(19)
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`(12)
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`Europäisches Patentamt
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`European Patent Office
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`Office européen des brevets
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`*EP001009176A2*
`EP 1 009 176 A2
`
`(11)
`
`EUROPEAN PATENT APPLICATION
`
`(43) Date of publication:
`14.06.2000 Bulletin 2000/24
`
`(21) Application number: 99309583.5
`
`(22) Date of filing: 30.11.1999
`
`(51) Int Cl.7: H04Q 7/24
`
`(84) Designated Contracting States:
`AT BE CH CY DE DK ES FI FR GB GR IE IT LI LU
`MC NL PT SE
`Designated Extension States:
`AL LT LV MK RO SI
`
`• Chuah,Mooi Choo
`Eatontown, New Jersey 07724 (US)
`• Yue, On-Ching
`Middletown, New Jersey 07748 (US)
`
`(30) Priority: 07.12.1998 US 206428
`
`(71) Applicant: LUCENT TECHNOLOGIES INC.
`Murray Hill, New Jersey 07974-0636 (US)
`
`(72) Inventors:
`• Budka, Kenneth Carl
`Marlboro, New Jersey 07746 (US)
`
`(74) Representative:
`Watts, Christopher Malcolm Kelway, Dr. et al
`Lucent Technologies (UK) Ltd,
`5 Mornington Road
`Woodford Green Essex, IG8 0TU (GB)
`
`(54) Methods and apparatus for route optimisation in a communications system
`
`(57)
`A route optimization technique in a GPRS net-
`work includes establishing a gateway GPRS support
`node in a visiting public land mobile network in which a
`roaming mobile station is currently located. Specifically,
`a tunnel is formed between the gateway GPRS support
`node and a serving GPRS support node to which the
`mobile station is in direct communication over a radio
`link. In this manner, external corresponding hosts may
`route packets to the gateway GPRS support node, rath-
`
`er than the GPRS support node in the mobile station's
`home public mobile network, as is done in conventional
`GPRS networks. Advantageously, a shorter path is es-
`tablished for transfer of packets between a mobile sta-
`tion and a corresponding host. A similar route optimiza-
`tion technique is provided in a CDPD network, wherein
`a home mobile data-intermediate system node (local
`HMD-IS) in the foreign (visiting) network serves as a
`gateway node to the roaming mobile-end system.
`
`Printed by Jouve, 75001 PARIS (FR)
`
`EP1 009 176A2
`
`1
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`HTC EXHIBIT 1007
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`Page 1 of 31
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`EP 1 009 176 A2
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`Description
`
`Field of the Invention
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`[0001] The present invention relates to methods and apparatus for providing mobility management in packet-based
`communications systems and, more particularly, to route optimization in General Packet Radio Service and Cellular
`Digital Packet Data systems.
`
`Background of the Invention
`
`[0002] Within the last decade, advances in computer systems, wireless communications and data networking have
`brought mobile data networking within reach of the masses. Mobile data networking enhances data applications such
`as, for example, email, client-server applications, electronic form, order entry systems, and other wired-line data com-
`munications applications. Mobile data adds a new dimension to Internet applications, a dimension developers of a new
`breed of mobility-empowered applications are beginning to probe.
`[0003]
`Introduction of Enhanced Throughput Cellular (ETC), MNP10, and other data link protocols specially tailored
`to the cellular environment have placed data rates on the order of 10 kbps. The high connect time charges characteristic
`of circuit switched cellular service, however, are not always well-suited for the bursty data transfers typical of many
`data applications. Addressing this need, the cellular industry has developed two wireless packet data systems to sup-
`port mobile computing with greater multiplexing efficiency. Cellular Digital Packet Data (CDPD) was designed as an
`overlay data network to the Advanced Mobile Phone System (AMPS), while General Packet Radio Service (GPRS)
`was developed for the Global System for Mobile Communications (GSM). The design of such wireless packet data
`networks spans the physical layer (frequency allocation, modulation and coding), the link layer (medium access control,
`error recovery and flow control), and the network layer (e.g., Internet Protocol (IP)).
