15.1 Paging

Paging involves messages sent over the radio informing the mobile user that an incoming call is pending. When the mobile station replies, the exact base station to which it is attached will be known to the network, and the call setup can proceed. The network knows the position of the mobile station only at the location area level. Because radio spectrum is scarce, these messages must be kept to a minimum by paging a minimum of cells. The trade-off, as mentioned previously, is that in order to minimize the number of cells that must be paged, location updates must be more frequent. It should be taken into account that because of the unpredictable nature of radio communications, paging messages may not arrive at the mobile with the first attempt, and there is usually some number of repetitions. Because the arrival of paging messages cannot be predicted, a mobile station must listen to the paging channel continuously or almost continuously, as explained in GSM.

For location management purposes, cells are usually grouped together into location areas (LAs) and paging areas (PAs) (see Figure 15.1). A location area is a set of cells, normally (but not necessarily) contiguous, over which a mobile station may roam without needing any further location updates. In effect, a location area is the smallest geographical scale at which the location of the mobile station is known. A paging area is the set of cells over which a paging message is sent to inform a user of an incoming call. In most operational systems, location area and paging area are identical, or paging areas are a subset of location area. For this reason, any grouping of cells for location management purposes is usually called a location area.

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Figure 15.1: Number of PAs within an LA.

15.1.1 Blanket Paging

Two major steps are involved in call delivery. These are (1) determining the serving VLR (visitor location register) of the called MT (mobile terminal) and (2) locating the visiting cell of the called MT. Locating the serving VLR of the MT involves the following database lookup procedures: ^[1]

The calling MT sends a call initiation signal to the serving MSC (mobile switching center) of the MT through a nearby base station.
The MSC determines the address of the HLR (home location register) of the called MT by global title translation (GTT) and sends a location request message to the HLR.
The HLR determines the serving VR of the called MT and sends a route request message to the MSC serving the MT.
The MSC allocates a temporary identifier called temporary local directory number (TLDN) to the MT and sends a reply to the HLR together with the TLDN.
The HLR forwards this information to the MSC of the calling MT.
The calling MSC requests a call setup to the called MSC through the CCS 7 network.

The procedure described here allows the network to set up a connection from the calling MT to the serving MSC of the called MT.

Because each MSC is associated with a location area, a mechanism is therefore necessary to determine the cell location of the called MT. In current cellular networks, this is achieved by a paging procedure so that polling signals are broadcast to all the cells within the residing LA of the called MT over a forward control channel. On receiving the polling signal, the MT sends a reply over a backward control channel, which allows the MSC to determine its current residing cell. This is called the blanket-paging method. In a selective paging scheme, instead of polling all the cells in an LA, a few cells are polled at a time. The cluster of cells polled at the same time constitutes the paging area. Here, a factor called granularity, K, is defined as the ratio of the number of cells in the PA to the number of cells in the LA. K denotes the fineness in the polling scheme. In a purely sequential polling scheme, K = 1/S_j, whereas the granularity factor is 1 in case of blanket polling and S_j is the number of cells in the j-th LA.

15.1.2 Different Paging Procedures

The work reported in Rose ^[2] developed methods for balancing call registration and paging. The probability distribution on the user location as a function of time is either known or can be calculated, the lower bounds on the average cost of paging are used in conjunction with a Poisson incoming-call arrival model to formulate the paging/registration optimization problem in terms of time-out parameters.

In another work by Rose and Yates, ^[3] efficient paging procedures are used to minimize the amount of bandwidth expended in locating a mobile unit. Given the probability distribution on user location, they have shown that optimal paging strategy, which minimizes the expected number of locations polled, is to query each location sequentially in order of decreasing probability. Because sequential search over many locations may impose unacceptable polling delay, they considered optimal paging subject to delay constraint.

Akyildiz and Ho ^[4] proposed a mobile user location mechanism that incorporates a distance-based location update scheme and a paging mechanism that satisfied predefined delay requirements. Akyildiz and coworkers ^[5] have introduced a mobility tracking mechanism that combines a movement-based location update policy with a selective paging scheme. This selective paging scheme decreases the location tracking cost under a small increase in the allowable paging delay.

Bar-Noy and Kessler ^[6] explored tracking strategies for mobile users in personal communications networks, which are based on topology of cells. The notion of topology-based strategies was introduced in a general form in this work. In particular, the known paging areas, overlapping paging areas, reporting centers, and distance-based strategies were covered by this notion.

Lyberopoulos et al. ^[7] proposed a method that aims at the reduction of signaling overhead on the radio link, produced by the paging procedure. The key idea is the application of a multiple-step paging strategy, which operates as follows: At the instance of a call terminating to a mobile user that roams within a certain location area, paging is initially performed in a portion of the location area (the paging area) that the so-called paging-related information indicates. Upon a no-paging response, the mobile user is paged in the complementary portion of the location area; this phase can be completed in more than one (paging) step. Several "intelligent" paging strategies were defined in this work. In Wang and coworkers, ^[8] various paging schemes were presented for locating mobile users in wireless networks. Paging costs and delay bounds are considered because paging costs are associated with bandwidth utilization and delay bounds influence call setup time. To reduce the paging costs, three paging schemes (reverse, semireverse, and uniform) were introduced to provide a simple way of partitioning the service areas and decreased the paging costs based on each mobile terminal's location probability distribution.

The several paging strategies mainly based on blanket paging were applied to reduce the paging costs, as well as update costs associated with constraints. The strategies briefly discussed here were widely used and few of them are applied in industry. In spite of having widespread use of those paging strategies, some disadvantages have been discovered. In the next section, we discuss new paging schemes to overcome the disadvantages in the different blanket-paging schemes.

^[1]Akyildiz, I.F. and Ho, J.S.M., On location management for personal communications networks, IEEE Communications Magazine, Sept. 1996, pp. 138–145.

^[2]Rose, C., Minimizing the average cost of paging and registration: a timer-based method, ACM J. Wireless Networks, 109–116, Feb. 1996.

^[3]Rose, C. and Yates, R., Minimizing the average cost of paging under delay constraints, Wireless Networks, 1, 211–219, 1995.

^[4]Akyildiz, I.F. and Ho, J.S.M., A mobile user location update and paging mechanism under delay constraints, Proc. of ACM SIGCOMM, Cambridge, Massachusetts, 1995, pp. 244–255.

^[5]Akyildiz, I.F., Ho, J.S.M., and Lin, Y.-B., Movement-based location update and selective paging for PCS networks, 1996.

^[6]Bar-Noy, A. and Kessler, I., Mobile users: to update or not to update? Proc. INFOCOM '94, June 1994, pp. 570–576.

^[7]Lyberopoulos, G.L., Markoulidakis, J.G., Polymeros, D.V., Tsirkas, D.F., and Sykas, E.D., Intelligent Paging Strategies for Third Generation Mobile Telecommunication Systems,

^[8]Wang, W., Akyildiz, I.F., and St ber, G.L., Effective paging schemes with delay bounds as QoS constraints in wireless systems, Wireless Networks, 7, 455–466, 2001.