Advanced wireless networks 4g technologies phần 7 ppsx

In order to support this functionality in an ad hoc network, the routing protocol must maintain and disseminate the following status information for each link: 1 the initial link activat

CLUSTERING PROTOCOLS 507

availability associated with each neighbor according to either a system default mobilityprofile or mobility information obtained through the network-interface layer protocol orphysical-layer sensing The precise methodology and the information required for the eval-uation of link availability is described later in this section

Finally, the neighbors, having discovered the unclustered status of the source node,automatically generate and transmit complete cluster topology information, which theyhave stored locally as a result of participating in the cluster’s intracluster routing protocol.This topology synchronization function is a standard feature of typical proactive routingprotocols when a router discovers the activation of a link to a new router The source nodedoes not immediately send its topology information to any of the neighbors

Link activation

A link activation detected by a clustered node that is not an orphan is treated as an intraclusterrouting event Hence, the topology update will be disseminated throughout the cluster.Unlike reactive routing that responds after path failure, the dissemination of link activation

updates is a key factor to an (c,t) cluster node’s ability to find new (c,t) paths in anticipation

of future link failures or the expiration of the timer

Link failure

The objective of a node detecting a link failure is to determine if the link failure has caused

the loss of any (c,t) paths to destinations in the cluster A node’s response to a link failure

event is twofold First, each node must update its view of the cluster topology and re-evaluatethe path availability to each of the cluster destinations remaining in the node’s routing table.Second, each node forwards information regarding the link failure to the remaining clusterdestinations

Expiration of c timer

The c timer controls cluster maintenance through periodic execution of the intracluster

routing algorithm at each node in a cluster Using the topology information available ateach node, the current link availability information is estimated and maximum availability

paths are calculated to each destination node in the cluster If any of the paths are not (c,t)

paths, then the node leaves the cluster

Node deactivation

The event of node deactivation encompasses four related events, namely, graceful vation, sudden failure, cluster disconnection and voluntary departure from the cluster Ingeneral, each of these events triggers a response by the routing protocol As a result, nodesdetermine that the node that has deactivated is no longer reachable

deacti-13.5.3.3 Ad hoc mobility model

The random ad hoc mobility model used in this section is a continuous-time stochastic

process, which characterizes the movement of nodes in a two-dimensional space Based on

Figure 13.40 Ad hoc mobility model node movement: (a) epoch random mobility vectors;

(b) ad hoc mobility model node movement.

the random ad hoc mobility model, each node’s movement consists of a sequence of random

length intervals called mobility epochs during which a node moves in a constant direction

at a constant speed The speed and direction of each node varies randomly from epoch to

epoch Consequently, during epoch i of duration T i

n , node n moves a distance of V i

n T i

n in

a straight line at an angle ofθ i

n The number of epochs during an interval of length t is the discrete random process N n (t) Figure 13.40(a) illustrates the movement of the node over

six mobility epochs, each of which is characterized by its direction,θ i

n , and distance V i

n T i

n

The mobility profile of node n moving according to the random ad hoc mobility model

requires three parameters:λ n , μ n andσ2 The following list defines these parameters and

CLUSTERING PROTOCOLS 509

states the assumptions made in developing this model:

(1) The epoch lengths are identically, independently distributed (i.i.d.) exponentiallywith mean 1/λ n

(2) The direction of the mobile node during each epoch is i.i.d uniformly distributedover (0, 2π) and remains constant only for the duration of the epoch.

(3) The speed during each epoch is an i.i.d distributed random variable (e.g i.i.d.normal, i.i.d uniform) with meanμ nand varianceσ2and remains constant only forthe duration of the epoch

(4) Speed, direction and epoch length are uncorrelated

(5) Mobility is uncorrelated among the nodes of a network, and links fail independently.Nodes with limited transmission range are assumed to experience frequent random changes

in speed and direction with respect to the length of time a link remains active between twonodes Furthermore, it is assumed that the distributions of each node’s mobility character-istics change slowly relative to the rate of link failure Consequently, the distribution of thenumber of mobility epochs is stationary and relatively large while a link is active Since

the epoch lengths are i.i.d exponentially distributed, N n (t) is a Poisson process with rate

λ n Hence, the expected number of epochs experienced by node n during the interval (0,t)

while a link is active isλ n t = 1.

