal receive the corresponding

UPDA~ (Section 5.2). Delaying the transmission of TORA routing

messages for aggregation, coupled with any queuing delay at the

network interface, allows these routing loops to last long enough that

significrmtnumbers of data packets me dropped.

The TORA and I~P specifications [15,5] do not define the pre-

cise semantics of rehable object delivery required by TORA, but

experimentation showed that very strong semantics must be pro-

vided in order to prevent the creation of long-lived routing loops.

k partictia, rdl TORA objects must be delivered reliably and in

order, without any duplication. Additiondly, rdl neighhring nodes

in the ad hoc network must have a consistent picture of the network

with regard to each destination. This impfies that anytime a node A

decides its link to a neighbor B hm gone down, B

mwt dso decide

that the link to A has gone down.

Wehave implemented ~P toprovide this functionrdity,although

the retransmission timeout and total number of attempts are not

specified by IMEP [5]. We chose a retransmission period of 500 rns

andatoti timeout of 1500rns, dthoughbaseduprr ourobsewations,

au adaptive retransmission timer should be added to the protocol. h-

order delivery is enforced by, at each receiver node B, ordypassing

————

TableU Constantsusd in theTORAsimulation.

I BMCONwriod

I Is I

Tme afterwhicha W is decked downif no BWCONor ~s

-o uacketswere exchanged

Tme afterwhichanobjectblockisre~nritted if no

500m

acknowledmnentisreceived

an object blwk from some node A to TORA if the block has the

sequence number that ~P at B next expects from A. Blocks with

Iowersequence numbers may generate another ACKbutae otherwise

dropped Blocks with higher sequence numbers are queued until the

missing blocks arrive or until the mtimum 1500 rns total timeout

expires, at which point B can be certain the object will never be

retransmitted. By this poin~ A will have declared its link to B down,

since it will not have received an ACKfor the missing packet. To

give the routing protocol at B a picture consistent with that seen by

the prot~ol at A, the ~P layer at B notifies its routing protocol

that the link to A is down, then notifies it the link is back up, and

then processes the queued packets.

Finally, we improved MFs method of link status sensing by

reducing it to a point that functions with minimum overhead yet

still maintains dl of the required fink status information. During

our experimentation with ~P, we found that nodes need ordy

send

BMCON messages when they are disconnected from dl other

nodes. Suppose two nodes A and B, both of which have neighbors,

transmit a single WO listing each of tieir neighbors once per

BMCON perid If abi-directiomd fink exists between A and B, both

nodes WUoverhear each other’s mLLOSand rdlother trmssrnissions,

causing each node to create a link to the other with “incoming”

status. In subsequent -O messages, A and B will list each other,

cotirming that a bi-directionti link exists between them.

Table ~ lists tie constants used in our TORA simtiation.

33 Dynamic Source Routing @SR)

DSR [9, 10,2] uses source routing rather than hopby-hop routing,

with each packet to be routed carrying in its header the complete,

ordered Est of nodes through which the packet must pass. The key

advantage of source routing is that intermediate nodes do not need to

maintain uptwdate routing information in order to route the packets

they forward since the packets themselves tieady contain dl the

routing decisions. This fact, coupled with the on-demmd nature of

the protocol, eliminates the need for the Priodic route advertisement

and neighbor detection packets present in other protocols.

3.3.1

Basic Mecbnisms

The

DSR protocol consists of two mechanisms: Route Discovery

and Route Maintenance. Route Discovery is the mechanism by

which a node S wishing to send a packet to a destination D obtains

a source route to D. To prforrn a Route Discovery, the source node

S broadcasts a Rom mQ~T packet that is flooded through the

network in a controlled manner and is answered by a Rom ~LY

packet from either the destination node or another node that hews a

route to the destination. To reduce the cost of Route Discovery, each

node maintains a cache of source routes it has learned or overheard

which it aggressively uses to limit the frequency md propagation of

Rom WQ~TS.

Route Maintenance is the mechanism by which a packet’s sender

S detects if the network topology has changed such that it can no

longer use its route to the destination D because hvo nodes listed

in the route have moved out of range of each other. When Route

Maintenance indicates a source route is broken, S is notified with

Table~ ConstarrKusti intheDSRsimuhtion.

fime between retransmittedROW WQ~S

(exponentitiybackedo~

500m

Ske

of source routeheader-g n addresses 4n + 4 bytes

~meout

fornonpropagatingsearch

30 m

Tne to holdpackets awaiting routes 30

Maxratefor sending ~tuitous ~LYs for a route 1/s

a Ro~ ERRORpacket. The sender S can then attempt to use any

other route to D tieady in its cache or crminvoke Route Discovery

again to find a new route.

3.3.2

Implementation Decisions

Using the suggestions from the published description of DSR [10],

we have optimized our implementation in a number of ways.

Although the DSR protocol supports unidirectional routes, EEE

802.11 requires an RTS/CTS~attiACK exchange for W unicast

packets, limiting the routing protocol to using ordy bidirectiomd

links in delivering data packets. We implemented DSR to discover

ordy routes composed of bidirectional finks by requiring that a node

return rdl Ro~ ~PLY messages to the requester by reversing the

path over which the Rom ~Q~ packet came. If the path taken

by a Ro~ ~Q~ contained unidirectionrd links, then the cor-

responding Ro~ ~LY wi~ not reach the requester, preventing it

from learning the unidirectional link route.

k Route Discovery, a node fist sends a Rom WQ~T with the

maximum propagation ~it @op Emit) set to zero, prohibhing its

neighbors from rebroadcmting it. At the cost of a singe broadcast

packet, this mechanism ~ows a node to query the route caches of

W its neighbors for a route ad rdso optimizes the case in which the

destination node is adjacent to the source. E this nonpropagating

search times out, a propagadrrg Rom NQWT is sent.

