Pre-Emptive Channel Assignment Scheme with Fair Queue Scheduling in

Evolving Mobile Networks

Atta-Ul-Qudoos, Asfand-E-Yar Aircom International Pakistan Private Limited (AIPPL),

P.O Box 2860, Pakistan

Irfan Ullah Awan Mobile Computing, Networks and Security Research Group

School of Informatics University of Bradford, UK

E-mail: {Atta.Qudoos, Asfand-E.Yar}@aircominternational.com,

{ I.U.Awan}@Bradford.ac.uk

Abstract

Due to rapid evolution in Mobile Cellular Industry

(MCI) there is an increase in complexity when using mobility management techniques during the design, commissioning and operations of mobile networks. 1 G methodology can no longer be adopted or applied for 2G and/or 3G mobile networks. Therefore, existing mobility management techniques need to evolve with the technological evolution in mobile networks which is offering different multimedia based services using internet and various other class based traffic as opposed to only conventional voice telephony with guaranteed Quality of Service (QoS) to the end user. Channel assignment schemes are key to the mobility management. In the literature, different channel assignment schemes have been devised for handovers occurring during multimedia based calls.

We propose pre-emptive channel assignment scheme with fair queue scheduling based on buffer occupancy under certain QoS constraints in this paper. The proposed scheme is evaluated in a simulation environment and three different complex models/scenarios are used to validate the results. The multimedia traffic has been modelled by using MMPP (Markov Modulated Poisson Process) whereas we use conventional Poisson Process to model the voice calls. The performance study shows that the proposed pre-emptive channel assignment scheme results in reduced dropping probability at higher handoff rates as compared to other channel reservation schemes and demonstrates how the performance trade off is obtained in terms of call dropping, call blocking and mean queue lenghts.

Keywords: Mobility Management; Quality of Service (QoS); Channel allocation; Handoff; Drop Call Rate (DCR); Key Performance Indicators (KPIs). 1. Introduction

The use of micro, macro and pico cells [1 - 6] (Cell -

a small geographical zone presented by hexagons to mark as cellular coverage area for user) in mobile cellular networks increases the frequency re-use rate which causes the mobile users to handoff or handover to a stronger dbm signal while moving closer to the cell boundaries or between the cells within a cluster. Figure 1.1 [1 - 3] shows a pictorial representation of a typical cellular coverage with a seven- cell reuse plan. Cellular coverage primarily depends on the user-defined parameters such as transmitting power, antenna height, antenna gain, antenna location, and antenna diversity. Several other parameters such as propagation environment, hills, tunnels, foliage and buildings greatly affect the overall RF coverage. Each base station is allocated a group of radio channels to be used within a cell. Base stations in the adjacent cells are assigned channel groups, which contain completely different channels than neighboring cells. The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning. 1.1 Channel Assignment Strategies

A variety of channel assignment strategies [1 - 3] have been developed to achieve the objectives of increasing capacity and minimizing interference. The

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choice of channel assignment strategy impacts the performance of the system, particularly as to how calls are managed when a mobile user is handed off from one cell to another. Channel assignment strategies can be classified as either fixed channel assignment strategy (where each cell is allocated a predetermined set of voice channels) or dynamic channel assignment strategy, (where voice channels are not allocated to different cells permanently. Instead, each time a call request is made, the serving base station requests a channel from the MSC).

Fig. 1.1: Cellular Coverage with a seven-cell reuse plan [1-3]

1.2 Handoff Strategies

Different systems have different policies and

methods for managing handoff requests. Some systems handle handoff requests in the same way they handle originating calls. In such systems, the probability that a handoff request will not be served by a new base station is equal to the blocking probability of incoming calls. However from user’s point of view, having a call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attempt.

When a mobile moves in to a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This handoff operation not only involves identifying a new base station, but also requires that the voice and control signal to be allocated to channels associated with the new base station.

