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AbstractThe advantages of high access speed and

differentiated service provisioning with WiMAX technology has

made it competent with current wired networks solutions. The

various service types have been defined in recent 802.16

standards. However, the detailed traffic scheduling algorithm

has been left empty to researchers. In this paper, the new

concept of urgency index (UI) is brought out as the system

parameter to monitor different traffic services. UI is a set of

system variables to reflect the urgency of bandwidth need for a

specific service flow. Traffic scheduling is done based on the UI

values. For different types of services, different UI changing

profiles have been proposed to achieve best scheduling outcome.

This scheme was implemented and simulation results have been

compared to other proposed scheduling algorithms to evaluateits performance. The results show the UI based scheduling

algorithm can achieve lower packets drop rate and also lower

mean delay time at all traffic loads.

I. INTRODUCTION

WiMAX (Worldwide Interoperability for Microwave Access)

networks are the new generation of wireless networks which

could support real-time multimedia services. It has received

much attention from the industry past a few years. The mainadvantage of WiMAX networks are high throughput, and

long range of access distance. WiMAX access technology is

defined in the IEEE 802.16 standards. It could provide

support to various types of traffic, such as multimediastreaming and internet data traffic [1]. In the standards four

types of traffic are defined, and an important issue in WiMAX

networks is to provide QoS (quality of service) among the

traffic services to meet delay time requirements. In order toachieve QoS support and efficient usage of system resources,

intelligent traffic scheduling algorithms are required. The

recent IEEE 802.16d standard which was published in 2004

has proposed a framework for service provisioning among the

four traffic types. However, the detailed scheduling algorithm

is left to the service providers and researchers. This leaves

wide space for researchers to design and evaluate various

traffic scheduling algorithms, and recently quite a few

research papers have been published on QoS support inWiMAX networks. Most of the published works focus on the

improvements of the QoS provisioning framework [2-5].

Some other proposals aimed to improve some issues in QoSsupport. As in [6], a synchronization scheme has been

introduced to reduce the system overhead in UGS

(Unsolicited Granted Service). In [7], it was suggested to

Maode Ma, Pengfei Xie, and Sanjay Kumar Bose are with the School of

Electrical & Electronic Engineering, Nanyang Technological University,Singapore, 639798 (Maode Ma is the corresponding author with phone:

65-6790-4385, e-mail: emdma@ntu.edu.sg).Stephane Maag is with the Telecom & Management SudParis, France

(e-mail: Stephane.Maag@it_sudparis.eu).

support VoIP (Voice over IP) traffic in both UGS and rtPS

(real-time Polling Service) in order to improve the utilization

of the uplink bandwidth. In [8], a hierarchical schedulingalgorithm was proposed to enhance the nrtPS (non real-time

Polling Service) which could combine the rtPS and nrtPS

together under a general polling framework. The proposed

algorithm in [9] has considered detail scheduling issues toensure the implementation of QoS differentiation. It

implemented an overall scheduling policy for all the four

traffic types. The proposed algorithm is a connection based

algorithm and strict priority has been assigned to the fourtypes. The solution employs the earliest deadline first (EDF)

policy for rtPS connections and the weighted fair queue(WFQ) policy for nrtPS. However, the strict priority

assignment of the four types of traffic makes the scheduling

scheme lack of flexibility. For example, there is no chance for

BE traffic, which has been delayed a long time until all the

rtPS and nrtPS flows have all been serviced. This algorithm

also requires rtPS traffic flows to delay in the network until

the last time frame before deadline. Though this helps to

reduce the possibility of starvation of bandwidth resources by

high priority traffic, it underestimates the burst nature of rtPS

traffic and results in high packets loss rate when the total bursttraffic rate is more than the system capacity.

In this paper, a novel connection oriented scheduling

algorithm is proposed by introducing the new concept ofurgency index (UI). UI is set of connection parameter

monitored by the system to describe connections scheduling

priority. The proposed solution breaks the strict priority

assignments to different traffic types. By introducing proper

UI changing profiles for each traffic type, it can effectively

accommodate all the four traffic types. Simulation shows

network performance of mean delay at low traffic loads and

overall packets loss rate can be significantly enhanced. The

main contribution of this paper is the new UI concept in

WiMAX traffic scheduling to break rigid service priority and

thus providing efficient QoS support. The remaining parts of

this paper are organized as follows. Section 2 describes theWiMAX network system specifications and service types

defined in standards; Section 3 describes the proposed UI

based scheduling algorithm; Section 4 specifies the

simulation system parameters and presents the simulation

results; and Section 5 concludes the paper with a brief

summary.