These assumptions reflect a network environment in which there are a large number of

heterogeneous nodes operating autonomously in an ad hoc fashion, which conceptually reflects the environment considered in the design of the (c,t) cluster framework In order to characterize the availability of a link between two nodes over a period of time (t0, t0+ 1),the distribution of the mobility of one node with respect to the other must be determined

To characterize this distribution, it is first necessary to derive the mobility distribution of asingle node in isolation The single node distribution is extended to derive the joint mobilitydistribution that accounts for the mobility of one node with respect to the other Using thisjoint mobility distribution, the link availability distribution is derived

The random mobility vector can be expressed as a random sum of the epoch randommobility vectors

The phase of the resultant vector Rn (t) is uniformly

distributed over (0, 2π) and its magnitude represents the aggregate distance moved by the

node and is approximately Raleigh distributed with parameter

Joint node mobility

Based on the assumption of random link failures, we can consider the mobility of twonodes at a time by fixing the frame of reference of one node with respect to the other.This transformation is accomplished by treating one of the nodes as if it were the basestation of a cell, keeping it at a fixed position For each movement of this node, the other

node is translated an equal distance in the opposite direction So, the vector Rm ,n (t)=

Rm (t)− Rn (t), representing the equivalent random mobility vector of node m with respect

to node n, is obtained by fixing m’s frame of reference to n’s position and moving m relative

to that point Its phase is uniformly distributed over (0, 2π) and its magnitude has Raleigh

distribution with parameterα m ,n = α m + α n

Random ad hoc link availability

If L m ,n (t) = 1 denotes an active and L m ,n (t) = 0 an inactive link, then for nodes n and m,

link availability is defined as

A m ,n (t) ≡ Pr[L m ,n 0+ t) = 1 | L m ,n (t0)= 1] (13.10)

Note that a link is still considered available at time t even if it experienced failures during one

or more intervals (t i , t j ); t0< t i < t j < t0+ t By definition, if m lies within the circular region of radius R centered at n, the link between the two nodes is considered to be active Depending on the initial status and location of nodes m and n, two distinct cases of link

availability can be identified

(1) Node activation – node m becomes active at time t0, and it is assumed to be at a

random location within range of node n In this case we have

A m ,n (t) ≈ 1 − 

1

2, 2, −R2/α m ,n



1

(2) Link activation: node m moves within range of node n at time t0by reaching the

boundary defined by R, and it is assumed to be located at a random point around the

boundary In this case we have

the links in the path are active at time t , and P k

m ,n (t)= 0 if one or more links in the path are

inactive at time t The path availability π k

m ,n (t) between two nodes n and m at time t ≥ t0

is given by the following probability

a probability≥ c.

Path availability cost calculation

The above discussion demonstrates how the link availability can be calculated, therebyproviding a link metric that represents a probabilistic measure of path availability Thismetric can be used by the routing algorithm in order to construct paths that support a lower

bound c on availability of a path over an interval of length t The availabilities of each of the links along a path are used by the (c,t) cluster protocol to determine if the path is an (c,t) path, and consequently, if a cluster satisfies the (c,t) criteria In order to support this functionality in an ad hoc network, the routing protocol must maintain and disseminate the

following status information for each link:

(1) the initial link activation time, t0;

(2) the mobility profiles for each of the adjacent nodes

λ i , μ i , σ2

i



, i = m, n;

(3) the transmission range of each of the adjacent nodes, R;

(4) the event which activated the link: (a) node activation at time t0or (b) nodes moving

into range of each other at time t0

Based on this information, any node in an (c,t) cluster can estimate, at any time τ, the

availability of a link at time t + τ This can be achieved because each node knows the initial link activation time t0; hence, link availability is evaluated over the interval (t0, t + τ).

