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					CSCD 433/533
Cellular Networks
Fall 2012

Lecture 9
Cellular Networks

Cellular Networks
 How do they work?
 Cell phone example


Cellular networks have been around longer
  than we think
They have been around since the late 70's
  Debuted in 1979, as analog systems, 1G networks
  Then, digital systems, 1990's, 2G networks
  Finally, with higher data rates, 3G networks
     Still being deployed
  Many standards and technologies defined along the
   way in evolving cellular networks
1G Cellular Networks
• Mobile phones began to spread 1980s with the
  introduction of "cellular" phones
• Cellular networks with multiple base stations located
  relatively close to each other, and protocols for the
  automated "handover" between two cells when a
  phone moved from one cell to the other
• At this time analog transmission was only option
• Mobile phones were somewhat larger than current
  ones, and at first, all were designed for permanent
  installation in vehicles
1G Cellular Networks
• Soon, some of these early phones were
  converted for use as "transportable" phones the
  size of a briefcase
• Motorola introduced the first truly portable, hand
  held phone
• These systems later became known as first
  generation (1G) mobile phone
• However, this first attempt at wireless
  was not without limitations ...
Problems of 1G Systems

   FDMA based sharing of all spectrum
         Inefficient use of bandwidth

   Purely Analog voice Channels
         Sensitive to interference from co-channel
         Impossible to add redundancy error
         Impossible to encrypt
         Impossible to add additional channels
   Analog FDMA technology
2G Cellular Networks

• Global System for Mobile Communications
  • Most popular standard for mobile phones in the
  • Its promoter, the GSM Association, estimates that
    80% of the global mobile market uses the
  • GSM is used by over 3 billion people across
    more than 212 countries and territories
  • 2G still dominates world cellular market today
3G Systems

• 3G networks enable network operators to offer
  users a wider range of more advanced services
  while achieving greater network capacity through
  improved spectral efficiency
  • Services include wide-area wireless voice telephony,
    video calls, and broadband wireless data
  • Additional features also include data transmission
    capabilities, speeds up to 14.4 Mbit/s on the downlink
    and 5.8 Mbit/s on the uplink
  • Basically, greatly increased speed of 2G networks
  • Greater emphasis on data!
3G based on CDMA
• Recall ... TDMA – Time Division Multiplexing and
  FDMA – Frequency Division Multiplexing
• Turns out, both these are not so efficient when
  large numbers of mobile users want channels
• Used in previous versions cellular networks
• Code division multiple access (CDMA),
  • Channel access method for simultaneously allowing
    communication by multiple calls over single
    communication channel
  • This allows several users to share a bandwidth of
• Basic version was used in 2G cellular networks
• CDMA modulates the signal with special coding
   • Each transmitter assigned a different code
   • Receiver demodulates signal with that code
   • Allows multiple users to be multiplexed over
     the same physical channel
3G Networks

• CDMA2000 represents a family of
  • International approved 3G standards
  • Deliver increased network capacity to meet growing
    demand for wireless services and high-speed data
     • Includes other sophisticated modulation
     • QOS, MIMO antenna technologies
  • CDMA2000 1X was the world's first 3G technology
    commercially deployed (October 2000)‫‏‬
Cells are Heart of Cell Networks
How Cellular Networks Operate

• Each mobile device uses separate, temporary
  radio channel to talk to cell site
• Cell site talks to many mobiles at once, using
  one channel per mobile
    Channels use pair of frequencies for
     One frequency for transmitting
     One frequency to receive calls from the users
How Cellular Networks Operate
Cellular Evolution

     In‫‏‬the‫‏‬old‫‏‬days‫‏…‏‬one‫‏‬main‫‏‬transmitter‫‏‬
    for large geographic area, like a TV
    broadcast system
     Cellular concept helped increase the
    power of mobile telephone networks
     Instead of using one powerful
    transmitter, many low-power
    transmitters were placed throughout a
    coverage area
           So, by dividing a metropolitan
Cellular Interference

• But, some communication issues with the
  cellular concept
• Interference problems from mobile units using
  same channel in adjacent cells
• Cells had to be skipped before the same
  channel could be reused
Cell Structure

