Approach for channel reservation and allocation to improve quality of ...

www.ietdl.org Published in IET Networks Received on 18th January 2013 Revised on 16th August 2013 Accepted on 21st August 2013 doi: 10.1049/iet-net.2013.0005

ISSN 2047-4954

Approach for channel reservation and allocation to improve quality of service in vehicular communications Vankadara Saritha, Vankadara Madhu Viswanatham School of Computing Science and Engineering, VIT University, Vellore, India E-mail: [email protected]; [email protected]

Abstract: Channel allocation plays an important role in next generation of wireless networks that require huge bandwidth support for various applications. It became very interesting and important research area in vehicular ad hoc network (VANET) as the channel allocation procedure needs to be efficient as the frequency of handoff is high because of the high mobility of the nodes. So, the authors propose a channel allocation algorithm that utilises channel reuse technique, channel borrowing process and the speed of the vehicle to reserve the channel for handoff calls to improve quality of service (QoS) in VANET. The calls are categorised as real-time originating calls, real-time handoff calls, non-real-time originating calls, non-real-time handoff calls and non-real-time transfer calls, and queue is maintained for non-real-time originating calls, which leads to nonreal-time transfer calls. The channel borrowing process is incorporated only for real-time originating calls and real-time handoff calls. The proposed system is modelled using two-dimensional Markov model. The proposed algorithm is simulated and evaluated in terms of QoS parameters as blocking probability, dropping probability and handoff latency. The performance is tested with varying system load, varying number of channels and is compared with legacy systems like cooperative reservation of service channels, CRaSCH, and a new dynamic channel allocation strategy which combines channel reservation with new call queuing, RQS.

1

Introduction

In recent years, the traffic on roads is increasing drastically and sometimes it becomes difficult in controlling it. The solution for this problem can be provided using vehicular ad hoc network (VANET). VANET is a network in which vehicles that move on road behave as nodes in the network. As the nodes in any network communicate with each other, the vehicles which are nodes in VANET communicate with each other by equipping a vehicle with an instrument or sensor which can receive or send messages. VANET has an important role in intelligent transportation system, which involves in making the users know about the information of the path which they wish to follow. There are two types of communication systems in VANET: Vehicle to vehicle (V2 V) communication, in which the communication takes place among vehicles and vehicle to infrastructure (V2I) communication, in which the communication is between the vehicle and the infrastructure (base station – BS) which is installed on roadside. Basically, there are three different types of channel allocation techniques, namely (i) fixed channel allocation (FAC) technique [1, 2] in which each BS is allocated a fixed number of channels, (ii) dynamic channel allocation (DCA) technique [3–5] in which all channels are kept under centralised pool and the channels are requested by BSs and are assigned to nodes dynamically and (iii) finally, the last 150 & The Institution of Engineering and Technology 2014

type of channel allocation technique is the hybrid channel allocation [6], which is a combination of fixed and DCA techniques in which some channels are kept in a central pool and some channels are allocated to BS. So, when all the channels under the control of BS are busy, BS requests the channels from central pool and returns back after the usage. Intersection violation warning, on-coming traffic warning, lane change warning are some of the applications of VANET. To make all these applications possible, an efficient channel allocation technique is required. Since the communication in VANET is given the high preference, and as the existing medium access control (MAC) protocols of MANET are not suitable for VANET, it needs an efficient channel assignment mechanism. The mobile ad hoc network (MANET) protocols are not suitable for VANET as the mobility of the nodes in VANET is very high when compared to MANET. This high mobility leads to high handoff as the period for which the nodes be in the same region is very less. The topology of the VANET also depends on the road topology whereas MANET is as per the range of the BS. IEEE802.11p is the MAC layer protocol developed for VANET which enables communication in ad hoc manner in a highly mobile network like VANET but has its own disadvantages like exposed node problem and so on [7]. Handoff occurs when the node in the network move from the range of one BS to the other. In VANETs, this process IET Netw., 2014, Vol. 3, Iss. 2, pp. 150–159 doi: 10.1049/iet-net.2013.0005