Nodes can use conditional probability to evaluate the availability of their own links becausethey have direct knowledge of such a link’s status at timeτ, whereas remote nodes do not.

Specifically, for an incident link that activated at time t0, a node will evaluate the availability

at time t, given that it is available at time τ ≥ t0

13.5.3.4 Performance example

A range of node mobility with mean speeds between 5.0 and 25.0 km/h was simulated inMcDonald and Znati [74] The speeds during each mobility epoch were normally distributed,and the direction was uniformly distributed over (0, 2π) A node activation rate of 250

nodes/h was used The mean time to node deactivation was 1 h Nodes were initiallyrandomly activated within a bounded region of 5× 5 km Nodes that moved beyond this

boundary were no longer considered to be part of the ad hoc network and were effectively deactivated (c,t) path availability was evaluated using Dijkstra’s algorithm.

For each simulation run, data was collected by sampling the network status once persecond over an observation interval of 1 h The first 2 h of each run were discarded toeliminate transient effects, and each simulation was rerun 10 times with new random seeds

Simulation results for cluster size and cluster survival times are given in Figures 13.40 andand 13.41 Finally logical relationships among MANET network-layer entities is given inFigure 13.42.

13.6 CASHING SCHEMES FOR ROUTING

A large class of routing protocols for MANETs, namely reactive protocols, employ someform of caching to reduce the number of route discoveries The simplest form of caching

is based on timeouts associated with cache entries When an entry is cached, a timer starts.When the timeout elapses, the entry is removed from the cache Each time the entry is used,the timer restarts Therefore, the effectiveness of such a scheme depends on the timeout

0 5 10 15 20 25

Mean mobile speed (km/h)

.4

0 5 10 15 20 25

t = 1 min

t = 5 min

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

(a)

(b)

Figure 13.41 Simulation results: (a) cluster size (R = 1000 m); (b) cluster size (R =

500 m); (c) cluster survival (R = 1000 m); and (d) cluster survival (R =

500 m) (Reproduced by permission of IEEE [74].)

0 10 20 30 40 50 60 70 80 90

t = 1 min

t = 5 min

0 5 10 15 20 25 30 35 40 45

0,4 = 0,2

0 10 20 30 40 50 60 70 80 90

Mean mobile node speed (km/h)

Mean mobile node speed (km/h)

t = 1 min

t = 5 min

0 5 10 15 20 25 30 35 40 45

= 0.4 = 0.2 a a

Internet protocol

Network-Interface Layer

Routing protocol

(α,t)-cluster

algorithm protocols

MANET encapsulation protocol

Internet protocol

Network-interface layer

Routing table

Figure 13.42 Logical relationships among MANET network-layer entities

513

value associated with a cached route If the timeout is well-tuned, the protocol performanceincreases; otherwise, a severe degradation arises as entries are removed either prematurely

or too late from the cache

13.6.1 Cache management

A cache scheme is characterized by the following set of design choices that specify cachemanagement in terms of space (cache structure) and time (i.e when to read/add/ delete acache entry): store policy, read policy, writing policy and deletion policy

The store policy determines the structure of the route cache Recently, two different cache

structures were studied [81], namely link cache and path cache, and applied to DSR In alink cache structure, each individual link in the routes returned in RREP packets is added

to a unified graph data structure, managed at each node, that reflects the node’s currentview of the network topology In so doing, new paths can be calculated by merging routeinformation gained from different packets In the path cache, however, each node stores aset of complete paths starting from itself The implementation of the latter structure is easiercompared with the former, but it does not permit inference of new routes and exploitation

of all topology information available at a node

The reading policy determines rules of using a cache entry Besides the straightforward

use from the source node when sending a new message, several other strategies are possible.For example, DSR defines the following policies:

r cache reply – an intermediate node can reply to a route request with information stored

in its own cache;

r salvaging – an intermediate node can use a path from its own cache when a data packet

meets a broken link on its source route;

r gratuitous reply – a node runs the interface in the promiscuous mode and it listens for

packets not directed to itself If the node has a better route to the destination node of apacket, it sends a gratuitous reply to the source node with this new better route

