delivery of wireless broadband traffic including voice over. IP(VoIP) and video or .... The MAC contatins a queuing and traffic-shaping engine that is ultimately ...
QoS guaranteed IPTV service over Wireless Broadband network A.H. Park, J.K. Choi Information Communications University(ICU) E-mail: {ahpark, jkchoi}@icu.ac.kr
Abstract IEEE 802.16 standard[1] was developed for the delivery of wireless broadband traffic including voice over IP(VoIP) and video or audio streaming, as well as low-data-rate applications, such as web surfing, and burst traffic over the Internet. In order to support such wide range of traffic on a network, the network must be capable of satisfying various Quality of service(QoS) requirements. In this paper, we consider multi threshold guard channel scheme(MTGC) for multimedia quality of service(QoS) support in wireless broadband network. A service model consisting of four service classes with different handoff-dropping requirements is presented. Appropriate call-admission control and resource reservation schemes are developed to allocate resource adaptively to the IPTV service classes with a stringent delay bound. Keywords IEEE802.16, QoS, IPTV, connection admission control, guard channel scheme, multi threshold IPTV: IPTV is defined as multimedia services such as television/video/audio/text/graphics/data delivered over IP based networks managed to provide the required level of QoS/QoE, security, interactivity and reliability.[2]
1. Introduction With increasing demands for mobile multimedia services such as audio, video, and data, next-generation wireless networks are expected to provide quality of service(QoS) for such multimedia applications to users on the move. Since the multimedia services have inherently different traffic characteristics, their QoS requirements may differ in terms of bandwidth, delay, and connection dropping probabilities. It is the networks’ responsibility to fairly and efficiently allocate network resources among different users to satisfy such differentiated QoS requirements for each type of service. In order to satisfy the level of QoS for each type of call, connection admission control policies usually use the prioritized methods. This priority is usually implemented through allocation of more resource to calls with higher level of QoS. The simplest connection admission policy is called guard channel(GC)[3]. Suppose that the given cell has C full duplex channels. The guard channel policy reserves a subset of channels, called guard channels, aloocated to the cell for sole use of handoff calls(say C-T channels). Whenever the channel occupancy exceeds a certain threshold, T, the guard channel policy rejects new calls until the channel occupancy goes below the threshold. The guard channel policy accepts handoff calls as long as channels are available. In order to have more
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control on both the dropping probability of handoff calls and the blocking probability of new calls, limited fractional guard channel policy(LFG) is proposed[[4]. Uniform fractional channel policy is introduced in [5], which accepts new calls with probability of π independent of channel occupancy. All of the above mentioned connection admission policies consider only one threshold to decide for accepting/rejecting of new calls. These policies cannot be used when there is several classes of traffic with different level of QoS. In such cases, we need multi-threshold scheme, which provides different thresholds for different classes. Dual-threshold reservation(DTR) scheme for integrated voice/data wireless networks is given in[6,7]. In this scheme, three classes of calls in ascending order of level of QoS are considered which are data calls(both new and handoff calls), new voice calls and handoff voice calls. In [6,7], DTR scheme is modeled using a two-dimensional Markov chain and the effect of different values for number of guard channels on dropping and blocking probabilities are plotted. Two-threshold guard channel(TTGC) scheme for wireless networks handling two classes of voice users is introduced in [8]. In this paper, the idea given in [8] is generalized to multi-classes, where are N classes of traffic each of which having different level of QoS. Each class may contain either new or handoff calls but having the same level of QoS. Our proposed model, referred to as multi-threshold guard channel(MTGC) scheme, builds upon guard channel scheme by using N-1 thresholds in which each threshold is used to reserve channels for satisfying the specified level of QoS for that class. The limiting behavior of MTGC is analyzed under stationary traffic using one dimensional Markov chain. The rest of this paper is organized as follows: Section 2 provides an overview of the IEEE 802.16-based BWA system. Section 3 describes the system model. Multi threshold guard channel scheme is presented in Section 4 considering the QoS constraints for different service types and Section 5 concludes the paper.
