Cellular Wireless Networks

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Cellular Wireless Networks
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Example of a Cellular Wireless Network
Picture http//www.scom.hud.ac.uk
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1G Cellular Networks

• 1st generation cellular networks are purely
  analog cellular systems.
• The transmission of data is sent via a
  continuously variable signal

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2G Cellular Networks

• 2nd generation cellular networks refer to digital
  cellular and PC wireless systems.
• voice and low speed data services.
• They consist of digital traffic channels, perform
  encryption, error detection correction
• Users share channels dynamically

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3G Cellular Networks

• 3rd generation refers to the next generation of
  wireless systems.
• This is digital with high speed data transfer
• It is voice quality comparable with a switched
  telephone network.
• Data transmission rates can be asymmetric or
  symmetrical
• It provides support for circuit switched and
  packet switched data services

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Cellular Operation

• Three basic devices
• A mobile station
• A base transceiver
• A Mobile Telecommunications Switching Office
  (MTSO)

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Cellular Operation
Picture www.Xanthippi.ceid.upatras.gr
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Cellular Network Organization

• Base Station (BS)
• includes an antenna, a controller, and a number
  of receivers
• Mobile Telecommunications Switching Office (MTSO)
• connects calls between mobile units
• Two types of channels available between mobile
  unit and BS
• Control channels
• used to exchange information having to do with
  setting up and maintaining calls (out-band or
  in-band through stealing bits)
• Traffic channels
• carry voice or data connection between users

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Cellular Operation

• Public Land Mobile Network (PLMN) refers to a
  cellular network that has land and radio based
  sections.
• This network consists of
• Mobile station (MS) A device used for
  communication over the network.
• Base station transceiver (BST) A
  transmitter/receiver used to transmit/receive
  signals over the network.

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Cellular Operation

• Mobile switching center (MSC) Sets up and
  maintains calls made over the network.
• Base station controller (BSC) Communication
  between a group of BSTs and a single MSC is
  controlled by the BSC
• Public switched telephone network (PSTN) Section
  of the network that is land based
•  

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Cellular Operation

• Outgoing from mobile
• input phone number and press send
• mobile links to base transceiver via control
  channel
• base to MTSO to PSTN
• MTSO routes connection back to mobile via voice
  channel
• mobile shifts from control to voice

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Cellular Operation

• Incoming to mobile
• call goes from PSTN to MTSO
• on control channel, MTSO searches for mobile by
  PAGING every active mobile
• If found, MTSO rings it and establishes voice
  channel connection
• uses transceiver with strongest signal from mobile

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Cellular Network Organization (Cells)

• Cells use low powered transmitters.
• Each cell is allocated a band of frequencies, and
  is served by its own antenna as well as a base
  station consisting of a transmitter, receiver and
  control unit.

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Hexagon Reuse Clusters
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Cellular Coverage Representation
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Frequency Reuse

• Each colour/letter uses the same frequency band

Picture netlab.cis.temple.edu/jmulik/teaching/85
50s03-slides/ 8550-Cellular-14.sxi.pdf
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3-cell reuse pattern (i1,j1)
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4-cell reuse pattern (i2,j0)
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7-cell reuse pattern (i2,j1)
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12-cell reuse pattern (i2,j2)
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19-cell reuse pattern (i3,j2)
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Relationship between Q and N
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Factors limiting frequency reuse

• Co-channel interference
• Adjacent channel interference

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Adjacent Channel Interference
Picture xanthippi.ceid.upatras.gr/courses/
mobi_net/Lecture1.ppt

• Adjacent channel interference can be controlled
  with transmit and receive filters

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Coping with increasing capacity

• Adding new channels
• Frequency borrowing
• frequencies are taken from adjacent cells by
  congested cells

Picture www.its.bth.se/courses/etc019/handouts/
ch10_Cellular_wireless_netw.pdf
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Coping with increasing capacity

• Cell splitting
• cells in areas of high usage can be split into
  smaller cells
• Cell sectoring
• cells are divided into a number of wedge-shaped
  sectors, each with their own set of channels
• Microcells
• antennas move to buildings, hills, and lamp posts

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Cell Splitting
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Site Configurations
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Handoffs

• Network protocols must refresh and renew paths as
  a mobile station host moves between cells.
• Handoffs are the function of one cell handing
  over the communication link between itself and a
  mobile station as the mobile station moves out of
  the boundary of its region into the boundary of
  an adjacent cell.