Nodes operate their network interfaces

inpromiscuow mode, dis-

abling theinterface’s address filtering and causing thenehvork proto-

col to received packets that the interface overhears. These packets

are scanned for use~ source routes or Rom

ERRORmessages and

then discarded. This optimization Wows nodes to learn potentially

usefl information, while causing no additiond overhead on the lim-

ited nehvork badwidth.

Furthermore, when a node overhears a packet not addressed to

itself, it checks the unprocessed portion of the source route in the

packet’s header. E the node’s own address is present, it knows that

this source route cotid bypass the unprocessed hops preceding it in

the route. The node then sends a gratuitous Rom ~LY message

to the packet’s source, giving it the shorter route without these hops.

Finrdly,when an intermediate node forwarding a packet discovers

that the next hop in the source route is unreachable, it examines its

route cache for another route to the destination. If a route exists, the

node replaces the broken source route on the packet with the route

from its cache and retransmits the packet. If a route does not exist in

its cache, the node drops the packet and does not begin anew Route

Discovery of its own.

Table ~ Ests the constants used in our DSR simtdation.

3.4 Ad Hoe On-Demand Distance Vector (AOD~

AODV [17] is essentially a combination of both DSR and DSDV.

It borrows the basic on-demand mechanism of Route Discovery and

Route Maintenance from DSR, plus the use of hopby-hop routing,

sequence numbers, and periodic beacons from DSDV.

3.4.1

Basic Mechnisrns

When a node S needs a route to some destination D, it broad-

casts a Ro~ mQmT message to its neighbors, including the last

known sequence number for that destination. The Rom mQ~T

is floodedin a controlled manner through the network until it reaches

—.

,..

. .

,<.. --

---—

-—

a node that has a route to the destination. Each node that forwards

the ROW WQ~T creates a

reverse route for itself back to node S.

When the Rom mQmT reaches anode with a route to D, that

node generates a Rom

WPLY that contains the number of hops

necessary to reach D and the sequence number for D most recenfly

seen by the node generating the

WPLY. Each node that participates

in fonvarding this

~PL>’ back toward the originator of the Ro~

~Qm (node S), creates a

foward route to D. The state created

in each node along the path from S to D is hopby-hop state that is,

each node remembers ody the next hop and not the entire route, as

wodd be done in source routing.

h order to maintain routes, AODV norrndly requires that each

node periodically transmit a ~Lo message, with a defadt rate of

once per second Failure to receive three consecutive -LO mes-

sages from a neighbor is taken as an indication that the fink to the

neighbor in question is down. Alternatively, the AODVspecification

briefly suggests that anode may use physical layer or link layer meth-

ods to detect Iinkbreakages to nodes that it considers neighbors [17].

When a hnk goes down, any upstrem node that has recenfly

forwarded packets to a destination using that link is notified via an

UNSOLIC~D Rom WLY containing an ifinite metric for that

destination. Upon receipt of such a Rom ~LY, a node must

acquire a new route to the destination using Route Discovery as

described above.

3.4.2

Implementation Decisions

Weinitially implemented AODV using periodic-O messages for

link breakage detection as described in the AODV specification [17].

Forcomparison, we dso implemented aversion of AODVthat we crdl

AODV-LL (link layer), instead using only ~i layer feedback from

802.11as inDSR, completely eliminating the standardAODV WLLO

mechanism. Such an approach saves the overhead of the periodic

~LLO messages, but does somewhat change the fundamentrd nature

of theprotocol; for example, dl Enk breakage detection in AODV-LL

is ordy on-demand, and thus a broken link cannot be detected until

a packet needs to be sent over the fink, whereas the periodic ~LO

messages in standard AODV may allow broken Enks to be detected

before a packet mustbe fonvmded. Nevertheless, we found our alte-

rnateversion AODV-LLto perform sigrrificanflybetter than standard

AODV,and so we report measurements from that version here.

h addition, we dso changed our AODV implementation to use a

shorter timeout of 6 seconds before retrying a Rom MQ~T for

which no Ro~ ~LY has been received &P-WAIT_T~).

The value given in the AODV specification was 120 seconds, based

on the other constants specified there for AODV. However, a Row

~LY can ody be returned if each node along the discovered route

still has areverse route along which to return iL saved from when the

Rom ~QH was propagatd. Sin@ the specified timeout fortbis

reverse route information in each node is ordy 3 seconds, the ongind

Rom ~LY timeout value of 120 seconds urmecesstily limited

the protocol’s abfity to recover from a dropped Rom ~QW or

Rom -LY packeL

Table N lists the cons~ts used in our AODV-LLsimdation.

4 Metiodolofl

The ovedl god of our experiments was to measure tie ability of

the routing protocols to react to network topology change while

continuing to successtily deliver data packets to their destinations.

To measure this ability, our basic methodology was to apply to a

simdated network a variety of wor~oads, in effect testing with each

data packet originated by some sender whether the routing protocol

can at that time route to the destination of that packet. We were

not attempting to measure the protocols’ performance on aparticdar

wor~oad taken from red life, but rather to measure the protocols’

performance under a range of conditions.

TableW ~nstanK usd in theAODV-LLsirmdation.

