WLAN GPRS Integration for Mobile Data Networks

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WLAN- GPRS Integration for Mobile Data Networks

• Chetan Thakker

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Introduction

• 2nd Generation cellular systems are adequate
  ONLY in meeting the demand for high quality voice
  services.
• There is an increasing demand for high speed and
  quality data services.
• 2.5G cellular systems provide data services
  (GPRS) but only at a speed of approximately
  100Kbps
• 3G promises higher data rates, but requires
  large investment for a new spectrum and also at
  present not readily available.
• WLAN technology is very famous and has been
  deployed worldwide.
• It also offer higher data rates compared to
  cellular systems.

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Need for Integration

• Higher Data rates provided by WLAN.
• Currently Packet Switched networks are not only
  used for data but also for voice (Example Voice
  over IP).
• Cost wise WLAN cheaper to deploy than a cellular
  network.
• WLAN technology exists and so does the cellular
  network, therefore just need to merge them.

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Main Requirements

• Mobile Subscriber should have a dual mode phone
  or a wireless card.
• User should be able to use both WLAN and GPRS
  functionalities.
• Common Billing for Voice and Data services.
• User can also use voice services over WLAN (Voice
  over IP).
• Session Mobility The user should be able to move
  easily in and out of the WLAN or Cellular domain
  without loosing connection. That is a smooth
  handover.

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Interworking Architectures

• ETSI acronym for European Telecommunications
  Standards Institute specifies two approaches for
  interworking.
• Tight coupling Here the access to the External
  packet data network is thru the GPRS core
  network.
• Loose coupling Access to the External packet
  data network is direct and not thru the GPRS core
  network.

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Tight Coupling Architecture

• Indirect Access to external Packet Data Network
• WLAN is just like another Radio Access Network
  (RAN) and the GPRS core thinks of it as another
  Routing Area.
• WLAN connects to the Serving GPRS Support Node
  (SGSN) of the core via the Gb interface or Iups
  Interface. (Paper assumes the Gb interface)
• User Authentication is carried on in the GPRS
  core network.
• Network Architecture includes a WLAN network with
  Access Points connected by means of a
  Distribution System.

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Tight Coupling Architecture
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GPRS Interworking Function

• Key element of the Infrastructure, connected to
  the DS and to the SGSN via the Gb Interface.
• GIF provides the GPRS core with a standardized
  interface and hides the WLAN interface
• GIF is the reason why WLAN appears as another
  Routing Area to the core network
• Since GIF is connected to the DS it has a MAC
  Address and can communicate with any Mobile
  Terminal associated with the DS.
• GIF also performs GPRS protocols specified on the
  Gb interface inorder to communicate with the
  SGSN. For example Frame Relay (FR), Network
  Service (NS), Base Station Subsystem GPRS
  protocol (BSSGP).
• Therefore uplink GPRS traffic is sent from MS to
  the GIF and downlink traffic is sent from the GIF
  to the MS.

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Tight Coupling Architecture
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WLAN Adaptation Function

• Another Key element of the infrastructure.
• A dual mode MS has two subsystems GPRS and WLAN
• WAF is present in the WLAN subsystem of the MS
  and also in the GPRS interworking Function.
• Once the MS associates with an AP, WAF identifies
  that the WLAN subsystem has been enabled and asks
  the LLC layer (above WAF) to redirect the
  Signaling and Data traffic to WLAN.

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WAF
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WAF

• Provides adaptation function between the LLC and
  the 802.11 MAC in the MS
• Also between the 802.3 and BSSGP layers in the
  GIF.

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WAF
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WAF Functionality

• Signals the activation of the WLAN interface once
  MS enters WLAN, also signals the change from RA
  to GMM (GPRS Mobility Management)
• Aids the GIF/RAI (Routing Area Identification)
  discovery procedure initiated by MS for MAC
  address discovery of the GIF and RAI of the WLAN
• Supports paging when SGSN needs to page a MS. It
  alerts the MS to respond to the page.
• Helps to transfer uplink PDUs from the MS to the
  GIF and downlink from the GIF to the MS.
• Transfers TLLI and QoS information in the WAF
  header.
• Temporary Logical Link Identifier (TLLI) is a
  temporary MS identifier used by the LLC Layer.
• TLLI which is included in the WAF header, is used
  by the GIF to update an internal mapping table,
  which co-relates TLLI with 802 MAC Address of the
  MS.
• Therefore downlink information sent by SGSN on
  the Gb interface to the GIF is forwarded to the
  correct MS in the WLAN network.

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WAF
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GIF/RAI Discovery

• GIF/RAI discovery is initiated by the WAF in the
  MS.
• To discover the MAC Address of the GIF and to
  send MSs IMSI or TLLI to the GIF
• Procedure initiated when 802.11 MAC is enabled,
  that is when MS gets associated to an AP.

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GIF/RAI Discovery Procedure

• Once the MS is associated, the WAF in the MS
  requests the 802.11 MAC to transmit a PDU with
  Source Address MSs MAC and Destination Address
  Broadcast. It also includes the MSs IMSI
  (TLLI). This PDU is the GIF/RAI Discovery Request
  message.
• The 802.11 MAC layer sends the message with
  appropriate address information.
• This message is sent to the AP with identity
  equal to BSSID.
• The AP then broadcasts this message to the DS
• The GIF in the DS receives the message and
  associates the MSs MAC with its IMSI (TLLI).

