Mutually Shared Bandwidth Allocation Mechanism in IEEE 802.16e

Siddu P. Algur1, Niharika Kumar2

1Department of Computer Science, Rani Chennamma University,

Belgaum, India email: siddu_p_algur@hotmail.com

2Department of Information Science and Engineering, RNS Institute of Technology,

Bangalore, India email: niharika.kumar@gmail.com

Abstract— IEEE 802.16e network implements a centralized

resource management system where the Base Station is responsible for allocating bandwidth to all the mobile stations. A fair bandwidth allocation algorithm ensures better quality of service and efficient utilization of precious wireless resource. In this paper a novel bandwidth allocation algorithm is proposed that aims to improve the throughput of the network by efficiently sharing the bandwidth among the users. A unique bandwidth allocation mechanism is proposed that tries to reduce the overhead in a frame to improve the overall throughput of the mobile station. Simulation results shows a 2-5% improvement in throughput of the MS. A reduction in the frame overhead is also observed in the simulation.

Keywords— IEEE 802.16e, WiMAX, Quality of Service, Medium Access Control

I. INTRODUCTION IEEE 802.16e also called WiMAX[1-3] is one of the major

broadband wireless access technology that has been adopted across various countries in the world. Countries like United States of America, Japan etc have WiMAX installed by many operators spanning across their country.

IEEE 802.16e specification deals with the last mile broadband access. Particularly it focuses on the Medium access control and the physical layer aspects of the last mile communication between a WiMAX enabled Mobile Station (MS) and the Base Station (BS). Users in a WiMAX network generate different types of data requiring different levels of quality of service. The quality of service depends on various factors like the throughput requirements of the traffic, the delay requirement of the generated data and the amount of jitter that the application can withstand without causing a discomfort to the user. WiMAX supports five different types of service classes to support graded quality of service. Unsolicited Grant Service (UGS) is a service class that supports fixed data packets generated at regular intervals of time. T1/E1 traffic is an example for the data that can be qualified as UGS traffic. Variable Sized data packets generated at regular intervals of time is handled by Real Time Polling Service (RTPS). Streaming video is an example for RTPS traffic. Extended Real Time Polling Service also called eRTPS supports variable sized data packets generated in real time, for example, voice over IP with silence suppression. Data streams that generate variable

sized data that can tolerate delay are categorized as Non Real Time Polling Service traffic (nRTPS). Compared to RTPS traffic, nRTPS traffic can withstand delay. For example, FTP traffic can be treated as nRTPS traffic. FTP traffic does not have real time user interaction. Hence such traffic can be deprioritized compared to video traffic. The final service calls, also called as Best Effort (BE) shall not be associated with traffic that can withstand significant delays. Traffic generated by Web Email clients, packets generated by Web Browser can be categorized as delay tolerant BE traffic.

Each of the above service class is associated with a queue at MAC layer. There shall be at least one queue per service class at MAC layer at the mobile station and the base station. When a user generates data the packets arrive at the MAC layer and the packet classifier shall classify the packet into one of the five service class and add the packet to the queue associated with the service class.

Each MS shall independently send the bandwidth requirement of each of its service flows. This is typically equal to the amount of outstanding traffic in the queue associated with the service flow. When the bandwidth request reaches BS, it shall allocate bandwidth to the user. Since there could be multiple users requesting for bandwidth, the BS shall have to allocate the bandwidth in an efficient manner so that the quality of service is maintained at an optimal level. Various bandwidth allocation algorithms have been proposed to tackle the demand-supply of the limited wireless bandwidth. A simple method based on type of service class is proposed in [4-5]. The type of service class is used to determine the order of bandwidth allocation. i.e. UGS is allocated bandwidth then RTPS followed by nRTPS and finally BE. This order ensures that real time traffic gets higher priority over non real time. A token or counter based bandwidth allocation mechanism is proposed in [7]. Since delay plays a crucial role in the overall quality of service, a bandwidth allocation algorithm that is sensitive to the delay requirements of the user is proposed in [9]. Classical weighted round robin algorithm is explored in [8]. [6] proposes a bandwidth allocation algorithm that takes the size of the queue into consideration. If the size of the queue for the service class is more then BS shall treat the service class of an MS on priority. The dynamic nature of the wireless medium results in changes to the wireless link between MS and BS. Hence bandwidth allocation algorithm should be sensitive

2013 IEEE Symposium on Wireless Technology and Applications (ISWTA), September 22-25, 2013, Kuching, Malaysia

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to the instantaneous state of the wireless medium. Authors in [10] propose a bandwidth allocation algorithm that considers the channel condition during bandwidth allocation. This is a credit based bandwidth allocation algorithm that gives a user credit when its channel condition is poor and lets other connections utilize the bandwidth. When the channel condition of the MS improves, it is entitled to redeem the credit and transfer its data in bulk. In [11] authors proposed a bandwidth allocation algorithm that takes carrier to interference ratio into consideration while allocating the bandwidth to the Mobile Station.