`[0004] Mobility management encompasses the tracking of mobile hosts as they move throughout a network and all
`interworking functions which mask mobility from Internet applications. Mobility management is one of the cornerstones
`of current and future wireless data networks. To satisfy the mobile users need for more bandwidth and more services,
`there are new wireless standards being proposed and evaluated, including PDC mobile packet data communication
`system (PDC-P) based on the Personal Digital Cellular system (PDC) in Japan, and Universal Mobile Telephone Service
`(UMTS). Understanding and contrasting the mobility management approaches used by current networks can help
`identify opportunities for improvement, improvements which may be incorporated into existing and future wireless data
`networking technologies.
`[0005] There are currently three mobility management approaches: the proposed Mobile-IP protocol developed by
`the Internet Engineering Task Force (IETF), CDPD and GPRS. Certain salient features are shared by all three mobility
`management approaches. A discussion of their respective approaches used to provide various mobility management
`features will now follow.
`
`I. Mobile IP
`
`[0006] An overview of the basic IETF Mobile IP protocol is described in IEFT RFC2002, "IP Mobility Support," C.
`Perkins (ed.), October 1996. The IETF Mobile IP protocol is not a complete mobility management solution: it merely
`provides a network layer solution. At a high level, the basic IETF Mobile IP sets up routing entries at appropriate nodes
`in the network to route packets to mobile hosts.
`[0007] Referring to FIG. 1A, a block diagram of the Mobile IP architecture is shown. There are four network entities
`in a network that supports IETF Mobile IP:
`
`• Mobile Host (MH) 2: A host or router that changes its point of attachment from one subnetwork to another. A mobile
`host may change its location without changing its IP address.
`
`•
`
`•
`
`•
`
`Home Agent (HA) 4: A router in the mobile host's home network 10 which tunnels datagrams for delivery to the
`MH when it is away from home. The HA maintains current location information for the mobile host.
`
`Foreign Agent (FA) 6: A router in a mobile host's visiting or foreign network 12 which provides routing services to
`the mobile host while registered. The foreign agent delivers datagrams to the mobile host that were tunneled by
`the home agent.
`
`Corresponding Host (MH) 8: A host or router with which a mobile host may communicate.
`
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`EP 1 009 176 A2
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`[0008] Mobile IP's protocol stack is shown in FIG. 1B. Key features of the protocol stack are:
`
`•
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`•
`
`•
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`Transport Layer - No assumptions on transport protocol were made during the design of Mobile IP.
`
`Network Layer - Mobile IP provides only native support of only IP. Mobiles are assigned a fixed home address by
`Mobile IP service providers.
`
`Link and Physical Layers - Mobile IP makes no assumption regarding the link and physical layers. It only requires
`a direct link between the foreign agent and the mobile host.
`
`[0009] The basic Mobile IP uses triangular routing to send forward IP packets to roaming mobile hosts. Each mobile
`host is assigned a unique home address. Hosts communicating with a mobile host (MH) are known as the corresponding
`or correspondent hosts (CH). In sending an IP packet to a mobile host, a corresponding host always addresses the
`packet to the mobile host's home address, regardless of the location of the mobile.
`[0010] Each mobile host must have a home agent (HA) on its home network that maintains the mobile host's current
`location. This location is identified as a care-of address, and the association between a mobile host's home address
`and its current care-of address is called a mobility binding. Each time the mobile host obtains a new care-of address,
`it must register the new binding with its home agent so that the home agent can forward upcoming traffic destined for
`the mobile host that serves.
`[0011] A mobile host, when connecting to a network away from its home network, may be assigned a care-of address
`in one of two ways:
`
`•
`
`•
`
`using Foreign Agent's IP address
`
`obtaining local address via a Dynamic Host Configuration Protocol (DHCP) server. DHCP consists of a protocol
`to deliver host specific configuration parameters from a DHCP server to a host and a mechanism for allocation of
`network addresses to hosts.
`
`Using a Foreign Agent's IP Address
`
`[0012] Normally, the mobile host will attempt to discover a foreign agent within the network being visited using an
`agent discovery protocol. The agent discovery protocol operates as an extension to the existing ICMP (Internet Control
`Message Protocol) router discovery protocol. It provides a means for a mobile host to detect when it has moved from
`one network to another, and to detect when it has returned home. The mobile host then registers with the foreign agent
`and one of the foreign agent's IP addresses is now used as the mobile host's care-of-address. The foreign agent acts
`as a local forwarder for packets arriving for the mobile host.