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Handoffs

• This practice must preserve end-to-end
  connectivity in a dynamically reconfigured
  network topology.

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Handoff Types (contd)
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Avoiding handoff Umbrella cells
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Encoding Modulation(1)

• Amplitude Modulation
• Frequency Modulation
• Phase Modulation
• are the three different methods of encoding
  binary information on a regular wave.

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Encoding Modulation(2)

• When using digital signals the methods are known
  as Amplitude Shift Keying (ASK), Frequency Shift
  Keying (FSK), and Phase Shift Keying (PSK).

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Encoding Multiplexing(1)

• Multiplexing allows many mobile users to use
  cellular radio transmission schemes at the same
  time. The different schemes are
• Frequency Division Multiplexing
• Time Division Multiplexing
• Code Division Multiplexing

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Encoding Multiplexing(2)

• Frequency Division Multiplexing involves a
  different frequency channel given to each user

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Encoding Multiplexing(3)

• Time Division Multiplexing involves a channel
  with a given number of time slots (per
  millisecond) where each user is assigned certain
  time interval.
• Code Division Multiplexing gives each user a
  code for differentiation purposes. The
  receiver picks out each channel from the noise
  using the code. Wide frequency band is used.
  Does not contain single frequencies or time
  slots.

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Differences between FDMA, TDMA, and CDMA.
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Advantages of Code Division Multiplexing

• better protection against interference
• good security
• signal difficult to jam

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Disadvantages of Code Division Multiplexing

• pseudo-random code sequences generated by the
  transmitters and receivers are not always random
• fast power control system needed so that strong
  signals dont overpower weaker signals.

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AnalogyMultiplexing

• Lectures at a learning institute
• Frequency Division takes place in different
  rooms
• Time Division taking turns in a single room
• Code Division lectures on different subjects.

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Wired vs. Wireless
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Wired vs. Wireless (1)

• A problem with wireless networks is that anyone
  with a wireless network card is able to access
  this network and is potentially harmful since
  they are able to corrupt and steal important
  files.
• These networks transmit data over an area such
  that the network signals may penetrate physical
  areas such as walls.

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Wired vs. Wireless (1)

• Although this problem is relevant to a wired
  network also, it exists to a greater degree in a
  wireless network.
• With regard to wired networks, the
  electromagnetic waves that are given off from the
  current traveling through the network cables.

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Advantages of wireless networks

• Flexible
• Cost is less (long term)
• Mobile user choice
• Accesses areas that wired networks cannot reach

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Disadvantages of wireless networks

• Compared to wired networks the data rates are
  slower
• User location determines performance
• Devices such as microwaves, cordless phones, etc
  may cause interference
• Can be accessed by hackers from the outside

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Wired Equivalent Privacy Algorithm (WEP) (1)

• Wireless networks may include additional security
  elements, which are not supported in wired
  networks.
• For eg. The use of security algorithms such as
  WEP (Wired Equivalent Privacy Algorithm), that
  uses an encryption algorithm which deals with
  unauthorized access to the network
  (eavesdropping).

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WEP (2)

• WEP is a implemented such that a block of
  plaintext (input text) is bitwise XORed with an
  equal length random key sequence. A random
  number generator is used on the initialization
  vector and the secret key and outputs a key
  sequence of random of octets.
• An integrity check value is produced to protect
  against data modification.
• The key sequence combined with plaintext combined
  with the integrity check value gives the
  enciphered message. The integrity check value and
  the ciphertext is the combination of the output.

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Block Ciphers

• Another security algorithm is Block Ciphers which
  is the most common of the encryption techniques.
  The Block Cipher consists of
• Data Encryption Standard (DES)
• Triple Data Encryption (TDEA)
• Advanced Encryption Standard (AES).

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The GSM Network

• The Global System for Mobile communications.-most
  widely used digital cellular communications
  system

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The GSM Network
Picture http//www.alphaorg.com/cellular/
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Mobile Station

• Mobile equipment or terminal
• 3 types fixed, portable, handheld
• Subscriber Identity Module (SIM).

