WO2004036948A2

WO2004036948A2 - Communication system and method of routing information

Info

Publication number: WO2004036948A2
Application number: PCT/EP2003/050695
Authority: WO
Inventors: Jin Yang; Jakub Tichy; Gerry Foster
Original assignee: Motorola Inc
Priority date: 2002-10-12
Filing date: 2003-10-06
Publication date: 2004-04-29
Also published as: AU2003282113A1; GB2394148B; AU2003282113A8; WO2004036948A3; GB2394148A; GB0223855D0; EP1554908A2

Abstract

A method of routing information in a communication system where a serving communicating unit serves a plurality of subscriber units with a communication resource and is identified by both a public and a private address. The method includes the steps of a subscriber unit (310) roaming from a home communication network (110) to a visited communication network (150) and requesting (350), using a GPRS transport protocol message, a communication resource. A visited serving communication unit (152) processes the request in order to extract a serving communication unit private address. In response to determining that the serving communication unit private address identifies a serving communication unit of a different communication network (110), the serving communication unit (152) incorporates its public address in the request and forwards (375) the request to a home serving communication unit (112) containing its public address (312). In this manner, subsequent messages to the subscriber unit can be routed via the visited serving communication unit using its public address. A communication system and serving communication units, for example a serving GPRS service node and a gateway GPRS service node, are also provided.

Description

- I -

COMMUNICATION SYSTEM, SERVING COMMUNICATION UNIT AND METHOD OP RQUTΪNS INFORMATION

Pi6ld of th Xwv<≥ntio»

This invention relates to the allocation of addresses in order for data to be routed to communication units. The invention is applicable to, but not limited to, addresses used by the Core Network entities when subscriber units roam between public land mobile networks {PLMN_S) .

Baαkgrotma of he iΛvβat oai

Present day communication systems, both wireless and wire-line, have a requirement to transfer data between communication units. ata, in this context, includes signalling messages, multimedia, speech communication, etc. Such data transfer needs to be effectively and efficiently provided for, in order to optimise use of limited communication resources.

For data to be trans rred across and between communication networks, a communication unit addressing protocol is required. The communication units are generally allocated addresses that are read by a communication bridge, gateway and/or router, in order to determine how to transfer the data to the addressed unit. The interconnection between networks is generally known as internetworking (or internet) .

Networks are often divided into sub-networks, with protocols being set up to define a set of rules that allow the orderly exchange of information. Currently, the two most popular protocols used to transfer data in communication systems are: Transfer Control Protocol (TCP) and Internet Protocol (IP). In all but the simplest of communication systems, these two protocols often work as a complementary pair. The IP portion corresponds to data transfer in the network layer of the well-known -osI model and the TCP portion to data transfer in the transport layer of the 0S1 model. Their operation is transparent to the physical and data link layers and can thus be used on any of the standard cabling networks such as Ethernet, FDDI or token ring_*

The Internet Protocol adds a data header on to the information passed from the transport layer. The resultant data packet is known as an Internet datagram. The header of the datagram contains information such as destination and source IP addresses, the version number of the IP protocol etc. An IP address is assigned to each node and network element. It is used to identify the location of the network and any sub-networks.

Each node using TCP-IP communications requires an IP address that is then matched to its token ring or Ethernet MAC address. The MAC address allows nodes on the same segment to communicate with each other. In order for nodes on a different network to_ communicate with one another, each node must be configured with an IP address.

Nodes on a TCP-IP network are either hosts or gateways. Any nodes that run application software, or are terminals, are defined as hosts. Any node which is able to route TCP-IP packets between networks is called a TCP/IP gateway node, A TCP/IP gateway node must have the necessary network controller boards to physically interface to other networks.

A typical IP address consists of two fields;

(i) The prefix field, where a network number identifies the network associated with that particular address, and

(ii) The suffix field, where a host number identifies the particular host within that network.

The IP address is 32 bits long and can therefore theoretically address 2³² (over four billion) physical networks. One problem, however, associated with using an IP address containing prefixes and suffixes lies in the decision on how large to make each field. If the prefix is too small, only a few networks will be able to be connected to the Internet. Eowever, if the prefix is made larger, then the suffix has to be reduced, which results in a network being able to support only a few hosts.