`
`Using a Temporarily Assigned Local Address
`
`[0013] Alternatively, if the mobile host can obtain a temporary local address within the visiting network, the mobile
`host may use this temporary address as its care-of address. This care-of address is referred to as the co-located care-
`of address. The mobile host in this case will register this co-located care-of address directly with the home agent.
`
`Data Forwarding
`
`[0014] When a mobile host is away from its home network, a mobile-host's home agent uses proxy ARP to intercept
`packets addressed to the mobile host. By proxy ARP, we mean the home agent will answer the ARP request sent to
`the home link on behalf of the mobile host. Then, the home agent forwards all packets for the mobile host to its current
`location. The home agent achieves this by tunneling each intercepted packet to the mobile host's current care-of ad-
`dress. By tunneling, we mean a new IP header is added to the original IP packet such that the source address is the
`home agent's address, the destination address is the mobile host's current care-of address.
`[0015]
`If the care-of address is provided by a foreign agent, the foreign agent removes any tunneling headers from
`the packet and delivers the packet locally to the mobile host by transmitting it over the local network on which the
`mobile host is registered. If the mobile host is using a locally obtained temporary address as a care-of address, the
`tunneled packet is delivered directly to the mobile host. The mobile host is expected to remove the tunnel headers
`before interpreting the content.
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`Beaconing Protocol: Agent Advertisement
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`EP 1 009 176 A2
`
`[0016] Home and foreign agents periodically advertise their presence by broadcasting an agent advertisement mes-
`sage on each network to which they are connected and for which they are configured to provide service. Home and
`foreign agents may be provided by separate nodes on a network. Alternatively, a single node may implement the
`functionality of both a home and a foreign agent.
`[0017] By listening to the periodic agent advertisements, a mobile host can determine if it is currently connected to
`its home or a foreign link, and whether it has moved from one link to another. In addition, a mobile host can also send
`Agent Solicitation messages to force any agents on the same link as the mobile host to immediately transmit an Agent
`Advertisement.
`[0018] Agent Advertisements and Agent Solicitations are extensions to the Router Advertisements and Router So-
`licitations messages as defined in IEFT RFC 1256, "ICMP Router Discovery Messages," S. Deering (ed.), September
`1991. ICMP Router Advertisement messages contain a list of router addresses and their preference values that any
`host at the same link can use as a default router. ICMP Router Advertisement messages are periodically broadcast.
`However, a host can solicit for ICMP Router Advertisement by sending a Router Solicitation message. Agent Solicitation
`messages look exactly the same as Router Solicitation messages except that the Time-to-Live filed is set to one. Agent
`Advertisement messages are longer than Router Advertisement messages because of the presence of Mobility Agent
`Advertisement extension. A host can use the IP total length field, the number of addresses and address entry size
`fields to determine if the received ICMP message is a Router Advertisement or an Agent Advertisement.
`[0019] There are two methods by which mobile hosts can determine that they have moved. The first method is using
`the lifetime field within the ICMP Router Advertisement portion of an Agent Advertisement. If a mobile host is registered
`with a foreign agent, and fails to hear an advertisement from that agent within the specified lifetime, then the mobile
`host can assume that it has moved. The second method for move detection uses network-prefixes. The mobile host
`compares the network prefix of the newly heard advertisement with that of the foreign agent with which it has registered.
`If they differ, the mobile host concludes that it has moved.
`[0020] For mobile hosts that use collocated care-of addresses, the mobile hosts can put their network-interface
`drivers into promiscuous mode. In this mode, a mobile host examines all packets on the link. If none of the packets
`flying across the link have network-prefixes that equal the mobile host's current collocated care-of address, then the
`mobile host may infer that it has moved and should acquire a new care-of address.
`
`Mobile Registration
`
`[0021] Mobile IP registration consists of an exchange of Registration Request and Reply messages. A registration
`message is carried within the data portion of a UDP packet. In Mobile IP, a registration is initiated by the mobile host.
`A registration is used by a mobile host for:
`
`•
`
`•
`
`•
`
`•
`
`requesting data forwarding services from a foreign agent
`
`informing its home agent of its current location
`
`renewing a registration which is due to expire
`
`de-registering the mobile host when it returns to its home link.
`
`[0022] A mobile host can register directly to the home agent or via the foreign agent. A Registration Request message
`is sent by a mobile host to begin the registration process. If the registration is via the foreign agent, the foreign agent
`examines the message and relays it to the home agent.
