Traffic-Aware Connection Admission Control Scheme ...

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.

Traffic-Aware Connection Admission Control Scheme for Broadband Mobile Systems A. Antonopoulos∗ , C. Skianis† , Senior Member, IEEE, C. Verikoukis∗ , Senior Member, IEEE ∗ Telecommunications Technological Centre of Catalonia (CTTC) Castelldefels, Spain Email: {aantonopoulos,cveri}@cttc.es † Department of Information and Communication Systems Engineering University of the Aegean, Karlovassi, Greece Email: [email protected]

Abstract—In this paper we introduce a connection admission control (CAC) mechanism for IEEE 802.16 broadband wireless access standard. Our scheme is based on the bandwidth reservation concept and has been developed considering the problem of “busy hour” in communications traffic variation during a typical day. The proposed solution, which is compatible to the IEEE 802.16 Standard, provides higher priority to VoIP calls compared to other types of traffic in the network. The performance of the proposed mechanism is evaluated by means of computer simulations and compared to simple traditional admission control schemes, while we provide an analytical solution as well. Index Terms—Admission Control; WiMAX; IEEE 802.16; Medium Access Control (MAC); Bandwidth Reservation.

I. I NTRODUCTION

I

EEE 802.16 Standard [1] was introduced in order to provide a Broadband Wireless Access (BWA), thus making it an attractive alternative solution to well-known wired technologies like xDSL and cable modem access. In the IEEE 802.16 architecture there are two types of stations: base station (BS) and subscriber station (SS). Each SS can serve a number of users who have access to the network. Both BS and SS are fixed, but inside the SS, users can be either static or mobile. Communication can take place in two different modes: Pointto-Multipoint (PMP) mode and Mesh mode. In PMP mode, the BS is the central entity that decides the transmission and reception schedules of the SSs, while in Mesh mode traffic can be routed directly between SSs without the need of a BS. Our study is based on PMP architecture, where the communication path between a BS and a SS has two directions: uplink (from SS to BS) and downlink (from BS to SS), multiplexed either in time or frequency domain (TDD and FDD respectively). Admission control is the mechanism that decides whether one new connection should be accepted or rejected, depending on network available resources. More sophisticated admission control algorithms respect the QoS requirements of the connections in terms of delay, jitter and other network characteristics in order to provide a more reliable service to the end users [2],[3]. The novelty of our proposed admission control mechanism lies mostly in two facts. First, our scheme does not adopt any fixed bandwidth allocation, as the bandwidth reservation takes place dynamically in respect to the arrival rate of VoIP

calls. Second, the bandwidth for the UGS class is reserved only under “busy hour” conditions, when the arrival rate of the connection requests exceeds one specific threshold. To the best of our knowledge, there is no proposed solution in the literature that tries to face the phenomenon of “busy hour”, while scheduling [4] and priority to the services after being admitted by the system [2],[3] are the two major concerns of the researchers. The rest of this paper is organized as follows. Section II provides an overview of IEEE 802.16 Medium Access Control layer. Section III outlines the related work on admission control schemes for WiMAX networks in the literature. In section IV we introduce our proposed connection admission control method and the analytical traffic model. The validation of the model and the numerical results are provided in section V. Finally, section VI concludes the paper. II. OVERVIEW OF IEEE 802.16 B ROADBAND W IRELESS ACCESS (W I MAX) MAC LAYER A. Introduction In this section we briefly review the mechanisms that IEEE 802.16 uses in order to provide the end user with quality of service. Even though the physical layer specifications and the signalling mechanism for information exchange between BS and SS are well defined in the standard, the admission control and the resource allocation strategies have been left open issues for the manufacturers, as it is shown in Figure 1. B. Medium Access Control (MAC) Layer IEEE 802.16 Medium Access Control layer, defined as connection-oriented, is designed to support different QoS requirements for different services. Based on that, the standard describes the following four types of services: • Unsolicited Grant Service (UGS) is designed to support real-time service flows that generate fixed-size data packets (CBR traffic) on a periodic basis, such as Voice over IP. • Real-Time Polling Service (rtPS) is designed to support real-time service flows that generate variable sized data

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Fig. 1.