The present version of the Internet protocol addressing scheme (IPv4) can accommodate a few very large networks or many small network . In reality, a reasonable number of networks of various sizes are required to be supported. However, most organisations tend to have their own IP addressing scheme, arranged to accommodate a larger network than they generally need, to allow for future network expansion.

As a consequence, the current version of Internet Protocol (IPv4) has scarce addressing space and future versions are currently being developed. It is envisaged that each Public Land Mobile Network (PLMN) will be unable to allocate a unique permanent IP address to each subscriber unit using IPv4. Moreover, even in the event that IPv6 were to be deployed in the future, many networks will still consist of legacy networks implementing IPv4,

An IP address can be defined in the form:

aaa' . 'bbb'.'ccc' . 'ddd';

Where: *aaa', *bbb^r , *ccc' and dd^f are integer values in the range 0 to 255.

On the Internet the ^Λaaa' , bbb^r , 'ccc' portions normally define the sub-network and the ' d ' portion defines the host. Such numbering schemes are difficult to remember. Hence, symbolic names {often termed domain names) are frequently used instead of IP addresses to identify individual communication units.

Normally, the DNS server is reachable by all the hosts on the network via the IP transport protocol. Therefore the DNS protocol for performing address lookup can be carried over IP.

The directory network services on the Internet determine the IP address of the named destination user or application program. This has the advantage that users and application programs can move around the Internet and are not fixed to a particular node and/or IP address.

In systems employing a limited number of addresses by which to identify Individual communication units, a technique called dynamic addressing is used. Dynamic addressing requires a pool of addresses to be maintained by an address allocation server, for example a Dynamic Host Configuration Protocol (DHCP) server. Whenever a host is connected to a network, a signalling process is performed between the host and DHCP server to assign an available IP address to the host. In order to do so, the host needs to send the DHCP server its unique ID. When the signalling process is de-activated, the IP address will be returned to the addressing pool and will wait to be assigned to other terminals.

If a packet data subscriber unit initiates an Internet connection, the DHCP server recognises the need to identify the subscriber unit and typically informs a domain name server (DNS) that a new Internet Protocol address assignment has occurred. Subsequently, the local DNS can then map the subscriber unit's domain name to an Internet Protocol address allocated by the DHCP, and pass the address information to an Internet Host.

Due to the static nature of typical devices that use IP, such as networked personal computers (PCs) and servers, DHCP has been widely used in the Intranet environment to allocate IP addresses dynamically to any hosts that are connected to a network.

However, it is clear that such an arrangement is unacceptable in a wireless domain when the communicating unit requiring an IP address, is not physically connected to the Internet. With such wireless technology, the subscriber unit needs to have previously established a logical connection with the Internet, in order to have been allocated an IP address and access Internet services, information and applications. This logical connection is generally referred to as a packet data protocol (PDP) context.

Furthermore, as wireless subscriber units will not be permanently connected to the Internet, there will be many occasions when the subscriber unit will be in a mode where no PDP context with the Internet has been established,

Due to the recent growth in data communication, particularly in Internet and wireless communications, there exists a need to provide TCP-IP data transfer techniques in a wireless communications domain.

An established harmonised cellular radio communication system is GSM (Global System for Mobile Communications) . An enhancement to this cellular technology can be found in the General Packet Radio System (GPRS) , which provides packet switched technology on a basic cellular platform, such as GSM. A further harmonised wireless communications system currently being defined is the universal mobile telecommunication system (UMTS) _r which is intended to provide a harmonised standard under which cellular radio communications networks and systems will provide enhanced levels of interfacing and compatibility with other types of communication systems and networks, including fixed communication systems such as the Internet .

Information to be transmitted across the Internet is packetised, with packet switching routes established between a source node and a destination node. Hence, GPRS and UMTS networks have been designed to accommodate packet switched data to facilitate Internet services, such as message service, information service, conversational service and casting service.

When a GPRS or UMTS user roams to a foreign network, in many cases the user needs to use the gateway GPRS service node (GGS ) function from the user's home network to access internet or intranet data, The traffic is transported across a Gp interface over an inter-PLMN backbone. Although, from a roaming support viewpoint, it would be better to use public IP addresses for the network elements such as the serving GPRS service node SGSN, the GGSN, and a Charging Gateway etc., notably in many cases Operators prefer to use private IP addresses.