`[0023] The home agent and mobile host authenticates one another via the mandatory Authenticator field within the
`Mobile-Home Authentication Extension which is part of the Registration messages. The Mobile-Foreign Authentication
`Extension is an optional feature in IEFT RFC2002, "IP Mobility Support," C. Perkins (ed.), October 1996.
`[0024]
`If the home agent accepts the Registration Request, it will update the mobile host's binding entry according
`to the specified care-of address, mobile host's home address, and the registration lifetimes. Then, the home agent
`sends a Registration Reply to inform the mobile host whether or not the attempted registration is successful. If the
`registration is done via the foreign agent, the foreign agent updates its list of known visiting mobile hosts and relays
`the Registration Reply to the mobile host. If a mobile host does not receive a Registration Reply within a reasonable
`period of time, then the mobile retransmits the Registration Request a number of times.
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`Data Forwarding to Mobile Host
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`EP 1 009 176 A2
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`[0025] Referring now to FIG. 1C, a block diagram of a Mobile IP network for illustrating data forwarding is shown.
`The home agent 4 intercepts packets destined to the home address of a registered mobile host 2 by advertising reach-
`ability to the mobile host's home address. Alternatively, the home agent can use gratuitous and proxy ARP. When a
`home agent receives a Registration Request message from a mobile host, it uses gratuitous ARP (unsolicited ARP
`reply) to inform hosts in the same home link that the current mapping in their ARP cache needs to be modified to reflect
`the mobile host's new link-layer address to be that of the home agent. After the mobile host's successful registration,
`the home agent is supposed to reply to any ARP request on behalf of the mobile host. Such an ARP reply is called a
`proxy ARP.
`[0026] All home agents 4 and foreign agents 6 are required to implement IP-in-IP Encapsulation (e.g., as described
`in "IP Encapsulation Within IP," C. Perkins, October 1996) for tunneling purposes. In addition, they may implement
`Minimal Encapsulation (e.g., as described in "Minimal Encapsulation Within IP," C. Perkins, October 1996) and Generic
`Routing Encapsulation (e.g., as described in "Generic Routing Encapsulation (GRE)," S. Hanks, R. Li, D. Farinacci,
`P. Traina).
`[0027] When the home agent receives a packet destined to one of its mobile hosts, it looks up the corresponding
`bindings. The home agent then tunnels the packet to the care-of address. The encapsulated inner packet is from the
`corresponding host 8 to the mobile host's home address. In the case of the foreign care-of address, when the foreign
`agent receives the tunneled packet, it removes the outer packet to recover the original inner packet. It sees that the
`destination address is that of a registered mobile host, looks up the appropriate interface, and sends the packet to the
`mobile host. In the case of collocated care-of address, the mobile host performs similar processing upon receiving the
`tunneled packet.
`
`Data Forwarding From Mobile Host
`
`[0028]
`If a mobile host registers via a foreign agent, the mobile host can either select the foreign agent as its router
`or any router whose address appears in the Router Address fields within the ICMP Router Advertisement portion of
`any node's Agent Advertisements or Router Advertisements.
`[0029] A mobile host that registers a collocated care-of address on a foreign link can use any of the addresses listed
`in the Router Address fields of the ICMP Router Advertisements if the mobile host can hear any Router Advertisements.
`Otherwise, it can rely on the same mechanism by which it acquires its collocated care-of address to provide the address
`of a suitable router.
`
`II. Cellular Digital Packet Data
`
`[0030] The Cellular Digital Packet Data system was designed as an overlay data network to existing 800 MHz cellular
`Advanced Mobile Phone System (AMPS) Networks. Typical maximum network layer throughputs are on the order of
`12 kbps per mobile, which serves as an airlink well-suited for the bursty traffic generated by light-weight client-server
`applications.
`[0031] Referring to FIG. 2A, a block diagram of a CDPD network is shown. At a high level, CDPD's network archi-
`tecture bears a strong resemblance to 800-MHz analog cellular Advanced Mobile Phone networks CDPD networks.
`To keep network deployment and operations costs low, CDPD's network side RF transmitters and receivers were
`designed to reuse much of an existing cellular voice network's infrastructure: antennae towers, RF amplifiers, cell site
`enclosures, and cell site - Mobile Telephone Switching Office trunks. The overlay architecture allows existing cellular
`service providers to leverage their sizable investment in voice infrastructure.