Micro or pico-cellular networks have been deployed to increase the spectrum utilization [4 - 6]. These cells have smaller cell size and coverage area. Due to smaller cell size, there is a possibility of rapid handoffs [4, 5, and 7].

Thus, a 2G/3G mobile user experiences frequent handovers at times while being on a call and moving, resulting in Quality of Service (QoS) degradation (voice chopping, echoes etc) due to repeated unsuccessful handovers at times that may even result in a call drop. So, it implies that Drop Call Rate (DCR), which is one of the Key Performance Indicators (KPIs) gets affected. Thus, mobility management is becoming one of the key concern and interest for mobile network deployment engineers, consultants and vendors these days. Integration of voice calls and packet based data services/technologies (like GPRS/EDGE technologies), which use internet traffic in one form or the other, have made mobility management even more complex. Multimedia Calls (voice call, video streaming, real-time video calls and other data calls over internet) are a common feature of network traffic generated in 3G cellular systems.

Moreover, the demand for multimedia services over the wireless communication has also increased. Multimedia traffic normally consists of two types of traffic; real-time (voice and video) and non real-time (data and graphics) [8, 9]. Real-time calls are delay sensitive, whereas non real-time calls are delay tolerant [8, 9]. Multimedia traffic has different QoS (Quality of Service) requirements [9]. Systems with multi-service traffic require more efficient resource management scheme to give guaranteed QoS. 1.3 Prioritization Schemes

The most common and simple prioritisation schemes used in literature include:

• Guard Channels • Queuing of Handoff Calls

In schemes, grouped under broader term of guard

channels, a set of channels are reserved for handover calls and handover calls are given priority over other new calls originating in the cell [4, 5, and 11]. In queuing of handoff calls, the handoff calls are queued when there is no free channel available [4, 5, 11, and 12]. When any channel becomes free, the queued handover call is given priority over other calls [4, 5, 11, and 12]. Queuing is possible due to the overlapping area between cells where a user can communicate with both serving and target base stations, but it must be noted that maximum queuing time should not be greater than mobile station’s dwell time in the overlapping region [12]. Guarded channels can also be

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used with buffers. When there is no free channel from reserved and shared pool of channels, handover calls can be queued [13].

The key objective of this study is to evaluate the behaviour of various channel assignment schemes for multimedia handover traffic under different load conditions, and to devise a better scheme that can perform well in terms of reduced handover call dropping probability. We have proposed pre-emptive channel assignment scheme with fair queue scheduling based on buffer occupancy. We have used four traffic classes; handover voice call, new voice call originating within the cell, handover delay sensitive data calls (video streaming etc.) and handover delay tolerant data calls (other data connections like e-mail).

Rest of the paper has been organized as: Section 2

presents a concise review of some resource management schemes for multimedia handover calls devised in literature. Section 3 contains brief description of proposed models, and Section 4 gives comparative performance evaluation on the basis of simulations results. Section 5 presents conclusion based on comparative evaluations.

2. Resource Management for Multimedia

Handover Calls In [7], a scheme is proposed to pre-assign reserved

channel to handoff calls. When all reserved channels are busy, the handoff call is either immediately dropped, called strict dropping policy, or handover call can compete with new call for common pool of available channels, called normal dropping policy. In case of multi-service handovers, each handover call will have its own reserved pool of channels depending upon its class type and QoS requirements. Handover dropping probability with normal dropping policy is less than that of with strict dropping policy, but at the expense of new call blocking probability.

Sheu and Yang [8], proposed a pre-emptive channel

allocation model for multimedia traffic in cellular networks using time division multiple access (TDMA). Authors have divided network traffic into three traffic classes. Class 1 represents the real time traffic with dedicated time slots e.g., voice; Class 2 represents the non-real time traffic with shared time slots e.g., email, background FTP and Class 3 represents the real time traffic with shared time slots e.g., streaming multimedia. Class 2 and class 3 operate in packet switch mode whereas class 1 operates in circuit switch mode. Threshold of reserved time slots (guard channels) for handoff calls is determined. If the total call requests do not exceed this already determined

threshold, both handoff and new calls are accepted. Otherwise, only handoff calls are accepted and new calls are blocked. Class 1 traffic can pre-empt class 2 traffic. This way the blocking probability of real time handoff traffic can be reduced to a significant level as compared to the non pre-emptive schemes.