2. IEEE 802.16 MAC Mechanism for QoS IEEE 802.16 uses a connection-oriented medium access control(MAC) protocol which provides a mechanism for the SSs to request bandwidth to the BS. IEEE 802.16 MAC supports two classes of SS: grant per connection(GPC) and grant per SS(GPSS). In the case of GPC, bandwidth is granted to a connection individually. In contrast, for GPSS, a portion
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of the available bandwidth is granted to each of the SSs and each SS is responsible for allocating bandwidth among the corresponding connections. The lengths of the downlink and uplink subframes for each SS are determined by the BS and broadcast to the SSs through downlink and uplink map messages(UL-MAP and DL-MAP) at the beginning of each frame. Therefore, each SS knows when and how long to receive from and transmit to the BS. In the uplink direction, each SS can request bandwidth to the BS by using BW-request PDU. The main feature of 802.16 QoS provisioning, and what distinguishes it from its competitors(i.e. 802.11 and 3G), is that it associates each packet with a service flow. 802.16 is a connection-oriented MAC. Each connection is assigned a unique Connection ID(CID) and a Service Flow ID(SFID) with an associated service class. The upper part of the MAC maps data into a QoS service class. Also, external applications can request service flows with desired QoS parameters using a named service class. The MAC contatins a queuing and traffic-shaping engine that is ultimately responsible for the reception and transmission of 802.16a packets according to the enforced QoS parameters. IEEE 802.16 provides four scheduling services, each with an associated service class: UGS, rtPS, nrtPS, and BE. These are detailed in Table 1. Table 1. IEEE 802.16 QoS Service Classes QoS Service Class Description Supports constant bit rate connection like voice Unsolicited Grant Service traffic BS provides fixed-size data grant burst (UGS) periodically Supports real-time services with variable size Real-Time Polling Service data on a periodic basis, such as MPEG (rtPS) BS provides SS the opportunity to request bandwidth on a regular basis Supports non-real-time services that require variable size data grant bursts on a regular Non-Real-Time Polling basis, such as FTP Service(nrtPS) BS provides SS opportunity to request bandwidth using unicast and contention methods For applications that do not require QoS, such as web surfing Best Effort (BE) BS allows SS to use all available mechanisms for transmission requests.
Each network application has to register with the network, where it will be assigned one of these service flow classifications with a Service Flow ID(SFID). QoS mapping in the form of classification of higher level layer data is provided in the upper part of the MAC. When the application wants to send data packet, the service flow is mapped to a connection using a unique CID.
3. System Model We consider a single BS serving multiple connection(from SSs) through a TDMA/TDD access mode using single carrier air-interface. For each of the rtPS and nrt PS connections, a
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separate queue(with size of X PDUs) is used for buffering the PDUs. In particular, for one connection, there are two queues for uplink and downlink transmissions from the SS and the BS, respectively. We consider an SS of type GPC. Therefore, during bandwidth allocation and connection admission control, a certain amount of bandwidth is reserved for each connection through that SS. The radio resource management model with the proposed joint bandwidth allocation and admission control framework is shown in Figure.2. In this model, there are two levels of optimization-one at the connection-level and the other at the packet-level. While the connection-level optimization is used to obtain the optimal setting for the complete partitioning thresholds for bandwidth allocation under connection-level QoS constraints, the packet-level optimization is used to allocation the available bandwidth among the ongoing and the newly arriving connections(when admitted) so that the corresponding packet-level QoS requirements can be satisfied.
