A Review of Curren t Routing Proto cols for Ad-Ho c Mobile - - PDF document

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A Review of Curren t Routing Proto cols for Ad-Ho c Mobile - - PDF document

A Review of Curren t Routing Proto cols for Ad-Ho c Mobile Wireless Net w orks Elizab eth M. Ro y er C-K T oh Dept. of Electrical & Computer Engineering Dept. of Electrical & Computer Engineering Univ


slide-1
SLIDE 1 A Review
  • f
Curren t Routing Proto cols for Ad-Ho c Mobile Wireless Net w
  • rks
Elizab eth M. Ro y er Dept.
  • f
Electrical & Computer Engineering Univ ersit y
  • f
California, San ta Barbara er
  • yer@alpha.e
c e.ucsb.e du C-K T
  • h
Dept.
  • f
Electrical & Computer Engineering Georgia Institute
  • f
T ec hnology , A tlan ta cktoh@e e.gate ch.e du Abstract An ad-ho c mobile net w
  • rk
is a collection
  • f
mobile no des that are dynamically and arbitrarily lo cated in suc h a manner that the in terconnections b et w een no des are capable
  • f
c hanging
  • n
a con tin ual basis. In
  • rder
to facilitate comm unicatio n within the net w
  • rk,
a routing proto col is used to disco v er routes b et w een no des. The primary goal
  • f
suc h an ad-ho c net w
  • rk
routing proto col is correct and ecien t route establishmen t b et w een a pair
  • f
no des so that messages ma y b e deliv ered in a timely manner. Route construction should b e done with a minim um
  • f
  • v
erhead and bandwidth consumption. This pap er examines routing proto cols for ad-ho c net w
  • rks
and ev aluates these proto cols based
  • n
a giv en set
  • f
parameters. The pap er pro vides an
  • v
erview
  • f
eigh t dieren t proto cols b y presen ting their c haracteristics and functionalit y , and then pro vides a comparison and discussion
  • f
their resp ectiv e merits and dra wbac ks. 1 In tro duction Since their emergence in the 1970s, wireless net w
  • rks
ha v e b ecome increasingly p
  • pular
in the com- puting industry . This is particularly true within the past decade whic h has seen wireless net w
  • rks
b eing adapted to enable mobilit y . There are curren tly t w
  • v
ariations
  • f
mobile wireless net w
  • rks.
The rst is kno wn as infrastructured net w
  • rks,
i.e., those net w
  • rks
with xed and wired gatew a ys. The bridges for these net w
  • rks
are kno wn as base stations. A mobile unit within these net w
  • rks
con- nects to, and comm unicates with, the nearest base station that is within its comm unication radius. As the mobile tra v els
  • ut
  • f
range
  • f
  • ne
base station and in to the range
  • f
another, a \hando "
  • ccurs
from the
  • ld
base station to the new, and the mobile is able to con tin ue comm unication seam- lessly throughout the net w
  • rk.
T ypical applications
  • f
this t yp e
  • f
net w
  • rk
include
  • ce
wireless lo cal area net w
  • rks
(WLANs). The second t yp e
  • f
mobile wireless net w
  • rk
is the infrastructureless mobile net w
  • rk,
commonly kno wn as an ad-ho c net w
  • rk.
Infrastructureless net w
  • rks
ha v e no xed routers; all no des are capable
  • f
mo v emen t and can b e connected dynamically in an arbitrary manner. No des
  • f
these net w
  • rks
function as routers whic h disco v er and main tain routes to
  • ther
no des in the net w
  • rk.
Example applications
  • f
ad-ho c net w
  • rks
are emergency searc h-and-rescue
  • p
erations, meetings
  • r
con v en tions in whic h p ersons wish to quic kly share information, and data acquisition
  • p
erations in inhospitable terrains. This pap er examines routing proto cols designed for these ad-ho c net w
  • rks
b y rst describing the
  • p
eration
  • f
eac h
  • f
the proto cols and then comparing their v arious c haracteristics. The remainder
  • f
the pap er is
  • rganized
as follo ws: Section 2 presen ts a discussion
  • f
t w
  • sub
divisions
  • f
ad-ho c routing proto cols. Subsection 2.1 discusses curren t table-driv en proto cols, while Subsection 2.2 describ es 1
slide-2
SLIDE 2

Ad-Hoc Routing Protocols Table Driven DSDV CGSR WRP Source-initiated On-Demand Driven AODV DSR LMR ABR TORA SSR

Figure 1: Categorization
  • f
Ad-Ho c Routing Proto cols. those proto cols whic h are classied as
  • n-demand.
Section 3 presen ts qualitativ e comparisons
  • f
table-driv en proto cols, follo w ed b y
  • n-demand-driv
en proto cols, and nally a general comparison
  • f
table-driv en and
  • n-demand
proto cols. Applications and c hallenges facing ad-ho c mobile wireless net w
  • rks
are discussed in Section 4. Finally , Section 5 concludes the pap er. 2 Existing Ad-Ho c Routing Proto cols Since the adv en t
  • f
D ARP A pac k et radio net w
  • rks
in the early 1970s [11 ], n umerous proto cols ha v e b een dev elop ed for ad-ho c mobile net w
  • rks.
Suc h proto cols m ust deal with the t ypical limitations
  • f
these net w
  • rks,
whic h include high p
  • w
er consumption, lo w bandwidth, and high error rates. As sho wn in Figure 1, these routing proto cols ma y generally b e categorized as: (a) table-driv en and (b) source-initiated
  • n-demand
driv en. Solid lines in this gure represen t direct descendan ts while dotted lines depict logical descendan ts. Despite b eing designed for the same t yp e
  • f
underlying net w
  • rk,
the c haracteristics
  • f
eac h
  • f
these proto cols are quite distinct. The follo wing sections describ e the proto cols and categorize them according to their c haracteristics. 2.1 T able-Driv en Routing Proto cols The table-driv en routing proto cols attempt to main tain consisten t, up-to-date routing information from eac h no de to ev ery
  • ther
no de in the net w
  • rk.
These proto cols require eac h no de to main tain
  • ne
  • r
more tables to store routing information, and they resp
  • nd
to c hanges in net w
  • rk
top
  • logy
b y propagating up dates throughout the net w
  • rk
in
  • rder
to main tain a consisten t net w
  • rk
view. The areas where they dier are the n um b er
  • f
necessary routing-related tables and the metho ds b y whic h c hanges in net w
  • rk
structure are broadcast. The follo wing sections discuss some
  • f
the existing table-driv en ad-ho c routing proto cols. 2.1.1 Destination-Sequenced Distance-V ector Routing (DSD V) The Destination-Sequenced Distance-V ector Routing proto col (DSD V) describ ed in [17 ] is a table- driv en algorithm based
  • n
the classical Bellman-F
  • rd
routing mec hanism [7 ]. The impro v emen ts made to the Bellman-F
  • rd
algorithm include freedom from lo
  • ps
in routing tables. Ev ery mobile no de in the net w
  • rk
main tains a routing table in whic h all
  • f
the p
  • ssible
desti- nations within the net w
  • rk
and the n um b er
  • f
hops to eac h destination are recorded. Eac h en try is mark ed with a sequence n um b er assigned b y the destination no de. The sequence n um b ers enable the 2
slide-3
SLIDE 3 mobile no des to distinguish stale routes from new
  • nes,
thereb y a v
  • iding
the formation
  • f
routing lo
  • ps.
Routing table up dates are p erio dically transmitted throughout the net w
  • rk
in
  • rder
to main- tain table consistency . T
  • help
alleviate the p
  • ten
tially large amoun t
  • f
net w
  • rk
trac that suc h up dates can generate, route up dates can emplo y t w
  • p
  • ssible
t yp es
  • f
pac k ets. The rst is kno wn as a \full dump." This t yp e
  • f
pac k et carries all a v ailable routing information and can require m ul- tiple net w
  • rk
proto col data units (NPDUs). During p erio ds
  • f
  • ccasional
mo v emen t, these pac k ets are transmitted infrequen tly . Smaller \incremen tal" pac k ets are used to rela y
  • nly
that information whic h has c hanged since the last full dump. Eac h
  • f
these broadcasts should t in to a standard size NPDU, thereb y decreasing the amoun t
  • f
trac generated. The mobile no des main tain an additional table where they store the data sen t in the incremen tal routing information pac k ets. New route broadcasts con tain the address
  • f
the destination, the n um b er
  • f
hops to reac h the destination, the sequence n um b er
  • f
the information receiv ed regarding the destination, as w ell as a new sequence n um b er unique to the broadcast [17 ]. The route lab eled with the most recen t sequence n um b er is alw a ys used. In the ev en t that t w
  • up
dates ha v e the same sequence n um b er, the route with the smaller metric is used in
  • rder
to
  • ptimize
(shorten) the path. Mobiles also k eep trac k
  • f
the settling time
  • f
routes,
  • r
the w eigh ted a v erage time that routes to a destination will uctuate b efore the route with the b est metric is receiv ed (see [17 ]). By dela ying the broadcast
  • f
a routing up date b y the length
  • f
the settling time, mobiles can reduce net w
  • rk
trac and
  • ptimize
routes b y eliminating those broadcasts that w
  • uld
  • ccur
if a b etter route w as disco v ered in the v ery near future. 2.1.2 Clusterhead Gatew a y Switc h Routing (CGSR) The Clusterhead Gatew a y Switc h Routing (CGSR) proto col diers from the previous proto col in the t yp e
  • f
addressing and net w
  • rk
  • rganization
sc heme emplo y ed. Instead
  • f
a \at" net w
  • rk,
CGSR is a clustered m ultihop mobile wireless net w
  • rk
with sev eral heuristic routing sc hemes [4]. The authors state that b y ha ving a cluster head con trolling a group
  • f
ad-ho c no des, a framew
  • rk
for co de separation (among clusters), c hannel access, routing and bandwidth allo cation can b e ac hiev ed. A cluster head selection algorithm is utilized to elect a no de as the cluster head using a distributed algorithm within the cluster. The disadv an tage
  • f
ha ving a cluster head sc heme is that frequen t cluster head c hanges can adv ersely aect routing proto col p erformance since no des are busy in cluster head selection rather than pac k et rela ying. Hence, instead
  • f
in v
  • king
cluster head reselection ev ery time the cluster mem b ership c hanges, a Least Cluster Change (LCC) clustering algorithm is in tro duced. Using LCC, cluster heads
  • nly
c hange when t w
  • cluster
heads come in to con tact,
  • r
when a no de mo v es
  • ut
  • f
con tact
  • f
all
  • ther
cluster heads.

