**1. Introduction**

16 Will-be-set-by-IN-TECH

110 Mobile Networks

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The service area of a cellular network is divided into cells. Users are connected to base stations in the cells via radio links. Channel frequencies are reused in cells that are sufficiently separated in distance so that mutual interference is below tolerable levels. When a new call is originated in a cell, one of the channels assigned to the base station of the cell is used for communication between the mobile user and the base station (if any channel is available for the call). If all the channels assigned to this base station are in use, the call attempt is assumed to be blocked and cleared from the system (blocked calls cleared). When a new call gets a channel, it keeps the channel until either the call is completed inside the cell or the mobile station (user) moves out of the cell. When the call is completed, the channel is released and becomes available to serve another call.

When a mobile station moves across the cell boundary and enters a new cell, a handover is required. Handover is also named handoff. If an idle channel is available in the destination cell, a channel is assigned to it and the call stays on; otherwise the call is dropped. Two commonly used performance measures for cellular networks are dropping probability of handover calls and blocking probability of new calls. The dropping probability of handover calls represents the probability that a handover call is dropped during handover. The blocking probability of new calls represents the probability that a new call is denied access to the network.

Call admission control (CAC) algorithms are used in order to keep control on dropping probability of handover calls and blocking probability of new calls. They determine whether a call should be accepted or rejected at the base station. Both the blocking probability of new calls and the dropping probability of handover calls are affected by the call admission algorithm used. The call admission algorithms must give priority to handover calls as compared to new calls.

Dedicated to the memory of Janis Sedols, Doctor of Mathematics (Dr.sc.comp.), 24.03.1939–11.08.2011. Dr. Sedols was active in writing this chapter.

Paper financed from EDRF's project SATTEH (No. 2010/0189/2DP/2.1.1.2.0./10/APIA/VIAA/019) being implemented in Engineering Research Institute "Ventspils International Radio Astronomy Centre" of Ventspils University College (VIRAC).

certain area is, the more channels are available for users (since the number of channels per cell

Call Admission Control in Cellular Networks 113

1. Pico-cells (range 10 – 50 m) are used inside buildings and lifts. The cell antennae are placed in corners of a room or in hallways. Pico-cells are used when the number of users in a building is large and signals from the outside cells cannot penetrate the building. A new

2. Micro-cells (range 50 m – 1 km) are cells used mainly in cities where there are a lot of users. 3. Macro-cells (range 1 km – 20 km) are used in rural areas since the number of users is small, and in populated areas where micro-cells are too small to handle frequent handovers of users that are moving fast while making calls. For example, if you are in a high speed car and connected to a micro-cell, and the car moves too fast for the call to be handed over

Having multi-tier cellular networks increases the number of cells, which means that more users are able to use the network without being blocked, and that users in cars or any high

The proliferation of computer laptops, personal digital assistants (PDA), and mobile phones, coupled with the nearly universal availability of wireless communication services is enabling the goal of ubiquitous wireless communications (Beigy & Meybodi, 2003); (Ramjee et al., 1997); (Leong & Zhuang, 2002); (Guerin, 1988). Unfortunately, to realize the benefits of omni-present connectivity, users must contend with the challenges of a confusing array of incompatible services, devices, and wireless technologies. Rice University, USA, is developing RENÉ (Rice Everywhere NEtwork) (Aazhang & Cavallaro, 2001), a system that enables ubiquitous and seamless communication services. The key innovations are a first-of-its-kind multi-tier network interface card, intelligent proxies that enable a new level of graceful adaptation in unmodified applications, and a novel approach to hierarchical and coarse-grained quality of service provisioning. The design of RENÉ requires a coordinated, collaborative effort across traditional layers and across different time scales of the system to maintain uninterrupted user

Future battlefield networks will consist of various heterogeneous networking systems and tiers with disparate capabilities and characteristics, ranging from ground ad hoc mobile, sensor networks, and airborne-rich sky networks to satellite networks. It is an enormous challenge to create a suite of novel networking technologies that efficiently integrate these

The key result of this chapter is the application part (Section 5) with the extension of the Equivalent Random Traffic method for estimation of throughput for networks with traffic splitting and correlated streams. The excellent accuracy (relative error less than 1%) is shown by numerical examples. The ERT-method has been developed for planning of alternate routing in telephone systems by many authors: (Wilkinson, 1956); (Bretschneider, 1973); (Fredericks, 1980) and others. In this chapter we propose an extension of the ERT-method

type having almost the same features as pico-cells are called femto-cells.

from one base station (cell antenna) to another, then the call will be dropped.

speed vehicles are able to talk without worrying about their calls being disconnected.

is fixed). We may consider four types of cells:

4. Satellites (world wide coverage).

**1.1.2 Hybrid networks**

connectivity.

**1.1.3 Battlefield networks**

disparate systems (Ryu et al., 2003).

Various priority based call admission algorithms have been reported in the literature, as for example (Beigy & Meybodi, 2003);(Ahmed, 2005);(Ghaderi & Boutaba, 2006). They can be classified into two basic categories: (1) reservation based, and (2) call thinning schemes.

1. Reservation based schemes:

In these schemes, a subset of channels is reserved for exclusive use by handover calls. Whenever the number of calls (new calls) exceeds a certain threshold, these schemes reject new calls until the number of simultaneous calls (new calls) decreases below the threshold. These schemes accept handover calls as long as the cell has idle channels. When the number of calls is compared with the given threshold, this scheme is called call bounding (Ramjee et al., 1997); (Hong & Rappaport, 1986); (Oh & Tcha, 1992); (Haring et al., 2001); (Beigy & Meybodi, 2005). When the current number of new calls is compared with the given threshold, the scheme is called new call bounding scheme (Fang & Zhang, 2002). Equal Access sharing with reservation schemes reserve an integral number of channels or a fractional number (Ramjee et al., 1997) of channels for exclusive use by handover calls. Schemes with fractional number of guard channels have better control of the blocking probability of the new calls and the dropping probability of the handover calls than schemes with integral number of guard channels.

2. Call thinning schemes:

These schemes accept new calls with a certain probability that depends on the number of ongoing calls in the cell (Ramjee et al., 1997); (Beigy & Meybodi, 2004). New call thinning schemes accept new calls with a probability that depends on the number of ongoing new calls in the cell (Fang & Zhang, 2002); (Cruz-Pérez et al., 2011). Both schemes accept handover calls whenever the cell has free channels.

In Sections 2 and 3, we compare four basic CAC strategies by examining new call and handover call blocking probabilities for the following schemes:


Section 4 deals with Dynamic reservation and Static reservation in two-tier networks. To a great extent, our purpose is a tutorial one because there are many papers on CAC schemes, but they usually contain incomparable numerical results developed by computer simulations. Similar research as our is done by (Ramjee et al., 1997). They show that the guard channel scheme is optimal for minimizing a linear objective function of call blocking and dropping probabilities. The scheme studied below appeals also to network providers in terms of maximizing the revenue obtained by simple mathematical means.