IEEE 802.16 QoS Architecture

packets (VBR traffic) on a periodic basis, such as Moving Pictures Expert Group (MPEG) video. • Non-Real-Time Polling Service (nrtPS) is designed to support non-real-time service flows that require variable size data grants, like File Transfer Protocol (FTP). • Best Effort service (BE) is designed to provide efficient service for best effort traffic like Hyper-Text Transfer Protocol (HTTP) The traffic scheduler located at the BS decides on the allocation of physical slots in each time frame by considering the following parameters for each of the active connections: • the scheduling service type that the connection belongs to; • the QoS parameter values of the connection; • the queue size of the data for transmission; • the capacity of the available bandwidth. C. Bandwidth Allocation and Request Mechanisms Requests refer to the mechanisms that SSs use in order to notify the BS that they need bandwidth allocation for their upstream transmissions. There are two methods for transmitting bandwidth requests to the BS: • Grant per Connection (GPC): Bandwidth is granted explicitly to each connection. • Grant per Subscriber Station (GPSS): All connections from the same SS are treated as a single unit and bandwidth is allocated accordingly by the BS. The latter case (GPSS) allows SS to distribute bandwidth among its connections, considering their QoS agreements, thus making it suitable for many connections per terminal, while the former case (GPC) is mostly appropriate for few users per SS. III. R ELATED W ORK Although bandwidth reservation strategies for wireless networks have been intensively studied in literature, there are only few related to IEEE 802.16 Standard.

In [5] the authors suggest a bandwidth reservation scheme for hybrid IEEE 802.16 wireless networks. The proposed schemes are classified into two (2) types: with or without hysteresis. However, there is no comparison with other admission control mechanisms. A Dynamic Bandwidth Reservation Admission Control (DBRAC) scheme is presented in [6]. The proposed scheme gives priority to real-time VBR services and to the handoff traffic as well. In [7] a dynamic, prediction-based, multi-class adaptive bandwidth reservation scheme for IEEE 802.16 networks is presented. The authors propose a handover prediction algorithm as well, as the scheme emphasizes in handover traffic. The authors in [8] introduce a mechanism for dynamic resource management in WiMAX networks. However, the proposed scheme operates at a lower offered load and there is no differentiation among different classes of traffic. Our proposed scheme overcomes the above problems, as it is adapted to CBR traffic and specifically in VoIP connections, while it is designed to work under heavy traffic as well. IV. A DMISSION C ONTROL A. Bandwidth Reservation Scheme As it derives from IEEE 802.16 service types, UGS flows that support voice services are the most common way for daily communication, while the non-UGS flows are used to support web applications. Furthermore, recent studies have shown that the proportion of VoIP users will continue to grow from 28% of users in 2008 (up from 20% of users in 2007) to more than 50% in 2010 [9]. Due to that fact, our proposed CAC focuses in UGS flows, giving them higher priority comparing to the other three types of flows of the standard. The problem becomes more intense if we take under consideration the variation of daily traffic volume, where there is a peak during the ”busy hour” [10]. In Figure 2 the mean number of calls per minute to a switching centre taken as an average for periods of 15 minutes during 10 working days (Monday - Friday) is depicted [11].

Fig. 2.

Typical 24-hour traffic variation

Our proposed admission control mechanism is based on the bandwidth reservation concept and is executed under “busy hour” conditions. Under these conditions (i.e. for high arrival rate of UGS connections), once a connection request arrives at

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the system, it is examined the class that it belongs to. In case of UGS connections, the request is accepted if the total available bandwidth (BWT ) suffices to serve the incoming connection. On the other hand, restricted bandwidth (BWT − BWR ) is provided to the service types of rtPS / nrtPS, as our aim is to prioritize VoIP calls over other types of connections. In order to deal with BE connections, the requests are always admitted. However, no bandwidth allocation is considered, since BE flows do not need any QoS guarantees. The portion of the reserved bandwidth for UGS connections is dynamically changed according to the traffic intensity of the VoIP calls: BWR = ρ1 × β × BW1

overall and the reserved bandwidth, while BW1 and BW2 represent the bandwidth that is needed in order to serve each UGS and rtPS/nrtPS connection, respectively. We also define other parameters as follows: Arrival rate of UGS connections λ1 Arrival rate of rtPS/nrtPS connections λ2 1/μ1 Service time for UGS connections 1/μ2 Service time for rtPS/nrtPS connections The state transmission diagram of the Markov model is shown in Figure 3.