1 2 3 4 5 6 7 8

Cluster head Gateway Node

Figure 2: CGSR: Routing from No de 1 to No de 8. CGSR uses DSD V as the underlying routing sc heme, and hence has m uc h
  • f
the same
  • v
erhead as DSD V. Ho w ev er, it mo dies DSD V b y using a hierarc hical cluster head-to-gatew a y routing approac h 3
slide-4
SLIDE 4 to route trac from source to destination. Gatew a y no des are no des that are within comm unication range
  • f
t w
  • r
more cluster heads. A pac k et sen t b y a no de is rst routed to its cluster head, and then the pac k et is routed from the cluster head to a gatew a y to another cluster head, and so
  • n
un til the cluster head
  • f
the destination no de is reac hed. The pac k et is then transmitted to the destination. Figure 2 illustrates an example
  • f
this routing sc heme. Using this metho d, eac h no de m ust k eep a \cluster mem b er table" where it stores the destination cluster head for eac h mobile no de in the net w
  • rk.
These cluster mem b er tables are broadcast b y eac h no de p erio dically using the DSD V algorithm. No des up date their cluster mem b er tables
  • n
the reception
  • f
suc h a table from a neigh b
  • r.
In addition to the cluster mem b er table, eac h no de m ust also main tain a routing table, whic h is used to determine the next hop in
  • rder
to reac h the destination. On receiving a pac k et, a no de will consult its cluster mem b er table and routing table to determine the nearest cluster head along the route to the destination. Next the no de will c hec k its routing table to determine the no de in
  • rder
to reac h the selected cluster head. It then transmits the pac k et to this no de. 2.1.3 The Wireless Routing Proto col (WRP) The Wireless Routing Proto col (WRP) describ ed in [14 ] is a table-based proto col with the goal
  • f
main taining routing information among all no des in the net w
  • rk.
Eac h no de in the net w
  • rk
is resp
  • nsible
for main taining four tables: (a) distance table, (b) routing table, (c) link-cost table, and (d) message retransmission list (MRL) table. Eac h en try
  • f
the MRL con tains the sequence n um b er
  • f
the up date message, a retransmission coun ter, an ac kno wledgmen t-required ag v ector with
  • ne
en try p er neigh b
  • r,
and a list
  • f
up dates sen t in the up date message. The MRL records whic h up dates in an up date message need to b e retransmitted and whic h neigh b
  • rs
should ac kno wledge the retransmission [14 ]. Mobiles inform eac h
  • ther
  • f
link c hanges through the use
  • f
up date messages. An up date message is sen t
  • nly
b et w een neigh b
  • ring
no des and con tains a list
  • f
up dates (the destination, the distance to the destination, and the predecessor
  • f
the destination), as w ell as a list
  • f
resp
  • nses
indicating whic h mobiles should ac kno wledge (A CK) the up date. Mobiles send up date messages after pro cessing up dates from neigh b
  • rs
  • r
detecting a c hange in a link to a neigh b
  • r.
In the ev en t
  • f
the loss
  • f
a link b et w een t w
  • no
des, the no des send up date messages to their neigh b
  • rs.
The neigh b
  • rs
then up date their distance table en tries and c hec k for new p
  • ssible
paths through
  • ther
no des. An y new paths are rela y ed bac k to the
  • riginal
no des so that they can up date their tables accordingly . No des learn
  • f
the existence
  • f
their neigh b
  • rs
from the receipt
  • f
ac kno wledgmen ts and
  • ther
messages. If a no de is not sending messages, it m ust send a hel lo message within a sp ecied time p erio d to ensure connectivit y . Otherwise, the lac k
  • f
messages from the no de indicates the failure
  • f
that link; this ma y cause a false alarm. When a mobile receiv es a hel lo message from a new no de, that new no de is added to the mobile's routing table, and the mobile sends the new no de a cop y
  • f
its routing table information. P art
  • f
the no v elt y
  • f
WRP stems from the w a y in whic h it ac hiev es lo
  • p
freedom. In WRP , rout- ing no des comm unicate the distance and second-to-last hop information for eac h destination in the wireless net w
  • rks.
WRP b elongs to the class
  • f
path nding algorithms with an imp
  • rtan
t exception. It a v
  • ids
the \coun t-to-innit y" problem [21 ] b y forcing eac h no de to p erform consistency c hec ks
  • f
predecessor information rep
  • rted
b y all its neigh b
  • rs.
This ultimately (though not instan taneously) eliminates lo
  • ping
situations and pro vides faster route con v ergence when a link failure ev en t
  • ccurs.
4
slide-5
SLIDE 5

N1 N2 N3 N4 N5 N6 N7 N8 N1 N2 N3 N4 N5 N6 N7 N8 N1 N2 N3 N4 N5 N6 N7 N8 N1 N2 N3 N4 N5 N6 N7 N8 N1 N2 N3 N4 N5 N6 N7 N8 N1 N2 N3 N4 N5 N6 N7 N8

(a) Propagation of the RREQ (b) Path of the RREP to the Source Destination Destination Source Source

Figure 3: A OD V Route Disco v ery . 2.2 Source-Initiated On-Demand Routing A dieren t approac h from table-driv en routing is source-initiated
  • n-demand
routing. This t yp e
  • f
routing creates routes
  • nly
when desired b y the source no de. When a no de requires a route to a destination, it initiates a route disco v ery pro cess within the net w
  • rk.
This pro cess is completed
  • nce
a route is found
  • r
all p
  • ssible
route p erm utations ha v e b een examined. Once a route has b een established, it is main tained b y some form
  • f
route main tenance pro cedure un til either the destination b ecomes inaccessible along ev ery path from the source
  • r
un til the route is no longer desired. 2.2.1 Ad-ho c On-Demand Distance V ector Routing (A OD V) The Ad-ho c On-Demand Distance V ector (A OD V) routing proto col describ ed in [19 ] builds
  • n
the DSD V algorithm previously describ ed. A OD V is an impro v emen t
  • n
DSD V b ecause it t ypically minimizes the n um b er
  • f
required broadcasts b y creating routes
  • n
an
  • n-demand
basis, as
  • pp
  • sed
to main taining a complete list
  • f
routes as in the DSD V algorithm. The authors
  • f
A OD V classify it as a pur e
  • n-demand
r
  • ute
ac quisition system, as no des that are not
  • n
a selected path do not main tain routing information
  • r
participate in routing table exc hanges [19 ]. When a source no de desires to send a message to some destination no de and do es not already ha v e a v alid route to that destination, it initiates a Path Disc
  • very
pro cess to lo cate the
  • ther
no de. It broadcasts a route request (RREQ) pac k et to its neigh b
  • rs,
whic h then forw ard the request to their neigh b
  • rs,
and so
  • n,
un til either the destination
  • r
an in termediate no de with a \fresh enough" route to the destination is lo cated. Figure 3a illustrates the propagation
  • f
the broadcast RREQs across the net w
  • rk.
A OD V utilizes destination sequence n um b ers to ensure all routes are lo
  • p-free
and con tain the most recen t route information. Eac h no de main tains its
  • wn
sequence n um b er, as w ell as a broadcast ID. The broadcast ID is incremen ted for ev ery RREQ the no de initiates, and together with the no de's IP address, uniquely iden ties a RREQ. Along with its
  • wn
sequence n um b er and the broadcast ID, the source no de includes in the RREQ the most recen t sequence n um b er it has for the destination. In termediate no des can reply to the RREQ
  • nly
if they ha v e a route to the destination whose corresp
  • nding
destination sequence n um b er is greater than
  • r
equal to that con tained in the RREQ. During the pro cess
  • f
forw arding the RREQ, in termediate no des record in their route tables the address
  • f
the neigh b
  • r
from whic h the rst cop y
  • f
the broadcast pac k et is receiv ed, thereb y establishing a rev erse path. If additional copies
  • f
the same RREQ are later receiv ed, these pac k ets are discarded. Once the RREQ reac hes the destination
  • r
an in termediate no de with a fresh enough route, the destination/in termediate no de resp
  • nds
b y unicasting a route reply (RREP) pac k et bac k to the neigh b
  • r
from whic h it rst receiv ed the RREQ (Figure 3b). As the RREP is routed bac k 5
slide-6
SLIDE 6