A problem arises when private IP addresses are used for intra-FMN backbone operation. In this scenario, normal IP routing between two PMNs cannot be performed, as there is no unique address for the network elements between the two respective PLMNs,

A known solution to this problem, in general, is to deploy Network Address Translation (NAT) technology at a border gateway (BG) within the PL N. In this manner, the source addresses of IP packets from SGSN are translated at the BG to public IP addresses. The IP packets are then forwarded to a GGSN in another P MN.

However, this solution cannot be easily used at the BG when a GPRS transport protocol (GTP) is implemented as described in RFC 1631 (The IP Network Address Translator (NAT). , Egevang, P, Francis. May 1994,), Basically, NAT technology changes only the source and/or destination IP address in the header of an IP packet. NAT may also be configured to change the source and/or destination port numbers in the header of an IP packet.

However, as the SGSN address of a PDP context is negotiated by relevant GTP messages (i.e. "Create PDP context"^' and "update PDP context"), the NAT will fail to cope with the addresses when the SGSN and GGSN IP addresses are negotiated in the data packet payload using application layer protocols.

The common practise to work around this problem is to develop Application Layer Gateway (ALG) software, which interpret the relevant protocol messages. The ALG software is then able to intercept packets and modify the packet addresses if necessary. The ALG is normally combined with the BG and shares a common platform.

The inventors of the present invention have recognised significant limitations and problems in the use of ALG to resolve the addressing problem when a data packet is communicated between two PL Ns. In particular, a new product, i.e. an ALG for GfP operation, has to be developed. This means that standard NAT products cannot be used directly. Furthermore, the performance of the BG is seriously impacted, as the ALG would need to check each GTP packet and determine if it includes a target message. Additionally, the use of the ALG would increase the system latency, as each packet will be delayed whilst being processed by the ALG. Even worse, when encryption on GTP control (GTP-C) messages is performed, the ALG has no way to decode the GTP-C messages. To enable the ALG to decode such messages, extra functionality has to be incorporated into the ALG to deal with issues such as encryption key management. Thus, the impact on the - a -

performance makes an ALG-based solution particularly unattractive.

As a result, a need exists to provide a communication system, a communication unit and method of routing information wherein the abovementioned disadvantages may be alleviated.

STrømaxs? of th<≥ Invention

In a first aspect of the preferred embodiment of the present invention, a method of routing information in a communication system is provided, in accordance with Claim 1.

In a second aspect of the preferred embodiment of the present invention, a communication unit is provided, in accordance with Claim 11,

In a third aspect of the preferred embodiment of the present invention, a communication system is provided, in accordance with Claim 12,

In a fourth aspect of the preferred embodiment of the present invention, a serving communication unit is provided, in accordance with Claim 13,

In a fifth aspect of the preferred embodiment of the present invention, a gateway GPRS Service Node (GGSN) is provided, in accordance with Claim 17.

In a sixth aspect of the preferred embodiment of the present invention, a communication system is provided, in accordance with Claim 18. In a seventh aspect of the preferred embodiment of the present invention, a serving communication unit is provided, in accordance with Claim 24.

In accordance with an eighth aspect of the present invention, there is provided a storage medium, as claimed in Claim 25,

Further aspects of the present invention are as claimed in the dependent Claims.

In summary, the inventors of the present invention propose that, instead of relying on ALG to intercept and modify the relevant P messages, the functionality of the SGSN within the network is enhanced. In particular, a visited SGSN determines when a PDP context message is destined for an alternative network, and in response to such a determination replaces the home network's SGSN private address with the visited SGSN^/s public address, so that subsequent messages can be routed to the subscriber unit when supported by the visited SGSN.

Brief Description! of the Drawings

Exemplary embodiments of the present invention will now be described, with reference to the accompanying drawings, in which:

FIG. 1 illustrates an architecture involving intra-PLMN and inter-PLMN networks, adapted to support the preferred embodiments of the present invention; FIG. 2 is a block diagram illustrating the address interaction between an SGSN and a BG operably coupled to a NAT adapted to support the inventive concepts of the preferred embodiments of the present invention; and

FIG. 3 illustrates a block diagram and associated method to support a subscriber unit performing inter-PLMN roaming, in accordance with the inventive concepts of the preferred embodiments of the present invention.