Timeforwhicha routeis consideredactive 3Ms

Lifetimeona Rom _LY sentby destinationnode 600S

Numberoftimesa Rom WQ_ is retied

Tme beforea ROW ~Qm is retried 6s

~me forwhichthebroadwt id foraforwardedRo=

WQ~ iskept

TimeforwhichreverserouteinformationforaRo~

~LY k kept

Tne beforebrokenfinkisdeleti fromroutingtable 3s

MC layerfinkbr~ge detection

yes

Ourprotocol evaluations arebasedon thesimtiationof 50wireless

nodes forming an ad hoc network, moving about over a rectan~ar

(1500m x 300m) flat space for 900 seconds of simdated time. We

chose a rectan@ar space in order to force the use of longer routes

between nodes than wotid occur in a square space with equal node

density. me physical radio characteristics of each mobile node’s net-

work interface, such as the antenna gain, transmit pwer, and receiver

sensitivi~, were chosen to approximate the Lucent WaveLAN [22]

direct sequence spread spectrum radio.

horder to enable direct, fair comparisons between the protocols,

it was cnticd to chrdlenge the protocols with identicd loads and

envirorunenti conditions. Each run of the simtiator accepts as input

scemn”o file that describes the exact motion of each node and the

exact sequence of packets originated by each node, together with the

exact time at which each change in motion or packet origination is

to occur. We pre-generated 210 different scenario files with varying

movement patterns and tic loads, and then ran dl four routing

protocols against each of these scenario files. Since each protocol

was challenged in an identicd fashion, we can direcfly compare the

performance restits of the four protocols.

4.1 Movement Model

Nodes in the simtiation move according to a model that we cdl the

“random waypoint” model [10]. The movement scenario files we

used for each simdation are characterized by

apause time. Each node

begins thesimtiationby remaining stationq

forpause time seconds.

It then selects a random destination in the 1500m x 300rn space and

moves to that destination at a speed distributed unifotiy between O

and some maximum speed. Upon reaching the destination, the node

pauses again forpause

time seconds, selects another destination, and

proceeds there as previously descnbe& repeating this behavior for

the duration of the simdation. Each simrdation ran for 900 seconds

of simdated time.

We ran our simulations with movement patterns generated for 7

different pause times: O,30, ~, 120, 300, 600, and 900 seconds.

A pause time of Oseconds corresponds to continuous motion, md a

pause time of 900 (the length of the simdation) corresponds to no

motion.

Because the~rfomrmce oftheprotocols isvery sensitive tomove-

ment pattern, we generated scenario fileswith 70 different movement

patterns, 10 for each vrdue of pause time. All four routing protocols

were run on the sme 70 movement patterns.

We experimented with NO different maximum s~eds of node

movement. We primarily report in this paper data horn simdations

using amaximum node speed of 20 meters per second (average s~d

10 meters per second), but dso compare this to simdations using a

maximum speed of 1 meter per second.

4.2 Communication Model

As the god ofoursimulation wasto compare theperformanceof each

routing protocol, we chose our trtic sources to be constant bh rate

(CBR) sources. When defining the parameters of the communication

model, we experimented with sending rates of 1, 4, and 8 packets

. .

456789 10

- mme m [m

1 D~tibution of the sho~t path ati~ble to each

apptimtionpacketori@natedoveratlsmnados.

~r secon~ networks containing 10, 20, and 30 CBR sources, and

packet sizes of 64 and 1024 bytes.

V@g the number of CBR sources was approximately equivalent

to varying the sending rate. Hence, for these simtdations we chose to

h the sending rate at 4 packets per second and used three different

communication patterns corresponding to 10,20, and 30 sources.

When using 1024byte packets, we found that congestion, due

to lack of spatial diversity, became a problem for dl protocols and

one or two nodes wodd drop most of the packets that they received

for forvfarding. As none of the studied protocols performs load

balancing, and the god of our amdysis was to determine ifthe routing

protocols codd consistently track changes in topology, we attempted

to factor out congestive effectsby reducing the packet size to 64bytes.

This sdler packet size still provides a good test of the routing

protocols, since we are sti~ testing their ability to determine routes

to a destination with the same frequency (a totrd of 40, 80, or 120

times per second).

All communication patterns were peer-tmpeer, and connections

were started at times tmifody distributed behveen Oand 1S0sec-

onds. The three communication patterns (10, 20, and 30 sources),

taken in conjunction with the 70 movement patterns, provide a totrd

of 210 different scenario files for each maximum node movement

speed (1 @s and 20 ds) with which we compared the four routing

protocols.

We did not use TCP sources because TCP offers a conforming

load to the network, meaning that it changes the times at which

it sends packets based on its perception of the networks ability to

carry packets. As a restdL both the time at which each data packet is

originated by its sender and the position of the node when sending

the packet wotid differ between the protocols, preventing a direct

comparison between them.

4S Scenario Charactetilm

To characterize the chrdlenge our scenarios placed on the routing

protocols, we measured the lengths of the routes over which the

protocols had to deliver packets, and the toti number of topology

changes in each scenario.

When each data packet is originate~ an intemd mechanism of our

simtdator (sepmte from the routing protocols) cdctdates the shortest

path between the packet’s sender md its destination. The packet is

labeled with this information, which is compared with the number

of hops actu~y taken by the packet when received by the intended

destination. The shortest path is cdctdated based on a nomimd

transmission range of 250m for each radio and does not account for

congestion or interference that any partictiar packet might see.

TableV Averagenrrrnberof ~ connectivitychangesdting

each~second simdationasa fmrctionofpawe time.

#ofcomrectivityChangu

%we Time

1ds 20ds

o 898 11857

30 908 8984

792 7738

120

732 5390

300 512 2428

600 245 1270

900 0

Figure 1 shows the distribution of shortest path lengths for dl

packets over the 210 scenario ties at 1ds and 20 tis that we used.

The height of each bar represents the number of packets for which

the destination was the given distance away when the packet was

originated. The average data packet in our simtdations had to cross

2.6 hops to reach its destination, and the farthest reachable node to

which the routing protocols had to deliver a packet was Shops away.

Table V shows the average number of link connectivity changes

that occurred during each of the sirntiations runs for each vahre

of pause time. We count one fink connectivity change whenever a

node goes into or out of direct communication range with another

node. For the specific scenarios we used the 3@second pause time

scenarios at 1 ds actudy have a shghfly higher average rate of link

cormectivi~ change than the @second pause time scenarios, due to

an artifact of the random generation of the scenarios. This artifact

is rdsovisible as a slight bump at a pause time of 30 seconds in the

performance graphs we present for 1tis in Section 5.

