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Evans, “Cross- Layer Design Approach for Multicast Scheduling over Satellite Networks”, in Proc. of IEEE International Symposium on Wireless Communication Systems 2005 (ISWCS 2005), ISBN 0-7803-9206-X, Siena, Italy, September 5-9, 2005. 6 CALL ADMISSION CONTROL Editors: Stylianos Karapantazis 1 , Petia Todorova 2 Contributors: Stylianos Karapantazis 1 , Petia Todorova 2 , Franco Davoli 3 , Erina Ferro 4 1 AUTh - Aristotle University of Thessaloniki, Greece 2 FhI - Fraunhofer Institute - FOKUS, Berlin, Germany 3 CNIT - University of Genoa, Italy 4 CNR-ISTI - Research Area of Pisa, Italy 6.1 Introduction to Call Admission Control RRM in multimedia satellite networks aims to guarantee the fair distribution of available resources, due to the fact that the total link capacity has to be divided among several users, as well as to fulfill certain pre-negotiated QoS requirements for the lifetime of the connection. RRM is one of the functions that are carried out in the Data Link Layer (DLL). A general DLL protocol stack that applies to satellite networks is depicted in Figure 6.1, while Figure 6.2 illustrates the most important RRM entities. One of the most important resource management functions is Call Ad- mission Control (CAC), which comprises the set of functions taken by the satellite network during the phase of connection establishment or connection re-negotiation to decide whether to accept or reject a user’s request for a connection. A new user’s request can be accepted provided that there 178 Stylianos Karapantazis, Petia Todorova are adequate network resources available to guarantee the QoS of both all already-existing connections and the new requested one. Generally, the CAC function results in the blocking of new calls or call dropping in the case of ongoing calls when the bandwidth required for the connection exceeds the available bandwidth. CAC, which turns out to be a crucial function to provide high utilization of network resources, is network-specific and is generally managed by the Network Control Center (NCC - recall that a description of the NCC functions is given in Chapter 1, sub-Section 1.4.3). However, in non-GEO satellite systems the CAC function has to be implemented on board of the satellite as well. Nevertheless, it should be mentioned that this approach requires satellites with on-board processing capabilities. Fig. 6.1: A general protocol stack for the main elements of a satellite network. Fig. 6.2: The main RRM entities. Chapter 6: CALL ADMISSION CONTROL 179 6.2 CAC and QoS management As noted in [1], the public data network provides a resource that could profoundly impact on high-priority activities of society, like defense and disaster recovery operations. Under stress, however, the public network turns out to be a virtually unusable resource, unless suitable traffic prioritization and CAC are applied to improve its performance. CAC has been extensively studied in the past as a general resource allocation mechanism in various networking contexts. Ross [2] is an excellent reference for CAC mechanisms in general, whereas reference [3] contains a recent survey on this topic in the context of wireless networks. In the simplest case of resource allocation, a connection is admitted simply if resources are available at the time the connection is requested. This policy is commonly called Complete Sharing (CS), where the only constraint on the system is the overall system capacity. In a CS policy, connections that request fewer resource units are more likely to be admitted (e.g., a voice connection will more likely be admitted compared to a video connection). A CS policy does not consider the importance of a connection when resources are allocated. At the other extreme, in a Complete Partitioning (CP) policy, every traffic class is allocated a set of resources that can only be used by that specific class. Other solutions are represented by Trunk Reservation (TR), where class i may use resources in a network as long as r i units remain available [4], and Guaranteed Minimum (GM) [5],[6], which gives each class its own small portion of resources; once used up, classes can then attempt to use resources from a shared pool. An Upper Limit (UL) policy was adopted in [1], and Virtual Partitioning (VP)wasproposedin[7]. As far as satellite systems are concerned, the architecture of the new satellite systems testifies the interest in ATM, IP and DVB technologies. A general architecture of a satellite system is illustrated in Figure 6.3. An Earth station (Gateway) is in charge of mapping ATM/IP traffic originated from terrestrial terminals over satellite connections, while the NCC performs CAC and DBA functions. The role of the aforementioned functions is to meet the QoS requirements of different service classes, i.e., delay, jitter and packet loss. A plethora of CAC algorithms were proposed in the literature for terrestrial ATM-based networks. Some of them require an explicit traffic model, while some others require traffic parameters such as peak and average rate. A classification of these schemes is provided in [8] along with the description of their salient features. Nevertheless, it should be noted that while some parameters can be easily specified (for instance, the peak rate), the actual average rate is difficult to estimate, since the source does not know it. Then, the user can declare an upper bound, which, however, results in low bandwidth efficiency. To cope with this issue, measurement-based CAC