WO2003056868A1

WO2003056868A1 - Time based paging in a cellular communication network

Info

Publication number: WO2003056868A1
Application number: PCT/GB2002/005906
Authority: WO
Inventors: Abdol Hamid Aghvami; Stephane Antoine
Original assignee: King's College London
Priority date: 2001-12-21
Filing date: 2002-12-23
Publication date: 2003-07-10
Also published as: GB0413269D0; AU2002356316A1; GB0130639D0; GB2403379B; GB2403379A

Abstract

A method of searching for a mobile node in a cellular communication network, which method comprises the steps of: (1) identifying a position of said mobile node at a first time in the cellular communication network; (2) when communication with said mobile node is desired at a second time later than said first time, searching firstly a first group of cells of the cellular communication network where the mobile node is more likely to reside, said first group of cells being determined on the basis of the time difference between said first and second times, and wherein said first group of cells are further from said first position the greater said time difference.

Description

TIME BASED PAGING IN A CELLULAR COMMUNICATION NETWORK

FIELD OF THE INVENTION

The present invention relates to a method of searching for a mobile node in a cellular communication network, to a cellular communication network, to a method, operable at a network node, of searching for a mobile node, to a computer program for carrying out such a method or part of a method, to network nodes for use in a cellular communication network, and to a method of paging a mobile node in a cellular communication network.

BACKGROUND OF THE INVENTION

The ever-increasing popularity of wireless communication is placing greater demands on the infrastructure supplying the service. More and more types of wireless device are being offered to the consumer: for example mobile telephones with increased functionality, notebook computers, personal digital assistants, digital cameras etc. Increasing data transfer rates over the wireless link have conditioned the consumer to expect fast reaction from such mobile devices.

Cellular communication networks rely upon the idea of dividing a physical space into a series of "cells" (or basic service set under IEEE 802.11), each cell being served by a base station that transmits and receives data to and from mobile stations within the area covered by the cell.

Of course, the fact that a user has a wireless device means that he or she is able to "roam" freely in the area covered by the network, whilst receiving service at substantially any time. Whilst a user is idle i.e. not sending or receiving data to or from the fixed part of the network, he or she may move from one cell to another. Locating an idle mobile station (or mobile host) quickly when the fixed part of the network wishes to deliver data to the mobile station has posed a problem. Although the mobile station is programmed to send a periodic update to the network, a considerable amount of searching by the network may still be required to find the mobile station.

The search strategy involves broadcasting paging signals from all base stations, one of which the mobile station will respond to if present in one of these cells. In a traditional cellular network a mobile switching centre stores a database called a Visitor Location Register (VLR) that stores information of the location of the mobile station. When communication with a mobile node is desired the mobile switching centre uses the VLR to instruct broadcast of a paging signal by base stations in a location area of the mobile node. The paging signal is transmitted over the forward control channels (FOCC), also know as the paging and access channels (PACH). Once the base station or cell serving the mobile station has been identified, the network can commence forwarding data to the mobile station or can set up a circuit for a telephone call.

Future cellular communication networks (fourth generation) may be almost wholly IP based i.e. each node of the network will have an IP address. Base stations in the existing cellular communication networks are not IP addressable.

A conventional paging method for a mobile station in a mobile communication system is described in "Public Land Mobile Network Interworking with ISDN and PSTN: CCITT Recommendations Q.1001 to Q.1032", Fascicle, VI.12, pps 5-16. In this system, the service area of a mobile communication system for mobile communication services consists of one or more location areas, and each location area consists of one or more base station areas. The base station area is defined as an area covered by the radio signal of one base station, and the location area is defined as that area in which a mobile station may move freely without updating its location information. The location information is used for identifying a location area in which a mobile station exists at present, and it is stored in a location register.

Therefore, when an incoming call is being sent to a mobile station, the location area where the mobile station is at that time is identified by the location information in the location register, and all base stations in the location area are instructed to broadcast a paging signal to locate the desired mobile station. As the location area is that area in which a mobile station may move freely without updating its location information, once it has moved to the other location area, the mobile station should report its change to the location register and update the location information to a new location area which has been notified by a location signal.

Since the location of each mobile station is identified by location information which is the same for all mobile stations in a given location area, base stations in the location area each broadcast a paging signal when a call is being sent to a mobile station present in that location area, even though the mobile station is present in only one base station area and not in the other base station areas.

In this conventional mobile station paging method, the number of paging signals broadcast into a location area is equal to the number of incoming calls to the mobile stations within the location area, multiplied by the number of base stations in the location area. This means that the number of paging signals broadcast by a given base station is greater than the number of incoming calls to the mobile stations in the base station area which can be covered by the radio signal of any one base station, and also that each base station must broadcast paging signals for mobile stations not only in its own base station area but also in other base station areas in the same location area.

Thus, there is a problem that each base station has to broadcast additional paging signals for mobile stations not within its own base station area. This wastes network resources, particularly on the wireless link.

Alternatively, it is possible to reduce the number of paging signals to be broadcast by reducing the number of base stations in a location area. In this case, however, there is a problem that the number of times of entering and leaving a given location area is increased because the location area must be reduced in size, so that the location information has to be updated frequently in the location register, and it causes an increased processing load at the network node storing and maintaining the location register.

US-A-5 390 234 discloses a method of tracking mobile stations in a cellular network. Each mobile station reports its location to the network after a minimum number of intercellular movements. When it is desired to search for the mobile station those cells that are in a "neighbourhood" of the last cell in which a location update was made are searched. This neighbourhood covers a number of cells around the last cell up to cells in which another location report would have been sent had the mobile station moved to any of them. All cells of the neighbourhood are searched. The problem with this method is that no consideration has been made of how the mobile station may be found more quickly within the neighbourhood nor how network capacity may be saved be performing less paging in the neighbourhood.

US-A-5 875 400 discloses a method of tracking mobile stations in a cellular network. Mobile stations are programmed to send location updates at regular intervals e.g. every 30 minutes. The last cell in which each mobile station sends a location update is recorded by the network. Furthermore the network builds a database of location update history for each cell against the number of times a mobile station sends the next location update from each other cell in the network. Thus it is possible to determine the respective probability, given the last cell from which a mobile station sent the last location update, of finding the mobile station in each other cell in the network. Cells are then paged in groups according to the probability of finding the mobile station based on the database. This method is complicated to administer and requires considerable network node resources.

It is an aim of the present invention to alleviate the aforementioned disadvantages and to provide a method of searching for a mobile station that increases the speed of locating a mobile station and that is less resource intensive at network nodes.