The writing policy determines when and which information has to be cached Owing to

the broadcast nature of radio transmissions, it is quite easy for a node to learn about newpaths by running its radio interface in the promiscuous mode The main problem for thewriting policy is indeed to cache valid paths Negative caches are a technique proposed inJohnson and Maltz [82] and adapted in Marina and Das [83] to filter the writing of cacheentries in DSR out A node stores negative caches for broken links seen either via the routeerror control packets or link layer for a period of time ofδt s Within this time interval, the

writing of a new route cache that contains a cached broken link is disabled

The deletion policy determines which information has to be removed from the cache

and when Deletion policy is actually the most critical part of a cache scheme Two kinds

of ‘errors’ can occur, owing to an imprecise erasure: (1) early deletion, a cached route is removed when it is still valid; and (2) late deletion, a cached route is not removed even if

it is no longer valid

The visible effect of these kinds of errors is a reduction in the packet delivery fractionand an increase in the routing overhead (the total number of overhead packets) [84] Latedeletions create the potential risk of an avalanche effect, especially at high load If a nodereplies with a stale route, the incorrect information may be cached by other nodes and, inturn, used as a reply to a discovery Thus, cache ‘pollution’ can propagate fairly quickly [83]

CASHING SCHEMES FOR ROUTING 515

Caching schemes in DSR

All such schemes rely on a local timer-based deletion policy [81, 84] The only exceptionhas been proposed in Marina and Das [83] They introduce a reactive caching deletionpolicy, namely, the wider error notification, that propagates route errors to all the nodes,forcing each node to delete stale entries from its cache

Simulation results reported in References [81, 83] show that performance of a based caching deletion policy is highly affected by the choice of the timeout associatedwith each entry In the path cache, for a value of timeout lower than the optimal one (i.e.early deletion), the packet delivery fraction and routing overhead are worse than cachingschemes that do not use any timeout In the link cache, late deletion errors increase therouting overhead while the packet delivery fraction falls sharply

timer-The cache timeout can obviously be tuned dynamically However, adaptive timer-baseddeletion policies have their own drawbacks This policy suffers from late or early deletionerrors during the transition time from one optimal timeout value to the successive one So,the more the network and the data load are variable, the worse the performance will be

To reduce the effect of such imprecise deletions, the adaptive timer-based cache schemehas been combined with the wide error notification deletion technique and studied for DSR

in Perkins and Royer [17] According to such a combined scheme, routes that become stalebefore their timeout expiration are removed reactively from all the sources using that route

In this combined technique, however, two more points remain unresolved: (1) Owing to thereactive nature of the deletions, if a cache entry is not used, it remains in the cache, even if

no longer valid, thus it can be used as a reply to a path discovery and (2) the effect of earlydeletions cannot be avoided

autonomously Therefore, a cache leader n is the only node that is authorized to advertise

route information inside its caching zone which is written into caches On receiving the

advertising message, a node proactively maintains a path to n so that it can be used as the

next-hop node to any of the advertised routes A cache leader is responsible for the validity

of the advertised routes Thus, it monitors such routes and forces each node in its cachingzone to remove a route as soon as it becomes stale, so the deletion policy is proactive Let

us note that, if we consider k∗ = k and each node of a ZRP interzone path as a cache leader,

we get the same underlying zone structure of ZRP (this implies that each active node is acache leader) However, more generally, a cache leader can decide to advertise paths only to

those nodes located at a distance k∗< k, and not all active nodes need to be cache leaders.

(3) Only paths to active nodes are advertised as external routes.