In this paper a bandwidth allocation algorithm is proposed that tries to maximize the utilization of the bandwidth by grouping the users and sharing the available bandwidth among the users in such a way that the overhead in a WiMAX frame is reduced. Reduction in the overhead results in more bandwidth being available for users thus improving the throughput of the users.

The paper is divided into following sections. Section II describes the proposed bandwidth allocation algorithm. Section III provides a theoretical analysis of the proposed algorithm. Section IV describes the simulation performed and describes the simulation output. Section V concludes the paper.

II. MUTUALLY SHARED BANDWIDTH ALLOCATION ALGORITHM

Data in WiMAX network, is transmitted in the form of Frames. WiMAX TDD OFDMA Frame can be visualized as shown in Fig. 1 [12].

Fig. 1. WiMAX TDD Frame structure.

DL Burst #i carries user data on the downlink (i.e. From BS to MS) for MSi and UL Burst #j represents the bandwidth allocated to MSj for its uplink traffic. Since it is a TDD frame, uplink and downlink data transmission shall occur on the same frequency band.

When an MS generates data of a particular kind, the data is classified and placed in one of the five queues representing the five service classes. MS shall in turn request for bandwidth based on the length of the queue. On receiving the bandwidth request, the BS calculates the new packet arrival for the connection “i” of MSj as given below:

If,

qt-f Length of the queue for a connection at time t-f.

ξt-f Bandwidth allocation to the connection in frame f at time t-f

Data left in the queue at time “t-f” is given by:

φt-f = qt-f - ξt-f (1)

During the bandwidth allocation for frame f, new packets would have accumulated in the queue in the timeframe {t-f, t}. Hence the amount of unique data generated in the time {t-f, t} is calculated as:

φunique = Queue Length at time t – Data left at time t-f.

φunique = qt - φt-f (2)

This unique/new data is associated with a deadline and needs to be transmitted by the deadline “d”. If the deadline is missed then the application can face delays. In case of visual applications, it can result in jitter. Hence,

δ Maximum delay tolerable by the service class

ξt+δ Bandwidth to be allocated to a connection at time t+ δ

ξt+δ ≥ φunique (3)

Since the new packet arrival at time t has a window up to t+δ, the BS has a breathing window from {t, t+δ} to allocate bandwidth for this new data

BS shall maintain a table to manage this window of opportunity. It shall maintain the table for all the connections of the users. Table I and Table II describe the tables for RTPS and nRTPS connections. In Table I and Table II, the columns represent the bandwidth needs that need to be satisfied before the end of frame

iFR . For example, the term RTPSMSFRX

,21+

ξ in column

1+XFR indicates the bandwidth need for RTPS connection on MS2 that needs to be satisfied before deadline frame

1+XFR .

TABLE I. DEADLINE REQUIREMENTS FOR RTPS

Connection XFR 1+XFR 2+XFR …

1MSRTPS RTPSMSFRX

,1ξ RTPSMSFRX

,11+

ξ RTPSMS

FRX

,12+

ξ

…

2MSRTPS RTPSMSFRX

,2ξ RTPSMS

FRX

,21+

ξ RTPSMS

FRX

,22+

ξ

…

… … … …

MSnRTPS RTPSMSnFRX

,ξ RTPSMSn

FRX

,1+

ξ RTPSMSn

FRX

,2+

ξ

…

TABLE II. DEADLINE REQUIREMENTS FOR NRTPS

Connection XFR 1+XFR 2+XFR …

1MSnRTPS nRTPSMS

FRX

,1ξ

nRTPSMSFRX

,11+

ξ

nRTPSMSFRX

,12+

ξ

…

143

2MSnRTPS nRTPSMS

FRX

,2ξ

nRTPSMSFRX

,21+

ξ

nRTPSMSFRX

,22+

ξ

…

… … … … …

MSnnRTPS nRTPSMSn

FRX

,ξ

nRTPSMSnFRX

,1+

ξ

nRTPSMSnFRX

,2+

ξ

…

MS shall be paired in groups. Each group shall contain two MS. Let MS1 and MS2 be one such pair in a group. Their bandwidth needs for each of their connections in the time interval {t, t+ δ} is already known to the BS. In the proposed algorithm, the BS shall allocate bandwidth in an alternate fashion. In any given frame only one user of the given group shall be allocated bandwidth as described below.