SUMMARY OF THE PRESENT INVENTION

Aspects of the present invention are based on the insight that a progressive searching strategy can be performed by the network, that is dependent on the amount of time since the mobile station last sent a location update.

According to one aspect of the present invention there is provided a method of searching for a mobile node in a cellular communication network, which method comprises the steps of:

(1) identifying a location of said mobile node at a first time in the cellular communication network;

(2) when communication with said mobile node is desired at a second time later than said first time, searching firstly a first group of cells of the cellular communication network where the mobile node is more likely to reside, said first group of cells being determined on the basis of the time difference between said first and second times, and wherein said first group of cells are further from said location the greater said time difference. The method is carried out by a number of network nodes. In one embodiment, the network nodes are all addressable by IP (Internet Protocol) address. A group of cells may comprise one or more cells of the network. The location of the mobile node is preferably given by the identity of the station of the network that received the last location update from the mobile node. Accordingly, the actual location of the mobile node at the second time may not be the same as at the first time

Preferably, the method further comprises the step of searching groups of cells so as to search progressively further away from or progressively nearer to said location depending on said time difference. If it has been some time since the location update, the method will start searching further away and search progressively inward as the mobile station is more likely to be further from the cell of the last location update. However, if it has been a comparatively short time since the location update the method will start searching cell of the last location update and search progressively outwards as the mobile station is more likely to be nearer or in the cell of the last location update.

Advantageously, the method further comprises the step of comparing said time difference against a reference value, selection of said first group of cells being based on whether or not said time difference is higher or lower than said reference value. In one embodiment a timer is run for each mobile station starting from the last location update and the reference value is half of a maximum lifetime of the location update. In another embodiment the lifetime is increased or decreased with time from said first time.

Preferably, said cellular communication network is divided into a plurality of sites, each of which comprises a plurality of cells, the method further comprising the step of searching only cells within the site containing said location. Thus the maximum number of possible cells to search is reduced, saving network resources.

Advantageously, the method further comprises the step of defining a location area for the mobile station that comprises a set of the cells of said cellular communication network, and wherein said first group of cells comprises one or more cells from said set. In one embodiment only groups of cells within said location area and within the site are searched. Preferably said location area is defined on the basis of the cell in which the mobile node last sent a location update.

Preferably, the method comprises the step of updating said location after a predetermined number of cell boundary crossings of the mobile node, and wherein said location area comprises cells reachable from said location with fewer cell boundary crossings than said predetermined number of cell boundary crossings. Thus, the number of cells to be searched is further reduced and these are searched starting with the cells in which the mobile station is most likely to reside.

Advantageously, the method further comprises the step of updating said location at expiry of a predetermined time period from said first time. In this way, a mobile node must send a periodic location update even if it is not moving, so that the entry in the network node responsible for mobile node is kept up to date.

Preferably, said location area comprises a centre cell containing said location, the method comprising the step of progressively searching either outwardly from said centre cell to cells located on the edge of the location area or inwardly from cells located on the edge of the location area to the centre cell. Which option is selected depends on the amount of time since the last location update sent by that mobile node. In the majority of cases this should reduce the amount of time required find the mobile node.

Advantageously, the method further comprises the step of sending a multicast message to said first group of cells to initialise a paging request at a base station in each cell to search for said mobile node. This further reduces load on the network node performing the method, saves network capacity as only one message needs to be sent to instruct all base stations, and all base stations will search substantially simultaneously, speeding response times.

According to another aspect of the present invention there is provided a cellular communication network comprising plurality of network nodes, one or more of those network nodes each comprising means for storing and executing a computer program comprising program instructions for causing said one or more network nodes to perform a method or part of a method as described above. In one embodiment, each of said one more network nodes is addressable with an Internet Protocol (IP) address.

According to another aspect of the present invention there is provided at a network node in a cellular communication network, a method of searching for a mobile node roaming in that network, which method comprises the steps of: ( 1 ) receiving a paging request;

(2) identifying a first group of cells in the cellular communication network to search first;

(3) sending instructions to a respective network node in each cell of said first group of cells to page for said mobile node, said first group of cells being determined on the basis of a time difference between a most recent location update sent by the mobile node at a location and approximately the time of receipt of said paging request, and wherein said first group of cells are further from said first location the greater said time difference.

Preferably, said cellular communication network is divided into a plurality of sites, each of which comprises a plurality of cells, the method further comprising the step of sending paging instructions to only those cells within the site containing said location.

Advantageously, the method further comprises the steps of storing identities of cells defining a location area for the mobile station, the location area comprising a set of the cells of said cellular communication network, and selecting said first group of cells from one or more cells of said set. In one embodiment the method further comprises the step of searching only groups of cells within said location area. Preferably, the method further comprises the steps of receiving an update of the location after a predetermined number of cell boundary crossings by the mobile node, updating the stored identities of cells in the location area in response to the updated location, said location area comprising cells reachable from said location (or said updated location) with fewer cell boundary crossings than said predetermined number of cell boundary crossings. In one embodiment identities of cells in said location area are valid for a predetermined time period after a location update.

Advantageously, said location area comprises a centre cell containing said location, the method comprising the step of instructing one or more base stations to progressively search either outwardly from said centre cell to cells located on the edge of the location area or inwardly from cells located on the edge of the location area to the centre cell.

Preferably, the method further comprises the step of sending a multicast message to said first group of cells to initialise a paging request at a base station in each cell to search for said mobile node.

Advantageously, said method further comprises the step of instructing transmission of computer implementable instructions to the mobile node for causing the mobile node to inform the cellular communication network of changes in location.

Preferably, said computer implementable instructions cause the mobile node to inform the cellular communication network of a change in location either after a predetermined number of cell boundary crossings have been made or on expiry of a predetermined time period, whichever occurs first.

Advantageously, said method further comprises the step of setting said predetermined number of cell boundary crossings for said mobile node and instructing said predetermined number of cell boundary crossings to be transmitted to said mobile node.

According to another aspect of the present invention there is provided a computer program comprising program instructions for causing a network node to perform the any steps of the method as set out above. The computer program may be embodied on a record medium, in a computer memory, in a read-only memory or on an electrical signal carrier for example.

According to another aspect of the present invention there is provided a network node for use in a cellular communication network, which network node comprises the steps of:

( 1 ) means for receiving a paging request;

(2) means for identifying a first group of cells in the cellular communication network to search first;

(3) means for sending instructions to a respective network node in each cell of said first group of cells to page for said mobile node, said first group of cells being determined on the basis of a time difference between a most recent location update sent by the mobile node at a location and approximately the time of receipt of said paging request, and wherein said first group of cells are further from said location the greater said time difference.