www.ietdl.org of handoff occurs frequently owing to high mobility of the nodes in the network [8, 9]. So, an efficient handoff procedure is very likely required in order to improve the performance of the system in terms of QoS parameters like blocking probability, dropping probability and handoff latency. Hence, we propose a channel assignment algorithm based on distributed DCA algorithm to improve QoS and make the handoff transparent to the user which in turn reduces the dropping probability. The capacity of the network in terms of channels is increased virtually by using reusability concept [10]. The channel can be reused in two different BSs at a time when the corresponding two BSs are at a reuse distant from each other such that co-channel interference is avoided. The reusability concept enables to make use of the same channel in different places at the same time, that is, when ‘C’ number of channels exists, then the reusability concept enables ‘U’ > ‘C’ users to utilise the channels simultaneously. The number of users using the channels simultaneously at the maximum is the number of channels increased virtually. When the handoff is made transparent, the call can be continued even when the user moves from the range of one BS to the other. This is further improved in this paper by reserving the channel for handoff call by determining the speed of the vehicle and identifying the vehicle crossing the range of the current BS and entering the target BS. Channel borrowing scheme is also incorporated in order to reduce the blocking and dropping probability. The rest of the paper is organised as follows: The existing research work done in this area is presented in Section 2. The proposed work with the mobility model, channel borrowing process, system model, the channel allocation algorithm, the performance analysis are presented in Section 3. Section 4 discusses the evaluation of the system and finally Section 5 concludes the paper.

2

Background

In this section we present the study of literature done in this area. In [11], the authors proposed a channel reservation and preemptive priority mechanism in baton handover strategy. The authors described how to identify the speed and direction of the mobile node during handover process from current BS to the target BS. They considered different traffics like voice and data and also assigned priorities based on speed as very important prioritised (VIP) high speed, high speed and ordinary in the case of handover voice traffic and least priority to data calls. The preemption method is incorporated such that the data calls are queued when there is no free channel available for the voice calls. The performance of the proposed work is measured in terms of blocking probability, average queue length and data transfer delay. However, the authors did not discuss the tradeoff between blocking and dropping probabilities. In [12], the authors concentrated on reserving the channels for handoff calls and reduce the dropping probabilities without much impact on the blocking probabilities. The channels are reserved dynamically based on the user characteristics. The authors utilised the concept of reusability to increase the channel capacity virtually. The learning automata concepts are used to improve the performance of the system and to break the tradeoff

IET Netw., 2014, Vol. 3, Iss. 2, pp. 150–159 doi: 10.1049/iet-net.2013.0005

between the blocking probability and dropping probability. The QoS parameters like blocking probability, dropping probability, handoff latency are used to measure the performance of the system and compared with legacy systems like RQS [13] and CraSCH [14]. Very fast handover scheme (VFHS) is an algorithm which is proposed by Chiu et al. [10]. It is a fast handover scheme and is built on cross-layer technique between physical layer and MAC layer. The cross-layer technique is used to share the data between these two layers and hence helps in reducing the handoff latency. But the drawback of this system is that the frequency of oncoming side vehicles (OSV) or relay vehicles must be good; otherwise, the performance of the system degrades. In other words, the VFHS algorithm attempts to reduce the handoff latency, but if there is any interruption in linking the relay vehicles, it fails in achieving its objective. In [15], the authors proposed a hybrid channel allocation technique that incorporated channel borrowing technique, and queues are maintained during handoff strategy. The advantages of FCA in low traffic conditions and DCA in high traffic conditions are utilised, and a scheme is proposed in [16] which switches between FCA and DCA based on the traffic conditions. This process of switching is done using the concept of fuzzy logic. This approach is aimed at reducing the signalling cost and failure rate which depends upon the blocking probability and dropping probability. In wireless environment, the importance of handoff is very high. So, the authors had made an extensive survey on different concepts and schemes related to handoff and presented in [17]. The concepts discussed in this paper are channel reservation as static and dynamic, queuing concept as constant queue size and dynamic queue size, transferring of a channel, concepts related to vertical handoff, and so on. The authors presented a detailed introduction to handoff types and the performance metrics to be taken care during the implementation of handoff process. More research has been carried out in the area of channel allocation in VANET [18–23] and more research in related area is carried out in [24–26]. The contributions of the proposed work include: † To develop channel assigning algorithm for typical VANET scenario. † Channel reservation for handoff calls is done based on the speed of the nodes in the network. † Incorporating channel borrowing procedure in channel assignment process. † Experimental evaluation of the proposed solution through simulation and analytical analysis. Our approach utilises reusability concept, considers different traffic like real-time originating calls, real-time handoff calls, non-real-time originating calls, non-real-time handoff calls, non-real-time transfer calls, maintains queues, incorporates channel borrowing scheme and finally, channel reservation for handoff calls based on speed. Our experimental results show that the proposed channel assignment strategy reduces the blocking probability, dropping probability and handoff latency. The performance of the proposed algorithm is compared with the legacy systems like RQS [13] and CRaSCH [14].

