A PRAGMATIC APPROACH FOR QOS IN WIRELESS MESH

NETWORKS

Andre Herms and Georg Lukas

Department of Distributed Systems

University of Magdeburg, Germany

Keywords:

Wireless Mesh Network, IEEE-802.11, QoS, clustering.

Abstract:

In this paper we present a QoS routing protocol for IEEE-802.11 based mesh networks. The main challenge

for providing QoS in terms of bandwidth and latency is that the medium is shared between all nodes in close

range, which complicates reservation of medium time. Furthermore, the use of standard compliant hardware

components requires an integration of the existing medium access mechanism, which is designed for best-

effort communication only. A cluster-based structure is used for representing the local domains of the shared

medium and allowing reservation of medium time. On top of this reservation an optimistic reactive algorithm is

used for discovery and reservation of routes that fulﬁl the application speciﬁed QoS requirements. Simulation

results are presented that prove the correctness of this approach.

1 INTRODUCTION

Wireless LANs based on the IEEE 802.11 stan-

dard (IEEE-802.11, 1999) are widely used for wire-

less communication. Based on this, mature multi-

hop wireless routing protocols like AODV or OLSR

(Liu and Kaiser, 2003) exist and the upcoming

IEEE 802.11s standard promises wide availability of

multi-hop technology. These multi-hop networks al-

low efﬁcient deployment and good usability due to

their self-organization abilities. Besides of typical

applications like web browsing, services like video

streaming or VoIP are commonly used today.

For use in automation the Wireless LAN Standard

seems to be a promising choice. It allows wireless

connectivity in a free frequency band, has a sufﬁ-

ciently large communication range, and allows rela-

tively high data rates. Furthermore, a huge number

of products exist that can be easily integrated into ex-

isting systems. The self-x properties of mesh rout-

ings – self-organizing, self-conﬁguring, self-healing –

would allow further ﬂexibility and better deployment

of such systems. However, applications in automation

typically require QoS, in terms of bandwidth guaran-

tees and timely delivery. This does not necessarily

mean a maximal average throughput of all connec-

tion. Instead, the packet loss and latency of individ-

ual packets should be limited by a known enforcible

bound.

Many approaches exist that aim to provide QoS

in wireless mesh networks (see section 7). However,

these are either based on different wireless technol-

ogy or require extensions and modiﬁcations that are

incompatible with the Wireless LAN standard. Thus,

they cannot be used with hardware off the shelf. In

this paper we present a pragmatic approach for pro-

viding QoS in wireless mesh networks, which allows

the use of standard hardware. It requires only a cor-

responding routing software. The protocol has to deal

with the behavior of standard compliant hardware that

was not developed to provide QoS at all. Of course,

this requires some compromises and workarounds so

that it will not outperform other, specialized solutions.

The main contributions of this paper is that it shows

the possibility of using standard hardware for QoS

routing in MANETS. Furthermore, various aspects of

the medium access are discussed that are crucial for

the provision of QoS.

The rest of the paper is organized as follows: Sec-

tion 2 describes the principles of medium access in

wireless networks and their consequences for the spa-

tial separation of the shared medium. The principle

of operation is explained in section 3. In section 4 our

clustering protocol is presented. This is followed by a

Herms A. and Lukas G. (2007).

A PRAGMATIC APPROACH FOR QOS IN WIRELESS MESH NETWORKS.

In Proceedings of the Second International Conference on Wireless Information Networks and Systems, pages 57-64

DOI: 10.5220/0002147000570064

 SciTePress

description of our routing protocol in section 5 and its

evaluation in section 6. The paper ends with a presen-

tation of related work in section 7 and a conclusion in

section 8.

2 MEDIUM ACCESS IN MANETs

In opposite to modern wired networks, which use

point-to-point duplex links, the wireless medium

is shared between the participating stations. The

MAC layer is responsible for managing access to

the medium and the IEEE 802.11 standard uses a

CSMA/CA mechanism, which is based on carrier

sensing:

Before a station starts sending, it checks if there

is an ongoing transmission on the medium and if nec-

essary defers the packet start. Two different carrier

sense strategies are combined for this – a physical car-

rier sense and a logical carrier sense. The logical car-

rier uses the transmission duration entry of packets

it receives. These are stored in a register and com-

pared to an internal timer. Combined with a four-way

handshaking (RTS/CTS/DATA/ACK) it can even be

used to prevent the hidden station problem (Tanen-

baum, 2002).