Description of Pre e e Embodiments

Referring now to FIG, 1, an architecture involving intra- PLϊxiN and inter-PL N networks is illustrated, where the architecture is adapted to support the preferred embodiments of the present invention. The preferred embodiment of the present invention is described with reference to communication between two PLMNs (PLMN A 11Q and PLMN B 150) via an inter-PLMN backbone 140 and a packet data network 130. However, it is within the contemplation of the invention that the inventive concepts described herein are equally applicable to interaction and address manipulation between other network types.

Every intra-PLMN backbone network 120, 160, is a private IP network intended for packet domain data and signalling only. A private IP network is an IP network to which an access control mechanism is applied in order to achieve a required level of security. As shown, the two intra-PLMN backbone networks 120, 160 are connected via the Gp interface 124 using Border Gateways (BGs) 118, 158 and the inter- LMN backbone network 140. The particular inter- LMN backbone network 140 functions under a roaming agreement that includes the security functionality of the respective BGs 118, 158. The BGs 118, 158 are not defined within the scope of the packet domain. The inter-PLMN backbone 140 may be a Packet Data Network such as PDN 130. An example of the PDN 130 would be the public Internet or a leased line,

SG Ns 112, 114, 152 are operably coupled to respective GGSNs 116, IS6 and BGs 118, 158 via the respective intra- P1,MN backbones 120, 160, as known in the art.

In accordance with the preferred embodiments of the present invention, one or more SGSN 112, 114, 152 are adapted to provide enhanced features. Let us assume that a subscriber unit is registered with PLMN A 110, but has roamed into PLMN B 150. Furthermore, let us assume that the subscriber unit wishes to communicate and transmits a create PDP contex ' message to its currently serving SGSN 152. The SGSN 152 in PLMN A 150 processes the PDP context to determine if the target GGSN 116 belongs to another PLMN A 110. Preferably, checking the Access Point Name (APN) within the PDP context makes this determination. Notably, the SGSN (source node) is addressed using a private IP address, where each SGSN is aware of a public IP address associated with it.

Therefore, if the SGSN 152 determines that the target GGSN 116 does belong to another PLMN, (PLMN A 110), the SGSN 152 incorporates the public IP address for the "SGSN address" field within the ^Create PDP context" message. In this manner, the public ^"IP address will be used by the NAT function at BG 158. In a similar manner, during inter-SGSN handover of a data communication unit such as a GPRS unit, if the GGSN 116 associated with a PDP context belongs to another PLMN (PLMN A 110), the SGSN 152 again uses its public IP address for the "SGSN address* field in the "Update PDP context" message. In this way, subsequent data packets may be routed to the subscriber unit supported by SGSN 152.

As known, the NAT is configured with a static mapping facility to map between the public IP address and the private address for the respective SGSNs, as illustrated in FIG. 2. Referring now to FIG. 2, the mapping arrangement 200 is illustrated in more detail, but with regard to PLMN A 110.

An SGSN 112 within PLMN A 110 includes a private IP address (10.1.1,1) 212 and an associated public address (195.1.1.1) 214. The SGSN 112 communicates PDP context messages to its respective BG 118, including the private IP address (10.1.1.1) 212 and an associated public address {195.1.1.1) 214. The NAT 220, operating with the BG 118, performs standard network address translations using these private and associated public IP addresses 212, 214. in this manner, the BG is able to route messages to/from the respective SGSN.

Referring now to TIG. 3, a system architecture diagram 300 illustrates the particular process messages/steps used in accordance with the preferred embodiment of the present invention. In particular, FIG, 3 illustrates a preferred example of how inter-PLMN roaming, between PLMN A 110 and PLMN B 150, is supported. The relevant PLMN configurations are: PLMN A 110:

The home GGSN 116 has a private IP address (10,1.1.1) 212, and is associated with a public IP address (195.1.1.1) 214.

The BG/NAT 118 of PLMN A 110 has a (permanent) static mapping from the private IP address (10.1.1,1) 212 to the associated public IP address (195.1.1.1) 214.