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www.ietdl.org 3

Proposed work

3.1

Mobility model

In the proposed work, the total number of available channels is divided into three equal groups. Actually, we can use only two groups also in VANET scenario like highway mobility model which is not possible in the case of MANET as the cells increases in all directions. But we are making use of three groups in order to make channel borrowing process simple. To illustrate why channel borrowing becomes typical if there are only two groups: Let us assume that the base stations BS1, BS3 and BS5 uses the G1 group of channels and the base stations BS2 and BS4 uses the G2 group of channels, as shown in Fig. 1a. When the base station BS3 wants to borrow a channel, it is supposed to request a channel from its neighbours, that is, BS2 or BS4. But if the channel is being used in group G2 by BS2 or BS4, then it is not possible to be borrowed because of the interference problem. If the channel is not being used, then it can be assigned to other BS. In this process, the system must keep track of the channel being utilised and should not be assigned to its neighbour during the channel borrowing process. If we consider that only particular channels will be used to assign to neighbouring cells in the borrowing process, then also the problem arises in the case of two groups. The problem is that if BS1 assigns a channel C1 to BS2, then there is a chance of C1 being under use in cell with BS3 as base station and interference may occur. To illustrate why channel borrowing becomes simpler if there are three groups: In the VANET scenario shown in Fig. 1b, it is assumed that the base stations BS1 and BS4 use G1 group of channels, BS2 and BS5 use G2 group of channels and BS3 uses G3 group of channels. Let us assume that at a particular situation, base station BS3 wants to borrow a channel, and then it can request and can acquire a channel from base station BS2 or BS4. If the channel is borrowed from BS2, the channel from G2 group will be assigned but still the interference problem

Fig. 1

occurs as similar to the case of two groups. But here, if we are limiting only some particular channels that can be used in the borrowing process, the problem of interference can be resolved. For example, even if C1 channel is assigned by BS1 to BS2, and as the other neighbour of BS2 is BS3 with group G3, the interference will not occur. The VANET scenario considered in this paper employs the BS with high range. It is not suggested to have BSs with high range inside the cities with respect to the health of the people but there will be no problem in the highway areas. The speed of the vehicles is calculated in regular intervals of time and estimates the time at which the vehicles may cross the range of the current BS. If the BS estimates that the vehicle cross the range within a very short duration like 1 or 2 minutes, then the channel will be reserved by the target BS. Here we consider the speed of the vehicle to reserve the channel for the handoff call because the signal strength might be less sometimes even within the range of the BS. So, it is preferred to consider both the signal strength and the time estimation of the vehicle to cross the range in order to estimate the handoff to occur. 3.2

System model

The calls which are initiated in a cell are referred as originating calls and the calls which transfer from the range of one BS to the other BS are referred as handoff calls. If the originating call is not able to acquire a channel, then it will be blocked. When the handoff call cannot acquire a channel, then it is dropped. In the proposed work, the calls are categorised as real-time originating calls, real-time handoff calls, non-real-time originating calls, non-real-time handoff calls and non-real-time transfer calls. The difference between the handoff calls and the transfer calls is the handoff call is an on-going call moving from the range of one BS to the other BS and the transfer call is the call in the queue and moved from the range of one BS to the other. The highest priority is given for real-time handoff calls, then real-time originating calls; next priority is

VANET scenarios

a VANET scenario using two groups b VANET scenario using three groups 152 & The Institution of Engineering and Technology 2014


www.ietdl.org

Fig. 2 System model

given to non-real-time handoff calls, then non-real-time transfer calls and least priority for non-real-time originating call. Let us assume that the total number of channels, ‘S’ is divided equally into three groups G1, G2 and G3, and there are ‘C’ channels in each group. These ‘C’ channels are divided into partitions as shown in Fig. 2. As the highest priority is given to the real-time handoff calls, these calls

can employ any of available ‘C’ channels; the real-time originating calls can employ any of available CO channels, non-real-time handoff calls can utilise any of the available CNH channels; non-real-time transfer calls can use any of the available CNT channels and the least prioritised non-real-time originating calls can employ only CNO channels. These CNO channels will be used only in the process of channel borrowing. So, the channel which is