The physical carrier is sensed using the receiver

unit for detecting the carrier signal and setting an

internal busy state. This mechanisms even detects

an ongoing transmission when the packet cannot

be correctly received due to bit errors. In experi-

ments with standard hardware Mahrenholz was able

to show that the range where the carrier can be de-

tected is about twice as large as the communication

range(Mahrenholz, 2006). In order to prevent that

multiple stations start a transmission immediately af-

ter the medium becomes available, a random back off

is used. Similar to the Ethernet standard, the time in-

terval it is selected from is exponentially increased,

when packet loss happens. This is detected by the

missing ACK of unicast data packets.

As our protocol uses standard hardware, the

medium is still controlled by this CSMA/CA mech-

anisms. We do not deﬁne a new MAC layer, but try to

cope with the existing one. In particular, this means

that packet collisions are already prevented and not

subject to further considerations. The main reason for

violation of QoS requirements in MANETs are con-

gestion effects and packet losses. While packet losses

are compensated by local retransmissions in the link

layer, congestion must be prevented by the routing.

It is not necessary to create an exact schedule to pre-

vent concurrent transmissions, because this is already

done by the MAC layer. Instead, it must ensure that

the medium time consumed by a station, its neigh-

bors, and its two-hop neighbors does not exceed the

maximum available one.

3 PRINCIPLE OF OPERATION

The protocol consists of two sublayers, the cluster

layer and the global routing layer. The aim of the clus-

ter layer is to provide a QoS-capable medium access.

It allows local reservation of bandwidth, is respon-

sible for local retransmissions, and has a predictable

packet delay. In order to achieve this, it creates lo-

cal domains of control, called clusters, which are

maintained by a local coordinator, the cluster head.

The cluster head initiates medium access in the clus-

ter by polling other stations. This is very similar to

the point coordination function (PCF) of the WLAN

standard. However, we operate in the ad hoc mode,

where such functionality is not available. Efﬁcient re-

transmissions and local two-hop routing are provided,

based on the corresponding microprotocols presented

in (Nett and Schemmer, 2003).

Above the clustering sublayer a routing sublayer

is used, which is tailored to the cluster structure. We

use a proactive link state routing. Three classes of

trafﬁc are available, control trafﬁc, best effort traf-

ﬁc, and QoS trafﬁc. Because of the strict schedul-

ing of control messages with the lowest priority, QoS

streams are never affected by control trafﬁc. The rout-

ing sublayer uses a hop-by-hop reservation of routes.

Bandwidth is reserved per cluster and the determin-

istic delay in the clusters allows an approximation of

the worst-case packet delay of the path. An appro-

priate path is discovered by an optimistic path search,

which tries to use the shortest path. If it fails in case of

insufﬁcient resources on a hop, alternatives are tried

by local, backtracking-like rerouting of the reserva-

tion requests. If a path with sufﬁcient resources ex-

ists, it is often found by the ﬁrst reservation. If the

ﬁst try of the shortest path is not usable, a different

usable path will be discovered. This helps to reduce

the number of required control messages.

4 CLUSTERING

In a typical WLAN infrastructure, two classes of sta-

tions exist – Access Points and Clients. In an ad hoc

mesh network all stations have the same role. They

act as router and communication endpoint at the same

time. We create a spatial partitioning of the network

into local domains (clusters) by assigning roles to the

stations. A station can either be a cluster head or a

cluster client. Every cluster head has a set of clients

in communication range. A client can belong to mul-

tiple clusters. In such a case it can act as gateway and

allows routing between clusters. A cluster head never

has other heads in communication range. An example

of such a structure is depicted in ﬁgure 1.

The clustering subprotocol is responsible for as-

signing these roles. It uses a periodic cluster an-

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

Figure 1: Cluster Structure.

nouncement packet, which is transmitted periodically

by every head and contains the list of all clients. The

following rules are applied:

1. A client that receives a cluster announcement

from a head, it is not associated with, adds the

cluster to its set and replies with a join packet.

Typically this is received by the head and the

client is included in the cluster.

2. A client that missed a deﬁned number of an-

nouncements removes the head from its set of

clusters.

3. A client that has no associated clusters changes its

role and becomes a cluster head.

4. A cluster head that receives announcements from

another head, changes its role to a client.