II. SYSTEM SPECIFICATIONS

A. System backgroundThe standard specifies the medium access control layer

An Efficient Scheduling Algorithm for

QoS Provisioning in WiMAX Networks

Maode Ma, Pengfei Xie, Sanjay Kumar Bose and Stephane Maag

978-1-4244-2858-8/08/$25.00 2008 IEEE

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(MAC) supports a primarily point to multipoint (PMP)

architecture. The MAC is structured to support various

physical layer (PHY) specifications, each suited to a

particular operational environment. For the common

operational frequencies of 10-66GHz, the WirelessMAN-SC

PHY, based on single-carrier modulation is specified. For

frequencies below 11GHz, propagation without a direct line

of sight must be accommodated, and alternative PHY likeWirelessMAN-OFDM should be used. The wide spectrum of

10-66GHz gives the system much bandwidth resources to

operate with. Baud rates and channel bandwidths are

specified in the standards. With different modulation

techniques, WiMAX networks can achieve data transmission

rates around 40 Mbps for each 25MHz bandwidth. There are

also detailed recommendations for the PHY frame durations.

The standards also specify QoS provisioning framework in

the MAC layer. Downlink traffic is relatively simple because

the BS (Base Station) broadcast messages to all the SS

(Subscriber Station). For uplink traffic, request and grant

scheduling is performed by BS with the intent of providing

each SS with bandwidth for uplink transmission and

opportunities to send bandwidth request. The uplink and

downlink traffic can be multiplexed either with Time

Division Duplex (TDD) or Frequency Division Duplex

(FDD). The system under study of this paper is using the

TDD mode. For TDD mode, the uplink and downlink

transmissions occur at different times and share the same

frequency. A TDD frame has fixed duration and is divided

into subframes for uplink and downlink traffic. The frame is

divided into an integer number of physical slots (PS), which is

used as the bandwidth unit in scheduling. After BS has done

the scheduling, the DL-MAP and UL-MAP fields which tell

the scheduling results will be included in the downlinksubframe. This information will be broadcasted to all the SS,

so that each SS knows the bandwidth allocation results for the

coming uplink subframe.

B. Traffic types supportMultiple types of traffic can be supported in WiMAX

networks. There are four types of services specified in the

standard. They are Unsolicited Grant Service (UGS),

Real-time Polling Service (rtPS), Non Real-time Polling

Service (nrtPS), and Best Effort (BE) service. These services

have different QoS requirements and represent different types

of network traffic. A good example for UGS traffic is VoIP,

with fixed size packets stream and periodic inter-arrival time.This type of service eliminates the overhead and latency of SS

requests and assures that grants are available to meet the

flows real-time needs. The rtPS traffic is characterized byvariable packet size and fixed inter-arrival time, such as video

streaming traffic. The service offers real-time, periodic,

unicast request opportunities, which meet the flows real-time

needs and allow the SS to specify the size of the desiredbandwidth grant. This service requires more requests

overhead, but can support variable grant size to achieve

optimum data transmission efficiency. The nrtPS offers

unicast polls on a regular basis, which assures that the service

flow receives request opportunities even during network

congestion. The SS can use both unicast request opportunities

and contention based request opportunities. This service

represents non real-time traffic which requires a minimum

bandwidth allocation, such as bandwidth intensive file

transfers. The intent of the BE service is to provide efficient

data transmission for best effort traffic. For this service, SS is

only allowed to use contention based request opportunities.

This service represents time insensitive burst type traffic likeinternet data traffic. There is no QoS guarantee for BEservice.

C. Traffic schedulingThe system works on a request-grant scheduling framework.

The SS register its traffic connections with the BS, and sendout traffic requests based on service specifications. The BS

performs scheduling and broadcast the bandwidth allocations

results to all SS.

For contention based bandwidth request messages, the

sending mechanism at SS follows a mandatory binary

exponential backoff mechanism for contention resolution.

There are some related parameters like initial window sizeand maximum windows size is configurable by the BS for

different network systems. The detailed contention resolution

mechanism can be found in [1].

The scheduler at BS takes account of all the connection

specifications and request messages to perform thescheduling. Important parameters which the scheduler should

consider include 1) the traffic service type; 2) the set of QoS

requirements of the connections; 3) the capacity of bandwidth

for data transmission; 4) the bandwidth requirements from theconnections; and 5) waiting time of bandwidth requests in the

system. The ideal scheduler should be able to make optimum

use of the available bandwidth to reduce traffic delays and

satisfy the QoS requirements to the best extent thus to reducepackets drop rate and sustain the QoS support.

III. PROPOSED UIBASED SCHEDULING ALGORITHM

A. The UI ConceptIn order to provide service to all the four types of service

flows, its necessary to differentiate each service type and

connection flow. Each service type has different QoSrequirements and traffic characteristics. The traffic burst time

is also different for each service flow. For the QoS scheduler

to work effectively, the usage of Urgency Index (UI) is

proposed as the only parameter set used in the schedulingscheme.

Basically, the UI is set of variables which reflect the

bandwidth requirement urgency of a specific connection.

Since the MAC protocol is connection oriented, SS nodes

request bandwidth on a connection basis. For optimal system

resource utilization, the traffic allocation at BS is also to

connections.

With this scheme, the BS allocates bandwidth on a

connection basis. The UI values represent traffic allocation

priority. This scheme breaks the rigid priority assignment of

the four service classes, and considers more parameters rather

than the service type only. The contributing factors to the UI

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values should include 1) Service type 2) Delay Bound 3)

Waiting time of requested traffic. 4) Connection mean traffic

rate.