Advantageously, the network node further comprising means for storing identities of mobile nodes roaming in said network, and associated with each mobile node identity:

(1) the identity of a centre base station network node or cell via which a mobile node last sent a location update message;

(2) the identity of at least one other base station network node or cell; wherein the identity of the at least one other base station network node or cell is grouped into one or more groups, member(s) of each group being common distance from said centre cell base station network node. In one embodiment said common distance is measured by the number of cell boundary crossings required to move from the centre cell to each other base station network node or cell. Alternatively, it may be desirable to group network nodes or base stations in bands of distance for example 0 - 2 cell boundary crossings from centre cell, 3 - 5, 6 - 8 etc.

According to another aspect of the present invention there is provided a network node comprising means for storing and transmitting computer implementable instructions for causing the mobile node to periodically update its location with the cellular communication network. According to another aspect of the present invention there is provided a method of paging a mobile node in a cellular communication network, which method comprises the steps of: (1) receiving a request for data transmission to the mobile node;

(2) determining whether or not said mobile node is idle;

(3) if idle, sending a paging request message to another network node responsible for maintaining location information of said mobile node;

(4) awaiting a paging response from said network node to said paging request;

(5) depending on said paging response, either commencing forwarding of data to said network node, or sending a message to a host attempting to send data to said mobile node that said mobile node is presently unreachable. In one embodiment step (1) comprises the step of receiving a packet having a header containing an IP address identifying said mobile node. At step (5) a virtual circuit connection with the mobile node may also be established.

According to another aspect of the present invention there is provided a computer program comprising program instructions for causing a network node to perform the method set out above. The computer program may be embodied on a record medium, stored in a computer memory, embodied in a read-only memory, or carried on an electrical signal carrier, for example.

According to another aspect of the present invention there is provided a network node for use in a cellular communication network, which network node comprises:

(1) means for receiving a request for data transmission to the mobile node;

(2) means for determining whether or not said mobile node is idle; (3) means for sending, if idle, a paging request message to another network node responsible for maintaining location information of said mobile node;

(4) means for awaiting a paging response from said network node to said paging request;

(5) the arrangement being such that, in use, depending on said paging response, data is either forwarded to said network node, or a message is sent to a host attempting to send data to said mobile node, said message indicating that said mobile node is presently unreachable.

According to yet another aspect of the present invention there is provided a method of locating a mobile station of a cellular network comprising (i) reporting the mobile station position to the network; (ii) updating its position each time the mobile station crosses a predetermined number of cell boundaries or crosses a site boundary and/or after a time interval if none of the two previous events have occurred; (iii) when communication with the said mobile station is desired, searching the cells where the mobile station is most likely to reside first; the cells where a mobile station is most likely to reside being determined by taking the Paging Entry Lifetime of a mobile station in a cell, τ and, using this value to select the cells to search first.

Preferably, the reporting of the mobile station position is carried out by sending a location update message via the center cell.

Advantageously, the cells to search first are, for low values of τ, the cells in the outer region of the paging region and for higher values of τ the cells in the inner region, the outer and inner region being based on the center cell of the mobile station.

Preferably, low values of τ are where (τ< T/2) where T is the maximum value of the Paging Entry Lifetime and τ is the current value of the Paging Entry Lifetime field and by high values of τ are values of τ where τ > T/2.

Advantageously, the outer region of a paging region is that region of paging region composed of cells that are far from the center cell starting by the outer-most cells and by inner region is meant that region of a paging region which is closer from the center cell starting at the center cell.

Preferably, a crossover site router controller controls each site and when the mobile moves from a first site to a second site, its entry at the first site router controller is be deleted. Advantageously, the searching areas are dynamically shaped around a centre cell wherever this centre cell is located in a site.

According to another aspect of the present invention there is provided an apparatus for tracking a mobile station in an IP network comprising

(i) means for reporting a first cell in which said mobile station is located to a base station;

(ii) means for updating its position each time the mobile station crosses a predetermined number of cell boundaries or crosses a site boundary and /or after a time interval if none of the two previous events have occurred;

(iii) means for searching the cells where the mobile station is most likely to reside first; the cells where a mobile station is most likely to reside being determined by taking the Paging Entry Lifetime of a mobile station in a cell, τ and, using this value to select the cells to search first.

By Paging Entry Lifetime of a mobile station in a cell, τ is meant the remaining amount of time associated with the paging entry. The remaining paging Entry decreases with the Time. After expiration of the Remaining Entry Lifetime the corresponding paging Entry will automatically be deleted from the paging cache.

To keep a paging Entry in the paging cache, it is necessary to refresh the entry by reassigning the paging Entry Lifetime its maximum value.

A crossover site router controller controls each site. The site router controller keeps a paging cache entry for each of its idle users roaming inside its site. Only one crossover site router controller keeps a valid paging entry for a mobile user because when the mobile moves to a different site, its previous entry will be deleted at the old site router controller. This technique makes the location tracking by the network accurate when an IP packet or stream of packets is to be delivered to the mobile user. Indeed one site uniquely identifies the area in which the mobile is residing. Therefore the probability of failing to locate the site where the mobile is residing is zero since the former entries are deleted at the former site controller router when the mobile crosses a site boundary.

Each crossover site router keeps the location information about a specific mobile user in a paging entry.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a network for wireless mobile IP access;

Fig. 2 is a schematic representation of an embodiment of a network topology in accordance with the present invention;

Fig. 3 is a schematic representation of two sites covered with a cellular communication network with each cell is approximated to a square;

Fig. 4 shows the path of a user with a mobile station across the two sites of Fig. 3 and those cells in which a mobile station sends a location update;

Fig. 5 is a flow chart of a method performed by a mobile node;

Fig. 6 is a flow chart of a method performed by a gateway router;

Fig. 7 is flow chart of a method performed by a crossover site router controller;

Fig. 8 is a flow chart of Algorithm 1 performed by the crossover site router controller;

Fig. 9 is a flow chart of Algorithm 2 performed by the crossover site router controller;

Fig. 10 shows the two sites of Fig. 1 with a first location area of a mobile station thereon; and

Fig. 11 shows the two sites of Fig. 1 with a second location area of a mobile station thereon. Referring to Fig. 1 a communication network is generally identified by reference numeral 1 that supports communication by wireless IP (Internet Protocol). The communication network 1 comprises a number of network nodes that are connected so as to permit a mobile station 2 to send and receive data over an IP network 3, such as the Internet, and over an SS7 (Signalling System number Seven) network 4, such as a Public Switched Telephone Network (PSTN). Generally, the mobile station will access the SS7 network 4 via a Mobile Switching Centre 5 to make and receive voice telephone calls, and will access the IP network 3 via gateway router 6 to send and receive data, for example to download a web page from a correspondent host 7. Data may be sent between the network nodes in packet form using the IP protocol, although the underlying protocols may be different between one pair of network nodes and another. For further details, in particular the identities of the acronyms in Fig. 1 , reference is made to "Wireless IP Network Standard", 3GPP2, Version 1.0.0 October 2002 (presently available at www.3gpp2.org/Public_html/Specs/P.S0001-B_vl.0.pdf), incorporated herein by reference.