4. Multi-threshold guard channel scheme In this section, the idea of two-threshold guard channel scheme is generalized to multi-classes and multi-threshold guard channel scheme is introduced. In this section, we first introduce a multi-threshold guard channel scheme for wireless broadband network where all cells have the same number of channel C and experience the same call arrival rates for all types of calls. The arrival process of new connection requests for UGS, rtPS and nrtPS is Poisson with rate λUGS , λrtPS and
λnrtPS respectively. λtotal = λUGS + λrtPS + λnrtPS The
service time of UGS, rtPS, nrtPS connections is exponentially distributed
with
mean
1
,
1
µUGS µrtPS
, and
1
µnrtPS
,
respectively. These assumptions have been found reasonable as long as the number of mobile users in a cell is much greater than the number of channels allocated to that cell. Assume that the calls of class k have a certain level of QoS such that its blocking probability must be less than qk . Without loss of generality, it is assumed that qUGS ≥ qrtPS ≥ qnrtPS ≥ qBE . To provide the specific level of QoS for calls, the allocated channels of each cell are partitioned into 4 subsets. In order to partition the channel sets, 3 thresholds, T1 , T2 , T3 (0 < T1 ≤ T2 ≤ T3 ) are used. For the sake of simplicity in presentation, we use two additional fixed thresholds T0 = −1 and T4 = C . The procedure for accepting calls in multi-threshold guard channel scheme (Algorithm 1) can be described as follows. A call from class k(for k=1,2,3,4) is accepted when the number of busy channels is smaller than Tk ;otherwise the call is blocked. Algorithm 1. Multi-threshold guard channel scheme [10] loop if Call of Class k then
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if c(t)< Tk then accept call else reject call end if end if end loop In the proposed multi-threshold bandwidth reservation scheme, C channels of each cell are divided into four regions by three thresholds T1, T2, and T3. It is assumed that T1 and T2, T3 are multiples of b, the number of channels needed to service a data call. These thresholds are adaptive according to the instantaneous cell traffic. UGS application, like E1/T1 and VoIP, is the most common way used by people for daily communication. The non UGS connections, such as rtPS, nrtPS and BE flows which support variable bit rate stream video, ftp, or HTTP applications are mostly used for entertainment and getting information. From the viewpoint of an end user, blocking a new UGS flow causes more serous problem than blocking a new non-UGS flow, since phone call is usually related to important commercial or personal conversation. People usually make phone calls to address certain urgent businesses needs. We therefore, give UGS connection a higher priority than non-UGS connection so that the request for UGS connection is accepted without restriction if bandwidth is available, while the request of non-UGS is accepted only if the total used bandwidth is not greater than the predetermined value. To guarantee the target handoff UGS connection dropping probability and to maintain low connection blocking probability, new nrtPS and rtPS connections can only use T1 channels out of the total C; handoff rtPS connections can only use T2 out of the total C channel; and new UGS connections can only use T3 channels out of the total C channels, thus handoff UGS connections are only dropped when no channels are available in the cell. Assume the number of channels needed to service a UGS connection and rtPS connection and nrtPS connection to be bUGS and brtPS and bnrtPS respectively. The required bandwidth must be satisfied in order to meet the QoS requirements of UGS and rtPS, nrtPS connections. Let l represent the number of UGS connections in the system and m represent the number of rtPS connections in the system, n represent the number of nrtPS connections in the system. The connection admission policy can be summarized as follows: - When a handoff UGS connections arrives if (l + 1)bUGS + mbrtPS + nbnrtPS ≤ C , it will be accepted. Thus a handoff UGS connection will only be dropped if there are no more channels available in the system. - When a new UGS connection arrives, if (l + 1)bUGS + mbrtPS + nbnrtPS ≤ T3 , it will be accepted. That is, if accepting a new UGS connection will increase the number of channels that are being used in the system to a number less than or equal to T3 , then the new UGS connection
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is accepted into the system. However, if this criterion is not met, the new UGS connection will be blocked. - When a handoff rtPS connection arrives, if lbUGS + (m + 1)brtPS + nbnrtPS ≤ T2 , it will be accepted. That is, if accepting the handoff rtPS connection will make the number of used channels in the system to be less than or equal to T2 , then the handoff rtPS connection will be accepted. However, if adding the handoff rtPS connection will cause the number of used channels to be greater than the predefined T2 threshold, then the handoff rtPS connection will be dropped. - When a new rtPS connection arrives, if lbUGS + (m + 1)brtPS + nbnrtPS ≤ T1 , it will be accepted. That is, if adding a new rtPS to the system will increase the number of used channels in the system to a number less than or equal to T1 , then new rtPS connection will be accepted. Otherwise, it will be blocked. - When a new nrtPS connection arrives, if lbUGS + mbrtPS + (n + 1)bnrtPS ≤ T1 , it will be accepted. That is, if adding a new nrtPS to the system will increase the number of used channels in the system to a number less than or equal to T1 , then new nrtPS connection will be accepted. Otherwise, it will be blocked.