The resulting structure has the following proper-

ties: A client node has only one-hop links to head

nodes, and vice versa. Every two-hop link between

two nodes has endpoints of the same role. A two-hop

neighbor of a client is always a client, which is con-

nected by a head node. Thus, both client nodes belong

to a common cluster deﬁned by the intermediate head

node. Two clusters can have common client nodes

that serve as gateway.

4.1 Timing

The medium access in a cluster is initiated by the

cluster head. It polls its clients with packet

Poll[x]

whereupon it must be answered by a new data packet

D[x]

. The data packet is not guaranteed to be re-

ceived by the other clients in the cluster. Therefore,

it is repeated by the cluster head. As this packets is

followed by the poll for the next client, both pack-

ets are combined. The head is allowed to communi-

cate by polling itself (

PollH

). An example is shown

in ﬁgure 2. Every such packet pair consisting of the

repetition by the head including the poll and the data

packet from the client deﬁnes one slot. The slots have

a ﬁxed duration l. The number of stations per cluster

is limited by a predeﬁned value C. The duration of

C+1 slots is one round. The head chooses a schedule

that guarantees every station to be polled at least af-

ter C slots, once per round. Clients can request more

slots per round, by sending a corresponding request

... +

Poll 1

D1 +

Poll 2

D2D1 D2 +

Poll H

DH DH +

...

Client 1

Client 2

Head

Figure 2: Polling scheme of a cluster.

... +

Poll 1

D1 D1 +

Poll 2

Client 1

Client 2

Head 2

Head 1

... +

Poll 1

D1 +

Poll H

D2 +

Poll ...

Figure 3: Example schedule with H = 2.

in response to the heads cluster announcement packet.

One slot per round is reserved for this packet.

Clients can belong to multiple clusters and receive

all packets from each of them. Therefore, it is not pos-

sible for a cluster head to use the whole medium time.

This is prevented by the carrier signal from adjacent

cluster heads, which are at most in two-hop range. Its

transmissions of poll packets would be deferred and

the output queue would ﬁll up and overﬂow. In order

to prevent this effect, the slots are widened by a fac-

tor of H, which corresponds to the maximum num-

ber of clusters with a common client. It can be eas-

ily shown that for stations with equal communication

ranges arranged in a plane, a value of H = 7 is suf-

ﬁcient. An example schedule of two clusters with a

common client (client 1) and H = 2 is shown in ﬁg-

ure 3.

The strict timing allows to determine the delay of

packets and further the detection of topology changes

in a bounded time. The polling period of a client is

deﬁned as CHl. When allowing a maximum number

of d lost equal packets, a client detects the loss of a

cluster head in dCHl and a cluster can detect the loss

of a client in 2dCHl.

4.2 Preventing Exponential Backoff

The exponential increase of the backoff in the WLAN

MAC layer is used to reduce the collision probabil-

ity during the contention based medium access. How-

ever, it also adds randomness to the transmission start,

which leads to unpredictable latencies. Our protocol

uses only broadcast packets that are never acknowl-

edged by the receiver and thus never increase the

backoff window. As a consequence, packet loss can-

not be detected by the MAC layer anymore and no lo-

cal retransmission are done. The Atomic Multicast in

A PRAGMATIC APPROACH FOR QOS IN WIRELESS MESH NETWORKS

(Nett and Schemmer, 2003) provides a Dynamic Time

Redundancy microprotocol that allows fault tolerant

and time bounded delivery to a group of stations. It

uses an omission degree that corresponds to the previ-

ously deﬁned parameter d. We use this microprotocol

for fault tolerant packet delivery to all stations in the

cluster. It requires a maximum duration of 2dCH slots

lengths per packet.

4.3 Properties

The clustering protocol has the following properties:

(1) The medium time is divided into slots that re-

spect the two-hop range of carrier sense: All one-

hop neighbors belong to a common cluster. Two-hop

neighbors are either both clients in a common clus-

ter or both heads of adjacent clusters. In a common

cluster the polling scheme controls the medium ac-

cess. For adjacent clusters the additional time be-

tween cluster slots prevents that the available medium

time is exceeded.

(2) Medium time can be reserved by requesting

additional slots from the cluster head. Consensus

about the assigned slots in a cluster is guaranteed be-

cause it is coordinated by a single station, the head.

(3) Packets are reliably transmitted to all nodes in

a cluster, as long as the cluster is not disbanded. A

transmission reaches all nodes of a cluster, even the

clients with a distance of two hops. This requires

at most 2dCH slot lengths, which corresponds to the

worst case latency in cluster. Furthermore, the guar-

anteed bandwidth of a client can be derived from the

maximum packet size, the worst-case transmit dura-

tion, and the number of assigned slots in a cluster.