In Fig. 1 the source and target radio networks (RN) each comprise a radio network controller (RNC) and a base station (BS), or node B in Universal Mobile Telecommunication System (UMTS) terminology. Intermediate the target PDSN and the target RN there may be one or more routers (not shown).

Referring to Fig. 2, a part of the communication network 1 is shown in more detail. The gateway router 6 serves a network such as a LAN (Local Area Network) or a WLAN (Wireless Local Area Network) that may cover a large town, city or university for example. The gateway router 6 receives all data packets coming from the IP network, in this case the Internet, and sends all data packets from the LAN to the Internet. The gateway router 6 distributes data to a relatively large geographical area as mentioned above, which is further divided by a network administrator into a number of sites or sub-domains in this case, site A and site B. Each site is controlled by the gateway router 6 (it would be possible to run the network with more that one gateway router 6). Respective crossover site router controllers 8 and 9 routes traffic to and from each site and stores location information of each mobile station 2 in the site. Below each crossover site router controller 8 and 9 (in hierarchical terms) there are a number of routers 10, 11 and 12, 13 respectively, although there may be more or less than two in each site of course. Routers 10, 11 and 12, 13 route data to and from respective base stations 14, 15, 16 and 17, 18, 19.

Each base station 14, 15, 16 and 17, 18, 19 is responsible for the transmission and reception of data to and from the mobile station 2 using electromagnetic waves, usually radio waves. Each base station 14, 15, 16 and 17, 18, 19 covers (in terms of transmission and reception) a geographical area the exact size and shape of which depends on a wide variety of parameters. In Fig. 2 each area that the base station 14,

15, 16 and 17, 18, 19 covers has been approximated to a circle when viewed from above. When a network administrator plans coverage of a town or city for example with base stations it is usual to approximate the coverage area of each base station with the same shape. Thus, for example, the area may be covered by a grid of squares, hexagons or triangles, or any other shape that permits substantially all of the ground area to be covered. An area covered by one base station is usually known as a "cell" (or "basic service set" under the 802.11 standard).

Referring to Fig. 3, the coverage of sites A and B is shown approximated by forty square cells, although each site may have a different number of cells. Those cells that are adjacent cells of another site will be known as "edge" or "border" cells. Between each site there is a site border.

The mobile station 2, away from its home network, is served by base station 15 and may communicate with the correspondent host 7 via the Internet 3 for example. The mobile station has a home agent (HA) further details of which can be found in the details of Mobile IP (see for example "Mobility Support in Ipv6" <draft- ietf-mobileip-ipv6-19.txt> and RFC 3220 both presently available at www.ietf.org). Either during a communication session or when the mobile station is idle (i.e. powered on but not sending or receiving data), the network permits the user to move about with the mobile station 2 and still remain "on-line".

As explained above, the network must maintain a record of the last known base station that the mobile station 2 communicated with or sent a paging (or location) update to. Each crossover site router controller 8, 9 maintains a table (or "paging cache entry") of the current IP address (e.g. care-of address under IPv6) of each mobile station 2 in its site against the last base station that each of those mobile stations sent a paging update from or that was used for communication purposes. Each entry has a respective lifetime, called Paging Entry Lifetime. The Paging Entry Lifetime is reduced with time and remains valid within the lifetime provided that the mobile station does not cross a predetermined number of cell boundaries or a site boundary. The Paging Entry Lifetime is set by the user's ISP and is typically of the order of approximately 10 mins. Of course, different values may be used and determination of the Paging_Entry_Lifetime may be based on the user's location update history for example. Ideally, the value of Paging Entry Lifetime should optimize (or at least not consume additional) network resources. It is believed that a Paging_Entry_Lifetime of approximately 10 mins neither utilizes too much of the mobile station's resources by demanding updates too frequently, nor leaves the network without a location update for too long.

The mobile station 2 maintains its own timer that is at least the maximum value of the Paging Entry Lifetime. The mobile station may be informed of the Paging Entry Lifetime in the broadcast beacon from base stations in each site. After the mobile station has sent a location update the timer is started. If, at or shortly after the expiry its timer, the mobile station 2 has not crossed more than d cell boundaries, a location update message has to be sent by the mobile station 2 to the network and the timer is reset.

The mobile station 2 will update its location with its current crossover site router controller when either of the following conditions is verified: 1. The mobile station has just crossed a site boundary; or 2. The mobile node has crossed d cell boundaries since the last location update was sent. The location update message is a paging update message. It will either refresh the existing paging cache entry for the mobile or create a new one if there is none at the crossover site router controller for that mobile station.

The cell in which the mobile station 2 sends a location update will be referred to hereafter as the "centre cell". Cells around the centre cell can be grouped by the distance λ in number of cell boundary crossings that they are away from the centre cell.

The mobile station 2 will be permitted a maximum number of cell boundary crossings d before it must send a location update to the crossover site router controller 8. The value of d will be assigned by the user's Internet Service Provider for example. The value of d may be static (for example d may be fixed in a given site or network) or may be set dynamically by the network. The mobile station may be assigned a value of d based on the user's mobility history over a previous time period such as hours, days, weeks etc. Where, for example, the user has a high mobility history, i.e. the mobile station has moved fast and crossed a large number of cell boundaries, the value of d should be large e.g. d = 10. Where, for example, the user has a low mobility history, i.e. the mobile station has moved slowly and crossed a small number of cell boundaries, the value of d should be small e.g. d= 4.

There are several ways in which the mobile station may be informed of the value of d to use. For example, when the mobile station moves into a new site or network, the mobile station can download the location update algorithm from its serving base station and the value of d that it should use. Alternatively, the value of d may be permanently fixed for the mobile station and is assigned at point of manufacture.