RTPSMSFRX

,1ξ - Deadline requirement for RTPS connection of MS1 to be satisfied by frame X.

RTPSMSFRX

,11+

ξ - Deadline requirement for RTPS connection of MS1 to be satisfied by frame X+1.

nRTPSMSFRX

,1ξ - Deadline requirement for nRTPS connection of MS1 to be satisfied by frame X.

nRTPSMSFRX

,11+

ξ - Deadline requirement for nRTPS connection of MS1 to be satisfied by frame X+1.

In the frame X, MS1 shall be allocated bandwidth as shown in Eq. (4):

nRTPSMSFR

nRTPSMSFR

RTPSMSFR

RTPSMSFR

MSFrX

XX

XXAllot

,1,1

,1,11

1

1

+

+

+

++=

ξξ

ξξξ (4)

MS1 receives bandwidth equal to its RTPS and nRTPS deadline requirements for frame X. Additionally, MS1 is allocated bandwidth required for its RTPS and nRTPS connections for frame X+1 as well. This additional bandwidth is obtained from MS2. So, in frame X, MS1 receives bandwidth equal to its need for frame X and X+1. Hence, during the bandwidth allocation for frame X+1, MS1 will not be allocated any bandwidth as its requirements for X+1 are satisfied in the frame X.

In Frame X+1, MS2 shall be allocated bandwidth as given below:

RTPSMSFRX

,21+

ξ - Deadline requirement for RTPS connection of MS2 to be satisfied by frame X+1.

RTPSMSFRX

,22+

ξ - Deadline requirement for RTPS connection of MS2 to be satisfied by frame X+2.

nRTPSMSFRX

,21+

ξ - Deadline requirement for nRTPS connection of MS2 to be satisfied by frame X+1.

nRTPSMSFRX

,22+

ξ - Deadline requirement for nRTPS connection of MS2 to be satisfied by frame X+2

In the frame X+1, MS2 shall be allocated bandwidth as in Eq. (5):

nRTPSMSFR

nRTPSMSFR

RTPSMSFR

RTPSMSFR

MSFrX

XX

XXAllot

,2,2

,2,221

21

21

++

++

+

++=+

ξξ

ξξξ (5)

Bandwidth allocated to MS2, in frame X +1, is equal to its requirements for frame X+1 and X+2. Hence, during the bandwidth allocation for frame X+2, MS2 shall not be considered as its X+2 requirements have been satisfied in frame X+1. In frame X+2, MS1 shall be allocated bandwidth equal to its need for X+2 and X+3. This alternate bandwidth allocation mechanism shall continue till MS1 and MS2 remain a pair. Eq. (4) and (5) shall apply to all MS that have been paired together.

So, in any given frame i, only one MS from a pair {MSa, MSb} shall be allocated bandwidth. The bandwidth thus allocated to an MS (say MSa) shall be equal to its deadline need for the current frame and the next frame. The other MS (MSb) shall be allocated bandwidth in the subsequent frame which, again, would be equal to the MS’s next frame requirement and the subsequent frame requirement. By employing this mechanism of bandwidth allocation, the number of MS allotted bandwidth reduces by half. Hence the number of UL Bursts in a frame gets reduced by half. We know that each UL Burst has an entry in the ULMAP (each entry is called an ULMAPIE). This entry indicates the starting and ending position of the UL Burst. So, reducing the number of UL MAP entries reduces the overhead in a frame. This reduced overhead translates to a saving in the bandwidth. The bandwidth thus saved can be allocated to the MS. Hence the effective bandwidth allocated to the MS shall be:

)(

,,

,,

1

1

UlMapIEsizeof

AllotnRTPSMSi

FRnRTPSMSi

FR

RTPSMSiFR

RTPSMSiFR

MSiFrX

XX

XX

++

++=

+

+

ξξ

ξξξ (6)

Since the MS could be in constant motion, an MS could move to another cell under a different BS. In such a scenario, the pair gets broken and the other MS from the pair is left without a partner. Alternately a new MS could come in to the cell under the current BS. This MS needs to be paired. All such MS are moved to a pool of unpaired MS. At regular intervals of time BS, scans through the pool and pairs the MS that are left unpaired.