4.4 Vrdidation of the Propagation Model and WC Layer

Our propagation model uses standard equations and techniques, and

it was vefied by an expert in radio propagation modeling. We

analyzed the power and simtdated radio behavior as a function of

distance between small groups of nodes to ensure that the propaga-

tion, capture, and carrier sense models were working as designed.

The S02.11 MAC implementation was studied in a variety of sce-

ntios and independency verified by two of the authors. We exper-

imentily tested that when dl nodes are in range of each other, no

data packets experience colhsions (regardess of offered traffic load),

and that each node is able to make progress sending packets. This

verified that the ctier sense, RTSICTS, and back-off mechanisms

ofS02.11 were working correctiy.

4.5 Vtidation of the Routing Protocol kplementations

Each protocol implementation was studied and verified by at least

hvo of the authors, and hvo independent implementations were made

of both AODV and DSDV.

The restdts of each simtiation were intemdly consistent. That

is, the percentage of packets originated by the “application Iayef’

sources that were not logged as either received or dropped at the end

of the simtdation was less than 0.0170(approximately 10packets per

simtdation). These packets were flmost certairdy in transit at the end

of the simulation, as the simtdation originates behveen 40 and 120

packets per simdated second and terminates at exactiy 900 seconds

without a cool-down period.

The restdts of our simulations are, in fact, different from the few

previously reported studies of some of these protocols. We explain

the reasons for these differences in Sections 5,6, and 7.

4.6 Metnm

h comparing the protocols, we chose to evahtate them according to

the following three metrics:

—_

.—

.—.

● Pacht deliveq ratio The ratio behveen the number of packets

originated by the “application layer” CBR sources and the num-

ber of packets received by the CBR sink at the find destination.

● Routing overhed The total number of routing packets trans-

mitted during the simdation. For packets sent over mukiple

hops,

each transmission of the packet (each hop) counts as one

transmission.

● Path optirt~ali~ The difference between the number of hops a

packet took to reach its destination and the length of the shortest

path thatphysically existed through the nehvork when the packet

was originated.

Packet delive~ ratio is important as it describes the loss rate that

will be seen by the transport protocols, which in turn effects the

maximum throughput that tie nehvork can support. This metric

characteties both the completeness and correctness of the routing

protocol.

Routing overhead is an important metric for comparing these pro-

tocols, asit measures the scalability of a protocol, the degree to which

it will function in congested or low-bandwidth environments, and its

efdciency in terms of consuming node battew power. Protocols that

send large numbers of routing packets can dso increase the prob-

ability of packet co~lsions and may delay data packets in nehvork

intefiace @ausmission queues. We rdso evaluated the number of

bytes of routing overhead caused by the source routing header re-

quired by DSR, and present those resuks in Section 6.1. We did

not include EEE 802.11 MAC packets or ARP packets in routing

overhead, since tie routing protocols cordd be run over a variety

of different medium access or addrtis resolution protocols, each of

which wotid have different overhead Our god was to compare the

routing protocols to each other, not to fid the optimaf performance

possible in our scenarios.

kr the absence of congestion or other “noisej’ pati optimrdiw

measures the ability of the routing protocol to efficiently use nehvork

resources by selecting the shortest path from asource to a destination.

We cdcrdate it as the difference between the shortest path found

intemdly by the sirudator when the packet was onginate~ and the

number of hops the packet actually took to reach its destination.

5 Stidation Rwulh

Asnotedin Section4. 1,we conducted simdations using twodifferent

node movement speeds: a maximum speed of 20 ds (average speed

10tis) and a maximum speed of 1 tis. We fist compme the four

protocols based on the 20 tis simrdations, and then in Section 5.5

present data for 1 tis for comparison. For dl simdations, the

communication patterns were peer-to-per, with each run having

either 10,20, or 30 sources sending 4 packets per second.

5.1 Comparison Summary

Figures 2 md 3 hi~ight the relative performmce of the four routing

protocols on our tic loads of 20 sources.

All of the protocols dehver a greater percentage of the originated

data packets when there is Etie node mobifity Q.e., at large pause

time), converging to 10070delivery when there is no node motion.

DSR and AODV-LLperform ptictiarly well, dehvering over 95%

of the data packets regmdess of mobility rate. In these scenarios,

DSDV-SQ ftils to converge at pause times less than 300 seconds.

The four routing protocols impose vastiy different amounts of

overhead, as shown in Figure 3. Nearly an order of magnitude

separates DSR, which has the least overhea~ from TORA, which

hm the most. The basic character of each protocol is demonstrated

in the shape of its overhead curve. TORA, DSR, and AODV-LL are

dl on-demand protocols, and their overhead drops as the mobility

rate drops. As DSDV-SQ is a largely periodic routing protocol, its

overhead is nearly constant with respect to mobility rate.

Figure2 Comparisonbetweenthefourprotocokofthefractionof

application&m packetssu=sfi~y defivered(packetdefiveryratio)

asa functionofpausetime.PausetimeOrepr=entsconstantmotihty.

....

a.m -

● w.

-+--

----

+ ----------- +------- ___

0 lm ao ao m 5W MO

700 m w

Pa,= he (s)

F-3 timparison ktwun the forrrprotocok of thenumkr of

routing packetssent (routing overhead)as a functionof pause time.

Pause time Orepresens constant motifi~.

The TORA resuks shown in Figures 2 and 3 at pause time 600 are

the average of ofly 9 scenarios, as the overhead for tie tenth scenario

was much higher than the others due to significant congestion caused

by the routing protocol. The complete resuks are included below and

explained in Section 5.3.