A paging cache entry in the crossover site router controller 8 for the mobile station 2 will be associated with the following information:

1. The IP address of the centre cell base station through which the mobile most recently updated its location. This base station will be the centre base station of the location area whose surrounding cells may need to be paged when an IP packet destined for the mobile station 2 arrives at the gateway router 6. 2. The set of IP addresses of the base stations which are situated at a distance λ=l (i.e. one cell boundary crossing) from the centre cell and are in the same site as the centre cell. 3. The set of IP addresses of the base stations which are situated at a distance λ =

2 from the centre cell and are in the same site as the centre cell. 4. The set of IP addresses of the base stations which are situated at a distance λ =

3 from the centre cell and are in the same site as the centre cell.

5. The set of IP addresses of the base stations which are situated at a distance λ =d - 1 from the centre cell and are in the same site as the centre cell.

6. The current Paging Entry Lifetime τ of the mobile station 2. Thus a likely location area of the mobile station is defined by reference to the centre cell that contains the centre cell and all surrounding cells located a maximum of d -1 cell boundary crossings away. This location area is further split into groups of cells, each group containing cells that are the same distance away from the centre cell. The location area is dynamically set with time as the user sends location update messages.

The base stations inside a paging area will dynamically join and prune multicast groups. Groups of base stations may be stored in a database at the crossover site router controller 8. Such a database may be created by a network administrator using a geographical map of the base stations in the network. From this an entry for each base station as a centre cell may be associated with a list of one or more IP addresses of base stations at one cell boundary crossing, two cell boundary crossings, three cell boundary crossings etc. from that centre cell. Each list may be assigned a different mulitcast address by the network administrator. From the database it will be possible to instruct each base station which multicast groups it should join. Alternatively, by looking up in the database the centre cell in which the mobile station is resident, the crossover site router controller 8 has access to the list of IP addresses of base stations that are different distances from the centre cell. Thus paging instructions could be unicast to the members of the group.

Referring to Fig. 4, a path 20 taken by a user with the mobile station 2 is shown over the two sites A and B. In this case d = 4 i.e. the mobile station 2 will send a location update message to its crossover site router controller 8 after crossing four cell boundaries, or if a site boundary is crossed, whichever occurs first. The cells in which the mobile station 2 sends a location update are shown by letters A, B, C, D and E. For example, when the mobile station 2 reaches location B it will send a location update message to its crossover site router controller 8 via one or more intermediate routers. On receiving the location update message the crossover site router controller 8 updates the paging cache entry for that mobile station to show the new IP address of the base station of the new centre cell in which the mobile station 2 is resident. From a database containing the IP addresses of base stations in the site, the crossover router controller also compiles the various groups of base stations at a given distance from the centre cell. As mentioned above, such a database may be compiled by the network administrator when setting up and maintaining the site. For each base station in the site the network administrator would need to record those adjacent base stations. The Paging Entry Lifetime for that entry is re-set to its maximum value.

Referring to Fig. 5 a flowchart of the steps in a method performed by a mobile station is generally identified by reference numeral 30. The method is initiated in response to the mobile station undergoing handover from one base station to another and being assigned a new care-of address for example. At step SI the mobile station 2 determines whether the user has crossed a cell boundary or a site boundary. A cell boundary crossing is determined if the mobile station is assigned a new care-of address (if utilizing IPv6) or if a new IP address is advertised in the broadcast beacon from the new serving base station. A site boundary crossing is determined by analysis of the subnet ID (for class B addresses) of the base station IP address in the broadcast beacon.

If the mobile station 2 has crossed a site boundary the method proceeds to step S2 where the mobile station sends a message to the new crossover site router controller 8 to inform it of its location in the new site. This is possible as the advertisement from the new base station contains the IP address and ID of the crossover site router controller serving the new base station. Alternatively, the new serving base station may inform the crossover site router controller that it has detected a new mobile station in its cell. All of the routers on the path between the mobile station 2 and the crossover site router controller will be updated to retain an entry for the mobile station so that subsequent paging requests from the site crossover router controller can be routed to the base station serving the mobile station. Associated with that entry in each router are the mobile station IP address, the IP address of the base station serving the mobile station and the incoming ID interface. At step S3 the cell boundary count stored by the mobile station 2 is reset to zero. At step S4 the mobile station 2 sends and ICMP (Internet Control Message Protocol: see RFC 792) message (Type and Code to be defined) to the old crossover site router controller so that it may remove the paging cache entry for that mobile station.

If the mobile station has crossed a cell boundary, the method proceeds to step

S5 where the cell boundary count stored in the memory of the mobile station 2 is increased by one. At step S6 the mobile station 2 determines whether or not the stored cell boundary count is equal to d i.e. the maximum number of allowable cell boundary crossings before a location update must be sent. If the cell boundary count is equal to d the mobile station 2 sends a location update message to the crossover site router controller 8 at step S7. If the cell boundary count is less than d the method returns to the start and awaits another indication that a cell or site boundary has been crossed.

Referring to Fig. 6 a flow diagram of a method of paging the mobile station 2 that is performed by the gateway router 6 is generally identified by reference numeral 40. At step SI the gateway router 6 receives a packet addressed to the mobile station 2. At step S2 the gateway router 6 determines whether or not the mobile station 2 is idle i.e. whether or not it is presently sending or receiving data . This is done by making a simple check for a routing cache entry for that mobile station 2. If a routing cache entry is present in the gateway router 6 the mobile station 2 is not idle. However, if the gateway router can find no routing cache entry, then it concludes that mobile station 2 must be idle. In the meantime further packets arriving at the gateway router 6 addressed to the mobile station 2 are placed in a queue. If the mobile station 2 is determined to be idle, the gateway router 6 looks up the paging entry for mobile station 2 stored in memory at step S3. From this paging entry the gateway router 6 can obtain the IP address of the crossover site router controller 8 that is responsible for the mobile station 2 at step S4. At step S5 the gateway router 6 sends a paging request message to that crossover site router controller 8 and starts a timer at step S6 in which it expects to receive a response. Ideally, the duration of this timer is a short as possible to preserve resources, but will ultimately be determined by the network running the routine. In future wireless IP networks a timer of one second or less should be achievable. If the gateway router 6 receives a response within the time limit, it starts forwarding packets destined for the mobile station 2 to the crossover site router controller 8 at step S7. If no response is received from the crossover site router controller 8, the gateway router 6 returns to step S2 at step S8. The routine may be repeated several times. If the mobile station 2 is not located after a given number of attempts e.g. three, the gateway router 6 will send a message to the host attempting communication with the mobile station 2 that the mobile station 2 is presently unreachable.