`[0032] The CDPD network is constructed from the following building blocks:
`
`•
`
`•
`
`•
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`CDPD's subscriber device, the Mobile-End Systems (M-ESs) 20: RF subsystem circuitry in the M-ES perform
`CDPD's Gaussian Modulated Shift Keying modulation over AMPS channels. Additional M-ES hardware and soft-
`ware run the CDPD protocol stack and user interface.
`
`CDPD's network side RF termination, the Mobile Data Base Stations (MDBSs) 22: The MDBS is responsible for
`CDPD radio resource management, termination of the reverse (M-ESfi network) link Medium Access Control pro-
`tocol, and relaying link layer frames to and from M-ESs. The MDBS is also responsible for the periodic broadcasting
`of CDPD-specific system information messages which inform M-ESs of network timers, protocol parameters, and
`system configuration information.
`
`CDPD's mobility-aware network layer router, the Mobile Data-Intermediate Systems (MD-ISs) 24: The Mobile Data-
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`EP 1 009 176 A2
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`Intermediate System masks M-ES mobility from mobility-unaware applications. The MD-IS delivers network layer
`packets to M-ESs, collects data used for usage accounting, mobility management.
`
`•
`
`Network Routers 26: The network router 26 is coupled to the MD-IS via a high speed data link in order to provide
`communications paths between the MD-IS and private networks 28, the Internet 30, and other CDPD service
`providers 32.
`
`[0033] CDPD networks also require a number of network support services - usage accounting, M-ES authentication,
`network management. For service interoperability, the CDPD specification, CDPD System Specification, Release 1.0,
`July 19, 1993, spells out standard interfaces for these support services.
`[0034] CDPD's protocol stack is shown in FIG. 2B. Key feature of the protocol stack are:
`
`•
`
`•
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`•
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`Network Layer - CDPD provides native support of IP and CLNP. Mobiles are assigned a fixed network layer address
`by CDPD service providers. To date, all M-ESs use IP. CLNP networks are used by MD-ISs for the exchange of
`control messages, forwarding of packets to roaming Mobiles, dissemination of raw accounting data and network
`management.
`
`Subnetwork Dependent Convergence Protocol (SNDCP) - The CDPD protocol stack was designed to make effi-
`cient use of airlink bandwidth. Compression of TCP/IP uses Van Jacobsen header compression. Header compres-
`sion is also defined for CLNP headers. Optional V.42bis compression is supported to compress payloads of SNDCP
`packets.
`
`Link Layer - CDPD's Mobile Data Link Protocol (MDLP) is similar to HDLC. Selective rejects are defined for efficient
`retransmission.
`
`CDPD Cell Selection
`
`[0035] Before a M-ES can register, it searches for an AMPS channel carrying a CDPD channel stream that is strong
`enough to lock on. Digital signatures sent over the forward link are used by the M-ES to determine that an AMPS
`channel has a CDPD channel stream on it. After locking on to a CDPD channel stream, the M-ES measures the block
`error rate of the forward channel. If the M-ES finds the measured block error rate acceptable, the M-ES listens to the
`forward channel for a CDPD channel Identification Message, a CDPD system overhead message containing the logical
`address of the CDPD channel stream and other configuration information.
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`[0036] Before a M-ES can gain access to the CDPD Network, the M-ES must register. By registering, a M-ES informs
`the CDPD network of the current CDPD channel that the M-ES is listening to, thereby allowing the CDPD network to
`forward any packets bound for the M-ES to the correct cell and CDPD channel. In addition, registration serves as a
`first line of defense against fraudulent network usage. During registration, a M-ES sends encrypted messages to the
`CDPD network containing shared secrets the network uses to authenticate the user. M-ESs presenting invalid creden-
`tials will be denied access to the CDPD network.
`[0037] During registration, a number of messages are transferred between the M-ES, the home and serving MD-
`ISs, and other CDPD network element. FIG. 2C shows a message flow diagram for a typical successful M-ES regis-
`tration attempt.