This is used for reservation.

(4) The communication is time-triggered. Every

station in the network has its exclusive slots that must

be used for communication. Only the announcement

slot of a cluster can be used by multiple stations. The

cluster head polls all members of a cluster and thereby

controls when a station has to transmit.

5 ROUTING

The purpose of a wireless mesh network is to al-

low nodes without a direct link to exchange data.

To achieve this, the nodes need to obtain knowledge

about the network structure, either as distance vectors

to other nodes or as link state data about the topology.

This information can either be gathered on demand,

when a connection is requested or in a proactive fash-

ion during regular operation. This paper presents

a proactive link state based approach, which allows

faster path searches and the calculation of alternative

routes around congested areas.

The following section describes a topology prop-

agation protocol, which exchanges global link state

data between interconnected nodes. This informa-

tion is used by a best effort routing protocol described

in section 5.2 to allow data exchange between nodes

not directly connected. The routing capability is used

in combination with the topology data for a directed

search of suitable QoS paths and for their monitoring

by a protocol described in 5.3.

5.1 Topology Propagation

Because every node only has data about its own di-

rect neighborhood, a cooperative approach is needed

to obtain information about the whole network, which

then can be used for routing purposes. To achieve this,

the neighborhood data, which is provided by the clus-

tering module on every node, must be propagated to

other stations and stored there. For storage a local

link graph is used, which contains representations of

nodes and lists of their respective neighbors.

Information about the network topology is trans-

mitted via broadcast packets to all neighbors. Every

such packet contains a list of elements, each consist-

ing of a node’s ID, a sequence number and that node’s

list of neighbors. When a node is not listed in the lo-

cal link graph or the transmitted sequence number is

higher than the last known one, the receiver merges

the element from the datagram into the local topology

database.

The propagation scheme is similar to the Fish-

eye State Routing protocol (Pei et al., 2000). Every

node has up-to-date information about its neighbor-

hood, while its knowledge about distant links is older,

and thus less accurate, because topology transmis-

sions from these nodes take more time. On the other

hand, speciﬁc knowledge is only needed for routing

when the target is near, for distant targets it is sufﬁ-

cient to have the approximate direction, because the

following gateway nodes have more current link state

data and are able to direct the packet.

A node has to send an answer every time it is

polled by the head, even when no application data is

available for transmission. These idle time slots are

used for topology propagation. Thus, the world mod-

els are most accurate when the network is not heav-

ily used, but the effect of trafﬁc load on the propa-

gation speed is only moderate, as will be shown in

section 6.1.

Because a single packet can only carry informa-

tion about approx. 30 nodes and their neighborhood,

a sophisticated algorithm is used to select parts of the

world model for transmission. This algorithm prefers

updated information while still preventing starvation

of older data. It allows a newly connected node to

be informed about the whole network topology af-

ter a relatively short time while tolerating packet loss,

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

which makes the protocol a reliable base for routing.

5.2 Best Effort Routing

To allow communication between nodes not directly

connected, a multi-hop routing mechanism must be

provided. This mechanism can be used not only for

best effort communication, it is also required for the

reservation and monitoring of QoS links.

Because every head reliably retransmits data

packets to all clients in its cluster, only gateway nodes

(which are member of two or more clusters) can be

used for routing. Thus, before transmitting a packet,

a node has to determine the cluster and the corre-

sponding local gateway leading to the destination. To

achieve this, the Dijkstra algorithm is applied to the

network topology stored in the link graph, and the

packet is sent to the gateway on the shortest path. On

the next node the same procedure is applied, where

the packet is routed to the next cluster according to its

local link graph.

Because a gateway node can only send a packet

into a cluster after being polled by the head, pack-

ets crossing cluster boundaries must be cached by the

gateway until they are requested. On highly utilized

links this can lead to congestion, when several dif-

ferent packets are sent over one gateway to one tar-

get cluster. This congestion is acceptable for best ef-

fort trafﬁc, but must be avoided for QoS connections.

The next section describes in detail how this is accom-

plished.

5.3 QoS Routing

Several requirements must be met to allow QoS com-

munication between distant nodes. First, the applica-

tion must be enabled to specify the target and its QoS

requirements. Then, a path must be found through

the network, which fulﬁls the requested parameters.