(1)

In the above expression, ρ1 , defined as traffic intensity, is a measure of the average occupancy of the base station during a specified period of time. It is denoted as ρ1 =λ1 /μ1 , where λ1 is the arrival rate for UGS connections and μ1 represents the mean service rate. Furthermore, BW1 is the bandwidth needed for each UGS connection, while β ∈ [0, 1] denotes the bandwidth reservation factor. Formula (1) implies that traffic intensity impacts the blocking probabilities both of UGS and non-UGS connections. It makes sense that applying this bandwidth reservation scheme, the available bandwidth for the connections of the rtPS and nrtPS service types is decreased and consequently the blocking probabilities for the specific service types are increased. On the contrary, the blocking probability for the UGS connections is decreased, as a portion of bandwidth is exclusively dedicated to this service type. In bandwidth reservation schemes, one main difficulty is to avoid the inefficient utilization of system resources. However, in our case, the daily traffic variation [11] establishes our ability to predict an increase in VoIP calls, thus enabling us to tackle this problem. Thus, our scheme outperforms classic bandwidth reservation mechanisms. B. Analytical Model In this section we develop an analytical model for our proposed bandwidth reservation scheme. Using this model we derive the blocking probabilities for the different class types and the results are further verified by extensive simulations in the following section. In order to simplify the analysis, we treat the connections that belong in rtPS and nrtPS class as one class type with the same characteristics (i.e. arrival rate, bandwidth demand). In this point we must clarify that this simplification concerns only the admission control process since, after being accepted, the two classes are treated according to their different priorities. Furthermore, BE connections are not included in the model as they are always accepted without QoS guarantees. Thus, the 2-dimensional continuous Markov model (Figure 3) can be used to analyze the performance of the proposed scheme. The state space of this Markov model is S = {(r, s)|0 ≤ r ≤ m, 0 ≤ s ≤ n, r · BW1 + s · BW2 ≤ BWT }, BWT −BWR T where m = BW . The number BW1 and n = BW2 of UGS and rtPS/nrtPS connections is represented by r and s, respectively. Additionally, BWT and BWR represent the

Fig. 3.

The two-dimensional Markov model’s state transition diagram

Its steady state equation is the following: pr,s · (λ1 · φr+1,s + λ2 · φr,s+1 · θr,s+1 + r · μ1 · φr−1,s + s · μ2 · φr,s−1 ) = λ1 · pr−1,s · φr−1,s + λ2 · pr,s−1 · φr,s−1 · θr,s + + (r + 1) · μ1 · pr+1,s · φr+1,s + (s + 1) · μ2 · pr,s+1 · φr,s+1

(2)

where pr,s denotes the steady state probability of the system lying in the state (r, s) and φr,s , θr,s denote characteristic functions: 1, (r, s) ∈ S φr,s = (3) 0, otherwise 1, r · BW1 + s · BW2 ≤ BWT − BWR (4) θr,s = 0, otherwise The above characteristic functions are used in order to prevent a transition into an invalid state, according to the restrictions that were previously defined. Furthermore, considering the normalizing condition pr,s = 1, the steady (r,s)∈S

state probability for each state can be obtained. The blocking probabilities for UGS and non-UGS connections are given by BPU GS = p(r, s) (5) (r+1)·BW1 +s·BW2 >BWT

BPnon−U GS =

p(r, s)

(6)

r·BW1 +(s+1)·BW2 >BWT −BWR

C. Operational Example In order to clarify the mathematical analysis above, we provide two possible states of the system’s Markov Chain. Figure 4 depicts the exact form of the chain in each of the two cases.

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First case: We assume that the system lies in the state that is subject to the following constraints: {(r, s), (r + 1, s), (r, s + 1), (r − 1, s), (r, s − 1)} ∈ S (7) r · BW1 + (s + 1) · BW2 > BWT − BWR

(8)

r · BW1 + s · BW2 < BWT − BWR

(9)

Under these assumptions and using the definitions of φr,s and θr,s , we derive the steady equation for the specific state: pr,s · (λ1 + r · μ1 + s · μ2 ) = λ1 · pr−1,s + λ2 · pr,s−1 + + (r + 1) · μ1 · pr+1,s + (s + 1) · μ2 · pr,s+1 (10) Second case: In this case we assume that the system lies in the state (r, s) which is subject to the following constraints: {(r, s), (r + 1, s), (r, s + 1), (r − 1, s)} ∈ S

