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Locating Users of Mobile Phones Tim Moors. Senior Lecturer School of Electrical Engineering and Telecommunications University of New South Wales Sydney, NSW, Australia t.moors@unsw.edu.au. Outline. Generations of mobile phones Phone identifiers Cellular location Cellular communication

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locating users of mobile phones tim moors

Locating Users of Mobile PhonesTim Moors

Senior Lecturer

School of Electrical Engineering and Telecommunications

University of New South Wales

Sydney, NSW, Australia

t.moors@unsw.edu.au

outline
Outline

2

  • Generations of mobile phones
  • Phone identifiers
  • Cellular location
    • Cellular communication
    • The communication process
    • Physical cell selection
  • Other locators
  • Summary and Questions
generations of mobile phones
Generations of mobile phones

3

Year

Names

G

Cellular?

Digital?

Provision for “data”

Rate

Support?

VOIP

1970s

MTS

0

No

No

No

No

8kHz

8kHz

1981

AMPS

Yes

No

No

No

1

2

1993

GSM

Yes

Yes

Yes

No

9.6kb/s

1990s

GPRS

Yes

Yes

Yes

No

114kb/s

2.5

2000

IMT-2000

Yes

Yes

Yes

No

384kb/s

3

3.5

today

HSPA

Yes

Yes

Yes

No

14.4Mb/s

?

?

4?

Yes

Yes

Yes

Yes

?

VOIP = Voice Over Internet Protocol

http://www.mobilebulgaria.com/uploads/mobiles/2004/01/pic_3_484.jpg

http://luciafreitas.tripod.com/umpoucodetudo/EricssonHotLine.jpg

www.belkin.com/skype/howitworks/

http://www.three.com.au

http://park.org/Japan/NTT/DM/images/RF004900_s.gif

phone identifiers
Phone identifiers

4

Handset serial number:International Mobile Equipment Identity (IMEI)

  • principally for locking phone to a service; theoretically also to block stolen handsets
  • press “*# 06 #” to view

SIM card (“dual SIM” in some phones)

16kB-512kB memory contains:

  • address book, personal settings, etc
  • codes for securing access to phone, network
  • International Mobile Subscriber Identity (IMSI): Identifies phone within mobile phone system

“Phone number”Mobile Station Integrated Services Digital Network (identifier) (MSISDN)

translated to IMSI at edge of mobile phone system

We usually want to locate people, not phones.



#

SIM = subscriber identity module

outline1
Outline

5

  • Generations of mobile phones
  • Phone identifiers
  • Cellular location
    • Cellular communication
      • Mobile vs cellular phones
      • Why cellular?
      • Base station snapshots
      • Cellular issues
    • The communication process
    • Physical cell selection
  • Other locators
  • Summary and Questions
mobile vs cellular phones
Mobile vs cellular phones

6

Radio range is limited by:

  • power (limited by mobility and health),
  • antennas,
  • noise/interference level, and
  • signal processing

Typical potential range: 10s of km in freespace (e.g. pastoral)

  • Usually don’t use full range due to congestive effects
    • Typical suburban effective range: 3km
    • Temporary base stations installed for large eventse.g. New Years Eve on Sydney Harbour
  • Real environments aren’t freespace; have “clutter”

e.g. terrain, buildings, vegetation, mobile vehicles/people

 may find blackspots in buildings only kms from base station

Location of mobile phones is often based on this limited range.

Current phones are cellular, despite being called “mobile” in Aust.

why cellular
Why cellular?