B. Index changing profileThe UI value describes allocation priority of request from

some connection. As an example, SS node m requestsbandwidth Si bytes for connection CID i. Upon receiving the

request message, BS denotes the request amount, and assigns

a starting UI value 0u to this connections traffic request.

Then at each scheduling instant, the UI value is updated

according to how long the traffic request has waited in the

system; traffic allocation decisions will be made based on the

new value. Since UI value essentially represents priority, it

should be increasing by waiting time. On the other hand, its

necessary to have a limiting value lu for the UI value to

converge to. This limiting value represents priority of a

connection in the long run if the request waits too long, and

this value lu should represent the service type priority of

connection.The scale of UI values can be arbitrary. For proper

representation of priority of traffic needs, the UI values are

limited to be normalized within 0.0 to 1.0, with 0.0 being least

urgent, and 1.0 being most urgent.

The UI value of a connections request needs to be

re-initialized periodically. It will appear when 1) the

requested amount Si is fully serviced and a new request Sj

comes; or 2) the requested Si has waited too long and misses

deadline for sure, and upon receiving a new request message,

the UI value is reset to 0u and starts a new period of change.

The time unit in scheduling process should be carefullychosen for optimal bandwidth allocation results. Since

scheduling is done at beginning of a time frame, let the time

variable be number of frames the connections request has

waited in system since request been denoted. For the purpose

of clear presentation, the frame duration is denoted as ft . The

UI values should be a joint function of time variable x, service

type T, delay boundD , mean traffic rateR, starting UI value

0u and limiting UI value lu , with x being from 1 to . The UI

profile can be formulated as below:

0.10.0,1),,,,,,( 0 == UIxuuRDTxfnUI l (1)

1) UI changing profile for UGS connectionsThe Standards specify UGS connections receive bandwidth

allocation periodically without need of request messages andbased on perceived traffic requirements. This type of traffic

has highest priority and should be delayed in minimum level.

Thus high values of 0u and lu are desired.

There are no request message needs for UGS connections,

and the bandwidth requirement is denoted based on registered

mean traffic rate. The UI changing curve is proposed to be a

logarithm curve for UGS connections, and changing from 0u

to lu across the packet inter-arrival time Ti. This gives

enough time for scheduler to allocate this traffic demand and

guarantees allocation to UGS connections. The choice of

Logarithm curve represents the highest priority assigned to

UGS service, as characteristic of logarithm curve is very high

rate of increasing at beginning, which means shorter time for

UI value to reach lu . Since the packet sizePis fixed for UGS

connections, Ti can be readily calculated fromR.

Expressed into formula, the UI changing profile for UGS can

be formulated as below:

>

=+

=

fil

fil

i

l

UGS

tTxforu

tTxforuxtT

uu

UI

/,

,/1,)ln()/ln( 0

0 (2)

where Ti=P/R is the packet inter-arrival time of UGS

connection i.

2) UI Changing Profile for rtPS connectionsThe bandwidth allocation between rtPS and BE connections

is the major optimization for efficient QoS provisioning.

Bandwidth should be used for rtPS connections while at rtPS

traffic burst time and used for BE services while rtPS traffic is

below mean rate. To achieve this, UI profiles for these two

services have overlap region. The UI changing curve shape isimportant to describe service type priority characteristics.

For rtPS traffic, the UI profile is proposed to be a liner

relation with time index number x and changing from 0u to

lu within half of the delay bound D of the connection flow.

Expressed in formula, it is:

=

+

=

fl

f

f

l

f

l

rtPS

tDxforu

tDxfor

t

D

uuux

t

D

uu

UI

2/,

)12/(1,

22

22

00

0

(3)

The choice of a linear relation for the curve shape is to reflectthe lower priority of rtPS service compared to UGS. Linear

curve means constant increasing rate, which implies less

aggressive bandwidth competition.

3) UI Changing Profile for BE connectionsFor BE traffic, the UI profile is proposed to be an exponential

curve. The characteristic of exponential curve is slow

increase at beginning and fast in the end, which aims to limit

the BE traffic to have low priority at the beginning, and gives

bandwidth resources for higher priority service classes.

Due to the fact BE connections are not guaranteed any QoSsupport, and the delay tolerance is generally long, the

changing time taken for UI value to change from 0u to lu is a

configurable scheduler parameter for system objectives. Let

this UI changing time of BE service be denoted as rt . This

parameter essentially determines whats the level of expected

BE connections delay time. The UI changing profile for BEconnections can be expressed into formula as:

>

=

+

=

frl

f

r

l

cct

t

ctt

l

cct

t

l

BE

ttxforu

t

txforuu

ee

eu

c

x

ee

uu

UIf

r

fr

f

r

/,

1),()exp(01

/

01

0

00

0

00

(4)

The constant 0c is chosen to be a suitable integer to make the

argument of exponential function bounded. The choice of

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REFERENCES

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