(11)

(r, s − 1) ∈ /S

(12)

r · BW1 + (s + 1) · BW2 > BWT − BWR

(13)

r · BW1 + s · BW2 = BWT − BWR

(14)

Considering again the definitions of φr,s and θr,s , we derive the respective steady state equation for this case, that is: pr,s · (λ1 + r · μ1 ) = λ1 · pr−1,s + (r + 1) · μ1 · pr+1,s + + (s + 1) · μ2 · pr,s+1 (15)

V. P ERFORMANCE R ESULTS In order to verify the analytical results about the performance of our CAC algorithm we have developed an event driven C++ simulator that executes the rules of the algorithm. Simulations have been carried out, assuming a WiMAX OFDMA radio interface between BS and SSs, using the TDD duplexing technique, a channel bandwidth of 10MHz and 1024 subcarriers. The frame duration is 5 msec and the OFDMA symbol duration of 102.86 μsec, while the modulation scheme for DL and UL direction is 16-QAM with code rate 3/4. In this section we present the simulation set up and results of our experiments. A. Simulation Scenario Based on the physical layer that was described, we assume that the overall bandwidth for the uplink traffic is 4032 kb/s. Considering that the non-UGS traffic consists mainly of audio and video data we assume that the average bandwidth that each rtPS / nrtPS connection uses is 128 kb/s [12]. The codec chosen to generate VoIP traffic is the G.711, resulting to a constant bit rate of 64 kb/s. Each result was produced by running the simulation 100 times using different seeds, while we simulate 3600 sec of real time in order to be in accordance with the definition of “busy hour”. Furthermore, in order to evaluate our proposed algorithm, we have conducted a research on the state-of-the-art admission control mechanisms for the IEEE 802.16 Standard. Several schemes in the literature [4],[13] accept one new connection when the following condition is satisfied: Creserved + T Riservice ≤ Ctotal

where Creserved represents the capacity reserved to the connections already admitted in the system, T Riservice denotes the traffic rate that should be guaranteed to the new connection i of service type service and Ctotal is the total capacity available. We refer to these methods as capacity-based (CB) algorithms in order to distinguish from our proposed algorithm which is based on bandwidth reservation (BR) concept. In order to evaluate our mechanism we have carried out simulation tests by varying the UGS arrival rate, thus providing a large range of VoIP traffic that fluctuates between 15 and 240 connections/min. The system parameters that are presented in TABLE I, define that the arrival rate of all connections follows a Poisson distribution, while the mean service time for the connections is exponentially distributed. TABLE I S YSTEM PARAMETERS

Fig. 4.

Two examples of possible states of the system

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Bandwidth λ2 1/μ1 1/μ2 B1 B2 Threshold β

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4032kb/s Poisson(1 connection/sec) Exponential (50 sec) Exponential (50 sec) 64kb/s (G.711) 128kb/s 0.2 calls/sec 1/3


Under these assumptions and considering λ1 =1 connection/sec, the system can serve about 98% of the UGS calls if all the requests of the other classes are rejected, which means that we have an over-loaded network. Furthermore, in the specific case we use a single admission control based on bandwidth availability where all the requests are accepted if there is enough bandwidth to serve them, no matter the class that they belong to, the system serves about 57% of the UGS flows and 34% of the other flows. Our aim is to serve more VoIP calls by reducing the Grade of Service (GoS) of the UGS connections. GoS corresponds to the blocking probabilities of the analysis mentioned in the previous section and may be defined as the probability of a call being blocked or delayed more than a specified interval: GoS = BP =

N umber of lost calls N umber of of f ered calls

ACKNOWLEDGMENTS

First, it can be observed that the simulation results verify the mathematical analysis, with the difference varying in a range of less than 5% (Figure 5). Comparing our proposed admission control to traditional schemes for different values of arrival rates for the UGS connections, we observe that our CAC outperforms single admission control methods, without any deterioration in the overall system performance. Figure 5 depicts the Grade of Service among various arrival rates of UGS connections. It is observed that, using our proposed CAC, a better system performance in terms of VoIP communication is achieved, as there is a significant enhancement in GoS (1039%) of UGS connections. Single CAC (UGS) BR_CAC (UGS Sim.) BR_CAC (Non-UGS An.)