Referring to Fig. 7 a flow chart showing the method for paging the mobile station 2 that is performed by the crossover site router controller 8 is generally identified by reference numeral 50. When the crossover site router controller 8 receives a paging request at step S 1 from the gateway router 6, it looks up the paging entry for that mobile station at step S2. This is done using the IP address of the mobile station (which may be a care-of address). As explained above, the paging cache entry contains inter alia the current Paging Entr _^Lifetime for that mobile station which, at step S3, is read by the crossover site router controller 8. At step S4 the crossover site router controller 8 compares the current Paging Entry Lifetime against a value T/2, where T is the maximum initial value of Paging Entry Lifetime.

If the current Paging Entry Lifetime is greater than T/2 the crossover site router controller 8 initialises Algorithm 1 at step S5, that will be described in greater detail below. If the current Paging Entry Lifetime is less than T/2 the crossover site router controller 8 initialises Algorithm 2 at step S6, that will also be described in greater detail below.

At step S7 the crossover site router controller 8 starts a timer of Is duration in which it expects to receive a reply from one of the base stations performing the paging. If a response is received, the crossover site router controller 8 informs the gateway router at step S8 that the mobile station 2 is located in its site and that the gateway router may begin forwarding packets. If no response is received, the crossover site router controller 8 informs the gateway router at step S9 that mobile station 2 has not been found in its site. This is done via a paging reply message.

As mentioned above, the crossover site router controller 8 initializes one of two algorithms in response to receipt of a paging request from the gateway router. Which of the two algorithms is run depends on the current value of the Paging Entry Lifetime for that mobile station.

Algorithm 1

If the current Paging Entry Lifetime is greater than T/2 i.e. the last location update was received from the mobile station relatively recently, then the likelihood is that the mobile station 2 is in the centre cell or in a cell near to it. In this case the crossover site router controller 8 initialises Algorithm 1 which is shown in flow diagram form in Fig. 8, generally referenced by numeral 60. Firstly at step SI the crossover site router controller 8 sends a unicast message to the centre cell base station to instruct a paging broadcast in that cell. At step S2 the crossover site router controller 8 starts a timer of Is duration in which it expects a reply from the centre cell base station. If a reply is received from the centre cell base station, the method proceeds to step S3 in which the paging entry for the mobile station 2 is updated in the memory of the crossover site router controller 8 and the Paging_Entry Lifetime is reset to maximum at step S4. At step S5 the crossover site router controller 8 sends a message to the gateway router 6 to inform it that the mobile station has been found (this step is step S8 in Fig. 7). This is done via a paging reply message.

If no response is received from the centre cell base station the method proceeds to step S6 where a value is initialised to 1. /^' is a variable used by the method to denote cells at different distances λ (in minimum number of cell boundary crossings) from the centre cell. For example, when i = 2, this refers to all cells that are two cell boundary crossings away from the centre cell. At step S7 the crossover router sends a message to each cell that is a distance λ = 1 from the centre cell. This may be done by sending a unicast message to each base station in the group, or sending a multicast message to the group of cells at λ = 1. At step S8 the timer is started for reception of a response from one of the base stations in the group. If a response is received the method returns to step S3 etc. as described above.

If no response is received from one base station in the group at λ = 1 , the method proceeds to step S9 where the current value of i is set to = 1. At step S10 the crossover site router controller 8 checks whether the new value of /^' is equal to d i.e. the number of cell boundary crossings before the mobile station must send a location update. If i = d the crossover site router controller 8 sends a message to the gateway router 6 that the mobile station 2 has not been found in the location area. If is not equal to d the method returns to step S7 and the next group of cells one further cell boundary crossing away from the previous group are paged. This part of the process is repeated either until the mobile station is located or when i = d as described above.

Thus it will be apparent that with Algorithm 1 , a progressive search is made for the mobile station over the likely location area, starting with those cells nearer to the centre cell since the mobile station sent a location update comparatively recently. If the mobile station is not found in the cells near to the centre cell, the method proceeds to search the next group of cells nearest to the centre cell. The method can be considered as an iterative search on groups of cells of the same distance from the centre cell, with the distance commencing with zero and increasing with each iteration, such that the location area is searched from the inside out.

Whether the crossover router controller unicasts each base station in a group or multicasts each group will determine how the cells are searched. For example a unicast method will result in a slightly staggered search pattern of cells in the group since the paging instruction cannot be sent to the cell base stations simultaneously. Such a method might result in cells being searched around the centre cell in a clockwise or anti-clockwise manner for example. However, such a unicast method is intensive on network resources. A multicast method reduces load on the network and enables a paging instructions to be sent to and executed by all base stations in each group substantially simultaneously.

Algorithm 1 may be expressed in the following terms:

1. START 1.1 Start MOBILE_FOUND 0

1.1.1 The crossover site router controller 8 will send a unicast- paging request to the cell.

1.1.2 Wait for paging reply at the crossover site router

1.1.3 If paging reply received before timeout expires then MOBILE_FOUND=l

1.1.4 Else MOBILE FOUND=O. Initiate i = 1.

1.2 While ((MOBILE_ FOUND= =0)&&( i< d - \ )) do

1.2.1 The crossover site router controller 8 creates a multicast group Gi that re-groups all the base stations of the paging area that are situated at a distance λ = from the centre cell and the relative interfaces

1.2.2 Send a multicast paging request in the cells of the multicast group G,

1.2.3 Wait for paging reply at the crossover site router 1.2.4 If paging reply received before timeout expires then MOBILE_FOUND=l 1.2.5 Else MOBILE _FOUND= 0. + + End 1.2 END 1

Algorithm 2

If the current I 'aging Entry _Lifetime is less than T/2 i.e. the last location update was not received from the mobile station relatively recently, then the likelihood is that the mobile station 2 is in a cell at or near the edge of the location area. In this case the crossover site router controller 8 initialises Algorithm 2 which is shown in flow diagram form in Fig. 9, generally referenced by numeral 70. At step S 1 the crossover site router controller 8 initialises to d - 1 i.e. the outer cells of the location area that are furthest away in number of cell boundary crossings from the centre cell. At step S2 the crossover site router controller 8 instructs the group of base stations with λ = d - 1 to broadcast a paging signal. This may be done by sending a plurality of unicast messages, one to each base station; alternatively it may be done by sending a multicast message to the group of base stations At step S3 the crossover router controller starts a timer of Is duration in which it waits for a reply from one of the base stations of the group. If a reply is received the method proceeds to step S4 where the crossover site router controller 8 updates the paging cache entry held in memory for that mobile station. The update includes recording the IP address of the current base station serving the mobile station, retrieving the multicast groups from the multicast group database associated with that IP address, and resetting the Paging Entry Lifetime at step S5. At step S6 the crossover site router controller 8 sends a message to the gateway router that the mobile station has been found so that the gateway may commence forwarding packets.