A bandwidth reservation must be performed on this

path and a monitoring mechanism has to continuously

check if it is still valid.

To create a QoS path, the application speciﬁes

the required bandwidth and latency. These values

are mapped to the number of slots to reserve (band-

width) and maximum hop count (latency). The QoS

protocol then uses the link graph to calculate a short-

est path to the destination and requests the speciﬁed

number of slots in the corresponding cluster. If the

request is granted, a packet is forwarded to the next

gateway, which again makes a path calculation and a

bandwidth request. Thus, the request packet is for-

warded from the requester to the target and the re-

quired bandwidth is reserved on the whole path. If

the request cannot be granted in one cluster (because

all time slots are already reserved), the corresponding

gateway performs the following steps: ﬁrst, it puts the

cluster into a black list in the request packet. Then, it

makes a new path search with the blacklisted clus-

ter(s) excluded from the link graph and reserves the

bandwidth in the newly calculated target cluster. If

no alternative route can be found, the gateway signals

its predecessor to blacklist the current cluster and to

dereserve the previous bandwidth request.

This optimistic reactive approach results in a dis-

tributed backtracking algorithm that ﬁnds a relatively

short route through clusters with enough bandwidth

to allow the requested communication channel.

Every node maintains a table of all QoS paths that

are routed through it. This table contains everything

needed for routing and monitoring the path, plus its

current status. When the reservation request reaches

the target, it is acknowledged on the exact same path

it was sent, and every participating gateway sets the

according entry to reserved.

Because the connectivity in a mobile ad-hoc net-

work changes steadily, it is not possible to guaran-

tee the consistency of a path. Thus, to meet the QoS

requirements, all links on a path must be monitored,

every failure must be detected and reported to both

ends of the QoS stream. Every node participates in

the monitoring process by several means. First, it for-

wards monitoring notiﬁcations (such as broken path

packets) from other nodes to the originator or desti-

nation of a QoS path. Second, it checks the data ﬂow

on the reserved path. When no data is transmitted

for a certain time, the connection is treated as closed

and silently dropped. The third part of the monitoring

process is a steady inspection of the local link graph.

When a neighbor disappears, the QoS path table is

checked if it was a source or gateway for one or sev-

eral of the links. In this case, monitoring notiﬁcations

are generated in the opposite direction, i.e. a notiﬁ-

cation is sent to the originator when a gateway dis-

appears, and a notiﬁcation is generated for the target,

when the previous hop node is disconnected. Because

of the strict clustering scheme and the bidirectionality

of every link, a disconnect is detected on both sides of

the broken connection, and both originator and target

are informed, while the reserved bandwidth is freed at

the same time.

5.4 Summary

Based on the reliable packet transmission in a cluster,

the combination of proactive world model propaga-

tion, best effort routing and optimistic reactive search

allows the fast and directed reservation of QoS paths

with application speciﬁed parameters. After the paths

are reserved, a sophisticated monitoring mechanism

observes their reliability and notiﬁes the application

when the given guarantees cannot be met because of

node failure, topology change or other reasons. This

combination allows the usage of the protocol in criti-

A PRAGMATIC APPROACH FOR QOS IN WIRELESS MESH NETWORKS

Table 1: Simulation Parameters.

simulation parameter value

area 1000 m × 1000 m

number of stations 100

initial positions uniform random

propagation model two-ray ground

transmission range 250m

interference range 500m

transmission rate 54Mbit/s

packet loss rate 5%

protocol parameter value

max. clients per cluster C 16

max heads per client H 7

slot length l 4ms

omission degree d 3

maximum packet size 1000byte

mobility paramter value

mobility model random waypoint

speed of stations 3− 5

cal domains like automation control.

6 EVALUATION

The described protocol was implemented based on

an abstraction layer (Herms and Mahrenholz, 2005)

that allows to use the same binary code under Linux

with real WLAN devices and in the network simula-

tor NS−2(NS2, 2007). In this paper we present re-

sults from the simulation. The software was also suc-

cessfully used in a real wireless mesh network, but an

extensive evaluation has not been done, yet. The rele-

vant simulation parameters are summarized in table 1.

6.1 Topology Propagation

The ﬁrst critical aspect of the protocol is the quality of

the link graph on the participating nodes. Inaccurate

information can be tolerated by the routing but may

lead to suboptimal path selections. If the divergence

between the local link graph and the real topology

grows too big, routing can even become impossible.