(b) Propagation of the Route Reply with the Route Record

N2

N3 N4 N5

N6 N7 N8

N1

Destination Source

N1-N2-N5-N8 N1-N2-N5-N8 N1-N2-N5-N8

N2

N3 N4 N5

N6 N7 N8

N1

Destination Source

N1 N1-N2 N1 N1-N3 N1-N2-N5 N1-N3-N4 N1-N3-N4 N1-N3-N4 N1-N3-N4-N6 N1-N3-N4-N7

(a) Building of the Route Record during Route Discovery

Figure 4: Creation
  • f
the R
  • ute
R e c
  • r
d in DSR. along the rev erse path, no des along this path set up forw ard route en tries in their route tables whic h p
  • in
t to the no de from whic h the RREP came. These forw ard route en tries indicate the activ e forw ard route. Asso ciated with eac h route en try is a route timer whic h will cause the deletion
  • f
the en try if it is not used within the sp ecied lifetime. Because the RREP is forw arded along the path established b y the RREQ, A OD V
  • nly
supp
  • rts
the use
  • f
symmetric links. Routes are main tained as follo ws. If a source no de mo v es, it is able to reinitiate the route disco v ery proto col to nd a new route to the destination. If a no de along the route mo v es, its upstream neigh b
  • r
notices the mo v e and propagates a link failur e notic ation message (a RREP with innite metric) to eac h
  • f
its activ e upstream neigh b
  • rs
to inform them
  • f
the erasure
  • f
that part
  • f
the route [19 ]. These no des in turn propagate the link failur e notic ation to their upstream neigh b
  • rs,
and so
  • n
un til the source no de is reac hed. The source no de ma y then c ho
  • se
to re-initiate route disco v ery for that destination if a route is still desired. An additional asp ect
  • f
the proto col is the use
  • f
hel lo messages, p erio dic lo cal broadcasts b y a no de to inform eac h mobile no de
  • f
  • ther
no des in its neigh b
  • rho
  • d.
Hello messages can b e used to main tain the lo cal connectivit y
  • f
a no de. Ho w ev er the use
  • f
hello messages is not required. No des listen for retransmissions
  • f
data pac k ets to ensure the next hop is still within reac h. If suc h a retransmission is not heard, the no de ma y use an y
  • ne
  • f
a n um b er
  • f
tec hniques, including the reception
  • f
hello messages, to determine whether the next hop is within comm unication range. The hel lo messages ma y list the
  • ther
no des from whic h a mobile has heard, thereb y yielding a greater kno wledge
  • f
the net w
  • rk
connectivit y . 2.2.2 Dynamic Source Routing (DSR) The Dynamic Source Routing (DSR) proto col presen ted in [10 ] is an
  • n-demand
routing proto col that is based
  • n
the concept
  • f
source routing. Mobile no des are required to main tain route cac hes that con tain the source routes
  • f
whic h the mobile is a w are. En tries in the route cac he are con tin ually up dated as new routes are learned. The proto col consists
  • f
t w
  • ma
jor phases: route disco v ery and route main tenance. When a mobile no de has a pac k et to send to some destination, it rst consults its route cac he to determine whether it already has a route to the destination. If it has an unexpired route to the destination, it will use this route to send the pac k et. On the
  • ther
hand, if the no de do es not ha v e suc h a route, it initiates route disco v ery b y broadcasting a r
  • ute
r e quest pac k et. This r
  • ute
r e quest con tains the address
  • f
the destination, along with the source no de's address and a unique iden tication n um b er. Eac h no de receiving the pac k et c hec ks whether it kno ws
  • f
a route to the destination. If it do es not, it adds its
  • wn
address to the r
  • ute
r e c
  • r
d
  • f
the pac k et and then forw ards the pac k et along its
  • utgoing
links. T
  • limit
the n um b er
  • f
r
  • ute
r e quests propagated
  • n
the
  • utgoing
links
  • f
a no de, a 6
slide-7
SLIDE 7

Destination Source

Ad Hoc Node ’height’ metric

(a)

(1) (2) (3) (4) A B C G F E D A B C D E F G A B C D E F G A B C D E F G Link Reversal Link Failure

(b)