• Is there an optimum way to allocate cells and
  users given limited frequencies? Yes.
• Cell is basic unit of a cellular system
• “Cellular” name comes from honeycomb
  shape of areas for a coverage region
  • Cells have base stations transmitting over small
    geographic areas represented as hexagons
  • Each cell size varies depending on landscape
  • Because of constraints imposed by natural terrain
    and man-made structures, true shape of cells is
    not a perfect hexagon
Cell Structure

• Because only a small number of radio
  channel frequencies were available for mobile
• Engineers had to find a way to reuse radio
  channels to carry more than one conversation
  at a time ... Frequency Reuse
  • Concept of frequency reuse is based on
    assigning to each cell a group of radio channels
    used within a small geographic area
  • Cells are assigned a group of channels that is
    completely different from neighboring cells
Cell Resuse

• Cells with the same number have the same
  set of frequencies
  • Here, because the number of available
    frequencies is 7,
    the frequency reuse
     factor is 1/7
  • Each cell is using 1/7
    of available cellular

Cellular Network Components

•    Cellular communications system consists of
    the following four major components that
    work together to provide mobile service to

1. Public switched telephone network (PSTN)‫‏‬
2. Mobile telephone switching office (MTSO)‫‏‬
3. Cell site with antenna system
4. Mobile subscriber unit (MSU)‫‏‬


• MTSO is the central office for mobile switching
  • Houses the mobile switching center (MSC), field
    monitoring, and relay stations for switching calls from
    cell sites to wireline central offices (PSTN)‫‏‬
• Mobile subscriber unit (MSU) consists of
  control unit and transceiver that transmits and
  receives radio transmissions to and from a cell
  site ... cell phones and other units

   More Definitions for GSM and Other
            Mobile Networks
Home network
  Network of cellular provider you subscribe to ( Sprint, Verizon)‫‏‬
Home Location Register (HLR)
   Database in home network containing permanent cell phone
#, profile information (services, preferences, billing), information
about current location
Visited Network
   Network in which mobile currently resides
Visitor Location Register (VLR)
    Database with entry for each user currently in network

So how does it all work?
When you first power up the phone, it listens for an SIDSystem
 Identification Code (SID) - a unique 5-digit number that is
 assigned to each carrier by the FCC on the control channel

 Control channel is a special frequency that the phone and base
  station use to talk to one another about things like call set-up
  and channel changing

If the phone cannot find any control channels to listen to, it knows
   it is out of range and displays a "no service" message

        So how does it all work?
   When it receives SID, phone compares it to SID
    programmed into phone
      Phone knows that cell it is communicating with is
       part of its home system.
   Along with SID, phone also transmits registration
    request, and MTSO (Mobile Telephone Switching
    Office) keeps track of your phone's location in a
    database --
      MTSO knows which cell you are in when it wants to
       ring your phone.
   MTSO gets call, and it tries to find you --- looks in its
    database to see which cell you are in
   MTSO picks a frequency pair that your phone will use in
    that cell to take call
Mobility in Cellular Networks

How do you handle roaming behavior in cellular
Think about, seamless handoff when you go
  between cell towers ... in a car or,
I have service in Spokane, travel to New York
   City ... still have service
Two issues:
    original local network?
Handoff of Call

• Since adjacent areas do not use the same
  radio channels,
  • Call must either be dropped or transferred from
    one radio channel to another when a user crosses
    the line between adjacent cells
• Because dropping the call is unacceptable,
  the process of handoff was created
Handoff of Call

• During a call, two parties are on one voice
  • When mobile unit moves out of the coverage area
    of a given cell site, the reception becomes weak ...
    like with 802.11 AP's
  • At this point, cell site in use requests a handoff
  • System switches call to a stronger-frequency
    channel in a new site without interrupting the call
    or alerting the user
  • Call continues as long as the user is talking, and
    the user does not notice the handoff ( if lucky!!)‫‏‬
  Handoff of Cell Phone Call
                  Really? More
                  blah, blah, blah
Hello Bill,
Blah, blah blah
Routing of a Call

• Second mobility issue, finding you and
  delivering the call when you are outside your
  local mobile network
• Say, I fly to New York City and turn on my
Details of Roaming

