Mobile data networks
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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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Mobile data networks

Mobile Data Networks


Topics

Topics

  • General Packet Radio Service (GPRS)

  • Short Messaging Service (SMS)

  • Mobility Management and Mobile IP

  • Application Protocols

  • Security


Gprs overview

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


Operations

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)


Gprs network services

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


Reference architecture

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


Mobility support attachment

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


Location management

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


Routing area updates

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


Handoff management

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


Uplink downlink transmissions

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


Short message services sms

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


Operations1

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


Reference architecture1

Reference Architecture

HLR

VLR

MS

SMSC

MSC

SMS-GMSC

SMS-IWMSC


Cases of short messages

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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