5.2 Packet Defivery Ratio Deti

Figure 4 shows the fraction of the originated application data packets

each protocol was able to deliver, as a function of both node moblfity

rate (pause time) and network load (number of sources). For DSR

and AODV-LL,packet delivery ratio is independent of offered trafdc

load, with both protocols delivering between 95% and 100% of the

packets in dl cases.

DSDV-SQ fails to converge below pause time 300, where it de-

livers about 9290 of its packets. At higher rates of mobility (lower

pause times), DSDV-SQ does poorly, dropping to a 70% packet de-

livery ratio. Nearly dl of the dropped packets are lost because a stie

routing table entry directed them to be forwarded over a broken link.

As described in Section 3.1.2, DSDV-SQ maintains ordy one route

per destination and consequently, each packet that the MAC layer is

unable to deliver is dropped since there are no akemate routes.

(a) DSDV-SQ

@) DSR

---+-

-* ------

Q9-

o=~

(d)

AODV-U

F-4 Packetdefiveryratio as a fonctionofpawe time. TORAisshow on a different vertid sde for ctity (see Figure 2).

TORA does well with 10 or 20 sources, delivering between 90%

md 9570of originated data packets even at the highest rate of node

mobility (pause time O). The majority of the packet drops are due

to the creation of short-hved routing loops that are a naturrd part

of its link-reversal prwess. Consider a node A routing packets to

C via B. H B’s link to C breaks, B will reverse its link to A,

transmit an UPDA~ to notify its neighbors it has done tis, and

be~n routing packets to C via A. Until A receives the UPDA~,

data packets to C will loop between A and B. Our implementation

of TORA detects when the next-hop of a packet is the same as the

previous-hop md drops the data packet, since experiments showed

that allowing these packets to loop until their TTL expires or the

loop resolves causes more packets to be dropped overall, as the

loopingdata packets interfere with the ability of other nearby nodes to

successtily exchange the broadcast UPDA~ packet that will resolve

their routing loop.

Whh 30 sources, TOWS average packet delivery ratio drops to

~% at pause time O, although upon examination of the data we

found that variability was extremely large, with packet delivery ra-

tios ranging from S90to 9190.

h most of these scenarios, TORA

fails to converge because of incremed congestion, as explained in

Section 5.3. A very recenfly released revision to the ~P speci-

fication [4] claims to improve the refiable control message delive~

semantics provided by ~P, which might eliminate some of the be-

haviors seen here. However, these new mechanisms add more packet

overhead to TO~P which, as shown in Section 5.3, is aheady

higher than the other protocols studied here.

5S Routing Overhead De-

Figure 5 shows the number of routing protocol packets sent by each

protocol in obtaining the defivery ratios shown in Figure 4. DSR and

DSDV-SQ are plotted on a the same scale as each other, but AODV-

LL and TORA me each plotted on different scales to best show the

effect of pause time and offered load on overhead TORA, DSR,

and AODV-LL are on-demand routing protocols, so as the number

of sources increases, we expect tie number of routing packets sent

to increase because there are more destinations to which the network

must maintain working routes.

DSR and AODV-LL,which use

only on-demand packets and very

similar basic mechanisms, have almost identically shaped curves.

Both protocols exhibit the desirable property that the incremental

cost of additiond sources decreases as sources are added, since the

protocol can use information learned from one route discovery to

complete a subsequent route discovery.

However, the absolute overhead required by DSR and AODV-LL

are very different, with AODV-LL requiring about 5 times the over-

head of DSRwhen there is constant node motion (pause time O).This

dramatic increase in AODV-LUS overhead occurs because each of

its route discoveries typically propagates to every node in the ad hoc

network. For example, at pause time Owith 30 sources, AODV-LL

_—________ ._

. .

--.-—-—

—.,

.—— —

(a) DSDV-SQ

j 180,&

,-’,

-“%. ~

Is,w

.’

‘.

,“

. .

,’

@) DSR

(d) AODV-LL

F~e 5 Routing overhad as a fanction of pause time. TORA md AODV-LLare show on differentvertid sales for ctity (see Figure 3)

initiates about 2200 route discoveries Wr 900-second simtiation

run, restiting in around 110,000 ROm-nQWT transmissions. In

contrast, DSR tits the scope and overhead of ROH nQmT

packes by using caching from forwarded and promiscuously over-

heard packets and using non-propagating Ro~ mQmTS as de-

scribed in Section 3.3.2, which restits in DSR sending ordy 950

non-propagating requests and 300 propagating requests per simtia-

tion run.

TOWS overhead is the sum of a constant mobility-independent

overhead and a variable moblli~-dependent overhead. The constant

overhead is the resdt of MP’s neighbor discovery mechrmism,

which requires each node to transmit at least 1 ~LLO packet per

BmCON period (1 second). For 900-second simdations with 50

nodes, this resdts in a minimum overhead of 45,000 packets. The

variable part of the overhead consists of the routing packets TORA

uses to create and maintain routes, multiphed by the number of

retransmission and ac~owledgment packets MP uses to ensure

their rehable, in-order delivery.

h many of our scenarios with 30 sources, TORA essentially un-

derwent congestive coflapse. A positive feedback loop developed in

TO~P wherein the number of routing packets sent caused nu-

merous MAC-layer collisions, wtich in turn caused data, ACK,and

~LLO packets to be lost. The loss of these packets caused IMEP to

erroneously beheve that links to its neighbors were breaking, even in

the pause time 900 scenarios when dl nodes were stationw. TORA

reacted to the perceived link breakages by sending more UPDA~,

which closed the feedback loop by directly causing more conges-

tion. More importantly, each UPDA~ sent required reliable delive~,

which increased the system’s exposure to additionrd erroneous link

failure detections, since the ftihrre to receive an ACKfrom retrans-

mitted UPDA~ was treated as a link failure indication. In the worst

runs, TORA generated over 10 million objects, which ~P aggre-

gated into 1.6 million packets requiring rehable defivery. h the few

3@source runs where congestion did not develop, the overhead var-

ied from 639,000 packets at pause time Oto 47,000 packets at pause

time 900.