PLΪ4N-B 150:

The visiting SGSN 152 also has a private IP address (10.1.1.1) 312, and is associated with a public IP address (196.1.1.1) 314.

The BG/NAT Of the visited PLMN B 150 has a (permanent) static mapping from (10.1,1,1) to (196.1.1,1).

A subscriber unit 310 associated with PLMN A 110 roams into PLMN B 150. The subscriber unit 310 requests, in tep 350, a PDP context indicating an APN in its home

PLMN A 110, Within PLMN B 150, the Visiting SGSN (VSGSN) 152 attempts to resolve the APN within the PDP context message to the IP address of the GGSN 116 to be used. In this regard, the VSGSN 152 checks, in step 355, with the local DNS server 330 associated with PLMN B 150.

If the local DNS server 320' does not include the required mapping, the local DNS server 330 sends a request to the DNS server 340 in PLMN-A 110. The request is, for example, based on the ^wOperator-lD" part of the APN, or "root" of the ".gprs" domain. Such requests can be supported by, for example, GSM Association, as known to those skilled in the art. The local DNS server 320 eventually resolves the mapping from the APN to the IP address of the DNS server 330 of the home GGSN (HGGSN) 116 in PLMN A 110, The local DNS server 320 and the home DNS server 330 preferably use the standard address resolution protocol (ARP) to inform the VSGSN 152 of the APN.

The VSGSN 152 then sends a ^ΛCreate PDP Context"^" request to the HGGSN 116. Notably, the VSGSN 152 processes the "Create PDP Context" request received from the subscriber unit and determines that the identified GGSN 116 belongs to another PLMN (PLMN A 110). In this regard, in implementing the preferred embodiment of the present invention, the SGSN includes a receiver portion (not shown) and a transmitter portion (not shown) for receiving and transmitting messages from/to other network elements or subscriber units. Furthermore, the SGSN includes one or more processors, for example digital signal processors or processing boards, to process and interpret signals/messages. The SGSN processor (s) is also operably coupled to a memory element (not shown) to store address data.

More generally, the adaptation of one or more SGSN to implement the aforementioned inventive concepts may be effected in any suitable manner. For example, new apparatus may be added to a conventional SGSN or alternatively existing parts of a conventional SGSN may be adapted, for example by reprogra ming one or more processors therein. As such, the required adaptation may be implemented in the form of processor-impiementable instructions stored on a storage medium, such as a floppy disk, hard disk, PROM, RAM or any combination of these or other storage multimedia. Thus, VSGSN 152 incorporates its public IP address ' (196.1.1.1) 312 into the value of SG N Address" field. This message, as a GTP message, is then sent to the address (195.1.1,1) 214 of the GGSN 116, in step 375. The message is routed via BG B 158, in step 365, that translates the source address. It is also routed via BG- A, in step 370, which translates the destination address ⁽from SGSN s public address (195.1.1.1) 214) to the home SGSN 112 private address (10,1,1.1) 212 of PLMN A 110.

The GGSN 116 records the VSGSN address (196.1.1.1) 312 as part of the PDP context. After the PDP context has been set up, the GGSN 116 is now able to forward data packets using the GPRS transport protocol (GTP) to the subscriber unit 310, in step 390. For example, let us consider downlink GTP packets addressed to the subscriber unit 310. The GTP packet is sent to VSGSN 152 using the public address (196.1.1,1) 314 of VSGSN 152. BG A 118 is able to replace the source address of SGSN 112, in _Step 380. BG B 158 then translates the destination address from the public address (196.1.1.1) 314 of VSGSN 152 to PLMN B's 150 internal (private) address (10.1.1.1) 312, in step 385,

In this manner, GTP data packets can be routed between ' PLMNs, for example for a roaming subscriber unit, without incurring the addressing problems that currently require development of specific ALGs.

Advantageously, the above-mentioned inventive concepts can be incorporated as enhancements on the SGSN using a software upgrade, by re-programming one or more processors as described above. - - A key benefit of the above-mentioned addressing methodology is that it allows the use of private address space for most addressing needs within a PLMN's network infrastructure. This minimises the use of public IP addresses, as only a few network components that are directly involved in inter-PLMN communication (including SGSN, GGSN and DNS server) are allocated with public IP addresses. Although the invention has been described with reference to inter-PLMN communication using GTP messages, with the address translation performed by the SGSN instead of the NAT, it is envisaged that the inventive concepts are equally applicable to any other wireless communication system supporting roaming of data communication units.