If no paging reply is received from any of the group of base stations that have been paged, the routine proceeds to step S7 where is set to - 1. At step S8 the crossover site router controller 8 determines whether the new value of is equal to zero. If not, the method returns to step S2 when the next group of cells further from the edge of the location area are paged to search for the mobile station. If /^' = 0 the crossover site router controller 8 instructs the centre cell base station to page for the mobile station at step S9. If a reply is received from the mobile station at step S 10 the routine returns to step S4 and proceeds as described above. If no reply is received from the centre cell base station the crossover site router controller 8 sends a message at step S 11 to the gateway router to inform it that the mobile station has not been located.

Thus it will be apparent that with Algorithm 2, a progressive search is made for the mobile station over the likely location area, starting with those cells further from the centre cell since the mobile station sent a location update some time ago. If the mobile station is not found in the cells furthest from the centre cell in the location area, the method proceeds to search the next group of cells nearest to the centre cell. The method can be considered as an iterative search on groups of cells of the same distance from the centre cell, with the distance commencing with d - 1 and decreasing with each iteration, such that the location area is searched from the outside in.

Whether the crossover router controller unicasts each base station in a group or multicasts each group will determine how the cells are searched. For example a unicast method will result in a slightly staggered search pattern of cells in the group since the paging instruction cannot be sent to the cell base stations simultaneously. Such a method might result in cells being searched around the centre cell in a clockwise or anti-clockwise manner for example. However, such a unicast method is intensive on network resources. A multicast method reduces load on the network and enables a paging instructions to be sent to and executed by all base stations in each group substantially simultaneously. This also reduces the amount of time the gateway router must wait before forwarding packets to a mobile station.

Algorithm 2 may be expressed in the following terms:

2. START PAGING

2.1 Start MOBILE_FOUND= 0 2.1.1 Initiate i=d-l

2.2 While ((MOBILE _FOUND= =0)&&( > 0)) do

2.2.1 The crossover site router controller 8 will create a multicast group G that groups all the base stations that are situated at a distance λ ~ i from the centre cell and the relative interfaces 2.2.2 Send a multicast paging request in the cells of the multicast group G,

2.2.3 Wait for paging reply at the site crossover site router

2.2.4 If paging reply received before timeout expires then MOBILE_ FOUND=l 2.2.5 Else MOBILE_ FOUND=0, /~

End 2.2 End 2

Referring to Figs. 10 and 11 two positions of the mobile station 2 are shown in site A. In this case d = 4 i.e. the mobile station may cross four cell boundaries after which it must send a location update to the crossover site router controller 8. The value of d defines the size of the location area which is indicated by those cells containing numbers in Figs. 10 and 11. Each number in the cell indicates how far it is away in number of cell boundary crossings from the centre cell. As explained above the centre cell is that cell in which the mobile station last sent a location update. However, the location area is only valid in the same site as the centre cell. Thus, when the centre cell is within d - 1 cells of a site boundary (i.e. cells of the adjacent site are within the potential location area), the location area only covers those cells in the same site as the centre cell. This is best illustrated by Fig. 10 where the location area is almost reduced in half because the centre cell is an edge cell i.e. one of its borders contains a site boundary. Thus network resources are further saved by making the location area site specific. It is not necessary to include cells in the adjacent site in the location area 80 since, if the mobile station moves into that site, it must send a location update so that the centre cell would be redefined.

If the user with mobile station 2 moves relatively fast across a site for example (such as by motorized transport) the location tracking method would enable the gateway router and crossover site router controller 8 to identify more quickly which base station is presently serving the user. This is because the user will cross cell boundaries comparatively quickly and thereby the time taken to reach the d limit is small compared to Paging_Entry_Lifetime. Accordingly the crossover site router controller 8 will be more likely to us Algorithm 1 and commence searching in cells near to the centre cell and work outwards.

If the user is comparatively slow moving (such as on foot) the time taken to Therefore this location tracking strategy allows the network to locate the user faster while also reducing the number of unsuccessful location tracking attempts.

The whole protocol has been designed on the assumption that the wireless access network is fully IP compatible (i.e. the nodes support IP and the TCP/IP stack). In preferred aspects the base stations are capable of dynamically joining and pruning multicast groups and of reporting their membership of a multicast group associated with an interface using protocols such as IGMP (Internet Group Management Protocol). The use of a new ICMP is proposed to set the new path by deleting the previous entries from the cache along the old path in the previous site. The wireless IP network is divided into sites. This subdivision into sites is compatible with the hierarchical organisation of IP networks. The sizes of the sites have to be defined by the ISP when deploying the network. One of the major entities in supporting mobility in the wireless IP network is the Home Agent as defined in mobile IP. The Home agent is a router on the mobile node's home network, which maintains current location information for each of the mobile nodes. The Home Agent for a mobile station in its home network can be either the gateway or one of the crossover site routers.

Although the embodiments of the invention described with reference to the drawings comprise computer apparatus and methods performed in computer apparatus, the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the methods according to the invention. The carrier may be any entity or device capable of carrying the program.

For example, the carrier may comprise a storage medium, such as a ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disc or hard disk. Further, the carrier may be a transmissible carrier such as an electrical or optical signal that may be conveyed via electrical or optical cable or by radio or other means. When the program is embodied in a signal that may be conveyed directly by a cable or other device or means, the carrier may be constituted by such cable or other device or means.

Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant methods.

For the avoidance of doubt the term "cellular communication network" as used herein is intended to cover all current and future forms of cellular networks, and more particularly, but not exclusively all-IP cellular networks.

Claims

1. A method of searching for a mobile node in a cellular communication network, which method comprises the steps of: (1) identifying a location of said mobile node at a first time in the cellular communication network;

(2) when communication with said mobile node is desired at a second time later than said first time, searching firstly a first group of cells of the cellular communication network where the mobile node is more likely to reside, said first group of cells being determined on the basis of the time difference between said first and second times, and wherein said first group of cells are further from said first location the greater said time difference.

2. A method as claimed in claim 1, further comprising the step of searching groups of cells so as to search progressively further away from or progressively nearer to said location depending on said time difference.

3. A method as claimed in claim 1 or 2, further comprising the step of comparing said time difference against a reference value, selection of said first group of cells being based on whether or not said time difference is higher or lower than said reference value.