DSDV-SQ hm approximately constant overhead, regardess of

movement rate or offered trafdc load. This constant behavior arises

because each destination D broadcasts a periodic update with a new

sequence number every 15 seconds. With 50 unsynchronized nodes

in the simulation, at least one node broadcasts aperiodic update dur-

ing each second. DSDV-SQ considers the receipt of a new sequence

number for a node to be important enough to distribute immediately

(Section 3.1), so each node that receives Ds periodic update gener-

ates a triggered update. These triggered updates flood the ne~ork,

m each node receiving one learns a new sequence number and so

dso generates a triggered update. Each node limits the rate at which

it sends triggered updates to one per second, but since there is at lemt

-——-- —

.. --

—..-. . . . . . . . . .

DSDV-SQ

TORA

W,m

OSR

AOOV-U

F-6 DifferencebeWwnthe numberofhops eachpacket

tooktoreachitsdestimtionandtheoptimalnum~r ofhops

required.Dataisfor20sources.

one new sequence number per second, every node transmits triggered

updates at themtimuru permitted rate. Therefore, although the base

wnodic action of DSDV-SQ is once per 15 seconds, the effective

rate of a group of nodes is one update per node per second, yielding

an overhead of 45,000 packets for a 9Wsecond, 50-node simulation.

5.4 Path OptirnrdityDeW

As described in Section 4, an intemd mechanism of our simdator

knows the length of the shortest possible path between dl nodes in

the network at any time and labels dl packets with this path length

when they are originated. Figure 6 shows the difference behveen

this shortest path length and the length of the paths actually taken

by data packets. A difference of Omeans the packet took a shortest

path, and a difference greater than Oindicates the number of extra

hops the packet took.

Both DSDV-SQand DSR use routes very close to optimal. TORA

and AODV-LL each have a significant tail, taking up to 4 or more

hops longer than optimal for some packets, although TORA was

not designed to find shortest paths. For space reasons, Figure 6

aggregates tie data from dl pause times into one graph. Men the

data are broken out by pause time, DSDV-SQ and DSR do very well

regardess of pause time, with no statistically significant change in

optimtity of routing with respect to node mobility rate. TORA and

AODV-LL,on the other hand each show asignificant difference with

respect to pause time in the length of the routes they use relative to

the shortest possible routes. men node motiliw is very low, they

use routes that are sigrtificantiy closer to the shortest possible routes

than when nodes are moving.

5.5 Lower Speed of Node Movement

In order to explore how the protocols scrde as the rate of topology

change varies, we changed the maximum node speed from 20 tis

to 1tis and re-evahrated dl four protocols over scenario files using

this lower movement speed Figures 7 and S show the resdts of this

experiment when using 20 sources. Ml of the protocols deliver more

than 9S.5% of their packets at this movement speed. Urdike in the

20 ds scenarios, where DSDV-SQ cotdd not converge, it delivers

exce~ent performance in the 1 ds scenarios.

Even a~thisslowerrate of movement, each of the routing protocols

generated very different amounts of overhead. Neither DSR nor

AODV-LL were seriously challenged by this set of scenarios, as

the overhead increases ordy mildy as pause time decreases. The

F@e 7 Comparisonof the fraction of apptimtion datapackets

sucmsfnlly detiveredas a function of pausetime. SPA is 1tis.

N.000

-- %--- --

----

{

..O

+*--+------+----------- --- ---

---

1w2M~@0~ 6W7W8WW

Pa= tme (S)

F@e S Comparisonof the number of routingpacke~ sent

as a fiction of pause time. Speedis 1tis.

separation between DSR and AODV-LL, however, has grown from

a factor of 5 to nearly a factor of 10 because DSWS caching is even

more effective at lower speeds where the cached information goes

stie more slowly.

Due to its largely periodic nature, DSDV-SQ continues to have a

constmt overhead of approximately 41,000 packets, while TOWS

overheads dorninatedby the litistatus sensing mechanism of I~P,

which amounts to one packet per node per second, or a totrdof45,000

packets per simdation (Section 5.3).

6 Additional Observations

6.1 Overhead in Source Routing Protocols

men comparing the number of routing overhead packets sent by

each ofthe protocols, DSR clearly has the lowest overhead (Figure 5).

The data for 20 sources is reproduced in Figure 9(a) on a semi-log

axis for cltity. However, if routing overhead is measured in bytes

and includes the bytes of the source route header that DSR places in

each packet, DSR becomes more expensive than AODV-LL except

at the highest rates of mobility, although it stiUtransmits fewer bytes

of routing overhead than does DSDV-SQ or TORA. AODV-LLuses

a Route Discovery mechanism based on DSFS, but it creates hop

by-hop routing state in each node along a path in order to elitinate

—.—

.—

_——

-. *.-

----

-,.

.. . .

.0..

-+. -

: .. .

g 104

---

‘-+--

-------

--+

. .

‘.

. .

. . .

““

. . . :]

J,, ,AA&

)SDV~

IODV-U

2Mmm

100 -- --- ---

mm m (-)

~.%

. . .

/“

. . .

-..

. . .

+----------

- + -----------,

...,

0. . . .

;

., .,....

106:

...““.0.,

(a) Routingoverheadh pzkeE.

~) Roudngovedrmdinby~.

F-9 tin~ting routing overhead

inpacketsandinbytes. Both~phs usesemi-logaxes.

the overhead of source routing from data packets. This reduction in

overhead bytes is shown in Figure 90).