1. When mobile device is turned on, this new
  "visited" network sees device, notices that it is
  not registered with its own system, and
  attempts to identify its home network.
    If no roaming agreement between two networks,
    maintenance of service is impossible, and service
    is denied by the visited network
2. Visited network contacts home network and
  requests service information about the roaming

Details of Roaming

3.If successful, visited network begins to
  maintain a temporary subscriber record for
  the device
   Likewise, home network updates its
  information to indicate that mobile is on host
  network so that any information sent to that
  device can be correctly routed

Details of Roaming
4. If a call is made to a roaming mobile, public
  telephone network routes call to phone's
  registered service provider
   Who then must route it to the visited network
   That network must then provide an internal
     temporary phone number to the mobile
   Once this number is defined, home network
     forwards the incoming call to temporary phone
     number, forwarded to mobile
     most national cellular providers

    Privacy in Cellular Networks
    Do you know what information your
    cell phone provider can collect and
    forward about you?
     How about our government including
    law enforcement?
    What laws protect our rights to
Laws Protecting Your Privacy

The Fourth
 (Amendment IV)
  Law Enforcement Tracking

The Fourth Amendment only protects you
  against searches that violate your reasonable
  expectation of privacy. A reasonable
  expectation of privacy exists if
1) you actually expect privacy, and
2) your expectation is one that society as a
  whole would think is legitimate

This rule comes from a decision by United
Law Enforcement Tracking
     Police‫‏‬are‫‏‬increasingly‫‏‬tracking‫‏‬people’s‫‏‬
    movements by tapping into GPS
    technology in modern cellphones, allowing
    in real time or to follow his or her tracks.
   “It’s‫‏‬accurate‫‏‬to‫‏‬say‫‏‬there‫‏‬are‫‏‬millions‫‏‬of‫‏‬
    requests (by police) to phone companies
    for tracking vehicles and people. Most of
    them, if not all of them, involve locating
    people through triangulation of signals
Law Enforcement Tracking

In cities in Nevada, North
  Carolina and other states,
  police departments have
  gotten wireless carriers to
  track cellphone signals back
  to cell towers as part of
Law Enforcement Tracking
And in Arizona, even small police
 departments found cell
 surveillance so valuable that they
 acquired their own tracking
 equipment to avoid the time and
 expense of having the phone
 companies carry out the
 operations for them
The police in the town of Gilbert, for
Law Enforcement Tracking

  eavesdropping powers in 2008
As part of the law, the Bush administration
  insisted that phone companies helping in the
  program be given immunity against lawsuits.

Since then, the wide use of cell surveillance has
  seeped down to even small, rural police
  departments in investigations unrelated to
  national security.
Law Enforcement Tracking

A federal appeals court in August, 2012 said
  authorities do not need a probable-cause

 6th U.S. Circuit Court of Appeals, ruling 2-1,
  upheld a 20-year term for a drug courier
  nabbed with 1,100 pounds of marijuana in a
  motorhome camper authorities tracked via his
  mobile phone pinging cell towers from
  Arizona to a Texas truck stop
    Law Enforcement Tracking

 Position is
  Obama administration told a federal court
  data, and hence the authorities may obtain
  from wireless carriers without a probable-
  cause warrant
Cell Phone Company Tracking

  In mid-October, Verizon Wireless changed its
    privacy policy to allow the company to record
    customers' location data and Web browsing
    history, combine it with other personal
    information like age and gender, aggregate it
    with millions of other customers' data, and
    sell it on an anonymous basis.
  Verizon (VZ, Fortune 500) is the first mobile
    provider to publicly confirm that it is actually
    selling information gleaned from its
    customers directly to businesses.
Cell Phone Company Tracking

All four national carriers use aggregated
  customer information to help outside parties
  target ads to their subscribers
AT&T, Sprint and T-Mobile insist that subscriber
  data is never actually handed over to third-
  party vendors; nevertheless, they all make
  money on it.
 Opt-out of Data Collection
What to do to insure privacy of your data?
  You can opt out of your provider's use of your data

Wireless networks whether WLAN's or cell
  networks are huge business with billions of
Positive aspects, no wires, ubiquitous
Negative, there may be loss of privacy,
  personal rights
Some negative health effects from use of cell
  phones, see Links on our class web page

Privacy Rights Clearinghouse

Ruling that Cell phone data can be used

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