(4) Caches are managed using explicit injection/deletion messages

(5) To stop redundant query threads, LT (loop back termination), QD2 (query tion) and ET (early termination) redundant filtering rules are used which have beendescribed earlier in this chapter

detec-When a node S, in Figure 13.43, executes a route request for a node D, an interzonepath from S to D is identified A node Bi belonging to an interzone path is an active nodefor the caching scheme In Figure 13.43 an interzone path between S and D is formed bynodes b, e, p and t Thus, an interzone path is also an active path An interzone path is storedaccording to a distributed next-hop fashion, where the next-hop node is an active node Bi

stores Bi+1as the next-hop active node for all the downstream nodes from Bi+2to BM+1and Bi−1as the next-hop active node for all the upstream nodes from B0to Bi−2 These two

Node S

2 a b 2 a c 1 a a IZT

2 a b 2 a c 1 a a IZT

5 b D 4 b t 3 b p 2 b e EZT

5 b D 4 b t 3 b p 2 b e EZT

2 h e 2 h l 2 f g 2 a c 2 a S 1 h h 1 f f 1 a a IZT

2 h e 2 h l 2 f g 2 a c 2 a S 1 h h 1 f f 1 a a IZT

4 e D

3 e t 2 e p EZT

4 e D

3 e t 2 e p EZT

IZP=(ID,S,B2) Node B1

2 h b

2 m n

2 m p

2 h l

1 m m

1 h h IZT

2 h b

2 m n

2 m p

2 h l

1 m m

1 h h IZT

3 p D

2 p t

2 b S EZT

3 p D

2 p t

2 b S EZT

2 q t

2 m e

1 q q

1 m m IZT

2 q t

2 m e

1 q q

1 m m IZT

2 t D 2 e b 3 e S EZT

2 t D 2 e b 3 e S EZT

2 r D

2 q p

1 r r

1 q q IZT

2 r D

2 q p

1 r r

1 q q IZT

2 p e 3 p b 4 p S EZT

2 p e 3 p b 4 p S EZT

2 r t 1 r r IZT

2 r t 1 r r IZT

5 t S

4 t b

3 t e

2 t p EZT

5 t S

4 t b

3 t e

2 t p EZT

IZP=(ID,B1,B3) Node B2

IZP=(ID,B2,B4) Node B3

IZP=(ID,B3,D) Node B4 Node D

S c

o

a f

i k

n

m q

r D

CASHING SCHEMES FOR ROUTING 517

active nodes will be referred to as companion nodes (as an example, the companion nodes

of node b, with respect to the interzone path from S to D are S and e)

All routing information concerning nodes belonging to an interzone path is advertisedinside the caching zone of each member of the path, which thus acts as cache leader for thatinformation Such routes are then maintained proactively by the IARP If a new node joins

Bi’s zone, it acquires, by means of the IARP, all previously advertised routing information

by Bi Since a node may belong to more than one overlapping zone, it can acquire morethan a single path to the same destination

When a node, say Bi+1, leaves Bi’s routing zone, not all the routing information gatheredduring the route request/reply is lost Roughly speaking, two active paths from S to Bi−1andfrom Bi+1to D are still up Hence, all the routing information concerning these subpaths isstill valid However, nodes B0, ,Bi−1(Bi−1, ,BM+1) notify the nodes inside their ownzones, using a delete control message, that the destinations Bi−1, ,BM+1(B0, ,Bi) are

no longer reachable

Data structures

Each node X uses the following local data structures:

r Internal zone routing table (IZT) – an entry of IZT is a triple (d, n, #h), where d is the

destination node, n is the next-hop node (located in X’s transmission range), and #h is

the path cost in number of hops

r External zone routing table (EZT) – a row of EZT is a triple (d, n, #z), where d is the

destination node, n is the next-hop active node (n belongs to X’s routing zone and is not

restricted to be in its transmission range), and #z is the cost of the path from X to d, given

as the number of active nodes that have to be traversed For example, in Figure 13.43,node b sets node e as the next-hop active node for p with cost two (nodes e and p)

r Interzone path table (IZP) – an interzone path corresponds to an entry in X’s IZP table

provided that X is an active node and (X= S,D) In this case, let the path id be ID and