It is unclear whether this improvement in bytes of overhead is

significant for red world protocol operation, because the majori~ of

AODV-LL overhead bytes me carried in many smrdl packets. The

cost to acquire the medium to transmit a packet is significantly more

expensive in terms of power and network utilization than the in-

cremented cost of adding a few bytes to an existing packet, so the

acturdcost of the source route header in DSR is less than the number

of bytes might indicate. A completely fair comparison based on

overhead in bytes wodd dso have to include the cost of physical

layer framing and WC protocol bytes, which we have deliberately

factored out sinw the routing protocols codd be N over many dif-

ferent ~C implementations, each of which wotid have a different

overhead.

6.2 The Effect of Triggered Updat= in DSDV

As noted in Section 3.1.2, DSDV can employ either of two strategies

for determining when to send triggered updates. h the fist strategy,

DSDV-SQ, a node sends a triggered update each time it receives a

new sequence number for some destination. As shown in Figure 10,

DSDV-SQdelivers over 99% of its packets for dl pause times when

the maximum node speed is 1 tis. h the 20 ds case, DSDV-SQS

packet delivery ratio falls to 95% at a pause time of 300 seconds and

further decreases at higher mobility rates as DSDV-SQ is unable to

converge. Figure 11 shows that for both the 1 tis and 20 tis dat%

DSDV-SQS routing overhead is approximately 45,000 packets for

M pause times, as discussed in Section 5.3.

The second scheme for sending triggered updates, which we cdl

simply DSDV, requires that they be sent ordy when a new metric

is received for a destination. h this case, Euk breakages are not

detected as quicNy as in DSDV-SQ, generally resdting in more

dropped packets.

For a movement speed of 1 tis, DSDV delivers fewer packets

than DSDV-SQ, with its packet delivery ratio decreasing to 95%

at pause time O. While DSDV’Srouting overhead is a factor of 4

smrdler, the fact that its routing overhead is constant indicates that

a movement speed of 1 tis does not exercise the routing protocol

tily. At 20 tis, both DSDV-SQ and DSDV failto converge, causing

alarge percentage ofdatapackets tobe dropped. However,theDSDV

triggering scheme reduces the relative routing overhead by a factor

of 4 at pause time 900 md by a factor of 2 at pause time O.

63 Retiabfity h= with Broadcast Packets

Because broadcast packets are not receiver directed there is no way

to reserve the wireless medium at the receivers before transmitting a

broadcast packet (e.g., with an RTSICTS exchange). Consequently,

broadcmt packets are inherently less reliable than unicast packets.

This difference does not exist in wired networks and represents a

fidamentrd limitation of wireless networks that must be accounted

for in the design of ad hoc nework routing protocols.

Upon

sampling a number of our scentios, we found that over any single

hop, 99.S% of unicast data packets are received successtily, while

ordy 92.670of broadcast packets are received, based on counting the

number of receivers within transmission mge of the broadcasting

node. The difference between the two numbers is due to col~sions.

h fiture work, we will examine how the difference varies with the

average degree of the nodes, the size of the broadcast packets, and

the relative proportion of broadcast packets.

6.4 bteraction of ARP with On-Demand Protocoh

When an on-demand routing protocol receives packets to a destina-

tion for which it does not have a route, the protocol typically buffers

the packets in the routing layer until it can discover a route for the

packets. Once the routing protocol finds a route, it sends the queued

packets down to the link-layer for transmission. k the course of our

early experiments, however, we observed a serious layer-integration

problem that wotid effect any on-demand protocol running on top

of an ARP implementation similar to that in BSD Unix [23].

OurARP code, like the BSD code, buffers one packet perdestina-

tion awaiting a link layer address. If a series of packets are passed by

the routing layer to the ARP code with a next-hop destination whose

link-layer address is urdmown, dl but the last of these packets will

be dropped by ARP. h our simtiation, we remedied this by pacing

the rate at which packets are passed from the routing queue, though

the implementation of ARP codd be modified to buffer additiond

packets, or the routing protocol codd be coded to check that the

ARP layer has a link-layer address for the next-hop before passing it

packets from the routing queue.

7 Related Work

Some simulation resdts for DSDV,TORA, and DSR have been pre-

sentedin earlier papers, although those simrdations used substautitiy

different input parameters than ours and did not simdate the wireless

network as accurately. We are not aware of any previously published

performance resdts for AODV.

——.

———

F-10 Fraction of originateddata packets

suwssfilly detiveredby DSDV-SQand DSDV.

-x-.--x- ----- *--- ----- -“--- ----- -

4D.W 3

q;::------E,;;,,,

1ds

ISDV 20 ds

10 a

DSDV 1 dS

Figsrre11 Routing overheadas a function of

pame drne for DSDV-SQmd DSDV.

7.1 Park and Corson

An earfier simtiation of TORA was done by Park and Corson [16],

who compared TORA to an “idetizd’ link state routing protocol.

Their restits are quite different from those presented in this paper,

showing TORA delivering over 9070of its packets in dl cases, but

these resdts are incompmble to ours because of the many simplifi-

cations they made in simtiating the environment. In order to avoid

congestion, their simdation used a packet transmission rate of ordy

4, 1.5, or 0.6 packets per

minute per node. Simtiations were run

for 2 hours, the meantime between failure for links was varied from

32 minutes to 1 minute, md the average nehvork connectivity was

ticidly held constant at 90%, 70%, or 50%.