Figure 5: (a) Route creation (sho wing link direction assignmen t), and(b) Route Main tenace (sho wing link rev ersal phenonemon) in TORA. mobile
  • nly
forw ards the r
  • ute
r e quest if the request has not y et b een seen b y the mobile and if the mobile's address do es not already app ear in the r
  • ute
r e c
  • r
d. A r
  • ute
r eply is generated when either the r
  • ute
r e quest reac hes the destination itself,
  • r
when it reac hes an in termediate no de whic h con tains in its route cac he an unexpired route to the destination [2 ]. By the time the pac k et reac hes either the destination
  • r
suc h an in termediate no de, it con tains a r
  • ute
r e c
  • r
d yielding the sequence
  • f
hops tak en. Figure 4a illustrates the formation
  • f
the r
  • ute
r e c
  • r
d as the r
  • ute
r e quest propagates through the net w
  • rk.
If the no de generating the r
  • ute
r eply is the destination, it places the r
  • ute
r e c
  • r
d con tained in the r
  • ute
r e quest in to the r
  • ute
r eply. If the resp
  • nding
no de is an in termediate no de, it will app end its cac hed route to the r
  • ute
r e c
  • r
d and then generate the r
  • ute
r eply. T
  • return
the r
  • ute
r eply, the resp
  • nding
no de m ust ha v e a route to the initiator. If it has a route to the initiator in its route cac he, it ma y use that route. Otherwise, if symmetric links are supp
  • rted,
the no de ma y rev erse the route in the r
  • ute
r e c
  • r
d. If symmetric links are not supp
  • rted,
the no de ma y initiate its
  • wn
route disco v ery and piggybac k the r
  • ute
r eply
  • n
the new r
  • ute
r e quest. Figure 4b sho ws the transmission
  • f
the r
  • ute
r eply with its asso ciated r
  • ute
r e c
  • r
d bac k to the source no de. Route main tenance is accomplished through the use
  • f
r
  • ute
err
  • r
pac k ets and ac kno wledgmen ts. R
  • ute
err
  • r
pac k ets are generated at a no de when the data link la y er encoun ters a fatal transmis- sion problem. When a r
  • ute
err
  • r
pac k et is receiv ed, the hop in error is remo v ed from the no de's route cac he and all routes con taining the hop are truncated at that p
  • in
t. In addition to r
  • ute
err
  • r
messages, ac kno wledgmen ts are used to v erify the correct
  • p
eration
  • f
the route links. Suc h ac kno wl- edgmen ts include passiv e ac kno wledgmen ts, where a mobile is able to hear the next hop forw arding the pac k et along the route. 2.2.3 T emp
  • rally-Ordered
Routing Algorithm (TORA) TORA (T emp
  • rally-Ordered
Routing Algorithm) is a highly adaptiv e, lo
  • p-free,
distributed rout- ing algorithm based
  • n
the concept
  • f
link rev ersal [16 ]. TORA is prop
  • sed
to
  • p
erate in a highly dynamic mobile net w
  • rking
en vironmen t. It is source-initiated and pro vides m ultiple routes for an y desired source/destination pair. The k ey design concept
  • f
TORA is the lo calization
  • f
con trol mes- 7
slide-8
SLIDE 8 sages to a v ery small set
  • f
no des near the
  • ccurrence
  • f
a toplogical c hange. T
  • accomplish
this, no des need to main tain routing information ab
  • ut
adjacen t (1-hop) no des. The proto col p erforms three basic functions: (a) route creation, (b) route main tainence, and (c) route erasure. During the route creation and main tenance phases, no des use a \heigh t" metric to establish a directed acyclic graph (D A G) ro
  • ted
at the destination. Thereafter, links are assigned a direction (upstream
  • r
do wnstream) based
  • n
the relativ e heigh t metric
  • f
neigh b
  • ring
no des, as sho wn in Figure 5a. This pro cess
  • f
establishing a D A G is similar to the query/reply pro cess prop
  • sed
in LMR (Ligh t w eigh t Mobile Routing) [5 ]. In times
  • f
no de mobilit y , the D A G route is brok en and route main tenace is necessary to re-establish a D A G ro
  • ted
at the same destination. As sho wn in Figure 5b, up
  • n
failure
  • f
the last do wnstream link, a no de generates a new reference lev el whic h results in the propagation
  • f
that reference lev el b y neigh b
  • ring
no des, eectiv ely co
  • rdinating
a structured reaction to the failure. Links are rev ersed to reect the c hange in adapting to the new reference lev el. This has the same eect as rev ersing the direction
  • f
  • ne
  • r
more links when a no de has no do wnstream links. Timing is an imp
  • rtan
t factor for TORA b ecause the \heigh t" metric is dep enden t
  • n
the logical time
  • f
a link failure; TORA assumes all no des ha v e sync hronized clo c ks (accomplished via an external time source suc h as Global P
  • sitioning
System). TORA's metric is a quin tuple comprised
  • f
v e elemen ts, namely: (a) logical time
  • f
a link failure, (b) the unique ID
  • f
the no de that dened the new reference lev el, (c) a reection indicator bit, (d) a propagation
  • rdering
parameter, and (e) the unique ID
  • f
the no de. The rst three elemen ts collectiv ely represen t the reference lev el. A new reference lev el is dened eac h time a no des loses its last do wnstream link due to a link failure. TORA's route erasure phase essen tially in v
  • lv
es
  • ding
a broadcast \clear pac k et" (CLR) throughout the net w
  • rk
to erase in v alid routes. In TORA, there is a p
  • ten
tial for
  • scillations
to
  • ccur,
esp ecially when m ultiple sets
  • f
co
  • rdi-
nating no des are concurren tly detecting partitions, erasing routes, and building new routes based
  • n
eac h
  • ther.
Because TORA uses in terno dal co
  • rdination,
its instabilit y problem is similar to the \coun t-to-innit y" problem in distance-v ector routing proto cols, except that suc h
  • scillations
are temp
  • rary
and route con v ergence will ultimately
  • ccur.
2.2.4 Asso ciativit y-Based Routing (ABR) A totally dieren t approac h in mobile routing is prop
  • sed
in [22 ]. The Asso ciativit y-Based Routing (ABR) proto col is free from lo
  • ps,
deadlo c k, and pac k et duplicates, and denes a new routing metric for ad-ho c mobile net w
  • rks.
This metric is kno wn as the de gr e e
  • f
asso ciation stability. In ABR, a route is selected based
  • n
the degree
  • f
asso ciation stabilit y
  • f
mobile no des. Eac h no de p erio dically generates a b eacon to signify its existence. When receiv ed b y neigh b
  • ring
no des, this b eaconing causes their asso ciativit y tables to b e up dated. F
  • r
eac h b eacon receiv ed, the asso ciativit y tic k
  • f
the curren t no de with resp ect to the b eaconing no de is incremen ted. Asso ciation stabilit y is dened b y connection stabilit y
  • f
  • ne
no de with resp ect to another no de
  • v
er time and space. A high degree
  • f
asso ciation stabilit y ma y indicate a lo w state
  • f
no de mobilit y , while a lo w degree ma y indicate a high state
  • f
no de mobilit y . Asso ciativit y tic ks are reset when the neigh b
  • rs
  • f
a no de
  • r
the no de itself mo v es
  • ut
  • f
pro ximit y . A fundamen tal
  • b
jectiv e
  • f
ABR is to deriv e longer-liv ed routes for ad-ho c mobile net w
  • rks.
The three phases
  • f
ABR are: (a) route disco v ery , (b) route re-construction (RR C), and (c) route deletion. The route disco v ery phase is accomplished b y a broadcast query and a w ait-reply (BQ-REPL Y) cycle. A no de desiring a route broadcasts a BQ message in searc h
  • f
mobiles that ha v e a route to the destination. All no des receiving the query (that are not the destination) app end their addresses and their asso ciativit y tic ks with their neigh b
  • rs
along with QoS information to the 8
slide-9
SLIDE 9

(b) Route Maintenance for a Destination Move SRC DEST’ DEST LQ[H] RN[0] RN[0] H=3 SRC SRC’ RN[1] BQ DEST (a) Route Maintenance for a Source Move