Because the topology information is propagated in the

idle slots of the clustering mechanism, it does not af-

fect application performance. However, the propaga-

tion speed depends on the medium usage. Here we

measure the dependency of propagation speed on the

application network load.

For measuring the accuracy of the link graph we

deﬁne a quality metric κ

that corresponds to the cor-

rectness of the local link graph of node i. It depends

on the number of really existing links L, the number

of false positives l

(links in the link graph without a

0 5 10 15 20 25 30 35 40 45 50

100

time [s]

link graph correctness κ [%]

70% load

80% load

90% load

95% load

100% load

Figure 4: Topology propagation with varying network load.

corresponding real link) and the number of false neg-

atives l

−

(real links not represented in the link graph).

For allowing comparison regardless of the number of

links, the value is normalized by dividing it by the

number of really existing links:

L− l

− l

−

(1)

The overall accuracy of the topology propagation

is represented by averaging all local accuracies: K =

∑

with K ∈ [0, 1]. A value of K = 1 indicates a

perfect propagation while smaller values mean lower

accuracy.

In ﬁgure 4 the accuracy of the link graph prop-

agation over time is depicted with varying network

load. It can be seen that the accuracy increases after

the start and that higher trafﬁc leads to slower prop-

agation of the link graph. If all slots are used for

data trafﬁc (load 100%) no link graph data is propa-

gated because no idle slots are available. This case

though is only theoretical, because it never will be

met with normal data trafﬁc where additional slots are

reserved for retransmissions and never entirely used.

Here, only high loads starting from 70% are consid-

ered and even under this conditions sufﬁcient accura-

cies are reached after a few seconds. It can also be

seen that the perfect propagation of 100 % is never

completely reached. This is caused by the mobility,

which changes the topology while older link state data

is still propagated. Due to the approach similar to FSR

(Pei et al., 2000), a completely correct link graph is

not actually needed for routing, because a node only

needs to have exact information about its neighbors

and approximate data about the direction to remote

nodes. Thus, we regard a link graph propagation of

K = 60% to be sufﬁcient for proper routing and QoS

path reservation, which is achieved after less than 12

seconds on a 90% saturated medium, starting with no

prior topology knowledge.

6.2 QoS Routing

Our QoS routing aims to prevent unpredictable tim-

ings in the network that are caused by congestion. An

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

0 10 20 30 40 50 60 70 80 90 100

100

number of background streams

packet delay [s]

best efford (max)

BE linear approx

QoS (max)

Figure 5: Comparison of QoS and Best effort trafﬁc.

admission control is used that requires prior reserva-

tion of QoS connections. This section shows the com-

pliance of the QoS protocol to the timing restrictions,

compared to data transmission times using the best

effort channel. We use a simulation setup with the

same parameters as in the previous experiments. The

trafﬁc was generated using different numbers of data

streams created from random source nodes to random

destinations. The number of streams was varied from

0 to 100. By using the QoS routing, data packets were

only sent after a successful path reservation. Best ef-

fort data was sent by the nodes in the same intervals

as if a path were reserved.

The comparison of best effort and QoS routing is

depicted in ﬁgure 5, where the maximum delays of

data packets are presented. It can be seen that with an

increasing number of streams the delay of best effort

streams rises. Without admission control the network

becomes congested and packets spend a lot of time

in the outgoing queues on gateway nodes or even can

be dropped. Using QoS routing with prior reservation

solves the problem by limiting the number of concur-

rently admitted streams. The maximum transmission

delays remain below a certain level. Compared to

other kinds of network, the latency of the routing is

still rather high. On the one hand, this results from

the quantity used for the experiments – the maximum

delay among all streams, which corresponds to the

maximum delay of the longest route and can be very

long due to the random selection of the paths destina-

tion. On the other hand the polling scheme of the clus-

ters is not optimal for short delays. Stations are only

allowed to transmit when they are polled and thus a

packet cannot immediately be forwarded. This means

a delay of up to one cluster round on every gateway,

accumulating on paths with a high number of hops

and resulting in long transmission times.

Still, it can be seen that the usage of a QoS reser-

vation mechanism completely evades congestion re-

lated latency. While the best effort transmission times

grow with the number of participating nodes, QoS

trafﬁc is not delayed when more nodes try to com-

municate. The only impact of a congested network is

either rerouting around saturated clusters or, when no

alternative paths can be found, rejection of reservation

requests.