Single CAC (Non-UGS) BR_CAC (Non-UGS Sim.)

100 90 80

GoS (%)

70 60 50 40 30 20 10 0 0,25

0,5

1

2

In this paper, a new dynamic connection admission control mechanism based on bandwidth reservation concept is presented. Compared to simple bandwidth-based admission control methods for IEEE 802.16 architecture, the proposed solution improves the Grade of Service of UGS connections, as the Base Station serves more VoIP calls by considering the “busy hour” phenomenon. Although the admission control algorithm has been designed considering a WiMAX infrastructure, the flexibility of the scheme allows the adaption in other similar technologies as Long Term Evolution (LTE). Furthermore, in order to achieve better results in terms of QoS requirements, CAC should be accompanied by an optimum transmission scheduling. In our future work we shall focus our research on such issues.

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B. Simulation Results

Single CAC (Total) BR_CAC (Total) BR_CAC (UGS An.)

VI. C ONCLUSION

4

UGS Arrival Rate (connections/sec)

Fig. 5. Grade of Service vs. UGS Connections Arrival Rate (capacity-based CB-CAC vs proposed bandwidth reservation BR-CAC including analytical results)

On the other hand, the GoS of the rtPS/nrtPS connections is increased as expected, but examining the system considering the total number of connection requests (UGS, rtPS, nrtPS) we achieve a more efficient utilization of system resources as we observe an enhancement in the total Grade of Service ratio for high arrival rates of UGS connections.

This work has been funded by the Research Projects MOBILIA (TSI-020400-2008-82) and CO2GREEN (TEC201020823). R EFERENCES [1] IEEE Standard, “IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, IEEE Std 802.16-2004, June 2004. [2] S. Chandra and A. Sahoo, “An efficient call admission control for IEEE 802.16 networks”, in Proc. of IEEE Workshop on Local and Metropolitan Area Networks (LANMAN), New Jersey, USA, pp. 188-193, June 2007. [3] K. Wongthavarawat, A. Ganz, “Packet Scheduling for QoS Support in IEEE 802.16 Broadband Wireless Access Systems”, International Journal of Communication Systems, Vol. 16, p.81-96, 2003. [4] J.F. Borin, N. da Fonseca, “Uplink Scheduler and Admission Control for the IEEE 802.16 Standard”, IEEE Global Telecommunications Conference (GLOBECOM), 2009. [5] Jianhua He, Kun Yang, Ken Guild, “A Dynamic Bandwidth Reservation Scheme for Hybrid IEEE 802.16 Wireless Networks”, IEEE ICC 2008: 2571-2575. [6] X. Guo, W. Ma, Z. Guo and Z. Hou, “Dynamic Bandwidth Reservation Admission Control Scheme for the IEEE 802.16e Broadband Wireless Access Systems”, IEEE Wireless Communications and Networking Conference, pp. 3418-3423, Mar. 2007. [7] Yi Sun, Yilin Song, Jinglin Shi, Eryk Dutkiewicz, “Research on Bandwidth Reservation in IEEE 802.16 (WiMAX) Networks”, 14th IEEE International Conference on Telecommunications (ICT2007), Penang, Malaysia, May 14-17, 2007. [8] Kamal Gakhar, Mounir Achir, Annie Gravey, “Dynamic Resource Reservation in IEEE 802.16 Broadband Wireless Networks”, 14th IEEE International Workshop on Quality of Service (IWQoS), pp.140-148, 19-21 June 2006. [9] Report Study, “Enterprise VoIP Market Trends 2009-2012”, Osterman Research Inc., Feb. 2009. [10] Weber, J., “Dictionary of English Language Traffic Terms”, IEEE Transactions on Communication Technology, vol.16, no.3, pp.365-369, June 1968. [11] Iversen, V.B. (1973): “Analysis of real teletraffic processes based on computerized measurements”. Ericsson Technics, No. 1, 1973, pp. 1-64. “Holbk measurements”. [12] R. Koenen, “Coding of Moving Pictures and Audio”, ISO/IEC JTC1/SC29/WG11 N4668, March 2000. [13] Seung-Eun Hong, Oh-Hyeong Kwon, “Considerations for VoIP Services in IEEE 802.16 Broadband Wireless Access Systems”, IEEE Vehicular Technology Conference (VTC) 2006-Spring, 7-10 May 2006.

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