Figure 6: Route Main tenance for Source and Destination Mo v emen t in ABR. query pac k et. A successor no de erases its upstream no de neigh b
  • rs'
asso ciativit y tic k en tries and retains
  • nly
the en try concerned with itself and its upstream no de. In this w a y , eac h resultan t pac k et arriving at the destination will con tain the asso ciativit y tic ks
  • f
the no des along the route to the destination. The destination is then able to select the b est route b y examining the asso ciativit y tic ks along eac h
  • f
the paths. In the case where m ultiple paths ha v e the same
  • v
erall degree
  • f
asso ciation stabilit y , the route with the minim um n um b er
  • f
hops is selected. The destination then sends a REPL Y pac k et bac k to the source along this path. No des propagating the REPL Y mark their routes as v alid. All
  • ther
routes remain inactiv e and the p
  • ssibilit
y
  • f
duplicate pac k ets arriving at the destination is a v
  • ided.
Route re-construction ma y consist
  • f
partial route disco v ery , in v alid route erasure, v alid route up dates, and new route disco v ery , dep ending
  • n
whic h no de(s) along the route mo v e. Mo v emen t b y the source results in a new BQ-REPL Y pro cess, as sho wn in Figure 6a. The RN[1] message is a route notication that is used to erase the route en tries asso ciated with do wnstream no des. When the destination no de mo v es, the immediate upstream no de erases its route and determines if the no de is still reac hable b y a lo calized query (LQ[H]) pro cess, where H refers to the hop coun t from the upstream no de to the destination (Figure 6b). If the destination receiv es the LQ pac k et, it REPL Ys with the b est partial route;
  • therwise,
the initiating no de times
  • ut
and the pro cess bac ktrac ks to the next upstream no de. Here an RN[0] message is sen t to the next upstream no de to erase the in v alid routes and inform this no de it should in v
  • k
e the LQ[H] pro cess. If this pro cess results in bac ktrac king more than halfw a y to the source, the LQ pro cess is discon tin ued and a new BQ pro cess is initiated at the source. When a disco v ered route is no longer desired, the source no de initiates a route delete (RD) broad- cast so that all no des along the route up date their routing tables. The RD message is propagated b y a full broadcast, as
  • pp
  • sed
to a directed broadcast, b ecause the source no de ma y not b e a w are
  • f
an y route no de c hanges that
  • ccurred
during route re-constructions. 2.2.5 Signal Stabilit y Routing (SSR) Another
  • n-demand
proto col is the Signal Stabilit y based Adaptiv e Routing proto col (SSR) presen ted in [6]. Unlik e the algorithms describ ed so far, SSR selects routes based
  • n
the signal strength b et w een no des and
  • n
a no de's lo cation stabilit y . This route selection criteria has the eect
  • f
c ho
  • sing
routes that ha v e \stronger" connectivities. SSR can b e divided in to t w
  • co
  • p
erativ e proto cols: the Dynamic Routing Proto col (DRP) and the Static Routing Proto col (SRP). The DRP is resp
  • nsible
for the main tenance
  • f
the Signal Stabilit y T able (SST) and the Routing T able (R T). The SST records the signal strength
  • f
neigh b
  • ring
no des, whic h is
  • btained
b y p erio dic b eacons from the link la y er
  • f
eac h neigh b
  • ring
no de. The signal strength ma y b e recorded as either a 9
slide-10
SLIDE 10 strong
  • r
w eak c hannel. All transmissions are receiv ed b y , and pro cessed in, the DRP . After up dating all appropriate table en tries, the DRP passes a receiv ed pac k et to the SRP . The SRP pro cesses pac k ets b y passing the pac k et up the stac k if it is the in tended receiv er
  • r
lo
  • king
up the destination in the R T and then forw arding the pac k et if it is not. If no en try is found in the R T for the destination, a r
  • ute-se
ar ch pro cess is initiated to nd a route. Route requests are propagated throughout the net w
  • rk
but are
  • nly
forw arded to the next hop if they are receiv ed
  • v
er strong c hannels and ha v e not b een previously pro cessed (to prev en t lo
  • ping).
The destination c ho
  • ses
the rst arriving r
  • ute-se
ar ch pac k et to send bac k b ecause it is most probable that the pac k et arriv ed
  • v
er the shortest and/or least congested path. The DRP then rev erses the selected route and sends a r
  • ute-r
eply message bac k to the initiator. The DRP
  • f
the no des along the path up date their R Ts accordingly . R
  • ute-se
ar ch pac k ets arriving at the destination ha v e necessarily c hosen the path
  • f
strongest signal stabilit y , as the pac k ets are dropp ed at a no de if they ha v e arriv ed
  • v
er a w eak c hannel. If there is no r
  • ute-r
eply message receiv ed at the source within a sp ecic timeout p erio d, the source c hanges the PREF eld in the header to indicate that w eak c hannels are acceptable, as these ma y b e the
  • nly
links
  • v
er whic h the pac k et can b e propagated. When a failed link is detected within the net w
  • rk,
the in termediate no des send an error message to the source indicating whic h c hannel has failed. The source then initiates another r
  • ute-se
ar ch pro cess to nd a new path to the destination. The source also sends an erase message to notify all no des
  • f
the brok en link. 3 Comparisons The follo wing sections pro vide comparisons
  • f
the previously describ ed routing algorithms. Sec- tion 3.1 compares table-driv en proto cols, and Section 3.2 compares
  • n-demand
proto cols. Section 3.3 presen ts a discussion
  • f
the t w
  • classes
  • f
algorithms. In T ables 1 and 2, Time Complexit y is dened as the n um b er
  • f
steps needed to p erform a proto col
  • p
eration, and Comm unication Complexit y is the n um b er
  • f
messages needed to p erform a proto col
  • p
eration [5], [23 ]. Also, the v alues for these metrics represen t worst c ase b eha vior. 3.1 T able-Driv en Proto cols Our discussion here will b e based
  • n
T able 1. As stated earlier, DSD V routing is essen tially a mo dication
  • f
the basic Bellman-F
  • rd
routing algorithm. The mo dications include the guaran tee
  • f
lo
  • p-free
routes and a simple route up date proto col. While
  • nly
pro viding
  • ne
path to an y giv en destination, DSD V selects the shortest path based
  • n
the n um b er
  • f
hops to the destination. DSD V pro vides t w
  • t
yp es
  • f
up date messages,
  • ne
  • f
whic h is signican tly smaller than the
  • ther.
The smaller up date message can b e used for incremen tal up dates so that the en tire routing table need not b e transmitted for ev ery c hange in net w
  • rk
top
  • logy
. Ho w ev er, DSD V is inecien t b ecause
  • f
the requiremen t
  • f
p erio dic up date transmissions, regardless
  • f
the n um b er
  • f
c hanges in the net w
  • rk
top
  • logy
. This eectiv ely limits the n um b er
  • f
no des that can connect to the net w
  • rk
since the
  • v
erhead gro ws as O(n 2 ). In CGSR, DSD V is used as the underlying routing proto col. Routing in CGSR
  • ccurs
  • v
er cluster heads and gatew a ys. A cluster head table is necessary in addition to the routing table. One adv an tage
  • f
CGSR is that sev eral heuristic metho ds can b e emplo y ed to impro v e the proto col's p erformance. These metho ds include priorit y tok en sc heduling, gatew a y co de sc heduling, and path reserv ation [4 ]. 10
slide-11
SLIDE 11 P arameter s DSD V CGSR WRP Time Complexit y (link addition / failure) O(d) O(d) O(h) Comm unicati
  • n
Complexit y (link addition / failure) O(x=N) O(x=N) O(x=N) Routing Philosoph y Flat Hierarc hical Flat 1 Lo
  • p
F ree Y es Y es Y es, but not instan taneou s Multicast Capabilit y No No 2 No Num b er
  • f
Required T ables Tw
  • Tw
  • F
  • ur
F requency
  • f
Up date T ransmissions P erio dically P erio dically P erio dically & as needed & as needed Up dates T ransmitted to Neigh b
  • rs
Neigh b
  • rs
Neigh b
  • rs
& cluster head Utilizes Sequence Num b ers Y es Y es Y es Utilizes \Hello" Messages Y es No Y es Critical No des No Y es (cluster head) No Routing Metric Shortest P ath Shortest P ath Shortest P ath T able 1: Comparisons
  • f
the Characteristics
  • f
T able-Driv en Routing Proto cols. Abbreviations: N = N umber
  • f
nodes in the netw
  • r
k d = N etw
  • r
k diameter h = H eig ht
  • f
r
  • uting
tr ee x = N umber
  • f
nodes af f ected by a topolog ical chang e The WRP proto col diers from the
  • ther
proto cols in sev eral w a ys. WRP requires eac h no de to main tain four routing tables. This can lead to substan tial memory requiremen ts, esp ecially when the n um b er
  • f
no des in the net w
  • rk
is large. F urthermore, the WRP proto col requires the use
  • f
hel lo pac k ets whenev er there are no recen t pac k et transmissions from a giv en no de. The hel lo pac k ets consume bandwidth and disallo w a no de to en ter sleep mo de. Ho w ev er, though it b elongs to the class