7

Confining signals:

  • allows a frequency to be concurrently used elsewhere, raising capacity
  • reduces power needed, shrink battery/elongate lifetime
  • contains base station faults

Hexagonal cells tessellateand approximate circles(distance = main determinant of quality)

Usually sectorise cells triplets of directional antennas in a triangular arrangement on tower

cellular issues
Cellular issues

9

  • Cell density should reflect subscriber density Better localisation in denser (e.g. urban) areas

Subscribers need to determine power level to use:

      • Mobiles measures strength and error rate
      • Mobile notifies Base Station
      • Base Station indicates what power level to use
  • Base Station position subject to available real estate
  • Signals don’t propagate in free space

 Real cells aren’t neat hexagons

 Complex to determine which tower covers a particular point

  • Phone system needs to
    • locate mobile user in order to establish call to it
    • “handover” mobile users between towers.Handover also used to shift load during congestionBut logs generally record first cell, before any handover
outline2
Outline

10

  • Generations of mobile phones
  • Phone identifiers
  • Cellular location
    • Cellular communication
    • The communication process
      • Frequencies
      • Signalling channels
      • Ephemeral location for call setup
      • What gets logged?
    • Physical cell selection
  • Other locators
  • Summary and Questions
frequencies
Frequencies

11

Bands of frequencies are divided into channels, assigned to different telcos, divided amongst calls.

Bands:

  • Main GSM bands: (others: 850MHz, 1900MHz)
    • 900MHz (890-915, 935-960MHz)
    • 1800MHz (1710-1785, 1805-1880)
  • Main 3G band: 2100MHz (Telstra on 850MHz)

Time-division of channel  8 calls

Effect on location:

  • # antennas: Separate antennas for each band more antennas  more directivity
  • Logged band: 900MHz propagates better than 1800MHz usually establish calls on 900MHz, then hand over to 1800MHz if quality OK most calls logged as established on a 900MHz cell

125 channels

375 channels

signalling channels
Signalling channels

12

  • Broadcast: Base Station announces identity and frequency parameters
  • Common Control Channels
    • Paging: for Base Station to notify phone of a call
    • Random access: for phone to request access; response comes on an...
    • “Access grant” channel: Indicates which Dedicated Control Channel to use
  • Dedicated Control Channel: Dedicated to a specific call (e.g. to control handset power)

Basic call processes:

  • Phone monitors Broadcast ch. to identify local Base Stations
  • Phone chooses Base Station, by strongest signal
  • Phone requests access using Random Access Channel

If (unlikely) can’t access best Base Station (due to interference or congestion on Random Access Channel); then try 2nd best

  • Base station indicates channels (DCCH and voice) to use

(Receiving call: Paging rather than request access + Access grant)

ephemeral location for call setup
Ephemeral location for call setup

13

Need to track location of phone in order to call it

“location areas” are non-overlapping groups of cells; larger areas:

  • Reduce frequency of location updates = mobile transmissions
  • Increase area covered when paging = mobile receipts and traffic

Location Registers:

  • Phone number  home MSC Home LR 
    • tracks subscriber’s location
    • records ‘service profile’ (e.g. caller ID, SMS, etc)
  • Each location area has a Visitor LR 

Location update process:

  • Mobile identifies new cell (through Broadcast)
  • Mobile reports to new Visitor LR for that cell
  • New Visitor LR notifies Home LR of location
  • HLR replies to new VLR with user’s ‘service profile’

then tells old VLR to delete record

Calls to mobile go through HLR to current location;page to determine cell within the location area

  • Location is updated whenever phone is on(not just during call setup)
  • Information is ephemeral

what gets logged
What gets logged?

14

Logs are kept for

  • both pre-paid and plan-based phones
  • voice (“regular”) calls and messages

Available (to telco) in various forms:

  • “cell dump”: All calls through a certain Base Station or cell
  • “Call Charge Record”: for a particular phone (IMEI or MSISDN)

What gets logged:

  • Date & time (when call was established), duration
  • Phone numbers (MSISDN) of both parties
  • Record Type: Numeric code indicating:
    • Voice or message
    • Which party initiated the call
  • IMEI of the mobile handset (served by this telco)
  • Cell IDs
    • First Cell ID: Where the call was established

[Cells traversed during call are not logged]

    • Last Cell ID: Where the call was released (A recent addition to logs.)
outline3
Outline

15

  • Generations of mobile phones
  • Phone identifiers
  • Cellular location
    • Cellular communication
    • The communication process
    • Physical cell selection
      • Sample antenna
      • Choosing which cell of a tower
      • Choosing which tower
  • Other locators
  • Summary and Questions
sample antenna
Sample antenna