Incoming traffic
λhUSG
Handoff UGS connection only
λUSG
Handoff and New UGS connections only
λUSG + λhrtPS λUSG + λnrtPS + λrtPS
Handoff UGS Connections, New UGS Connections, and Handoff rtPS Connections Only Handoff UGS Connections, New UGS Connections, Handoff rtPS Connections, and New rtPS, nrtPS Connections
T2
T3
T1
Figure 1. Multi-threshold guard channel scheme
5. Conclusion We have investigated a dynamic admission control scheme based on the traffic characteristics of services defined in the IEEE 802.16 wireless broadband network. The proposed system is based on a service model designed for both connection- and application-level QoS. Wireless broadband multimedia applications are classified into different service classes in the service model by their QoS requirements. Based on the service model, adaptive resource allocation is performed for each service class by employing the appropriate connection admission control and bandwidth allocation tailored to the QoS requirements of the service class. We have assumed that the traffic arrival process and service time of all service are exponential distributions in our traffic model, while knowing fact that the holding time of certain traffic is not an exponential distribution. We want to continue the research in analyzing traffic model for non-exponential holding time traffic. We want to continue evaluating the system performance using proposed scheme.
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The request/grant mechanisms as well as the definitions of scheduling services in the IEEE802.16 standard only give us a scheme of packet transmission for admitted connections. The detailed packet scheduling scheme, such as the criteria for determining unicast request intervals, granting or not granting the request, has not been referred to in the standard and are not mentioned in the paper either. ACKNOWLEDGEMENT This work was supported in part by the MIC, Korea under the ITRC program supervised by the IITA and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government(MOST) (No.R11-2000-074-02002-0) REFERENCES [1] IEEE 802.16 Working Group, 2004, IEEE Std 802.16-2004(Revision of IEEE Std 802.16-2001): IEEE Standard for Local and Metropolitan Area Networks Part 16: Air interface for Fixed Broadband Wireless Access Systems, IEEE Press [2] ITU-T IPTV Focus Group, 2006, FG IPTVINTERIM-DOC-001REV.2: IPTV SERVICES REQUIREMENTS, FG-IPTV WG1 Editor [3] D.Hong, S. Rappaport, “Traffic modeling and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoffs procedure”, IEEE Transactions on Vehicular Technology 35”, 1986, pp.77-92. [4] R. Ramjee, D. Towsley, R. Nagarajan, “On optimal call admission control in cellular networks”, Wireless Newtorks 3, 1997, pp.29-41. [5] H. Beigy, M.R. Meybodi, A new fractional channel policy”, Journal of High Speed Networks 13, 2004, pp.25-36. [6] L.Yin, B.Li, Z.Zhang, Y.Lin. “Performance analysis of a dual threshold reservation(DTR) scheme for voice/data integrated mobile wireless networks”, Proceedings of the IEEE Wireless Communications and Networking Conference, 2000, pp.258-262 [7] B.Li, L.Li,B.Li, X.Cao, “On handoff performance for an integrated voice.data cellular system”, Wireless Networks 9, 2003, pp.393-402. [8] H.Beigy, M.R.Meybodi, A two-threshold guard channel scheme for minimizing blocking probability in communication network”, June 2004 [9] L.-Z.Li, B.LI, B.Li, X.-R.Cao, “Call level performance analysis for multi-services wireless cellular networks”, IEEE ICC’2003, Anchorage, USA, May 2003, pp.11-15. [10] H. Beigy, M.R.Meybodi, “A general call admission policy for next generation wireless networks”, Computer Communications 28, January 2005, pp.1798-1813.
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