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Cellular technologies

Properties

  • Communication range
    • Lower frequency → less power required to gain longer distance
  • Latency
  • Data transfer rates
  • Architecture
  • Cost of infrastructure

Introduction

  • AKA Mobile phone technologies
    • TDMA, GSM, GPRS, EDGE, UMTS
  • Supported mainly by mobile phones
    • Add capability to device
      • USB modems (mokkula)
    • Connection through mobile phone
      • Serial connection over wire
      • Wireless Bluetooth connection
  • Different standards supported in different world regions
    • Some standards use different frequencies in different area
    • Compatibility problems for mobile terminal
    • Solution may require conversions to work in different areas
  • Topology
    • Terminals connected to Base Station at fixed location
    • Base stations interconnected with wires
  • Base stations
    • AKA Access Points (AP) in other technologies
    • Coverage area termed cell
    • Cell sizes vary
    • Interconnected using dedicated lines
  • Regulated radio frequency
    • Creating own network requires a license

GSM network

  • mobile station
    • Device has IMEI code (International mobile equipment code)
    • Contains Subscriber Identity Module (SIM)
  • BSS, basestation subsystem
  • MSC Mobile switching center
  • HLR Home locator register
  • VLR Visitor locator Register
  • PSTN Public switched telephone network

Connectivity

  • Two types of connections
    1. Voice connections - (circuit switched)
    2. Data connections - (packet switched)
  • Connection oriented (GSM Data, IP connections)
  • Connectionless messaging (SMS, MMS)
  • Control channels (CCH) for signaling
    • Handling connections
    • Messaging capabilities
  • Traffic channels (TCH) for payload, speech or data
  • Connection forming between BS & mobile
    1. Mobile requests free channel for signalling
    2. BS allocates the signalling channel
    3. MS requests call setup & gains traffic channel from BS
    4. Mobile authentication based on Subscriber Identity Module (SIM) - contains:
      • International Mobile Subscriber Identity (IMSI)
      • Personal Identification Number (PIN)
      • Identifying key
      • Cryptographic algorithms
  • Latency
  • From hundreds of milliseconds to seconds
  • Connection processing increases latency
    • The more users they are the higher the latency
  • Good support for mobility
  • Handover & roaming

Messaging

  • Connectionless transfer of messages
  • 1 message contains a maximum of 140 octets (bytes) → 1120 bits
    • GSM alphabet is 7 bits → 1120/7 = 160 characters maximum
    • SMS concatenation allows many smaller messages be delivered as 1 long messge
    • Can also be used to deliver binary content
  • Message deliverly not guaranteed
    • Best effort delivery
  • Class 1 messages pop on screen on arrival
    • Meant for urgent information
  • Billing is message based

GSM DATA

  • Mobile phone used as a modem
  • Circuit-switched
  • Bandwidth 9600 bit/s
  • High-Speed Circuit-Switched Data (HSCSD)
    • Allocates several data channels into one logical link
    • Bandwidth up to 43.3 kbit/s
    • Realistic bandwidth ~30 kbit/s when stationary & 10 kbit/s when moving
  • Billing is time based

GPRS

  • General Packet Radio Service
  • Enhancement to GSM
    • Separate GPRS network added behind BS controller (BSC)
    • BTS, BSC and database require software update
    • BSC requires Packet Control Unit
    • New mobile terminal required (terminals backward compatible)
  • Packet based communication
  • Support typical up to 100 users
    • User might use 1 -8 channels

GPRS ...

  • Support for IP & X.25 packets
  • TCP over X.25
  • UDP & TCP over IP
  • Application can be created on top of TCP / UDP
  • GRPS connection
  • Mobile has to activate the packet data address
  • Mobile initiates connection to the service
  • Billing is based on amount of data transferred

EDGE

  • Enhanced Data rates for GSM Evolution (EDGE / EGPRS)
    • Enhancement of GPRS
    • Can utilise existing GSM core network
      • Only a software upgraded needed
  • Data rates up to 384 kbit/s
    • Attained using 8PSK (8 Phase Shift Keying) + GMSK (Gaussian-Minimum-shift keying)
    • 3 bit word for every change in carrier phase

UMTS

  • Universal Mobile Telecommunications System
    • 3G in Europe
  • Standardised by 3GPP (3G Partnership Project)
  • Uses WCDMA (Wideband Code Division Multiple Access)
  • Supports up to 1920 kbit/s data transfer rates
    • Real rates ~384 kbit/s
  • Support for different cell sizes
    • from pico to satellite
  • Support for quality of service
    • Conversation class
    • Streaming class
    • Interactive class
    • Background class

UMTS network

UMTS_structures.svg

  • GERAN : GSM Edge radioa Access network
  • HSS: home subscriber service
  • CSCF: Call state control function
  • MGCF: Media gateway control function
  • SGSN: Serving GPRS support node
  • GGSN: gateway GPRS support node
  • MGW: Media gateway

UMTS components

  • USIM (user and service identity module
    • Can contain also among other things credit card information
  • Mobile equipment
  • UMTS radio access network (UTRAN)
    • Basestations (BS)
    • Radio network controller (RNC)
      • Controls ME BS radiointerface
      • Connects BS to UMTS core network
    • Radio network Sub-system
      • One RNC+ basestations

UMTS core network

  • Interworking Unit (IWU)
    • packet to circuit switched and vice versa
  • MSC/VLR (Mobile switching center/ visitor location Register)
    • GMSC = Gateway MSC (gateay to telephone networks)
  • SGSN (Service GPRS support Node)
  • GGSN (Gateway GPRS support Node)
  • HLR (Home Location Register)
  • SCP (Service Controlling point)

LTE

Considerations

  • The network is owned by network operator
    • Operator charges on network usage
    • Messaging based service can use operators charging infrastructure
      • Use of service is charged in phone bill
    • P2P communication is problematic

Self SMS based service

  • Details + c-code is available
    • Ikonen, Porras, Hämäläinen: Mobile Network Services with Linux, Embedded Linux Journal, march-april, 2001
  • Parts
    • Desktop Linux box
    • Nokia communicator phone
    • Serial cable between phone & desktop
      • Communicator visible in >$/dev/ttysx
      • Where x depends on used communicaton port

Self SMS based service ...

  • Phone controlled using AT commands
    • Phone has to be set to “fax modem” to accept these commands & activate
    • Connect phone with terminal program
    • Set the number to message center: AT+CSCA=“number”, 145
  • Recieving message
    • Set phone to online mode: AT+CNMI=1,2
      • Messages given directly to serial line
    • Listen to the serial port
  • Sending message
    • AT+CMGS=“number”
    • Write message & press ctrl-Z to send message
  • Combing these methods → different kinds of service can be created
Last modified: 2013/07/01 14:42