In [14], the authors have given the idea of integrated

partial and full pre-emptive channel allocation for multimedia traffic. It is more focused on multimedia traffic handling instead of handover voice calls. Partial and full pre-emptive notion can be successfully modelled for handover voice calls.

What lacks in the existing work is the appropriate

traffic modelling with real traffic characteristic and an optimised use of scarce resources at heavy traffic load in order to guarantee various QoS requirements for different services. This study attempts to address these issues.

3. Proposed Models 3.1 Priority Based Channel Reservations and Sharing Scheme with Weighted Fair Queue Scheduling and Pre-emptive Channel Assignment

The model allows handover voice calls to pre-empt other handover delay tolerant data calls (low priority data connection calls). The pre-empted calls are queued at the top of the queue and will be assigned channels again.

Fig. 3.1 Pre-emptive Channel Assignment Strategy

IF All Shared Channels are Busy Then Pre-empt in-service Delay Tolerant Data Call

IF All Shared Channels are Busy Then Multimedia Call is queued.

IF Buffer is Full Then Call is Lost.

Handover Voice Call

Handover Delay Tolerant Data Call

Handover Delay Sensitive Data Call

Reserved for Handover Voice Calls

Shared Channels

In-service Delay Tolerant Data Call is pre-empted by Handover Voice Call and queued at top of buffer.

IF All Reserved are Busy Then Go for Shared One

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For the sake of simplicity, the size of IP packet after pre-emption has been ignored.

It is assumed that it is a normal packet being queued at the top and served when channel becomes free. Along with pre-emptive assignment, fair scheduling is also used to reduce the degradation of delay tolerant data calls caused by pre-emption.

Weighted Fair Scheduling scheme used is simple too and like other Priority Queue Scheduling, it gives priority to queued delay sensitive data calls (handover video streaming calls) but with a weight of 3:1.

It means that after serving 3 packets from the buffer of delay sensitive data call, the model will serve 1 packet from the buffer of delay tolerant data call, even if there is still a call in the buffer of delay sensitive data call. This has been done to make system pretty much fair with delay tolerant data calls.

Following pseuodocode illustrate core idea of how weight counter works to give weight to delay sensitive data call.

IF Num_in_Q_Delay_Sensitive > 0 THEN IF (Weight_Counter <= 3) THEN

Serve Queued_Delay_Sensitive_Data_Call

Increment Weight_Counter ELSE

IF Num_in_Q_Delay_Tolerant > 0 THEN

Serve Queued_Delay_Tolerant_Data_Call

Weight_Counter = 1

3.2 Priority Based Channel Reservations and Sharing Scheme with Queue Scheduling Based on Buffer Occupancy and Pre-emptive Channel Assignment

In previous scheme, the behavior of model with pre-

emption and weighted fair queue scheduling was studied. Due to pre-emption, model was successful in reducing the dropping probability of handover voice calls but at the expense of queuing delays of other multimedia handover calls especially delay tolerant data calls. An alternative scheduling scheme has been devised to reduce the queuing delays of other multimedia handover calls especially delay sensitive data calls at higher arrival rates. Same pre-emption scheme as before is also used here.

Queue Scheduling Based on Buffer Occupancy checks the buffers of both multimedia handover traffic classes, and depending upon that, it will schedule next service from queue. Previous scheme can perform

better when buffers are not heavily occupied, but when buffers are getting filled rapidly, this scheme cannot perform better, causing more queuing delay of delay sensitive and delay tolerant data calls.