III. THEORETICAL ANALYSIS Assuming a total bandwidth of 20 Mbps with uplink

bandwidth being 10 Mbps, RTPS arrival rate at 100 Kbps. The ULMapIE is of size nine bytes. If the network contains MS with RTPS traffic then the maximum number of MS that the network can support is 100. Suppose all the hundred MS are grouped in pairs then the amount of overhead saved per frame shall be 450 bytes which is 3.5% of the total frame size. The bandwidth thus saved is about 720kbps. BS can admit seven more RTPS connections using the saved bandwidth or assuming a voice call with data rate of 64kbps (including

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headers), BS can accept eleven additional UGS connections from the saved bandwidth. If Poisson arrival pattern is assumed for BE traffic with an average data arrival rate of 32Kbps (including headers) then the BS can accept twenty two new BE connections.

IV. RESULTS AND DISCUSSION Simulations were carried out on MATLAB. Table III lists

the simulation parameters:

TABLE III. SIMULATION PARAMETERS

Parameter Value

Uplink Bandwidth 10 Mbps

Frame Duration 5 ms Average RTPS arrival rate 100 kbps Average nRTPS arrival rate 100 kbps

Simulation results for the proposed Shared Bandwidth

Allocation (SBA) mechanism is compared with Earliest Deadline First (EDF) algorithm. Since EDF algorithm caters to real time traffic and is widely used for scheduling real time queues, the proposed SBA algorithm was compared with EDF.

Simulation was carried out to find the average frame overhead. Frame Overhead is calculated as given in (7)

zationFrameUtilieadFrameOverh −= 1 (7) Fig. 2 shows the simulation results for EDF and the

proposed SBA.

Fig. 2. Average Frame overhead (%) for SBA and EDF v/s number of MS.

Fig. 2 reveals that as the number of MS increases, the frame over for both EDF and SBA increases. However the average frame overhead for the proposed SBA algorithm is always less than that of EDF. For example when the number of MS is 40, the frame overhead for SBA is roughly half of EDF. This is observed because in the proposed algorithm, the MS are paired and at any given time only one MS from a pair participates in the bandwidth allocation procedure. In the best case scenario, since at any given time a maximum of n/2 MS are allocated bandwidth, the number of ULMAP IE entries also reduces to n/2. Hence the reduction in frame overhead is observed in Fig. 2.

Simulation was carried out to find the uplink overhead for every bit of uplink data. Fig. 3 shows the simulation results.

Fig. 3. Uplink overhead per bit of UL data v/s number of MS.

Fig. 3 reveals that the uplink over-head observed for SBA is roughly half of what is observed for EDF.

Since the overhead is reduced, this results in saving of precious bandwidth. The bandwidth thus saved by reducing the overhead is redistributed among the MS. Fig. 4 shows the improvement in throughput when an MS uses this saved bandwidth to transmit additional nRTPS data.

Fig. 4. Average Throughput for nRTPS connections (kbps) v/s number of MS.

Fig. 4 reveals that each MS achieves throughput improvement of up to 8Kbps for its nRTPS connections using SBA. Till the number of MS is about 20 there is sufficient bandwidth to cater to all nRTPS connections. But beyond 20 MS, the incoming traffic exceeds the available bandwidth. In such a scenario, the saved bandwidth is used to transmit the additional data. Hence the improvement in throughput is observed for the proposed algorithm.

Since there is an improvement in throughput, there is a reduction in the data drop. Simulation was carried out to find the amount of data drop for nRTPS connections. The results of simulation are plotted in the graph as shown in Fig. 5.

Fig. 5 reveals that initially there is less or no data drop as there is sufficient bandwidth to cater to the needs of the MS. As the number of MS increases the traffic increases and allotted bandwidth decreases. In case of SBA, the nRTPS connections receive additional bandwidth. Hence the data drop in case of SBA is less compared to EDF.

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Fig. 5. Data Drop rate (kbps) v/s number of MS.

V. CONCLUSION In this paper a unique shared bandwidth allocation algorithm is proposed that pairs the users into groups and allocated bandwidth in an alternate fashion. This reduces the overhead data i.e. the number of ULMapIE which when allotted back to users, results in improved throughput for the users. Simulations results show the reduction in frame overhead. An improvement in user throughput and overall system throughput is also observed in the simulations. Further improvements are planned to observe the impact of the algorithm on the system when the MS get added and removed from the network dynamically. Simulations are also planned to observe the throughput on repairing the MS at different time intervals.

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