Fig. 3 Flowchart for real-time calls at BS IET Netw., 2014, Vol. 3, Iss. 2, pp. 150–159 doi: 10.1049/iet-net.2013.0005

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www.ietdl.org assigned to another cell will be shown as busy channel and will be used in this current cell by any type of call. We are incorporating the channel borrowing process only for real-time originating and handoff calls. When there is a real-time handoff call, the BS verifies for a free channel among all the ‘C’ channels and assigns if available. Otherwise, if the BS is able to borrow a channel from its neighbouring BSs then the channel is assigned to the call otherwise it is dropped. When there is a real-time originating call, it is served with a free channel from the available CO channels. If the free channel is not available in CO channels and a channel can be borrowed from the neighbouring cells, then also the call is served, otherwise it is blocked. The non-real-time handoff call is served when there is a free channel in CNH channels, otherwise it is dropped. The non-real-time transfer call is that the call from the queue is served if

there is available free channel in CNT channels, otherwise it will still remain in queue until it acquires a channel or its time is expired. The non-real-time originating call is served if there is a free channel available in CNO channels, otherwise it is queued if the queue is not full. If the queue is full and there is no free channel in CNO channels, then the non-real-time originating call is blocked. This process is shown in Figs. 3 and 4. 3.2.1 Performance analysis: The arrival processes of all the type of calls are assumed to be according to the Poisson distribution. Some of the assumptions made as the exact scenario cannot be modelled are: † The speed of the vehicles is random and the direction depends on the mobility model. † The nodes are GPS enabled.

Fig. 4 Flowchart for non-real-time calls at BS

Fig. 5 State transition diagram for Fig. 2 154 & The Institution of Engineering and Technology 2014


www.ietdl.org † Let the arrival rate of the real-time originating calls, real-time handoff calls, non-real-time originating calls, and non-real-time handoff calls be λRO, λRH, λNO, λNH, respectively. † Let the service rate be uniform and is represented as µ. † Let the non-real-time originating queue size be Q. If P(i) is the probability of i channels to be busy, then P(i) can be determined from the state transition diagram shown in Fig. 5. The state balance equations can be obtained as imP(i) = lNO + lNH + lRO + lRH P(i − 1),

can be given as BNO =

imP(i) = lNO P(i − 1), CNT , i ≤ Q imP(i) = lNH + lRO + lRH P(i − 1), CNT , i ≤ CNH imP(i) = lRO + lRH P(i − 1), imP(i) = lRH P(i − 1),

CNH , i , CO CO , i ≤ C

(8)

The dropping probability of non-real-time handoff calls can be given as BNH =

CNH

P(i)

(9)

i=0

(1) (2)

P(i)

i=0

0 ≤ i ≤ CNT

C NT +Q

The blocking probability of real-time originating calls can be given as

(3)

BRO =

CO

P(i)

(10)

i=0

(4) (5)

The steady-state probability P(i) can be obtained as follows (see (6)) where (see (7)) The blocking probability of non-real-time originating calls

The dropping probability of real-time handoff calls can be given as (see (11))

4

Performance evaluation

The QoS parameters used to evaluate the performance of the proposed system are:

⎧ i ⎪ lNO + lNH + lRO + lRH ⎪ ⎪ ⎪ P(0), ⎪ ⎪ i!mi ⎪ ⎪ ⎪ i−CNT C ⎪ ⎪ lNO + lNH + lRO + lRH NT ⎪ ⎪ lNO ⎪ P(0), ⎪ ⎪ i!mi ⎪ ⎪ ⎪ C ⎨ i−CNT lNH + lRO + lRH lNO + lNH + lRO + lRH NT P(i) = P(0), ⎪ i!mi ⎪ ⎪ ⎪ i−CNH C −C C ⎪ ⎪ ⎪ lRO + lRH lNH + lRO + lRH NH NT lNO + lNH + lRO + lRH NT ⎪ ⎪ P(0), ⎪ ⎪ i!mi ⎪ ⎪ ⎪ i−CO C −C C −C C ⎪ ⎪ ⎪ lRH lRO + lRH O NH lNH + lRO + lRH NH NT lNO + lNH + lRO + lRH NT ⎪ ⎪ P(0), ⎩ i!mi