So, in order to reduce the queuing delay of delay sensitive data calls, queue scheduling based on buffer occupancy is used. This scheme checks the queue size of delay tolerant data calls, and if current queue size of delay tolerant data calls is more than one third of its buffer capacity and at that time, buffer of delay sensitive data call is not full, system will schedule delay tolerant data call to be served next. Otherwise, delay sensitive data calls will be scheduled for service (if queue is not empty). This way both multimedia traffic classes will have fair chance to get channel.

Following algorithm explains how buffer occupancy

works in this model. IF Num_in_Q_Delay_Sensitive > 0 THEN IF Num_in_Q_Delay_Tolerant >= (Buffer_Size_Delay_Tolerant / 3) AND Num_in_Q_Delay_Sensitive < Buffer_Size_Delay_Sensitive

THEN Serve Delay_Tolerant_Data_Call

ELSE Serve Delay_Sensitive_Data_Call ELSE IF Num_in_Q_Delay_Tolerant > 0 THEN

Serve Delay_Tolerant_Data_Call

Fig. 3.2 Channel Assignment Sequence when Channel becomes Free

If Reserved Channel completes service, it is set Free for other new voice calls

If Reserved Channel completes service, it is set Free for other handover voice calls

If Shared Channel Completes service, first priority is handover voice call

If no handover voice calls to be served, it will check the buffer of delay sensitive data calls. If not empty, channel will be assigned to it but according to Buffer Occupancy.

If buffer of delay sensitive data call is empty, system will serve any buffered delay tolerant data call.

If no buffered delay tolerant data call, this channel is set free for other calls.

Shared Channels

Reserved Channels for Handover Voice Calls

Reserved Channels for New Voice Calls

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4. Performance Evaluation This section presents performance evaluation of the proposed schemes for channel assignments to multiple class traffic with different QoS requirements. In order to model network traffic, multimedia and data are generated by a two-state MMPP process [19], whilst voice calls have been generated by a traditional Poisson process with the average arrival rate λ . Two different states of the MMPP-2 correspond to two independent Poisson arrival processes with rate 1λ and 2λ , respectively. 1δ and 2δ are the intensities of transition between two states. MMPP-2 is generally parameterized by the infinitesimal generator

−

−=

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matrix

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00 λ

λΛ . The average arrival rate of

the MMPP can be calculated by )/()( 211221 δδδλδλλ ++= . The service time of both traffic classes is assumed to be exponentially distributed with mean µ/1 . Channels Reserved for Handover Voice Calls = 3 Channels Reserved for New Voice Calls = 3 Shared Channels = 10 New Call Arrival Rate = 0.6 Delay Sensitive (Data Call) Handover Rate = 0.2 for bursty and 0.01 for non-bursty Delay Tolerant (Data Call) Handover Rate = 0.1 for bursty and 0.01 for non-bursty Service Rate = 0.1 Buffer size of delay sensitive data calls = 10. Buffer size of delay tolerant data calls = 20.

Handover Voice Call Dropping Probability

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Handover Voice Call Arrival Rate

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Dropping Probability of Handover Voice Calls in Pre-emptive ChannelAllocation Scheme with fair Queue Scheduling

Dropping Probability of handover Voice Calls in basic Guard ChannelScheme with Priorty Queue

Fig. 4.1 Comparison of Handover Voice Call Dropping Probability in Pre-emptive

Channel Allocation with Fair Queue Scheduling and Basic Guard Channel Scheme with Priority Queue.

These results have been taken at medium to very

heavy traffic load conditions especially of handover calls. It is obvious from graph that handover voice calls dropping probability can be significantly reduced with

pre-emption of delay tolerant data calls (non-real time calls).