X= Bi The entry is the triple (ID, Bi−1, Bi+1).

r Reachable nodes (RN) list – this is a sequence of pairs (d,#z), where d is an active node

belonging to an interzone path and #z is the cost of the path from X expressed as number of

active nodes that must be traversed to reach d A node X advertises RN to nodes belonging

to Zk(X) RN includes the projection of EZT along the first and third components For

example, node b of Figure 13.43 will include the pairs (p, 2), (t, 3), and (D, 4) in RN.

r Unreachable nodes (UN) set – this set of nodes is used to advertise destinations that

become unreachable

Interzone path creation

A single interzone path from S to D is created during a route request/reply cycle by allowingonly the destination D to send a single reply for a given request The path is tagged with aunique identifier ID, for example, obtained by using increasing sequence numbers generated

by the requesting node When S triggers a new route discovery for a node D, it bordercasts

a query message to all its border nodes The message contains the identifier ID and a route

accumulation vector AV[ ], initialized with AV[0]= S Let M be the number of active nodes

(not including S and D)

(1) When a border node X= D receives a query message, if the message is received forthe first time and the redundant query filter rules are satisfied:

(a) It adds its own identification into the accumulation vector As an example, if thenode X corresponds to node Bj in the interzone path, then AV [ j ] = X.

(b) If D belongs to X’s routing zone, then the latter unicasts the query message to

D Otherwise, it executes a bordercast

(2) When the destination node D receives a query message with identifier ID for thefirst time:

(a) It stores the tuples (AV[i ] , AV[M], M + 1 − i, for 0 ≤ i ≤ M − 1 in EZT.

(b) It prepares the list RN= (AV[i], M + 1 − i)], for 0 ≤ i ≤ M.

(c) It sets AV[M+ 1] = D

(d) It sends a reply message to AV[M] The message contains the AV vector

accu-mulated in the query message

An example of path creation is given in Figure 13.44(a)

(3) When a border node Bj receives a reply message:

(a) If Bj = S, then it stores the triple (ID,AV[ j − 1], AV [ j + 1]) in the IZP table,

thus becoming an active node

(b) It stores the following tuples in EZT: (AV [i], AV[j − 1], j − i), for 0 ≤ i ≤ j − 2; (AV [i], AV[j +1], j − i), for j +2 ≤ i ≤ M + 1.

AV=[S] AV=[S,B1] AV=[S,B1,B2]

(c) (b)

(a)

active node

AV=[S, ,D] AV=[S, ,D] AV=[S, ,D] AV=[S,B1,B2,B3,B4,D]

Reply Query

UN=[B3,B4,D] UN=[B3,B4,D] UN=[B3,B4,D] UN=[B3,B4,D]

3 B3 D

2 B3 B4

2 B1 S EZT

3 B3 D

2 B3 B4

2 B1 S EZT

IZP=(ID,B1,B3) RN=[(S,2),(B1,1),(B3,1),(B4,2),(D,3)]

2 B1 S

CASHING SCHEMES FOR ROUTING 519

(c) It prepares RN= [(AV[j + i], |i|)] for − j ≤ i ≤ M + 1.

(d) If Bj = S, then it forwards the reply message to the node AV [j − 1].

Figure 13.43(b) shows the state at node B2 after the reception of the reply message with

AV [S,B1,B2,B3,B4,D] that caused the execution of the following actions:

(1) B2becomes a member of an interzone path [it stores the triple (ID,B1,B3) in IZP].(2) B2adds the entries (S,B1,2), (B4,B3, 2), (D,B3, 3) in EZT

(3) B2prepares the list of reachable nodes, RN= [(S,2),(B1,1),(B3,1),(B4, 2),(D,3)].(4) B2forwards the reply to B1

Interzone path deletion

An interzone path is broken at node B j when B j−1(or B j+1) is no longer in B j’s routing zone

In this case, the path is divided in two subpaths and the source node is notified with an error

message An active node B j executes the following actions (in the following notation ‘–’means any):

(1) Deletes the entry (–,Bj−1,–) or (–,Bj+1,–) from EZT.