7 RELATED WORK

The most similar structure to our approach can be

found in CGSR (Chiang et al., 1997). It also uses

scheduling for controlling the medium access. The

intracluster schedule is done by the head polling its

clients (TDMA). For avoiding collisions between ad-

jacent clusters, every cluster uses its own CDMA

code. Because CDMA cannot be realized on typical

IEEE 802.11 devices, this can only be implemented

with special hardware. Furthermore, the routing does

not consider QoS, although this seems possible. In

(Dong et al., 2002) a supernode-based reverse label-

ing algorithm is presented that can give guarantees

on bandwidth and delay of links. Supernodes are

elected that act as coordinators for the routing and

coordinate the search for routes that satisfy the QoS

requirements. The routing assumes that the MAC pro-

vides information about available bandwidth and de-

lay. However, it is not discussed how this is accom-

plished. CEDAR(Sinha et al., 1999) is a link state

based routing that creates a logical structure based on

so called core nodes that are very similar to our clus-

ter heads. For providing bandwidth guarantees, the

corresponding information is ﬂooded in the network.

The core nodes are responsible for ﬁnding routes that

meet the bandwidth requirements. It is also not ex-

plained, how the available bandwidth is calculated,

but assumed that the MAC layer can provide this in-

formation.

Besides of cluster based routings, some ﬂat rout-

ing protocols exist that aim to provide QoS. Here, the

problem of managing the bandwidth also exists and

there are no spatial structures (like clusters) that cor-

respond to the groups of neighbor nodes. In (Liao

et al., 2002) and (Zhu and Corson, 2002) slot-based

bandwidth reservation mechanisms for MANETs are

presented that use coordination with 1-hop neigh-

bors. However, the protocols are not applicable to

CSMA/CA-based MAC layers. In (Chakeres and

Belding-Royer, 2004) and (de Renesse et al., 2005)

admission control for bandwidth reservation is done

by measuring the ratio of busy and idle state of the

MAC layer. This gives an exact view of the current

usage of the local bandwidth, including the effects of

physical carrier sense. However, it requires modiﬁ-

cations in the MAC layer to export this information

to the upper layers and can only report the previously

used medium time, which does not consider ongoing

reservations. The admission control in (Kuo et al.,

2005) uses one-hop beaconing for exchanging infor-

mation about the available bandwidth with neighbor

nodes. It is designed for the IEEE 802.11 MAC layer

and considers the effects of CSMA/CA on the band-

width, but the extended physical carrier sense of the

MAC layer is not regarded. In (Chen and Heinzel-

man, 2005) the effects of carrier sense are correctly

A PRAGMATIC APPROACH FOR QOS IN WIRELESS MESH NETWORKS

considered. A two-hop beaconing is used for reaching

all affected nodes and calculating the available local

bandwidth. This is used for verifying the suitability

of a path returned by an AODV routing. If the ﬁrst

guessed path is not suitable, no alternatives are used

in contrast to our backtracking based protocol.

8 CONCLUSION AND OUTLOOK

In this paper we addressed the problem of providing

QoS in terms of bandwidth and latency in wireless

mesh networks based on standard compliant WLAN

hardware. We have shown that it is essential to con-

sider the MAC layer with its extended carrier sense

range. A cluster-based local coordination protocol

is presented that does two-hop coordination between

nodes and allows reservation of medium time. On

top of this a routing and a path reservation mecha-

nism are used that allow bandwidth reservation and

latency limitation per stream. Because the consumed

medium time must be coordinated even with two-hop

neighbors, a relatively expensive mechanism is ap-

plied. The resulting routing gives strict guarantees

for individual streams, but these are paid with the

high overhead, which results in low usable bandwidth.

Therefore, it is not suitable for applications like mul-

timedia streaming or Internet connections. Instead it

can be used in automation scenarios where bounded

latencies with strict guarantees are required.

Further work will include a more detailed evalua-

tion of the protocol, especially by using a real wire-

less mesh network instead of simulations. This will

help to estimate the properties of the routing in real

scenarios. The current approach still has points for

possible improvements. The latency of multihop con-

nections can be reduced by synchronizing the polling

times of gateway nodes. The usable bandwidth can

be increased by overcoming the strict partitioning of

medium time between adjacent clusters or maybe by

using a solution without clusters. Some approaches

mentioned in section 7 seem promising, but would re-

quire some rework for being applicable to standard

hardware.

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