Mobile Data Networks Topics General Packet Radio Service (GPRS)

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Mobile Data Networks Topics General Packet Radio Service (GPRS)

  1. 1. Mobile Data Networks
  2. 2. Topics <ul><li>General Packet Radio Service (GPRS) </li></ul><ul><li>Short Messaging Service (SMS) </li></ul><ul><li>Mobility Management and Mobile IP </li></ul><ul><li>Application Protocols </li></ul><ul><li>Security </li></ul>
  3. 3. GPRS - Overview <ul><li>GPRS is an overlay on top of the GSM physical layer and network entities </li></ul><ul><li>Extends data capabilities of GSM </li></ul><ul><li>Provides connections to external packet data networks through the GSM infrastructure with short access time to the network for independent short packets (500-1000 bytes) </li></ul><ul><li>No hardware changes required for Base Station </li></ul><ul><li>Scalable, high throughput (21.4 kbps), support for voice and data </li></ul>
  4. 4. Operations <ul><li>GPRS uses same physical radio channels, only new logical GPRS radio channels are defined </li></ul><ul><li>Active users share timeslots using TDMA; uplink and downlink are allocated separately </li></ul><ul><li>Capacity allocation in GPRS is based on the “on-demand” principle </li></ul><ul><li>GPRS terminals: </li></ul><ul><ul><li>Class A: Operates GPRS and GSM services simultaneously </li></ul></ul><ul><ul><li>Class B: Operate either GPRS or GSM service at one time </li></ul></ul><ul><ul><li>Class C: Only GPRS service </li></ul></ul><ul><li>Limitations: </li></ul><ul><ul><li>Limited cell capacity </li></ul></ul><ul><ul><li>No store-and-forward service (SMS provides this service) </li></ul></ul>
  5. 5. GPRS Network Services <ul><li>Point-to-point (PTP): packet data transfer </li></ul><ul><ul><li>Connectionless based on IP </li></ul></ul><ul><ul><li>Connection oriented based on X.25 </li></ul></ul><ul><li>Point-to-multipoint (PTM-M): multicast service to all subscriber in one area </li></ul><ul><li>Point-to-multipoint (PTM-G): multicast source to a predetermined group </li></ul><ul><li>Short messaging service (SMS) </li></ul><ul><li>GPRS has parameters that specify a QoS based on service precedence, priority, reliability and required transmission characteristics </li></ul>
  6. 6. Reference Architecture <ul><li>Refer to handout </li></ul><ul><li>Uses GSM architecture </li></ul><ul><li>GPRS support nodes (GSN): responsible for delivery and routing of data packets between the MS and the external network </li></ul><ul><li>Serving GPRS support node (SGSN) </li></ul><ul><ul><li>Controls access to MSs that are attached to a group of BSCs (routing area (RA) of SGSN) </li></ul></ul><ul><li>Gateway GPRS support node (GGSN) </li></ul><ul><ul><li>Logical interface to the Internet </li></ul></ul><ul><li>GPRS Register (GR) </li></ul><ul><ul><li>Colocated with HLR and stores routing information </li></ul></ul>
  7. 7. Mobility Support: Attachment <ul><li>Before accessing GPRS services, the MS must register with the GPRS network </li></ul><ul><li>MS performs an attachment procedure with an SGSN that authenticates it by checking the GR </li></ul><ul><li>The MS is allocated a temporary logical link identity (TLLI) </li></ul><ul><li>A packet data protocol (PDP) context is created for the MS for each session and is stored at the MS, SGSN, and GGSN </li></ul><ul><li>PDP context: PDP type, address, QoS, GGSN address </li></ul><ul><li>A user may have several PDP context enabled. The PDP address may be statically or dynamically assigned </li></ul><ul><li>PDP context is used to route packets </li></ul>
  8. 8. Location Management <ul><li>Location management depends on three states as shown in the figure </li></ul><ul><li>In the IDLE state, the MS is not reachable </li></ul><ul><li>In the STANDBY state, movement across routing areas is updated to the SGSN but not across cells </li></ul><ul><li>In the READY state, every movement of the MS is indicated to the SGSN </li></ul>READY STANDBY IDLE GPRS Dettach GPRS Attach Timer Expiry Packet Tx Timer Expiry
  9. 9. Routing Area Updates <ul><li>Intra-SGSN Update </li></ul><ul><ul><li>The SGSN already has the user profile and PDP context </li></ul></ul><ul><ul><li>The home location register (HLR) need not be updated </li></ul></ul><ul><ul><li>A new temporary mobile subscriber identity is issued as a part of the RA update </li></ul></ul><ul><li>Inter-SGSN Update </li></ul><ul><ul><li>The new RA is serviced by a new SGSN </li></ul></ul><ul><ul><li>The new SGSN requests the old SGSN to send the PDP context </li></ul></ul><ul><ul><li>The SGSN informs the home GGSN, the GR, and other GGSNs about the user’s new routing context </li></ul></ul>
  10. 10. Handoff Management <ul><li>The MS listens to the broadcast control channel (BCCH) and decides which cell to connect using the RSS, cell ranking, path loss, etc. </li></ul><ul><li>The location is updated using the routing update procedure (see handout) </li></ul><ul><li>The SGSN updates the GGSN of the home network with the new SGSN and the tunneling information </li></ul>
  11. 11. Uplink/Downlink Transmissions <ul><li>The uplink and downlink transmissions are independent </li></ul><ul><li>The Mac protocol is called “Master-Slave Dynamic Rate Access” or MSDRA </li></ul><ul><li>The organization of the time-slot assignment is done centrally by the BSS </li></ul><ul><li>Refer to the handout </li></ul>
  12. 12. Short Message Services (SMS) <ul><li>Extremely popular service, similar to the peer-to-peer instant messaging services in the Internet </li></ul><ul><li>Allows exchange of alphanumeric messages up to 160 characters </li></ul><ul><li>Two types of services: </li></ul><ul><ul><li>Broadcast </li></ul></ul><ul><ul><li>Peer to peer </li></ul></ul><ul><li>Uses the same infrastructure as GSM </li></ul><ul><li>SMS has instant delivery service as well as store-and-forward service </li></ul>
  13. 13. Operations <ul><li>SMS makes use of the GSM infrastructure, protocols, and the physical layer to manage the delivery of messages </li></ul><ul><li>Each message is treated individually, and is maintained and transmitted by the SMS center (SMSC) </li></ul><ul><li>Short messages (160 char mapped into 140 bytes) are transmitted through the GSM infrastructure using SS-7 </li></ul><ul><li>Short messages are transmitted in time slots that are freed up in the control channels </li></ul>
  14. 14. Reference Architecture HLR VLR SMS-GMSC SMS-IWMSC MSC MS SMSC
  15. 15. Cases of Short Messages <ul><li>SM originating from an MS </li></ul><ul><ul><li>Goes to MSC for processing </li></ul></ul><ul><ul><li>SMS-interworking MSC (SMS-IWMSC forwards the SM to the SMSC </li></ul></ul><ul><li>Mobile terminated short message </li></ul><ul><ul><li>SM is forwarded by the SMSC to the SMS-gateway MSC (SMS-GMSC) </li></ul></ul><ul><ul><li>Either the HLR or VLR is queried </li></ul></ul><ul><ul><li>SM is either delivered to the BSC or forwarded to another MSC </li></ul></ul>

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