(2) Checks for the companion node Bj+1or Bj−1in the IZP table.

(3) If the companion node is found, then it prepares the following list of

unreach-able nodes: N = [B0,B1, ,Bj−1] (UN = [Bj+1,Bj+2, ,BM+1]); and sends aDelete Path message, containing UN and the path identifier ID, to the companionnode

(4) Deletes the entry (ID,Bj−1,Bj+1) from IZP after the successful transmission of themessage

When an active path is broken, the source node either receives the Delete Path messagefrom B1[if the link is broken between (Bj,Bj+1), with j > 0], or is able to detect the break

autonomously via IARP The source node thus triggers a new route discovery if required

to send other packets, while the two subpaths (B0,B1, ,Bj−1and Bj+1,Bj+2, ,BM+1)remain active Figure 13.44(c) shows the case when the ‘link’ between B2and B3is broken

(i.e their distance becomes higher than k) Two interzone subpaths, (S,B1,B2) and (B3,B4,D),are generated In the figure, B2’s EZT data structure is also shown

When an active node receives a Delete Path message from one of its companion nodes

X, it deletes the entries stored in the UN list from EZT and forwards the message to theother companion node If the receiving node has some another route to a node stored in UN,then it does not include such a node when forwarding UN

Cache management

In order to allow all the nodes of B j ’s routing zone to use the acquired information, B j broadcasts RN inside its zone Such a message is referred to as the inject message On receiving an inject message carrying the reachable node list RN from a node X = Bj, a

node Y creates a set of entries (RN[i ].d,X,RN[i ].#z) into its own EZT, 0 ≤ i ≤ |RN|, where RN[i ].d is the first component (destination node) of the i th pair of RN, RN[i ],#z, the second

component (i.e the length), and|RN| is the number of elements of RN Figure 13.44(a)

(a) (b)

1 B2 B1

2 B2 S

1 B2 B1

2 B2 S

3 B2 D

2 B2 B4

1 B2 B3

1 B2 B1

2 B2 S

3 B2 D

2 B2 B4

1 B2 B3

1 B2 B1

2 B2

Y’s EZT

Y’s EZT

Y’s routing zone zone

Y

Delete

RN=[(S,2),(B1,1), (B3,1),(B4,2),(D,3)]

UN=[(B3,B4,D)]

B2’s routing

Figure 13.45 An example of (a) injection and (b) deletion of external nodes

shows node B2injecting the external routes to nodes S,B1,B3,B4,D into its zone Note that

Y now has two routes to node B1since such a node is in Y’ routing zone

Deleting external routes

When a node Bjeither detects a path breakage or receives a Delete Path message, it casts a Delete message into its zone containing the list of unreachable nodes UN When

broad-an internal node receives a Delete message it deletes all the matching entries from EZT.Figure 13.45(b) shows the delete mechanism on node Y

13.7 DISTRIBUTED QoS ROUTING

In this section a distributed QoS routing scheme for ad hoc networks is discussed Two routing problems are presented, delay constrained least cost routing (DCLC) and bandwidth

constrained least cost routing (BCLC) As before, the path that satisfies the delay (or

bandwidth) constraint is called a feasible path The algorithms can tolerate the imprecision

of the available state information Good routing performance in terms of success ratio,message overhead and average path cost is achieved even when the degree of informationimprecision is high Note that the problem of information imprecision exists only for QoSrouting; all best-effort routing algorithms, such as DSR and ABR, do not consider thisproblem because they do not need the QoS state in the first place Multipath parallel routing

is used to increase the probability of finding a feasible path In contrast to the flooding-basedpath discovery algorithms, these algorithms search only a small number of paths, which