  • f
path nding algorithms, WRP has an adv an tage
  • v
er
  • ther
path nding algorithms b ecause it a v
  • ids
the problem
  • f
creating temp
  • rary
routing lo
  • ps
that these algorithms ha v e through the v erication
  • f
predecessor information, as describ ed in Section 2.1.3. Ha ving discussed the
  • p
eration and c haracteristics
  • f
eac h
  • f
the existing table-driv en based routing proto cols, it is imp
  • rtan
t to highligh t the dierences. During link failures, WRP has lo w er time complexit y than DSD V since it
  • nly
informs neigb
  • ring
no des ab
  • ut
link status c hanges. During link additions, hel lo messages are used as a presence indicator suc h that the routing table en try can b e up dated. Again, this
  • nly
aects neigh b
  • ring
no des. In CGSR, b ecause routing p erformance is dep enden t
  • n
the status
  • f
sp ecic no des (cluster head, gatew a y
  • r
normal no des), time complexit y
  • f
a link failure asso ciated with a cluster head is higher than DSD V, giv en the additional time needed to p erform cluster head reselection. Similarly , this applies to the case
  • f
link additions asso ciated with the cluster head. There is no gatew a y selection in CGSR since eac h no de declares it is a gatew a y no de to its neigh b
  • rs
if it is resp
  • nding
to m ultiple radio co des. If a gatew a y no de mo v es
  • ut
  • f
range, the routing proto col is resp
  • nsible
for routing the pac k et to another gatew a y . In terms
  • f
comm unication complexit y , since DSD V, CGSR and WRP use distance v ector shortest-path routing as the underlying routing proto col, they all ha v e the same degree
  • f
com- plexit y during link failures and additions. 1 While WRP itself uses at addressing, it can b e used hierarc hicall y [15]. 2 The proto col itself curren tly do es not supp
  • rt
m ulticast; ho w ev er, there is a separate proto col describ ed in [3], whic h runs
  • n
top
  • f
CGSR and pro vides m ulticast capabilt y . 11
slide-12
SLIDE 12 P erformance P arameters A OD V DSR TORA ABR SSR Time Complexit y O(2d) O(2d) O(2d) O(d+z) O(d+z) (initializatio n) Time Complexit y O(2d) O(2d)
  • r
O(2d) O(l+z) O(l+z) (p
  • stfailure)
(cac he hit) Comm unication Complexit y O(2N) O(2N) O(2N) O(N+y) O(N+y) (initializatio n) Comm unication Complexit y O(2N) O(2N) O(2x) O(x+y) O(x+y) (p
  • stfailure)
Routing Philosoph y Flat Flat Flat Flat Flat Lo
  • p
F ree Y es Y es Y es Y es Y es Multicast Capabilit y Y es No No 3 No No Beaconing Requiremen ts No No No Y es Y es Multiple Route P
  • ssibilities
No Y es Y es No No Routes Main tained in route route route route route table cac he table table table Utilizes Route Cac he/T able Y es No No No No Expiration Timers Route Reconguratio n Erase Route; Erase Route; Link Rev ersal; Lo calized Erase Route; Metho dology Notify Source Notify Source Route Repair Broadcast Query Notify Source Routing Metric F reshest & Shortest Shortest P ath Asso ciativit y & Asso ciativit y & Shortest P ath P ath Shortest P ath & Stabilit y
  • thers
4 T able 2: Comparisons
  • f
the Characteristics
  • f
Source-Initiated On-Demand Ad-Ho c Routing Pro- to cols. Abbreviations: l = D iameter
  • f
the af f ected netw
  • r
k seg ment y = T
  • tal
number
  • f
nodes f
  • r
ming the dir ected path w her e the RE P LY pack et tr ansits z = D iameter
  • f
the dir ected path w her e the RE P LY pack et tr ansits 3.2 Source-Initiated On-Demand Routing Proto cols T able 2 presen ts a comparison
  • f
A OD V, DSR, TORA, ABR and SSR. The A OD V proto col emplo ys a route disco v ery pro cedure similar to DSR; ho w ev er, there are a couple imp
  • rtan
t dis- tinctions. The most notable
  • f
these is that the
  • v
erhead
  • f
DSR is p
  • ten
tially larger than that
  • f
A OD V since eac h DSR pac k et m ust carry full routing information, whereas in A OD V pac k ets need
  • nly
con tain the destination address. Similarly , the route replies in DSR are larger b ecause they con tain the address
  • f
ev ery no de along the route, whereas in A OD V route replies need
  • nly
carry the destination IP address and sequence n um b er. Also, the memory
  • v
erhead ma y b e sligh tly greater in DSR b ecause
  • f
the need to remem b er full routes, as
  • pp
  • sed
to
  • nly
next hop information in A OD V. A further adv an tage
  • f
A OD V is its supp
  • rt
for m ulticast [18 ]. None
  • f
the
  • ther
algorithms considered in this pap er curren tly incorp
  • rate
m ulticast comm unication. On the do wnside, A OD V requires symmetric links b et w een no des, and hence cannot utilize routes with assymetric links. In this asp ect, DSR is sup erior as it do es not require the use
  • f
suc h links, and can utilize assymetric links when symmetric links are not a v ailable. The DSR algorithm is in tended for net w
  • rks
in whic h the mobiles mo v e at a mo derate sp eed with resp ect to pac k et transmission latency [10 ]. Assumptions that the algorithm mak es for
  • p
eration are that the net w
  • rk
diameter is relativ ely small and that the mobile no des can enable a promiscuous receiv e mo de, whereb y ev ery receiv ed pac k et is deliv ered to the net w
  • rk
driv er soft w are without ltering b y destination address. An adv an tage
  • f
DSR
  • v
er some
  • f
the
  • ther
  • n-demand
proto cols 3 Lik e CGSR, TORA also do es not supp
  • rt
m ulticast; ho w ev er, there is a separate proto col, LAM [9], whic h runs
  • n
top
  • f
TORA and pro vides m ulticast capabilt y . 4 ABR also uses the Route Rela ying Load and Cum ulativ e F
  • rw
arding Dela y as routing metrics. 12
slide-13
SLIDE 13 is that DSR do es not mak e use
  • f
p erio dic routing adv ertisemen ts, thereb y sa ving bandwidth and reducing p
  • w
er consumption. Hence the proto col do es not incur an y
  • v
erhead when there are no c hanges in net w
  • rk
top
  • logy
. Additionally , DSR allo ws no des to k eep m ultiple routes to a destination in their cac he. Hence, when a link
  • n
a route is brok en, the source no de can c hec k its cac he for another v alid route. If suc h a route is found, route reconstruction do es not need to b e rein v
  • k
ed. In this case, route reco v ery is faster than in man y
  • f
the
  • ther
  • n-demand
proto cols. Ho w ev er, if there are no additional routes to the destination in the source no de's cac he, route disco v ery m ust b e reinitiated, as in A OD V, if the route is still required. On the
  • ther
hand, b ecause
  • f
the small diameter assumption and b ecause
  • f
the source routing requiremen t, DSR is not scalable to large net w
  • rks.
F urthermore, as previously stated, the need to place the en tire route in b
  • th
r
  • ute
r eplies and data pac k ets causes greater con trol
  • v
erhead than in A OD V. TORA is a \link rev ersal" algorithm that is b est-suited for net w
  • rks
with large, dense p
  • pu-
lations
  • f
no des [16 ]. P art
  • f
the no v elt y
  • f
TORA stems from its creation
  • f
D A Gs to aid route establishmen t. One
  • f
the adv an tages
  • f
TORA is its supp
  • rt
for m ultiple routes. TORA and DSR are the
  • nly
  • n-demand
proto cols considered here whic h retain m ultiples route p
  • ssibilities
for a single source/destination pair. Route reconstruction is not necessary un til all kno wn routes to a destination are deemed in v alid, and hence bandwidth can p
  • ten
tially b e conserv ed b ecause
  • f
the necessit y for few er route rebuildings. Another adv an tage
  • f
TORA is its supp
  • rt
for m ulticast. Al- though, unlik e A OD V, TORA do es not incorp
  • rate
m ulticast in to its basic
  • p
eration, it functions as the underlying proto col for the Ligh t w eigh t Adaptiv e Multicast Algorithm (LAM), and together the t w
  • proto
cols pro vide m ulticast capabilit y [9 ]. TORA's reliance
  • n
sync hronized clo c ks, while a no v el idea, inheren tly limits its applicabilit y . If a no de do es not ha v e a GPS p
  • sitioning
system
  • r
some
  • ther
external time source, it cannot use the algorithm. Additionally , if the external time source fails, the algorithm will cease to
  • p
erate. F urther, route rebuilding in TORA ma y not
  • ccur
as quic kly as in the
  • ther
algorithms due to the p
  • ten
tial for
  • scillations
during this p erio d. This can lead to p
  • ten
tially length y dela ys while w aiting for the new routes to b e determined. ABR is a compromise b et w een broadcast and p
  • in
t-to-p
  • in
t routing and uses the connection-
  • rien
ted pac k et forw arding approac h. Route selection is primarily based
  • n
the aggregated asso cia- tivit y tic ks
  • f
no des along the path. Hence, although the resulting path do es not necessarily result in the smallest p
  • ssible
n um b er
  • f
hops, the path tends to b e longer-liv ed than
  • ther
routes. A long- liv ed route requires few er route reconstructions and therefore yields higher throughput. Another b enet
  • f
ABR is that, lik e the
  • ther