It will be understood that the mechanism for resolving non-unⁱque addresses between two networks, as described above, additionally provides at least the following advantages:

⁽i) No new (ALG) product needs to be developed. Only currently available technology is required, such as an off-the-shelf NAT product used in combination with a BG and enhanced SGSN functionality.

⁽ii⁾ There is _no impact on the BG performance, as the BG does not need to perform any additional functions such as provision of ALG,

⁽iii⁾ Encryption, for example on GTP-C, can be used without any limitation.

(iv⁾ There is no impact on the standardisation programs.

⁽v⁾ The enhancement to the SGSN functionality can be performed using software upgrade. The present invention finds particular application in wireless communication systems such as the UMTS or GPRS systems, employing GTP for packet data communication. However, a skilled person would readily recognise that the inventive concepts contained herein are equally applicable to alternative fixed and wireless communications systems.

Whilst the specific, and preferred, implementations of the present invention are described above, it is clear that one skilled in the art could readily apply variations and modifications of such inventive concepts.

Thus, a communication system, serving communication units and a method of routing information between communication units, has been provided that alleviates some of the above entioned disadvantages .

Claims

1 A method of routing information in a communication system that includes a serving communicating unit serving a plurality of subscriber units with a communication resource, wherein said serving communication unit is identified within the communication system by both a public address and a private address, the method comprising the steps of: roaming from a home communication network ₍lio₎ to a visited communication network (150) by _a subscriber unit (310); requesting ⁽350⁾ a communication resource from a serving communication unit in said visited communication network (150⁾, by said subscriber unit (310₎ using a general packet radio system (GPRS) transport protocol ⁽GTP⁾ formatted message; the method characterised by the steps of: processing said request by said visited serving communication unit ⁽152) in order to extract a serving communication unit private address; determining that said serving communication unit private address identifies a serving communication unit of a different communication network ₍110) ; incorporating, in response to such a determination, a public address (312) of said visited serving communication unit (152) in said request_; and forwarding (375), by said visited serving communication unit (152), said request for a communication resource to a serving communication unit (112) in said home communication network ₍110₎ of said subscriber unit.

2. The method of routing information in a communication system according to Claim 1, wherein said step of roaming, includes roaming, by said subscriber unit (310⁾ from a home public land mobile network (110) to a visited public land mobile network (150) that identifies a number of communication elements using private addresses.

3. The method of routing information in a communication system according to Claim 2, wherein said step of requesting (350) a communication resource includes sending a ^create packet data protocol (PDP) context' GTP message to said home public land mobile network (110) »

4. The method of routing information in a communication system according to Claim 3, wherein said step of sending a "create PDP context' message is sent to an access point name <APN) in the home public land mobile network (110) ,

5. The method of routing information in a communication system according to Claim 4, the method further characterised by the step of: attempting to resolve said APN within said PDP context message to a public address of a home serving communication unit (156).

6. The method of routing information in a communication system according to Claim 5, wherein said step of attempting includes sending a request for address information from a visited domain name server (320) to a home domain name server (340) located in said home public land mobile network (110) . ^*?. The method of routing information in a communication system according to any preceding Claim, wherein said step of processing said request includes processing said request to determine a private address of a home serving general packet radio system (GPRS₎ service node,

8. The method of routing information in a communication system according to any preceding Claims, wherein said step of forwarding includes sending the request message, as a GPRS Transport Protocol message to a home gateway GPRS Service Node (HGGSN) (116₎ to record said public address of said visited serving communication unit (152) as part of said request message.

9. The method of routing information in a communication system according to any preceding Claims, the method further characterised by the steps of_; routing ⁽370) a message from said home communication network (110) to said subscriber unit via a border gateway (BG⁾ in said visited communication network (150); and translating ⁽385), by said BG, a destination address of said message from said public address of said visited serving communication unit (152) to its private address.