Mean Queue Length of Delay Sensitive Data Calls

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Handover Voice Calls Arrival Rate

MQL

M QL of Delay Sensit ive Data Calls in Pre-emtive Channel Allocat ion Scheme withFair Queue SchedulingM QL of Delay Sensit ive Data Calls in Basic Guard Channel Scheme with PriorityQueue

Fig. 4.2 Comparison of MQL of Handover Delay Sensitive Data Calls in Pre-emptive Channel Allocation with Fair Queue Scheduling and Basic Guard Channel Scheme with Priority Queue

Mean Queue Length of Delay Tolerant Data Calls

00.10.20.30.40.50.60.70.8

0 0.2 0.4 0.6 0.8 1 1.2Handover Voice Calls Arrival Rate

MQ

L

MQL of Delay Tolerant Data Calls in Pre-emtive Channel A llocation Scheme with FairQueue SchedulingMQL of Delay Tolerant Data Calls in Basic Guard Channel Schemes with Priorit yQueue

Fig. 4.3 Comparison of MQL of Handover Delay Tolerant Data Calls in Pre-emptive Channel Allocation with Fair Queue Scheduling and Basic Guard Channel Scheme with Priority Queue.

Dropping Probability of Delay Tolerant Data Calls in Pre-emptive Channel Allocation Scheme w ith Fair Queue

Scheduling

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0.0000020.0000040.0000060.0000080.00001

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0 0.2 0.4 0.6 0.8 1 1.2Handover Rateof Voice Calls

Dro

ppin

g

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abili

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Dropping Probability of Delay Tolerant Data Calls in Pre-emptiveChannel A llocation Scheme wit h Fair Queue Scheduling

Fig. 4.4 Dropping Probability of Handover Delay Tolerant Data Call in Pre-emptive Channel Allocation with Fair Queue Scheduling.

Above results show that at very high handover rates

there is a slight increase in mean queue lengths of data calls but dropping probability is very low due to fair queue scheduling.

Another scheme (see section 3.2) has been proposed to further reduce the mean queue lengths of handover data calls.

MQL of Delay Sensitive Data Calls

0

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MQL

M QL of Delay Sensit ive Data Calls in Pre-empt ive Channel A llocation with FairQueue SchedulingM QL of Delay Sensit ive Data Calls in Pre-empt ive Channel A llocation Scheme withQueue Scheduling Based on Buffer Occupancy

Fig. 4.5 Comparison of MQL of Handover Delay Sensitive Data Calls in Pre-emptive Channel Allocation with Fair Queue Scheduling and Pre-emptive Channel Allocation with queue scheduling based on Buffer Occupancy.

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MQL of Delay Tolerant Data calls

00.10.20.30.40.50.60.70.8

0 0.2 0.4 0.6 0.8 1 1.2

Handover Rate of Voice Calls

MQ

L

M QL of Delay Tolerant Data Calls in Pre-empt ive Channel Allocat ion Scheme with FairQueue SchedulingM QL of Delay Tolerant Data Calls in Pre-empt ive Channel Allocat ion Scheme with QueueScheduling Based on Buffer Occupancy

Fig. 4.6 Comparison of MQL of Handover Delay Tolerant Data Calls in Pre-emptive Channel Allocation with Fair Queue Scheduling and Pre-emptive Channel Allocation with queue scheduling based on Buffer Occupancy.

It is apparent from above graphs that scheduling scheme on the basis of buffer occupancy can further reduce mean queue lengths of handover data calls.

5. Conclusion

This paper presents two different models for the effective channel assignments to provide seamless connectivity to the roaming users generating heterogeneous traffic with certain QoS constraints. Pre-emptive channel allocation with weighted fair Queue scheduling model has been devised where handover voice calls are allowed to pre-empt in-service delay tolerant data calls. This way, dropping probability of handover voice calls is significantly reduced as compared to other guard channel schemes but at the expense of delay in delay tolerant data calls (non-real time calls). A further improvement on this Pre-emptive channel allocation scheme is a new scheduling scheme based on buffer occupancy. This scheduling scheme takes buffer sizes into account before selecting and scheduling queues. Simulation results show that this scheduling scheme performs better for delay sensitive data calls, and also for delay tolerant data calls at higher rates. In summary, Pre-emptive channel allocation scheme based on buffer occupancy, has improved performance for handover calls.

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