proto cols, it is guaran teed to b e free from pac k et duplicates. The reason is that
  • nly
the b est route is mark ed v alid while all
  • ther
p
  • ssible
routes remain passiv e. ABR, ho w ev er, relies
  • n
the fact that eac h no de is b eaconing p erio dically . The b eaconing in terv al m ust b e short enough so as to accurately reect the spatial, temp
  • ral,
and connectivit y state
  • f
the mobile hosts. This b eaconing requiremen t ma y result in additional p
  • w
er consumption. Ho w ev er, exp erimen tal results
  • btained
in [24 ] rev eal that the inclusion
  • f
p erio dic b eaconing has a min ute inuence
  • n
the
  • v
erall battery p
  • w
er consumption. Unlik e DSR, ABR do es not utilize route cac hes. The SSR algorithm is a logical descendan t
  • f
ABR. It utilizes a new tec hnique
  • f
selecting routes based
  • n
the signal strength and lo cation stabilit y
  • f
no des along the path. As in ABR, while the paths selected b y this algorithm are not necessarily shortest in hop coun t, they do tend to b e more stable and longer-liv ed, resulting in few er route reconstructions. One
  • f
the ma jor dra wbac ks
  • f
the SSR pro cotol is that, unlik e in A OD V and DSR, in termediate no des cannot reply to route requests sen t to w ards a destination; this results in p
  • ten
tially long dela ys b efore a route can b e disco v ered. Additionally , when a link failure
  • ccurs
along a path, the route disco v ery algorithm m ust b e re-in v
  • k
ed from the source to nd a new path to the destination. No attempt is made to use partial route reco v ery (unlik e ABR)
  • i.e.
to allo w in termediate no des to attempt to rebuild the route themselv es. A OD V and DSR also do not sp ecify in termediate no de rebuilding. While this ma y 13
slide-14
SLIDE 14 P arameters On-Demand T able-Driv en Av ailabilit y
  • f
Av ailable Alw a ys a v ailable Routing Information when needed regardless
  • f
need Routing Flat Mostly at Philosoph y except for CSGR P erio dic route Not Y es up dates required Coping with Using lo calized Inform
  • ther
no des mobilit y route disco v ery to ac hiev e consisten t as in ABRand SSR routing table Signaling trac Gro ws with increasing Greater than generated mobilit y
  • f
activ e that
  • f
  • n-demand
routes (as in ABR) routing Qualit y
  • f
Service F ew can supp
  • rt
QoS Mainly Shortest P ath Supp
  • rt
as QoS metric T able 3: Ov erall Comparisons
  • f
On-Demand v ersus T able-Driv en Based Routing Proto cols. lead to longer route reconstruction times since link failures cannot b e resolv ed lo cally without the in terv en tion
  • f
the source no de, the attempt and failure
  • f
an in termediate no de to rebuild a route will cause a longer dela y then if the source no de had attempted the rebuilding as so
  • n
as the brok en link w as noticed. Th us it remains to b e seen whether in termediate no de route rebuilding is more
  • ptimal
than source no de route rebuilding. 3.3 T able-Driv en vs On-Demand Routing As discussed earlier, the table-driv en ad-ho c routing approac h is similar to the connectionless approac h
  • f
forw arding pac k ets, with no regard to when and ho w frequen t suc h routes are desired. It relies
  • n
an underlying routing table up date mec hanism that in v
  • lv
es the constan t propagation
  • f
routing information. This is, ho w ev er, not the case for
  • n-demand
routing proto cols. When a no de using an
  • n-demand
proto col desires a route to a new destination, it will ha v e to w ait un til suc h a route can b e disco v ered. On the
  • ther
hand, b ecause routing information is constan tly propagated and main tained in table-driv en routing proto cols, a route to ev ery
  • ther
no de in the ad-ho c net w
  • rk
is alw a ys a v ailable, regardless
  • f
whether
  • r
not it is needed. This feature, although useful for datagram trac, incurs substan tial signaling trac and p
  • w
er consumption. Since b
  • th
bandwidth and battery p
  • w
er are scarce resources in mobile computers, this b ecomes a serious limitation. T able 3 lists some
  • f
the basic dierences b et w een the t w
  • classes
  • f
algorithms. Another consideration is whether a at
  • r
hierarc hical addressing sc heme should b e used. All
  • f
the proto cols considered here, except for CGSR, use a at addressing sc heme. In [1], a discussion
  • f
the t w
  • addressing
sc hemes is presen ted. While at addressing ma y b e less complicated and easier to use, there are doubts as to its scalabilit y . 4 Applications and Challenges Akin to pac k et radio net w
  • rks,
ad-ho c wireless net w
  • rks
ha v e an imp
  • rtan
t role to pla y in military applications. Soldiers equipp ed with m ulti-mo de mobile comm unicators can no w comm unicate in an ad-ho c manner, without the need for xed wireless base stations. In addition, small v ehicular devices equipp ed with audio sensors and cameras can b e deplo y ed at targetted regions to collect imp
  • rtan
t lo cation and en vironmen tal information whic h will b e comm unciated bac k to a pro cessing no de via ad-ho c mobile comm unications. Ship-to-ship ad-ho c mobile comm unication is also desirable since it 14
slide-15
SLIDE 15 pro vides alternate comm unication paths without reliance
  • n
ground-
  • r
space-based comm unication infrastructures. Commerical scenarios for ad-ho c wireless net w
  • rks
include: (a) conferences/meetings/lectures [8], (b) emergency services, and (c) la w enforcemen t. P eople to da y attend meetings and conferences with their laptops, palm tops and noteb
  • ks.
It is therefore attractiv e to ha v e instan t net w
  • rk
formation, in addition to le and information sharing without the presence
  • f
xed base stations and systems administrators. A presen ter can m ulticast slides and audio to in tended receip en ts. A ttendees can ask questions and in teract
  • n
a commonly-shared white b
  • ard.
Ad-ho c mobile comm unication is particularly useful in rela ying information (status, situation a w areness, etc.) via data, video and/or v
  • ice
from
  • ne
rescue team mem b er to another
  • v
er a small handheld
  • r
w earable wireless device. Again, this applies to la w enforcemen t p ersonnel as w ell. Curren t c hallenges for ad-ho c wireless net w
  • rks
include: (a) m ulticast, (b) QoS supp
  • rt,
(c) p
  • w
er-a w are routing [20 ], and (d) lo cation-aided routing [12 ]. As men tioned ab
  • v
e, m ulticast is desirable to supp
  • rt
m ulti-part y wireless comm unications. Since the m ulticast tree is no longer static (i.e., its top
  • logy
is sub ject to c hange
  • v
er time), the m ulticast routing proto col m ust b e able to cop e with mobilit y , including m ulticast mem b ership dynamics (suc h as l eav e and j
  • in).
In terms
  • f
QoS, it is inadequate to consider QoS merely at the net w
  • rk
lev el without considering the underlying media access con trol la y er [13]. Again, giv en the problems asso ciated with the dynamics
  • f
no des, hidden terminals, and uctuating link c haracteristics, supp
  • rting
end-to-end QoS is a non- trivial issue that requires in-depth in v estigation. Curren tly , there is a trend to w ards an adaptiv e QoS approac h instead
  • f
the \plain" resource reserv ation metho d with hard QoS guaran tees. Another imp
  • rtan
t factor is the limited p
  • w
er supply in handheld devices whic h can seriously prohibit pac k et forw arding in an ad-ho c mobile en vironmen t. Hence, routing trac based
  • n
no des' p
  • w
er metric is
  • ne
w a y to distinguish routes that are more long-liv ed than
  • thers.
Finally , instead
  • f
using b eaconing
  • r
broadcast searc h, lo cation-aided routing uses p
  • sitioning
information to dene asso ciated regions so that the routing is spatially-orien ted and limited. This is analogous to asso ciativit y-orien ted and restricted broadcast in ABR. Curren t ad-ho c routing approac hes ha v e in tro duced sev eral new paradigms, suc h as exploit- ing user's demand, the use
  • f
lo cation, p
  • w
er, and asso ciation parameters. Adaptivit y and self- conguration are k ey features
  • f
these approac hes. Ho w ev er, exibilit y is also imp
  • rtan
t. A f l exibl e ad-ho c routing proto col could resp
  • nsiv
ely in v
  • k
e table-driv en approac hes and/or
  • n-demand
ap- proac hes based
  • n
situations and comm unication requiremen ts. The \toggle" b et w een these t w
  • approac
hes ma y not b e trivial since concerned no des m ust b e \in-sync" with the toggling. Co- existence
  • f
b
  • th
approac hes ma y also exist in spatially clustered ad-ho c groups, with in tra-cluster emplo ying the table-driv en approac h and in ter-cluster emplo ying the demand-driv en approac h
  • r
vice v ersa. F urther w
  • rk
is necessary to in v estigate the feasibilit y and p erformance
  • f
h ybrid ad-ho c routing approac hes. Lastly , in addition to the ab
  • v
e, further researc h in the areas
  • f
