Call Admission Control In Umts Networks Computer Science Essay

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Abstract-UMTS network is anticipated to seamlessly support different applications and avails, such as multimedia services with altering quality of service (QoS) demands. Real-time multimedia like audio and video require strict bounds on throughput, disturbance and delay. The main advantage is flexibility in resource management allocated by the W-CDMA (Wideband Code Division Multiple Access) access technique. However, to exploit this flexibility it is essential to design an effective call admission control (CAC) scheme able to accommodate the number of active calls in each cell according to interference levels and power availability. We compare the performance of two CAC styles for voice services, one based on the number of active calls and one on run-time measures of the power emitted by the base station or of the total received interference.

INTRODUCTION:

UMTS network is anticipated to support different services such as voice, video, interactive multimedia and data. these services have varying QoS demands. CAC is a provisioning strategy that is used to limit the number of calls allowed into the networks in order to reduce the network congestion and provide the desired QoS to users in service.

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The second generation cellular systems like GSM the decision if a new call can be accepted or not be considered an easy task since it only depends on the usable number of channels in the cell. On the other side, the planning of the number of channels and the frequencies to be alloted to each cell is not an easy job. In the next generation cellular system UMTS the two problems have changed their characters. The network planning has no longer to deal with the frequency allocation problem, so in a sense it is an easier task. The call admission control can become a very difficult problem due to the soft capacity of W-CDMA systems. The system is mainly interference partial and the number connections can not specify the actual capability of each cell. The acceptance of a new connection depends on the SIR (signal-to-interference ratio) values achievable by each existing connection once the new one is activated. These values are function of the emitted powers which, due to power control (PC) mechanisms, depend on the mobile user positions. Since the power available at each base station (BS) is bounded, the number of users that can be assisted is the former are close to the BS and the small if they are far away.

The PC mechanism aquired by UMTS controls the power emitted on each channel in order to keep the SIR at the receiver a target value. In normal checks, an equilibrium point is achieved after some algorithm loops and all channels achieve the SIR target. The acceptance of a new call can create two possible situations: the new call is safely activated since a new equilibrium can be reached, or the newcall is erroneously admitted since a new equilibrium cannot be reached due to the interference levels and the power constraints. In this last case, the PC raises the power levels until some power limit is reached and some call is dropped due to the low value of SIR.

Ideally, call admission control should be able to accept a call. only if a new equilibrium of the power control can be reached and to decline it otherwise. This ideal behaviour can be found with a complete knowledge of the propagation conditions or allowing the new call to enter the system for a trial period. More practical schemes implemented with a distributed control must cope with partial knowledge of the system status and may mistakenly accept or reject a call. As performance metrics it is useful considering the average accepted load and the call dropping probability separately. A good CAC scheme must be able to guarantee a dropping probability below a quality threshold even at high offered loads and to keep the accepted load as high as possible.

In this paper we compare the execution of the two CAC styles for voice services, one based on the number of active calls and on run-time measures of the power emitted by the base station or of the total received disturbance. For this last class of CAC algorithms we propose a simple scheme that allows to perform measures when silence inhibition is adopted. The results are obtained by simulation considering uniform and non-uniform traffic distributions. We define criteria for setting the parameters of the CAC algorithms to meet the dropping probability restraint and then compare the different methods on the basis of the assumed load.

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Evolution: from 2G to 3G

Call Admission Control Schemes

Interactive CAC algorithms are based on the idea of accepting the new call with a low power level and then estimate if a new power control equilibrium can be reached. These algorithms can be considered best admission policies since they admit a new call if and only if the system can really provide the required SIR level to all calls. Unfortunately, their implementation in real systems presents some drawbacks. First they present serious convergence speed problems and, above all, they can normal work only with always active connections and can not effort discontinuous transmission, which is one of the most essential issues of UMTS.

A different approach to the CAC problem is based the idea that the CDMA ability is strictly related to power limits which forbids the PC to reach a new equilibrium when the load is too high. In fact, the power levels are increased by the PC mechanism when interference growth to keep the SIR at the target value and, as a result, the level of power emitted with respect to the limit can be assumed as load indicator in the admission decision. The decision rule can be quite simple like considering a threshold, Pthr, on the emitted power and admitting new calls only if the powers considered are below the threshold.

