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Mobile Data Networks Topics General Packet Radio Service (GPRS)

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