media access con trol, securit y , service disco v ery , and in ternet proto col
  • p
erabilit y is required b efore the p
  • ten
tial
  • f
ad-ho c mobile net w
  • rking
can b e realized. 5 Conclusion In this pap er w e ha v e pro vided descriptions
  • f
sev eral routing sc hemes prop
  • sed
for ad-ho c mobile net w
  • rks.
W e ha v e also pro vided a classication
  • f
these sc hemes according to the routing strategy , i.e., table-driv en and
  • n-demand.
W e ha v e presen ted a comparison
  • f
these t w
  • categories
  • f
routing proto cols, highligh ting their features, dierences and c haracteristics. Finally , w e ha v e iden tied p
  • ssible
applications and c hallenges facing ad-ho c mobile wireless net w
  • rks.
While it is not clear 15
slide-16
SLIDE 16 that an y particular algorithm
  • r
class
  • f
algorithm is the b est for all scenarios, eac h proto col has denite adv an tages and disadv an tages and has certain situations for whic h it is w ell-suited. The eld
  • f
ad-ho c mobile net w
  • rks
is rapidly gro wing and c hanging, and while there are still man y c hallenges that need to b e met, it is lik ely that suc h net w
  • rks
will see wide-spread use within the next few y ears. 6 Ac kno wledgmen t W e w
  • uld
lik e to thank C.-C. Chiang, J. J. Garcia-Luna-Acev es, Da vid Maltz, Vincen t P ark, and Charles P erkins for their assistance in assuring accurate descriptions
  • f
the
  • p
eration
  • f
their proto- cols. References [1] D. Bak er, M. S. Corson, P . Sass, and S. Ramanatham, \Flat vs. Hierarc hical Net- w
  • rk
Con trol Arc hitecture," AR O/D ARP A W
  • rkshop
  • n
Mobile Ad-Ho c Net w
  • rking,
h ttp://www.isr.umd.edu/Courses/W
  • rkshops/MANET/pro
gram. h tml, Marc h 1997. [2] J. Bro c h, D. B. Johnson, D. A. Maltz, \The Dynamic Source Routing Proto col for Mobile Ad Ho c Net w
  • rks,"
IETF Internet Dr aft draft-ietf-manet-dsr-01.txt, Decem b er 1998 (W
  • rk
in Progress). [3] C.-C. Chiang, M. Gerla, and L. Zhang, \Adaptiv e Shared T ree Multicast in Mobile Wireless Net w
  • rks,"
Pr
  • c
e e dings
  • f
GLOBECOM '98, pp. 1817{1822, No v em b er 1998. [4] C.-C. Chiang, H.K. W u, W. Liu, and M. Gerla, \Routing in Clustered Multihop, Mobile Wireless Net w
  • rks
with F ading Channel," Pr
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e e dings
  • f
IEEE SICON'97, pp. 197{211, April 1997. [5] M. S. Corson and A. Ephremides, \A Distributed Routing Algorithm for Mobile Wireless Net- w
  • rks,"
A CM/Baltzer Wir eless Networks Jouornal, V
  • l.
1, No. 1, pp. 61{81, F ebruary 1995. [6] R. Dub e, C. D. Rais, K.-Y. W ang, and S.K. T ripathi, \Signal Stabilit y based Adaptiv e Routing (SSA) for Ad-Ho c Mobile Net w
  • rks,"
IEEE Personal Communic ations, pp. 36{45, F ebruary 1997. [7] L. R. F
  • rd
Jr. and D. R. F ulk erson, Flows in Networks. Princeton Univ ersit y Press, Princeton N.J., 1962. [8] M. Gerla, C.-C. Chiang, and L. Zhang, \T ree Multicast Strategies in Mobile, Multihop Wireless Net w
  • rks,"
T
  • app
ear in A CM/Baltzer Mobile Networks and Applic ations Journal, 1998. [9] L. Ji and M. S. Corson, \A Ligh t w eigh t Adaptiv e Multicast Algorithm," Pr
  • c
e e dings
  • f
GLOBE- COM '98, pp. 1036{1042, No v em b er 1998. [10] D. B. Johnson and D. A. Maltz, \Dynamic Source Routing in Ad-Ho c Wireless Net w
  • rks,"
Mobile Computing, ed. T. Imielinski and H. Korth, Klu w er Academic Publishers, pp. 153{181, 1996. [11] J. Jubin and J. T
  • rno
w, \The D ARP A P ac k et Radio Net w
  • rk
Proto cols," Pr
  • c
e e dings
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the IEEE, V
  • l.
75, No. 1, pp. 21{32, 1987. 16
slide-17
SLIDE 17 [12] Y. B. Ko and N. H. V aidy a, \Lo cation-Aided Routing (LAR) in Mobile Ad Ho c Net w
  • rks,"
Pr
  • c
e e dings
  • f
A CM/IEEE MOBICOM '98, Octob er 1998. [13] C. R. Lin and M. Gerla, \MA CA/PR: An Async hronous Multimedia Multihop Wireless Net- w
  • rk,"
Pr
  • c
e e dings
  • f
IEEE INF OCOM '97, Marc h 1997. [14] S. Murth y and J. J. Garcia-Luna-Acev es, \An Ecien t Routing Proto col for Wireless Net- w
  • rks,"
A CM Mobile Networks and Applic ations Journal, Sp ecial Issue
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Routing in Mobile Comm unication Net w
  • rks,
pp. 183{197, Octob er 1996. [15] S. Murth y and J. J. Garcia-Luna-Acev es, \Lo
  • p-F
ree In ternet Routing Using Hierarc hical Rout- ing T rees," Pr
  • c
e e dings
  • f
INF OCOM '97, April 7-11, 1997. [16] V. D. P ark and M. S. Corson, \A Highly Adaptiv e Distributed Routing Algorithm for Mobile Wireless Net w
  • rks,"
Pr
  • c
e e dings
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INF OCOM '97, April 1997. [17] C. E. P erkins and P . Bhagw at, \Highly Dynamic Destination-Sequenced Distance-V ector Rout- ing (DSD V) for Mobile Computers," Computer Communic ations R eview, pp. 234{244, Octob er 1994. [18] C. E. P erkins and E. M. Ro y er, \Ad Ho c On Demand Distance V ector (A OD V) Routing," IETF Internet Dr aft, draft-ietf-manet-ao dv-02.txt, No v em b er 1998 (W
  • rk
in Progress). [19] C. E. P erkins and E. M. Ro y er, \Ad-ho c On-Demand Distance V ector Routing," Pr
  • c
e e dings
  • f
2nd IEEE Workshop
  • n
Mobile Computing Systems and Applic ations, F ebruary 1999. [20] S. Singh, M. W
  • ,
and C. S. Ragha v endra, \P
  • w
er-Aw are Routing in Mobile Ad Ho c Net w
  • rks,"
Pr
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e e dings
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A CM/IEEE MOBICOM '98, Octob er 1998. [21] A. S. T anen baum, Computer Networks, Thir d Edition. Pren tice Hall, Englew
  • d
Clis, Chapter 5, pp. 357{358, 1996. [22] C-K. T
  • h,
\A No v el Distributed Routing Proto col T
  • Supp
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Ad-Ho c Mobile Computing," Pr
  • c
e e dings
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the 1996 IEEE Fifte enth A nnual International Pho enix Confer enc e
  • n
Computers and Communic ation, pp. 480{486, Marc h 1996. [23] C-K. T
  • h,
\Asso ciativit y-Based Routing for Ad-Ho c Mobile Net w
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Wir eless Personal Com- munic ations, V
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4, No. 2, pp. 1{36, Marc h 1997. [24] C-K. T
  • h
and George Lin, \Implemen ting Asso ciativit y-Based Routing for Ad Ho c Mobile Wireless Net w
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Unpublished Article, Marc h 1998. 17
slide-18
SLIDE 18 Authors' Biographies ELIZABETH M. R O YER (ero y er@alpha.ece.ucsb.edu) receiv ed her M.S. degree in Electrical and Com- puter Engineering from the Univ ersit y
  • f
California, San ta Barbara in Decem b er 1997 and her B.S. degrees in Computer Science and Applied Mathematics from Florida State Univ ersit y in April 1996. She is curren tly a PhD studen t in the Electrical and Computer Engineering Departmen t at the Univ ersit y
  • f
California, San ta Barbara. A t UCSB she is a mem b er
  • f
the Computer Net w
  • rks
and Distributed Systems Lab
  • ratory
. Her researc h in terests include routing and m ulticast for ad-ho c mobile net w
  • rks.
Elizab eth is the recipien t
  • f
a National Science F
  • undation
Graduate F ello wship and a Univ ersit y
  • f
California Do ctoral Sc holar F ello wship. CHAI-KEONG TOH (C-K.T
  • h@acm.org)
w as b
  • rn
in Singap
  • re
in 1965. He receiv ed his Diploma in Electronics and Comm uni cation Engineering with a Certicate
  • f
Merit a w ard from the Singap
  • re
P
  • lytec
hnic in 1986, his BEng degree in Electronics Engineering with rst class honors from the Univ ersit y
  • f
Manc hester Institute
  • f
Science and T ec hnology (UMIST) in 1991 and his Ph.D. degree in Computer Science from the Computer Lab
  • ratory
, Univ ersit y
  • f
Cam bridge, Cam bridge, England in 1996. Dr. T
  • h
founded and c haired the Mobile Sp ecial In terest Group (Mobile SIG) from 1994-1996. Before joining Cam bridge Univ ersit y , C-K w as a net w
  • rk
sp ecialist, R&D engineer and tec hnical sta mem b er. A t Cam bridge, C-K w as an Honorary Cam bridge Commo n w ealth T rust Sc holar and a King's College Cam bridge Researc h Sc holar. He authored the b
  • k
  • n
`Wireless A TM and Ad Ho c Net w
  • rks:
Proto cols and Arc hitectures', whic h w as published b y Klu w er Academic Press in 1996. Dr. T
  • h
serv es
  • n
the editorial b
  • ard
for A CM Mobile Computing and Comm uni cations Review (MC2R), IEEE Net w
  • rk
and P ersonal T ec hnologies Journal (Springer V erlag). He is a mem b er
  • f
IEE, IEEE, USENIX, A CM, Sigma Xi Honor So ciet y , New Y
  • rk
Academ y
  • f
Science, American Asso ciation for the Adv ancemen t
  • f
Science (AAAS) and is a F ello w
  • f
the Cam bridge Philosophical So ciet y and Cam bridge Common w eal th So ciet y . He is curren tly an assistan t professor with the Sc ho
  • l
  • f
Electrical and Computer Engineering at the Georgia Institute
  • f
T ec hnology , A tlan ta, Georgia, directing the Mobile Multimedia and High Sp eed Net w
  • rking
Lab
  • ratory
. 18