4. A method as claimed in claim 3, wherein said reference value is based on a lifetime associated with said location of said mobile node, the method further comprising the step of increasing or decreasing said lifetime with time from said first time.

5. A method as claimed in any preceding claim, wherein said cellular communication network is divided into a plurality of sites, each of which comprises a plurality of cells, the method further comprising the step of searching only cells within the site containing said location.

6. A method as claimed in any of claims 1 to 5, further comprising the step of defining a location area for the mobile station that comprises a set of the cells of said cellular communication network, and wherein said first group of cells comprises one or more cells from said set.

7. A method as claimed in claim 6, further comprising the step of searching only groups of cells within said location area.

8. A method as claimed in claim 6 or 7, further comprising the step of updating said location after a predetermined number of cell boundary crossings of the mobile node, and wherein said location area comprises cells reachable from said location with fewer cell boundary crossings than said predetermined number of cell boundary crossings.

9. A method as claimed in claim 6, 7 or 8, further comprising the step of updating said location at expiry of a predetermined time period from said first time.

10. A method as claimed in claim 6, 7, 8 or 9, wherein said location area comprises a centre cell containing said location, the method comprising the step of progressively searching either outwardly from said centre cell to cells located on the edge of the location area or inwardly from cells located on the edge of the location area to the centre cell.

11. A method as claimed in any preceding claim, further comprising the step of sending a multicast message to said first group of cells to initialise a paging request at a base station in each cell to search for said mobile node.

12. A cellular communication network comprising plurality of network nodes, one or more those network nodes comprising means for storing and executing a computer program comprising program instructions for causing said one or more network nodes to perform a method or part of a method of any of claims 1 to 11.

13. A cellular communication network as claimed in claim 12, wherein each of said one more network nodes is addressable with an Internet Protocol (IP) address.

14. At a network node in a cellular communication network, a method of searching for a mobile node roaming in that network, which method comprises the steps of: ( 1 ) receiving a paging request;

15. A method as claimed in claim 14, further comprising instructing or performing the steps of any of claims 2 to 4.

16. A method as claimed in claim 14 or 15, wherein said cellular communication network is divided into a plurality of sites, each of which comprises a plurality of cells, the method further comprising the step of sending paging instructions to only those cells within the site containing said location.

17. A method as claimed in any of claims 14, 15 or 16, further comprising the steps of storing identities of cells defining a location area for the mobile station, the location area comprising a set of the cells of said cellular communication network, and identifying said first group of cells comprises one or more cells from said set.

18. A method as claimed in claim 17, further comprising the step of searching only groups of cells within said location area.

19. A method as claimed in claim 17 or 18, further comprising the steps of receiving an update of the location after a predetermined number of cell boundary crossings by the mobile node, updating the stored identities of cells in the location area in response to the updated location, said location area comprising cells reachable from said location (or said updated location) with fewer cell boundary crossings than said predetermined number of cell boundary crossings.

20. A method as claimed in claim 17, 18 or 19, further comprising the step of marking identities cells in said location area as valid for a predetermined time period after a location update.

21. A method as claimed in claim 17, 18, 19 or 20, wherein said location area comprises a centre cell containing said location, the method comprising the step of instructing progressive searching either outwardly from said centre cell to cells located on the edge of the location area or inwardly from cells located on the edge of the location area to the centre cell.

22. A method as claimed in any of claims 14 to 21, further comprising the step of sending a multicast message to said first group of cells to initialise a paging request at a base station in each cell to search for said mobile node.

23. A method as claimed in any of claims 14 to 22, further comprising the step of instructing transmission of computer implementable instructions to the mobile node for causing the mobile node to inform the cellular communication network of changes in location.

24. A method as claimed in claim 23, wherein said computer implementable instructions cause the mobile node to inform the cellular communication network of a change in location either after a predetermined number of cell boundary crossings have been made or on expiry of a predetermined time period, whichever occurs first.

25. A method as claimed in claim 23 or 24, further comprising the step of setting said predetermined number of cell boundary crossings for said mobile node and instructing said predetermined number of cell boundary crossings to be transmitted to said mobile node.

26. A computer program comprising program instructions for causing a network node to perform the method of any of claims 14 to 25.

27. A computer program as claimed in claim 26, embodied on a record medium.

28. A computer program as claimed in claim 26, stored in a computer memory.

29. A computer program as claimed in claim 26, embodied in a read-only memory.

30. A computer program as claimed in claim 26, carried on an electrical signal carrier.

31. A network node for use in a cellular communication network, which network node comprises:

(1 ) means for receiving a paging request;

32. A network node as claimed in claim 31, further comprising means for performing any of the method steps of any of claims 14 to 22.

33. A network node as claimed in claim 31 or 32, further comprising means for storing identities of mobile nodes roaming in the cell covered by that node, and associated with each mobile node identity:

(2) the identity of at least one other base station network node or cell; wherein the identity of the at least one other base station network node is grouped into one or more groups, member(s) of each group being common distance from said centre cell base station network node.

34. A network node as claimed in claim 33, wherein said common distance is measured by the number of cell boundary crossings required to move from the centre cell to each other base station network node or cell.

35. A network node for use in a cellular communication network, which network node comprises means for storing and transmitting the computer implementable instructions of any of claims 23 to 25.

36. A method of paging a mobile node in a cellular communication network, which method comprises the steps of:

(1) receiving a request for data transmission to the mobile node;

(2) determining whether or not said mobile node is idle;

(3) if idle, sending a paging request message to another network node responsible for maintaining location information of said mobile node; (4) awaiting a paging response from said network node to said paging request;

(5) depending on said paging response, either commencing forwarding of data to said network node, or sending a message to a host attempting to send data to said mobile node that said mobile node is presently unreachable.

37. A method as claimed in claim 36, wherein step (1) comprises the step of receiving a packet having a header containing an IP address identifying said mobile node.

38. A computer program comprising program instructions for causing a network node to perform the method of any of claims 36 or 37.

39. A computer program as claimed in claim 26, embodied on a record medium, stored in a computer memory, embodied in a read-only memory, or carried on an electrical signal carrier.

40. A network node for use in a cellular communication network, which network node comprises:

(1) means for receiving a request for data transmission to the mobile node;

(2) means for determining whether or not said mobile node is idle;

(3) means for sending, if idle, a paging request message to another network node responsible for maintaining location information of said mobile node;

(4) means for awaiting a paging response from said network node to said paging request; (5) the arrangement being such that, in use, depending on said paging response, data is either forwarded to said network node, or a message is sent to a host attempting to send data to said mobile node, said message indicating that said mobile node is presently unreachable.