10. method of routing information in a communication system according to any preceding Claim, wherein said communication system supports^' the universal mobile telecommunication standard (UMTS) or general packet radio system ⁽GPRS) communication and said public address is an Internet Protocol address. 11, A communication unit adapted to perform the steps of any of method claims 1 to 10,

12. A communication system, for example one supporting a universal mobile telecommunication standard ⁽UMTS⁾ or a general packet radio system ^GPRS), adapted to facilitate the operation of the steps of any of Claims 1 to 10.

13. A serving communication unit (152) for serving a plurality of subscriber units with one or more communication resources, wherein said serving communicating unit is identified by both a public address and a private address, said serving communication unit (152) comprising: a receiver for receiving a general packet radio system (GPRS) transport protocol (GTP) formatted message request (350) for a communication resource from a subscriber unit (310); the serving communication unit (152) characterised hy a processor that performs at least the following functions: processing said request in order to extract a serving communication unit private address; determining whether said serving communication unit private address identifies a different serving communication unit (112) of a different communication network (110) ; incorporating, in response to such a determination, its public address (312) in said request; and a transmitter to forward (375) sai request for a communication resource to said different serving communication unit (112).

14^, The serving communication unit (152) according to Claim 13, wherein said serving communication unit ₍152₎ operates in a public land mobile network ₍150₎ ,

15. The serving communication unit (152) according to Claim 13 or Claim 14, wherein said communication resource request is a ^create packet data protocol (PDP₎ context' message.

16. The serving communication unit (152) according to any of preceding Claims 13 to 15, wherein said serving communication unit ⁽152) is a serving general packet radio system ⁽GPRS) service node (SGSN), for example supporting universal mobile telecommunication standard ⁽UMTS⁾ or general packet radio system ₍GPRS₎ communication.

17. A gateway general packet radio system (GPRS₎ Service Node ⁽GGSN⁾ (ll₆) adapted to receive a message, for example a GPRS Transport Protocol message, having a public address of said serving communication unit ₍152₎ and configured to record said public address of said serving communication unit (152) as part of said message.

18. A communication system comprising a plurality of networks having respective serving communication un_its that serve a plurality of subscriber units with one or more communication resources, wherein a number of said plurality of serving communication units are identified by both a public address and a private address and said communication system supports roaming of subscriber units between different networks, wherein a subscriber unit ⁽310⁾ roams from a hom& communication network (110) to a visited communication network (150) and requests (350) a communication resource from a visited serving communication unit using a general packet radio system ⁽GPRS) transport protocol (GTP) formatted message, said communication system characterised in that; said visited serving communication unit (152) processes said request in order to extract a serving communication unit private address and determines that said serving communication unit private address identifies a serving communication unit (112) of a different communication network (110), and in response to such a determination, said visited serving communication unit ⁽152⁾ incorporates a public address (312) of said visited serving communication unit (152) in said request; in order that data can be routed to said subscriber unit via said visited serving communication unit (152).

19. The communication system according to Claim 18, said communication system further characterised by said subscriber unit (310) roaming from a home public land mobile network (110) of said subscriber unit (310) to a visited public land mobile network (150).

20. The communication system according to Claim 18 or Claim 19, said communication system further characterised by said message from said subscriber unit (310) being a 'create packet data protocol (PDP) context' message transmit to a home public land mobile network (110) .

21. The communication system according to any of preceding Claims 18 to 20, said communication system further characterised ^'by said serving communication unit being a serving general packet radio system (GPRS) service node.

22. The communication system according to any of preceding Claims 18 to 21, said communication system further characterised by said serving communication unit being operably coupled to a border gateway employing a network address translation function such that said border gateway (BG) in said visited communication network ⁽150) routes a subsequent message transmit from said home communication network (110) to said subscriber unit (130) and translates (385) a destination address of said - subsequent message from a public address of said visited serving communication unit (152) to a private address.

23. The communication system according to any of preceding Claims 18 to 22, wherein said communication system supports the universal mobile telecommunication standard (UMTS) or general packet radio system (GPRS) communications .

24. A serving communication unit (152, 156) for operating in the communications system of any one of claims 18 to 23.

25. A storage medium storing processor-implementable instructions for controlling a processor to carry out the method steps of any of Claims 1 to 10, or facilitate an operation of the serving communication unit (152, 156) of Claims 13 to 17 or Claim 24.