0 ≤ i ≤ CNT CNT , i ≤ Q CNT , i ≤ CNH CNH , i ≤ CO CO , i ≤ C (6)

⎡

⎤−1 i lNO + lNH + lRO + lRH ⎢ ⎥ i!mi ⎢ i=0 ⎥ ⎢ ⎥ i−CNT CNT ⎢ ⎥ Q lNO lNO + lNH + lRO + lRH ⎢ ⎥ ⎢+ ⎥ ⎢ i=CNT +1 ⎥ i!mi ⎢ ⎥ i−CNT CNT ⎢ ⎥ C NH ⎢ ⎥ lNH + lRO + lRH lNO + lNH + lRO + lRH ⎥ + P(0) = ⎢ i ⎢ i=C +1 ⎥ i! m ⎢ ⎥ NT ⎢ ⎥ i−CNH CNH −CNT CNT C ⎢ ⎥ O lRO + lRH lNH + lRO + lRH lNO + lNH + lRO + lRH ⎢+ ⎥ ⎢ ⎥ i!mi ⎢ i=CNH +1 ⎥ ⎢ i−CO CO −CNH CNH −CNT CNT ⎥ ⎢ C ⎥ lRO + lRH lNH + lRO + lRH lNO + lNH + lRO + lRH ⎣ lRH ⎦ i i!m i=CO +1 C NT

BRH =

lRH

C−CO C −C C −C C lRO + lRH O NH lNH + lRO + lRH NH NT lNO + lNH + lRO + lRH NT P(0) C!mC


(7)

(11)

155


www.ietdl.org Table 1 Simulation parameters

Table 3 Network parameters

Parameter number of channels number of groups number of cells/BSs time limit to estimate the handoff, α number of channels in each group, C number of channels that can be accessed by real-time originating calls, CO number of channels that can be accessed by non-real-time handoff calls, CNH number of channels that can be accessed by non-real-time transfer calls, CNT number of channels that can be accessed by non-real-time originating calls, CNO size of queue

Value 30 3 6 1 min, 45 s 10 7 5 3

Parameter

Value

channel type radio-propagation model network interface type MAC type interface queue type link layer type antenna model routing protocol topology

wireless channel two way round wirelessPhy 802_11 drop tail/priority queue LL Omni antenna AODV 602*702

2 5

Table 2 Traffic parameters Parameter

Value

agent packet size application CBR rate

UDP 1200 CBR 64 kbps

Number of calls blocked: It is the number of calls (real-time originating calls + non-real-time originating calls) that are not able to acquire a channel. Number of calls dropped: It is the number of calls (real-time handoff calls + non-real-time handoff calls) that are not able to acquire a channel.

Handoff latency: The delay in acquiring a channel by handoff calls when the call crosses the range of current BS and enters the range of the target BS. Blocking probability: The probability with which the originating call (real time + non-real time) obtains blocked without attaining a channel. Dropping probability: The probability with which the handoff call (real time + non-real time) obtains dropped without attaining a channel. 4.1

Experimental evaluation

The simulation of the proposed work is carried out using the tools such as MOVE (mobility model generator for vehicular networks) [27], which integrate the traffic generator, SUMO (simulation of urban mobile wireless networks) [28] and network simulator, NS-2 [29]. Using MOVE, we can model the mobility model of the network with traffic generated and can be viewed using SUMO. The algorithm is implemented in NS-2. The algorithm is tested on the

Fig. 6 Traffic pattern using SUMO 156 & The Institution of Engineering and Technology 2014