`[0038] After receiving a Channel Identification message, the M-ES sends a request for a Terminal Endpoint Identifier
`(TEI), a link layer address that will be used to identify link layer frames sent to and from the MD-IS. The TEI request
`message is received by the MDBS and forwarded to the Serving MD-IS. The serving MDIS generates a TEI for the M-
`ES, and sends the value to the M-ES. The MD-IS begins the Diffie-Hellman key exchange by sending an Intermediate
`System Key Exchange (IKE) message to the M-ES. The M-ES then responds with an End System Key Exchange
`(EKE) message. After this point, all communication between the Serving MD-IS and the M-ES is encrypted. To request
`access to the CDPD network, the M-ES sends an End System Hello message containing the M-ES's IP or CLNP
`address and its credentials. The Serving MD-IS forwards the credentials to the M-ES home MD-IS via a CLNP network
`(if the M-ES is roaming). The Home MD-IS compares the M-ES's credentials with those stored in a database, and
`responds to the Serving MD-IS whether access should be granted. The serving MD-IS sends an Intermediate System
`Confirm (ISC) message to the M-ES indicating whether the M-ES may begin transmitting and receiving data over the
`CDPD network.
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`Data Transfer
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`EP 1 009 176 A2
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`[0039] Referring to FIG. 2D, a block diagram of a CDPD network illustrating network data flow is shown. CDPD uses
`triangular routing to send forward IP packets to roaming M-ESs 20. Each M-ESs IP address maps to a home MD-IS
`24A. The home MD-IS keeps track of the serving MD-ISs 24B of all its homed M-ESs. Packets sent to M-ESs using a
`serving MD-IS that is not their home are routed to the M-ES's home MD-IS. The home MD-IS then forwards the traffic
`to the serving MD-IS over CLNP tunnels. This way of forwarding traffic means that all MD-ISs in a CDPD network need
`to know the CLNP addresses of the home MD-ISs for all mobiles they offer service to. Carriers with roaming agreements
`share this information. The IP-CLNP mapping is maintained manually. Reverse IP packets sent by roaming M-ESs
`follow the normal IP/CLNP routing.
`[0040] Overhead messages sent over the forward link of each CDPD channel give the channel identity as well as
`the identity of the cellular service provider. Additional overhead messages inform the M-ESs of where to find CDPD
`channels on neighboring cells to assist in handoffs.
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`[0041] General Packet Radio Service (GPRS) is the packet data service developed by the European Telecommuni-
`cations Standards Institute (ETSI) for the Global System for Mobile Communications (GSM). The GSM/GPRS standard
`is found in GSM 03.60: Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS);
`Service Descriptions, stage 2, Version 5.3.0, 1998.
`[0042] Referring to FIG. 3A, a block diagram of a GPRS network architecture is shown. In the GPRS architecture,
`there are four logical elements: mobile stations (MS) 40, base station subsystems (BSS) 42, location register: visiting
`location register (VLR) 44 and home location register (HLR) 46, and GPRS support nodes: serving GPRS support
`node (SGSN) 48 and gateway GPRS support node (GGSN) 50. FIG. 3A illustrates the case of an MS roaming away
`from its home public land mobile network (PLMN) 52 into a visiting PLMN 54. The GSN connected to the MS is called
`the serving GSN (SGSN) 48 which has access to the visiting location register (VLR) 44 located in a mobile switching
`center or MSC (not shown). However, the MS is registered at the home location register (HLR) 46 which can be ac-
`cessed by the gateway GSN (GGSN) 50. A corresponding host (CH) 56 in a packet data network (PDN) 58 sends the
`IP packet to the MS through the GGSN first.
`[0043] Referring to FIG. 3B, a GPRS protocol stack is shown. The packet data network (PDN) is an IP network
`providing connectivity from the corresponding host (CH) to the gateway GSN (GGSN). Between GGSN and the serving
`GSN (SGSN), IP packets are transported via the GRPS tunneling protocol (GTP), GSM 09.60: Digital Cellular Tele-
`communications System (Phase 2+); General Packet Radio Service (GPRS); GPRS Tunneling Protocol (GTP) Across
`the Gn and Gp Interface, which is used for both data and signaling. The network connecting the GSNs within a PLMN
`and between PLMNs is a private IP network. In the case of IP packets encapsulated by GTP, UDP (User Datagram
`Protocol) is used to carry the GTP PDUs (Protocol Data Units). At the SGSN, the original IP packet is recovered and
`encapsulated again according to the subnetwork dependent convergence protocol (SNDP) for transporting to the MS.