Their simtiatormodeled the nehvork as a“densely-connectedhon-

eycomW with constant node density. There is no notion of node mo-

bifity. fich node is connected to a fixed set of neighbors by sepmate

finks that cycle behveen an active and an inactive state independent

of dl other links. These links are error free, and no dynamics below

the nehvork layer are modeled. Radio propagation, medium access,

collisions, and physic~ node mobili~ are completely ignored, it is

et’enpossible for a node to correcdy receive hvo simtitaneous trans-

missions. h their simdation, link transitions cause interrupts that

give the protocols immediate feedback whenever a link goes up or

down. In reality, though, a node can ordy detect that a link has broken

if it is wing to use the link or if it fails to receive expected Priodlc

beacons over it, and anode can ordy recognize the existence of a new

neighbor when it receives a packet from that neighbor.

An earlier paper [3] presented simtiation resrdts for the protocol

on which TORA is in part based, but the sirntiator used in that study

had the same problems as the simdator used by Park and Corson,

and the metrics used are incomparable to ours.

7.2 Johnson ana Mdti

We have previously simdatea DSR [10] using the same mobility

model as in this paper. h this previous work, movement speeds

ranged from 0.3 to 0.7 tis, with the nodes moving about in a

9m x 9m space using shorter range infrared wireless transmitters

with a 3-meter range.

This simulation lacked a realistic model of radio propagation and

a MAC layer such as fEEE 802.11. These missing pieces gready

simplify tie problem faced by the routing protocol, as propagation

delay, capture effects, MAC-layer collisions, and the effects of con-

gestion due to large packet sizes are unaccounted for. Furthermore,

broadcast and unicast packets were deliverea with the same proba-

bility, and, as noted in Section 6.3, this is not a realistic assumption.

This earlier simtiation study rdso did not consider ad hoc nehvorks

of more thm 24 nodes.

hr the earher simtiation, we characterized the path optimdity

of the routing protocol by reporting the

ratio of the average route

length used to the optimal route length that codd have been used

if the routing protocol had perfect information. However, we did

not report the actual route lengths used, ana since both numbers are

averages, this ratio tendea to blur the aynamics of the protocol. As a

restit, in this paper we present path optimdity data as the

dt~erence

behveen the optimal and acturdpath lengths uses.

7S Freisleben and Jansen

DSDV and DSR were recentiy simdatea ana comparedby Freisleben

and Jansen [7]. Their sirntiations used configurations of 10 or 25

mobtie nodes, with movement speeds relative to transmission range

approximately 6 times faster than in our simrdations, making some

of their resdts incomparable to ours. Furthermore, dthougb their

restits coda be explainea by many factors, such as congestion,

collisions, or the failure of the routing protocols to converge, the

authors do not analyze or interpret the restits and it is not possible

to assess the impact of these factors from the data presented.

All of their restits rdso suffer due to a number of deficiencies in

their sirmdation and implementation of the protocols. Ml events in

their simtiator take place in re~ar time steps, restiting in perfectiy

synchronized behavior of a number of separate mobile nodes in some

cases. For example, upon receipt of a broadcast packet, W nodes

who attempt to send a response packet wi~ experience a collision on

their RTS,requiring a binary exponentird backoff and retrarrsrnission

attempt. Furthermore, they do not buffer any packets while waiting

for a Rom ~LY to be returnea auring a DSR Route Discovery,

causing large numbers of packets to be dropped needessly.

8 Conclmions

The area of ad hoc nehvorkbrg has been receiving increasing at-

tention among researchers in recent years, as the avrdlablewireless

nehvorking and mobile computing hardware bases are now capable

of supporting the promise of this technology. Over the past few

years, a variety of new routing protocols targeted specifically at the

aa hoc nehvorking environment have been proposed, but Iittie per-

formance information on each protocol and no detailed performance

comparison between the protocols has previously been available.

This paper makes contributions in hvo areas. First, we describe

our modifications to the m nehvork simtiator to provide an accu-

rate simulation of the MAC and physical-layer behavior of the EEE

.. -7-. .- ...—-

=.... .. —

..-=

_ .— --

S02.11 wireless LN standara including a retistic wireless trans-

mission channel model. This new simtiation environment provides

apowefl tool for evahtating ad hoc nehvorking protocols and other

wireless protocols and apphcations. Semn& using this simtiation

environrnen~we present the resrdts of a detailed packet-level sirnda-

tioncomparing four recent mtdti-hop wireless adhocnehvorkrouting

protocols. These protocols, DSDV,TORA, DSR, and AODV,cover

a range of design choices, including periodic advertisements vs. on-

demand route discovery, use of f~dback from the MAC layer to

indicate a failure to forward a packet to the next hop, and ho~by-

hop routing vs. source routing. We simdated each protocol in ad hoc

networks of 50 mobile nodes moving about and communicating with

each other, and presented the resdts for a range of node mobitity

rates and movement speeds.

Each of the protocols studied performs well in some cases yet

has certain drawbacks in others. DSDV performs quite predictably,

delivering virtually rdl data packets when node mobitity rate and

movement speed are low, and faihg to converge as node mobility

increases. TORA, although the worst prforrner inour experiments in

terms ofrouting packet overhea~ still defiveredover90% ofthepack-

ets in scenarios witi 10or20 sources. At 30 sources, the network was

unable tohande dl of the trafdc generated by therouting protocol and

a si~cant fraction of data packets were dropped. The performance

of DSR was very good at M mobifity rates and movement speeds,

although its use of source routing increases the number of routing

overhead bytes required by tie protocol. Findy, AODV performs

almost aswe~ as DSR at rdlmobility rates and movement speeds ad

accomphshes its god of e~inating source routing overhea~ but it

still requires the transmission of many routing overhead packets and

at high rates of node moblfity is acturdly more expensive than DSR.

Acbowled~enk

Many thanks are due to Dan Stancfl, who reviewed our propaga-

tion model for correctness and provided insights on the details of

radio propagation. We wordd dso Eke to thank Pravin Bhagwat,

Tracy Camp, Scott Corson, Vincent Park, and Charles Perkins for

answering our questions about the detils of the operation of DSDV,

TO~P, and AODV.

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