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GIF\RAI Discovery Procedure

• The WAF in GIF responds with a GIF\RAI Discovery
  response which includes the RAI of the WLAN.
• The MS receives the message and stores the RAI of
  the WLAN and the GIF address.
• The MS notifies the GMM layer above the LLC that
  the current GPRS RA has changed.
• The GMM layer initiates a regular GPRS Routing
  Area Update (RAU) and notifies the SGSN that the
  MS has changed the RA.
• Now GPRS data and signaling is carried over WLAN.

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Loosely Coupled Architecture

• Direct access to external packet data network.
• WLAN traffic does not go thru GPRS Core but thru
  cellular operators IP Network.
• Cellular access Gateway (CAG), which is located
  in the cellular operators IP Network
  authenticates the MS.
• CAG provides AAA functionality in the cellular
  operators IP network.
• CAG gets authentication credentials from the Home
  Location Register and compares it with the ones
  obtained from the MS.
• Follows SIM based authentication techniques to
  authenticate a MS. (USIM for UMTS subscribers)

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LA Session Mobility

• Tight Coupling uses GPRS Mobility Management
  (GMM) for session mobility
• In Loose Coupling Mobile IP (MIP) can be used.
• MIP framework has 3 clients MS, Foreign Agent
  (FA), Home Agent (HA).
• FA located at 2 places FA in the GGSN and FA in
  the access router of the WLAN.
• HA located in the cellular operators IP Network.

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LA Session Mobility

• When MS moves in from GPRS to WLAN, it performs a
  MIP registration with the FA in the WLAN.
• The FA then completes the registration with the
  HA in the operators IP network, by providing a
  care of address to forward packets destined for
  that MS.
• The FA then associates the care of address to
  that of the MS and acts as a proxy on behalf of
  the MS.

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Loosely Coupled ArchitectureAuthentication
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Loosely Coupled Architecture
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EAP SIM

• Loosely Coupled Architecture very similar to EAP
  SIM technique, that we had talked about earlier
• In this paper we have the CAG, which communicates
  with the HLR. Before we had the GSM/MAP/SS7
  gateway.
• Here the paper assumes that the AP sends radius
  messages to the CAG.
• Before we had the Radius Server located in the DS
  communicating with the Gateway
• In this paper the CAG compares the authentication
  vectors from the HLR and MS to authenticate the
  MS.
• Before we had the Radius server doing the
  authentication

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EAP-SIM
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LC Advantages Vs Disadvantages

• Based on IETF protocols that are already
  implemented in the WLANs
• WLAN requires minimal updates.
• Supports 3rd party WLANs. A WLAN owned by the
  3rd party can also be used to integrate
  WLAN-GPRS.
• Less cost compared to Tight Coupling
• Requires provisioning of AAA servers in the
  operators network for interworking.
• Requires MIP for session mobility.
• High Latency an issue with MIP.

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TC Advantages Vs Disadvantages

• Provides firm coupling and enhanced mobility
  between the 2 domains.
• Offers reuse of GPRS authentication, accounting.
• Protects operators investment by using GPRS core
  services.
• Supports GPRS services like MMS and SMS.
• Not meant for 3rd party WLAN owners.
• Cost associated with connection of WLAN to SGSN.
  For example the throughput capacity of an SGSN
  could be sufficient for supporting thousands of
  low-bit-rate GPRS terminals but could not be
  sufficient for supporting hundreds of
  high-bit-rate WLAN terminals.
• Does not support legacy WLANs that do not
  implement GPRS protocols.

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Additions to the Network

• WLAN should not only support Data Services but
  also voice services.
• VOIP is an existing protocol that can be used for
  voice services when MS in WLAN.
• VOIP is cheap and can not only help customers but
  also services providers to cut costs.
• Session Mobility would be a issue but is being
  addressed By UMA.

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Conclusion

• WLAN owned by 3rd Party
• Loose coupling seems to be the most cost
  effective
• No upgrades at WLAN required
• But can only be used for data services
• UMA needs to be implemented for VOIP
• Places with existing WLAN can upgrade to UMA and
  charge the service provider per calls made to
  recover costs.
• Latency can be reduced in loose coupling by
  having user information in the AAA server.
• Therefore no need to goto the core network for
  authentication.
• Username / Password authentication can also be
  followed with every user trying to connect to
  WLAN entering a unique username and password.
• Example Nextel customer can have
  number_at_nextel.com as username and password.
• Users can register with locations providing data
  services thru their service providers.

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Conclusion

• WLAN owned by service provider
• UMA best solution as it offers all the 3
  services.
• Tight coupling can be upgraded by introducing the
  UNC controller instead of the GIF.
• Cost shall be more than a legacy WLAN as new
  infrastructure to be added.
• Roaming agreements between service providers
  shall enable any user irrespective of his/her
  service provider to use the WLAN services.
• Shall not only provide cheap and fast data but
  also cheap voice calls.

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References

• WLAN-GPRS Integration for Next Generation Mobile
  Data Networks, AK Salkintzis, Chad Fors, Rajesh
  Pazhyannur, IEEE Wireless Communication Oct 2002.
• UMA Architecture Stage 2, Specification

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Questions

• Thanks..!!