16

Argus Antennas JPX310D

JPX310D

Elevation

Azimuth

Pictures from http://www.argusantennas.com/main/?c=custom/argus&custom/argus_task=view_product&product_code=JPX310D&mode=generic

choosing which cell of a tower
Choosing which cell of a tower

17

Environmental factors equally affect all

antennas on a tower since they are effectively co-located(c.f. choosing which tower)

 Choice of cell affected only by:

  • Radiation patterns typically symmetrical
  • Resolution with which phonecan distinguish signal strengths
    • Uncertain: No worse than bar display,but unsure of requisite and signalling accuracy
    • Grey area of +/- 10 degrees around demarcation line
choosing which tower
Choosing which tower

18

Signal strength diminishes with distance,as energy is dissipated and absorbed 

Signals can be

 absorbed by obstructions (terrain, buildings, people)

 reflected, particularly by metal (rooves, vehicles)

    • generally weaker than direct signal, butmay be all that is available in a dense environment.

 received through multiple paths, which may interfere(constructively or destructively,depending on multiples of wavelength  30cm)

Telcos

  • Predict coverage, based onantennas and propagation & terrain models
  • Measure actual coverage, e.g. with cherry picker tower /monitoring & positioning equipment in boots of taxis
  • Produce “Cell Coverage Maps” indicating the expected“dominant cell” for each point

Note: Devices are ambivalent to distance; only aware of signal strength

outline4
Outline

19

  • Generations of mobile phones
  • Phone identifiers
  • Cellular location
    • Cellular communication
    • The communication process
    • Physical cell selection
  • Other locators
    • Who cares where you are?
    • Sources of location info
    • E911
  • Summary and Questions
who cares where you are
Who cares where you are?

20

Consumers

  • Where am I?
  • Where is the closest xyz?
  • Where is my daughter?

Telco

  • Derived from consumer demand
  • To customise content (e.g. ads for local businesses)
  • Capacity planning
  • New services, e.g. road traffic conditions
  • Obliged to help:

Emergency services

e.g. for people who are lost, or unable to

Law enforcement

sources of location info
Sources of location info

21

Cell identity

e.g. Optus FindA service

Measurements of phone emanations

  • strength
  • timing
  • direction (in future, using steerable antennas)

Direction finding well established for wildlife, military; but needs to be done live  traditionally exceptional;only now potentially normal.

Positioning systems, e.g. GPS

  • Works best outdoors
  • Handset-based  privacy throughhandset control of disclosure

Image from optus.com.au: Homepage / Personal / Mobile / 3G Mobile / FindA; drawing from http://www.acma.gov.au/webwr/consumer_info/location.pdf

slide22
E911

22

Telco provides address associated with caller to direct emergency services

FCC has Enhanced 911 (E911) for mobile & VOIP users

Legislation gives carriers choices:

Location Popularity 67% within 95% within

handset 60% 50m 150m

network 20% 100m 300m

hybrid 20%

Timing:

1996: Development started (pre 9/11, but never only for emergency services)

1999: Wireless Communication and Public Safety Act

2003: 100% of handsets due to be compatible; few carriers met schedule

Similar efforts in

  • Europe: “Coordination Group on Access to Location Information for Emergency Services”; more concern for privacy
  • Australia: ACMA discussion paper in 2004
summary
Summary

23

  • Most current capacity to locate mobile phones due is to the cellular nature of communication
    • cells are a couple of km wide
    • 120O sectors around towers, with 20O grey areas
    • Choice between towers heavily influenced by terrestrial “clutter”
    • Logs only record first (& recently last) cell for a call
    • System tracks, but doesn’t preserve, location area of phones that are on
  • Location services are increasing in importance
    • to customers
    • to telcos
    • to law enforcement...
locating users of mobile phones tim moors1

Questions?

Locating Users of Mobile PhonesTim Moors

Senior Lecturer

School of Electrical Engineering and Telecommunications

University of New South Wales

Sydney, NSW, Australia

t.moors@unsw.edu.au