`The logical link control (LLC) between the SGSN and the MS provides a highly reliable connection. The base station
`system GPRS protocol (BSSGP) is used to convey the routing and Quality of Service-related information between the
`SGSN and the BSS. In the BSS, the LLC PDUs are recovered and sent to the MS using the radio link control (RLC)
`function.
`
`GPRS Cell Selection
`
`[0044]
`In a GPRS network, the cells are organized into routing areas (RA), which are in turn grouped into location
`areas (LA). When an MS wishes to use the GRPS service, it first performs the GPRS Routing Area and GPRS cell
`selections. These selections are done autonomously by the MS using procedures similar to GSM phone subscribers.
`The procedures include the measurement and evaluation of signal quality from nearby cells, and the detection and
`avoidance of congestion within candidate cells. The base station system (BSS) can also instruct the MS to select a
`certain cell.
`
`Mobile Registration
`
`[0045] The mobile registration in GPRS can be divided into two procedures: attach and activation.
`
`Attach Procedure
`
`[0046] When an MS is roaming in a visiting PLMN, it first needs to attach itself to a SGSN. The MS initiates the attach
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`EP 1 009 176 A2
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`procedure by sending to the SGSN its International Mobile Subscriber Identity (IMSI) which is unique to each GPRS/
`GSM subscriber. Based on the IMSI, the SGSN informs the HLR in the home PLMN about the IP address of the SGSN,
`and the VLR in the visiting PLMN about the location area of the MS. The HLR transmits the subscriber data to both
`the SGSN and the VLR. After the databases in the SGSN, HLR and VLR have been updated, the attach procedure is
`complete.
`
`Activation Procedure
`
`[0047] After the MS has been attached to the SGSN, it can negotiate the packet data protocol (PDP) which is used.
`The MS sends to the SGSN the MS's IP address, if one exists, otherwise, an IP address will be assigned by the home
`or visiting PLMN. Based on the information in the subscriber data, the SGSN determines the GGSN address in the
`home PLMN. Then the SGSN sends a message to the GGSN with the IP address of the MS and the GTP tunnel
`identifier (TID). The GGSN creates a new entry in its PDP context table which allows the GGSN to route IP packets
`between the SGSN and the external IP network. The entry is similar to binding information for Mobile-IP. Now the SGSN
`is able to route IP packets between the GGSN and the MS.
`
`Data Transfer
`
`[0048] Referring to FIG. 3C, a block diagram illustrating GPRS data transfer is shown. After the MS has been attached
`to the GPRS and the PDP Context Activation procedure has been completed, the GPRS network transparently trans-
`ports IP packets between external packet data networks and the MS. When a corresponding host (CH) has a packet
`to be sent to the MS, it will send an ARP request (IP address of the MS) to which the GGSN will respond. When the
`IP packet is routed to the GGSN, the IP packet is encapsulated with a GPRS Tunnel Protocol (GTP) header. The GTP
`PDU is inserted into an UDP PDU which is again inserted in an IP PDU. The IP header contains the address of the
`SGSN. At the SGSN, the original IP packet is recovered and re-encapsulated for transmission to the MS.
`[0049] For packets sent by the MS to the CH, a reverse tunnel is used. In this case, the SGSN does the encapsulation
`using GTP and transmits the GTP PDU to the GGSN. At the GGSN, the original IP packet is recovered and sent to
`the CH via regular IP routing.
`[0050] While it is known that Mobile IP implements certain route optimization techniques, CDPD and GPRS networks
`do not do the same. Accordingly, it would be highly advantageous to implement route optimization techniques in CDPD
`and GPRS networks.
`
`Summary of the Invention
`
`[0051] The present invention provides methods and apparatus for providing route optimization in GPRS and CDPD
`networks. In one aspect of the invention, a route optimization technique in a GPRS network includes establishing a
`gateway GPRS support node in a visiting public land mobile network in which a roaming mobile station is currently
`located. Specifically, a tunnel is formed between the gateway GPRS support node and a serving GPRS support node
`to which the mobile station is in direct communication over a radio link. In this manner, external corresponding hosts
`may route packets to the gateway GPRS support node, rather than the GPRS support node in the mobile station's
`home public mobile network, as is done in conventional GPRS networks. Advantageously, a shorter path is establ