methods have been proposed. In [9], the authors present a taxonomy as well as a detailed survey of measurement-based CAC techniques. In that study, different measurement-based CAC methods were compared against each other 180 Stylianos Karapantazis, Petia Todorova Fig. 6.3: General architecture of a satellite system. in the light of bandwidth efficiency, Cell Loss Ratio (CLR), implementation complexity, scalability and dependency on traffic model. The authors were led to the conclusion that those methods that are based on effective bandwidth are the most suitable for high-speed communication systems, since they are simple enough to be implemented in real systems, they attain high bandwidth efficiency and last, but not least, they assume fewer traffic parameters. The rationale behind this category of CAC schemes is rather simple. First, the effective bandwidth for the aggregate connections is measured, namely the equivalent bandwidth needs of ongoing connections. Then, a request for a new connection is accepted provided that the requested bandwidth is smaller than the residual bandwidth, that is, the total link bandwidth minus the effective bandwidth. Concerning ATM-based satellite networks, they are able to meet different QoS requirements at the ATM layer [10]. These requirements are defined in terms of objective values of the network performance parameters, as specified in ITU-R Recommendation S.1420 [11]. Some of the QoS parameters (Peak-to-Peak Cell Delay Variation, Max Cell Transfer Delay and Cell Loss Ratio) may be offered on a per-call/connection basis and negotiated between the end-system and the network, whereas some other QoS parameters (Cell Error Ratio, Severely Errored Cell Block Ratio and Cell Misinsertion Rate) cannot be negotiated. For each direction of the call/connection, a specific QoS is negotiated, based on a traffic contract between the network and the user. At call set-up time, the user declares the source traffic descriptors and the Chapter 6: CALL ADMISSION CONTROL 181 QoS class by means of signaling or subscription. The traffic descriptors in the set-up signaling message include a generic list of traffic parameters, specific for each user connection. For each connection request, the CAC function derives the following information: • The source traffic descriptors, including the traffic characteristics of the ATM source; • The Cell Delay Variation Tolerance (CDVT) value; • The requested and acceptable values of each QoS parameter, and the QoS class. In particular, the idea of endowing LEO satellites with on-board ATM switching capabilities (Figure 6.4) combines the advantages of LEO systems, like significantly reduced propagation delay, rendering them suitable for real-time applications, with those offered by ATM, including faster trans- mission rate, bandwidth on demand, compatibility with existing protocols and guaranteed QoS [12],[13]. By supporting statistical multiplexing, priority queuing and multicasting, ATM technology can accommodate all QoS features requested by the user and therefore, becomes a suitable solution for broad- band multimedia communications. However, as LEO satellites’ coverage area changes continuously over time, in order to maintain connectivity, end-users must switch from beam to beam and from satellite to satellite, resulting in frequent intra- and inter-satellite handovers. Fig. 6.4: An on-board ATM switching/processing architecture. See reference [12]. Copyright c 2003 IEEE. 182 Stylianos Karapantazis, Petia Todorova The functions of the individual modules in Figure 6.4 are as follows: • Switch Fabric: switching cells from input ports to appropriate output ports. • Input Processor: scheduling, buffer monitoring. • Output Processor: scheduling, buffer monitoring and cell discarding. • Control Module : CAC, handover monitor & control, resource allocation, routing table update, signaling protocol, etc. The ATM switch uses different input/output ports for the uplink/downlink and for the Inter-Satellite Links (ISLs). This is because of the different bandwidth and signaling protocols used. The functions of the Control Module (CM) are shown in Figure 6.5, assuming that signaling and routing table updating are implemented [13]. For intra-satellite handover, the Handover Monitor & Control module has to monitor and measure the handover status of all beams belonging to the satellite. Fig. 6.5: The anatomy of an ATM switch with CAC/handover control module. See reference [13]. Copyright c 2004 IEEE. It is assumed that the mobile user initiates the intra-satellite handover process based on physical link quality measurements. Then, the mobile user will send a handover request message to the LEO satellite, indicating the new beam identification and the QoS requirements. The satellite CM has to implement the handover/CAC process in order to decide whether or not the new beam could provide the QoS requirements. If the handover is . Transmission Scheduling with Resource- Sharing Constraints in Wireless Networks , IEEE Journal on Selected Areas in Communications, Vol. 19, No. 10, pp. 205 3 -206 4, October 200 1. [38] G. Giambene,. LEO satellites’ coverage area changes continuously over time, in order to maintain connectivity, end-users must switch from beam to beam and from satellite to satellite, resulting in frequent intra-. Multicast Scheduling over Satellite Networks , in Proc. of IEEE International Symposium on Wireless Communication Systems 200 5 (ISWCS 200 5), ISBN 0-7803- 9206 -X, Siena, Italy, September 5-9, 200 5. 6 CALL