The downlink direction the only power level that can be considered is that emitted by the base station even if also limits to the power used for each individual channel can be acquired. In the uplink direction the emitted power levels on each channel require to be considered. each mobile station must inform the base station on the real level of power so that the base station can take the access decision on new call requests. The downlink and uplink directions can be measured independently since the W-CDMA interface of UMTS acquired a FDD (Frequency Division Duplexing) scheme in which separate bandwidths are used for the two directions. To avoid the periodic reports of the power levels from the mobile stations to the base stations in the uplink direction, the total interference level at the base station can be acquired. this interference level is that considered by all the PC loops controling on each channel to place the power emitted by each mobile station.

CAC schemes can not guarantee a safe access and call dropping can occur. They can not maximize either the accepeted traffic since a call can be sometime rejected even if it could be safely accepted. In the simple version messured in the literature they do not solve the problem of quitted transmissions. CAC is to derive an average cell capacity in number of connections so that a simple CAC based on the accessible number of circuits can be adopted. The admission controller can know also the number of active connection in neighbours cells, this extra information can be used in the decision criteria to make the supply management more flexible.it is possible to define the load of a cell as the weighted sum of the number of alive call in that cell and in its neighbours.

CALL ADMISSION CONTROL TECHNIQUES

To evaluate the system performance we have considered new call blocking possibility and handoff dropping probability due to unavailable of wireless capacity later a handoff. To force termination of an accepted call due to a handoff failure is less popular than blocking a new call, so we have made a variable resource reservation for handoff calls depending on the MS quality behavior. We compare the results with a complete sharing access although it is know that complete sharing and compete partitioning are not the most convenient strategies to be used.

Call admission result is made in a distributed manner near to that used in for fixed capacity systems. Each BS makes an admission decision by exchanging state information with the adjusent cell regularly. BS estimate the future MS handoffs using information of power and quality measurement reported by each MS. We imagine that the power dimension will come from the current and adjacent cells. The control system knows which of the adjacent cells are potential candidates to handoff, especially when channel degradation becomes important. A MS placed in a degradation area, defined in the limits of the cell, has high possibility to handoff within a time interval. Using information about the evolution in the power levels received by a MS in this region, it could be possible to direct in some cases the direction

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Of movement of the MS and avoid in some cases unnecessary resource reservation. When a MS is predicate to handoff, an indication of a capacity reserve equal to Ci,ef is sent to the predicated destination in order to apportion resources for the expected handoff. a reservation may be no longer necessary and cancelled.

QoS AND CALL ADMISSION CONTROL IN UMTS NETWORKS

A CAC guarantees that the admittance of a new call into a resource controlled network does not violate service commitment made by the network to already accepted calls, while still making the best use of the network source. in the wireless networks the design of CAC is more difficult due to users mobility. An accepted call that has not been complicated in the cell may not get a channel in the nearby cell to continue its service due to limited radio source in wireless networks. This will finally lead to the call being dropped. Since users are more intolerant to dropping a handoff call that blocking a new call, handoff calls are usually given a higher importance.

CAC can be grouped into two types: parameter-based admission control schemes use a priori

traffic requirement to determine the parameters of deterministic or random models. On the other hand measurement-based admission control offers QoS to users without requiring priori traffic qualifications or online policing. It depends on the measurement of actual traffic load in the network in making admission decisions. As a result it shifts the task of traffic requirement from the user to the networks and relieves the network from the load of traffic policing.

The design of UMTS networks, QoS was taking into account from the very starting. UMTS networks support various services, such as voice and video, over its packet switching domain. The QoS architecture is UMTS is based on the idea of Bearer Service (BS) which follows a layered based approach. Each layer of one or more BSs that are added together to provide the hoped QoS support on an end-end basis. The 3GPP defines four classes for the UMTS bearer service taking into account the behaviour of the traffic created by the various applications that are used over the network.

CAC techniques are great importance in UMTS networks in order to fulfil the QoS requirements of different classes and use the network resource in a capable manner. CAC determines whether a requested bearer access can be accomplished or not.

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