www.ietdl.org mobility model using MOVE. The simulation is run for 10 min and 25 runs and the average is considered to plot the graph and estimate the performance of the system. The experiment is carried out for various numbers of channels, at various time limit, which is used to reserve the channel in the target BS. Simulation parameters, traffic parameters and network parameters are shown in Tables 1–3, respectively, and the traffic pattern is shown in Fig. 6. The probability with which the handoff call or on-going call obtains disconnected is by losing the signal from the current BS and unable to acquire channel from the target BS. It can be observed from Fig. 7 that the probability of forced termination of on-going calls is reduced by the proposed system, SBC as it is reserving the channel for on-going call on estimating the time at which the handoff occurs, by calculating the frequency of mobility of the node. The dropping probability results shown in Fig. 7 are evaluated for both real-time handoff calls and non-real-time handoff calls. When the target BS is reserving the channel and if free channel is not available, then there is a chance of borrowing a channel from its neighbours and assign the channel to handoff call which is another reason for the reduction in dropping probability. The difference between the RQS and CRaSCH with SBC is the resuability and channel borrowing technique. In CRaSCH, there is no channel reservation based on speed. Overall, by considering different factors, the proposed protocol is able to reduce the dropping probability. The probability with which the newly generated call which is referred as originating call do not acquire a channel is referred as blocking probability. The proposed algorithm, SBC improves the performance in terms of blocking probability and the results are shown in Fig. 8. The results are shown with the combination of both real-time originating calls and non-real-time originating calls. The improvement is just because of the reusability concept and channel borrowing process which is missing in the other comparative systems like RQS and CRaSCH. The system is evaluated with varying system load, that is, random generation of real-time originating calls and non-real-time

Fig. 7 Comparison of RQS, CRaSCH and the proposed system, SBC in terms of dropping probability IET Netw., 2014, Vol. 3, Iss. 2, pp. 150–159 doi: 10.1049/iet-net.2013.0005

Fig. 8 Comparison of RQS, CRaSCH and the proposed system, SBC in terms of blocking probability

originating calls. The speed of the vehicles and the starting position of the vehicles are also considered randomly. The other reason for reducing the blocking probability is the maintenance of the queue for non-real-time originating calls. As the preference is given for handoff calls and the tradeoff between dropping probability and the blocking probability may lead to rise in blocking probability. So, we tried to reduce that by making use of the queue concept. The size of the queue is limited and the time limit is maintained for the calls in the queue. The delay in acquiring a channel from the target BS by an on-going call is referred as handoff latency. It can be observed from Fig. 9 that the proposed system performs well when compared to the systems like RQS and CRaSCH in terms of handoff latency. The results of handoff latency shown in

Fig. 9 Comparison of RQS, CRaSCH and the proposed system, SBC in terms of handoff latency 157


www.ietdl.org of cells increases either horizontally or vertically. So, the channel borrowing technique becomes a bit tedious, but in this paper, we made the channels that can be accessed by non-real-time originating calls only can be used in channel borrowing process. The performance of the system is implemented and evaluated in terms of QoS parameters like blocking probability, dropping probability and handoff latency. The performance of the system is compared with CRaSCH and RQS and is proved to be providing better performance.

6

Fig. 10 Comparison of analytical results and simulation results of the proposed algorithm in terms of blocking probability and dropping probability

Fig. 9 is evaluated by considering both real-time and non-real-time handoff calls. This is because of the estimation of the time to cross the current BS and the time to enter the target BS with the help of speed calculation and reserving the channel on before hand. Let us assume that the non-real-time originating call arrives at the target BS just before the real-time handoff call arrives. In this case, if channel reservation process is not there, then the channel is allocated to non-real-time originating and the real-time handoff call will be dropped. However, the channel reservation process avoids such cases and reduces the handoff latency and at the same time the dropping probability as the channel is reserved or if free channel is not available, channel is borrowed from its neighbours. The proposed algorithm is evaluated in terms of blocking probability and dropping probability using the analytical model presented in Section 3.3.1, compared with the simulation results and is shown in Fig. 10. It can be observed that both the analytical results and simulation results are almost similar.

5

Conclusions

The limited availability of channels leads to an active research in channel allocation field of wireless networks. In VANET, it plays more important role because of the high mobility of nodes in the network. So, we proposed an efficient channel allocation algorithm in VANET, SBC which reserved channels for handoff calls by estimating the time at which the handoff may occur with the calculation of the speed of the nodes in the network. The channel borrowing technique is also incorporated in the case of real-time originating calls and real-time handoff calls. The reusability concept is used and the number of groups considered is three to make channel borrowing simple. The calls are categorised as real-time originating calls, real-time handoff calls, non-real-time originating calls, non-real-time handoff calls and non-real-time transfer calls. The queue is maintained for non-real-time originating calls. The mobility model considered is the highway mobility model; so, the number 158 & The Institution of Engineering and Technology 2014

References

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