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The Base Station Subsystem (BSS)

• The Base Station Subsystem connects the Mobile
  Station and the Network and Switching Subsystem.
  It is responsible for transmission and reception
  and can be divided into two parts
• The Base Transceiver Station (BTS) or Base
  Station.
• The Base Station Controller (BSC).

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The Base Transceiver Station (BTS)

• The BTS corresponds to the transceivers and
  antennas used in each cell of the network. It is
  usually placed in the center of a cell. The size
  of a cell defines the transmitting power. A BTS
  can have up to 16 transceivers. It all depends on
  the density of users in the cell.

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The Base Station Controller (BSC)

• The BSC controls a group of BTS.
• A BSC is primarily responsible for handovers,
  frequency hopping, exchange functions and control
  of the frequency power levels of the BTSs.

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The Network and Switching Subsystem (NSS)

• Its main role is to manage the communications
  between the mobile users and other users, such as
  mobile users, ISDN users, landline users, etc.
• It also has databases that store information
  about the subscribers and to manage their
  mobility.
• The different components of the NSS are described
  below.

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The Mobile services Switching Center (MSC)

• Central component of the NSS.
• It performs switching functions and also provides
  connection to other networks.

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The Gateway Mobile services Switching Center
(GMSC)

• A gateway is a node that interconnects two
  networks.
• The GMSC is the interface between the mobile
  cellular network and the Public Switched
  Telephone Network (PSTN).
• It is responsible for routing calls from the
  fixed network towards a GSM user.

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Home Location Register (HLR)

• The HLR is an important database as it stores
  information about the subscribers belonging to
  the covering area of a MSC.
• It stores the current location of these
  subscribers and the services to which they have
  access.
• The location of the subscriber corresponds to the
  SS7 address of the Visitor Location Register
  (VLR) associated to the terminal.

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Visitor Location Register (VLR)

• The VLR contains information from a subscriber's
  HLR.
• It is necessary for the provision of subscribed
  services to visiting users.
• When a subscriber enters the covering area of a
  new MSC, the VLR associated to this MSC will
  request information about the new subscriber to
  its corresponding HLR.
• The VLR will then have enough information to
  assure the subscribed services without needing to
  confirm with the HLR each time a communication is
  established.

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The Authentication Center (AuC)

• The AuC register provides the parameters needed
  for authentication and encryption functions.
• These parameters help verify the user's identity.

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The Equipment Identity Register (EIR)

• The EIR is also used for security purposes. It is
  a register containing information about the
  mobile equipment.
• It particularly contains a list of all valid
  terminals.
• A terminal is identified by its International
  Mobile Equipment Identity (IMEI).
• The EIR uses this to forbid calls from stolen or
  unauthorized terminals.

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The GSM Interworking Unit (GIWU)

• The GIWU corresponds to an interface to various
  networks for data communications.
• During these communications, the transmission of
  speech and data can be alternated.

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The Operation and Support Subsystem (OSS)

• The OSS is connected to the different components
  of the NSS and BSC in order to control and
  monitor the GSM system.
• It also takes care of controlling the traffic
  load of the BSS.

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• In GSM, there are five main functions
• Transmission.
• Radio Resources management (RR).
• Mobility Management (MM).
• Communication Management (CM).
• Operation, Administration and Maintenance

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GSM

• GSM uses an intriguing method to send data. By
  simply sending computer data as it would send
  voice data, GSM allows every phone to be "data
  enabled."
• Since the GSM network is already a packet network
  of sorts because of its frequency hopping, it
  requires no additional hardware to support data.
• GSM allows data rates in multiples of 300 bits
  per second, up to 64 kilobits per second.

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Advantages of Cellular Networks

• Cellular networks use "small" cells with low
  powered transceivers instead of one large area
  with a high powered transceiver.
• Using cellular networks increases overall call
  handling capacity.
• Avoids central point of failure.
• Allows dynamic distribution of capacity based on
  demand.
• Less interference with other wireless
  communications

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Conclusion (1)

• Mobile phone contracts provide a useful and
  relatively cheap service.
• Cellular radio network infrastructures are
  growing at a tremendous rate.
• Cellular networks are becoming high speed data
  networks.

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Conclusion (2)

• Speeding up the development of mobile
  communication technologies is the focus of the
  industry. The ideal communication system where
  both voice and data services can be delivered
  regardless of location, network, or terminal.