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Mobile Data Networks

Mobile Data Networks. Topics. General Packet Radio Service (GPRS) Short Messaging Service (SMS) Mobility Management and Mobile IP Application Protocols Security. GPRS - Overview. GPRS is an overlay on top of the GSM physical layer and network entities Extends data capabilities of GSM

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Mobile Data Networks

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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 Packet Tx Timer Expiry GPRS Attach GPRS Dettach STANDBY IDLE 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 MS SMSC MSC SMS-GMSC SMS-IWMSC

  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

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