DISTRIBUTED QoS ROUTING 521

limits the routing overhead In order to maximize the chance of finding a feasible path, thestate information at the intermediate nodes is collectively utilized to make intelligent hop-by-hop path selection The logic behind this is very much equivalent to using the Viterbiinstead of ML (maximum likelihood) algorithm in trellis-based demodulation processes.The algorithms consider not only the QoS requirements, but also the optimality of therouting path Low-cost paths are given preference in order to improve the overall networkperformance In order to reduce the level of QoS disruption, fault-tolerance techniques arebrought in for the maintenance of the established paths Different levels of redundancyprovide tradeoff between the reliability and the overhead The dynamic path repairingalgorithm repairs the path at the breaking point shifts the traffic to a neighbor node, andreconfigures the path around the breaking point without rerouting the connection along acompletely new path Rerouting is needed in two cases One case is when the primary pathand all secondary paths are broken The other case is when the cost of the path grows largeand hence it becomes beneficial to route the traffic to another path with a lower cost

13.7.1 Wireless links reliability

One element of the cost function will be reliability of the wireless links The links betweenthe stationary or slowly moving nodes are likely to exist continuously Such links are called

stationary links The links between the fast moving nodes are likely to exist only for a short

period of time Such links are called transient links A routing path should use stationary links

whenever possible in order to reduce the probability of a path breaking when the network

topology changes A stationary neighbor is connected to a node with a stationary link As

in Chapter 7, delay of path P between two nodes equals the sum of the link delays on the path between the two nodes and will be denoted as delay(P) Similarly bandwidth(P) equals the minimum link bandwidth on the path P, and cost(P) equals the sum of the link costs.

13.7.2 Routing

Given a source node s, a destination node t, and a delay requirement D, the problem of delay-constrained routing is to find a feasible path P from s to t such that delay(P) ≤ D.

When there are multiple feasible paths, we want to select the one with the least cost Another

problem is bandwidth-constrained routing, i.e finding a path P such that bandwidth(P)≥

B, where B is the bandwidth requirement When there are multiple such paths, the one with

the least cost is selected Finding a feasible path is actually the first part of the problem.The second part is to maintain the path when the network topology changes

13.7.3 Routing information

The following end-to-end state information is required to be maintained at every node i for every possible destination t The information is updated periodically by a distance-vector

protocol discussed in Section 13.1:

(1) Delay variation –D i (t) keeps the estimated maximum change of D i (t) before the

next update That is, based on the recent state history, the actual minimum end-to-end

delay from i to t is expected to be between D i (t) − D i (t) and D i (t) + D i (t) in

the next update period

(2) Bandwidth variation –B i (t) keeps the estimated maximum change of B i (t) before the next update The actual maximum bandwidth from i to t is expected to be between

B i (t) − B i (t) and B i (t) + B i (t) in the next period.

(3) The cost metric C i (t) is used for optimization, in contrast to the delay and bandwidth

metrics used in QoS constraints

Consider an arbitrary update ofD i (t) and D i (t) Let D i (t) and D

i (t) be the values of

D i (t) before and after the update, respectively Similarly, let D i (t) and D i (t) be the values

of D i (t) before and after the update, respectively D i (t) is provided by a distance-vector

protocol.D

i (t) is calculated as follows:

D i (t) = αD i (t) + (1 − α) D

i (t) − D i (t) ...

wireless networks, in Proc INFOCOM ‘ 97, April 19 97.

[11] M.S Corson and A Ephremides, A distributed routing algorithm for mobile wireless

networks, ACM/Baltzer Wireless Networks. ..

point, the differences between sensor networks and ad hoc networks (see Chapter 13, and

Advanced Wireless Networks: 4G Technologies< /small> Savo G Glisic... TechnicalReport, 1 971

[43] T.-W Chen and M Gerla, Global state routing: a new routing schemes for ad-hoc wireless networks, in Proc IEEE ICC’98, 7? ??11 June 1998, pp 171 – 175 .

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