INTRODUCTION TO WIRELESS NETWORKS

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INTRODUCTION TO WIRELESS NETWORKS

• By
• Subramanian E

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WIRELESS NETWORK COMPONENTS

• Basic Architecture consists of
• Radio Network components
• Mobile Station (MS) Any Mobile Equipment
• Base Station (BS) Responsible for relaying
  calls to and from mobile station.
• Wire-line Transport Network
• Mobile Switching Centre (MSC)
• - a special switch configured for wireless
• wireless applications.

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Wireless Network Components
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Wireless Network Components

• DATABASES INVOLVED
• Home Location Register (HLR)
• It is a permanent record. It is present in the
  Home System of the mobile user It contains
  entries of all valid mobile stations.
• It stores
• user profile information
• Subscription details
• Current location of user
• Validation period

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Databases Involved contd

• Visitor Location Register (VLR)
• It is a temporary record in the visited system.
  It is used to store and retrieve information
  necessary to handle the calls of a visiting
  mobile user.

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TWO ASPECTS OF MOBILITY IN A WIRELESS NETWORK

• Handoff It is a Automatic Link Transfer Scheme
  due to
• (i) Cell to cell migration
• (ii) Need for load balancing
• (iii) Emergency call handling
• (iv) Poor transmission or channel quality
• In these cases the handoff occurs within the
  same cell.
• Roaming System should be informed about the
  current whereabouts of MS to deliver the services
  in spite of the MSs mobility.

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ROAMING MANAGEMENT

• Roaming management essentially involves two
  issues
• a. Registration or Location Update
• b. Location Tracking or Call Delivery.

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REGISTRATION

• REGISTRATION
• Registration is a process where the system
  updates the present location of the mobile user
  when the MS moves from one service area to
  another.
• BASIC STEPS INVOVLED IN THE REGISTRATION PROCESS
• 1. MS enters area controlled by MSC2.
• 2. The registration request is forwarded from the
  MS to BS (new) then to the MSC2 and finally to
  the VLR2.
• 3. VLR2 sends message containing MSs identity
  called MIN (Mobile Identification Number) to a
  signal transfer point (STP).

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REGISTRATION PROCESS CONTD.

• 4. At the STP, translation of MIN to home
  location register (HLR) address occurs using a
  table look up technique called Global Title
  Translation (GTT). Then the registration request
  is forwarded to the HLR.
• 5. The HLR verifies the MS with its records and,
  authorizes or denies registration.
• 6. After registration HLR sends acknowledgement
  (ACK) back to VLR2 authorizing the registration.
• 7. HLR sends a Deregistration message to the old
  VLR1 which cancels the obsolete record.

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REGISTRATION PROCESS

• NOTE Deregistration adds to the Network
  traffic. So to avoid this we may use the
  following schemes
• Implicit Deregistration VLR deletes the
  obsolete records automatically once it becomes
  full without prompting from HLR.
• Periodic Reregistration If MS does not register
  within a particular time out period then the VLR
  will delete that particular MSs record.

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POINTER FORWARDING SCHEME
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FIND OPERATION
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Pointer Forwarding Scheme

• In this we define two operations
• MOVE When MS moves from one VLR area to another
  VLR area a pointer is created from old VLR to new
  VLR. Registration is done every K steps where K
  is limited by the cost of pointer creation and
  should be chosen properly.
• (ii) FIND This operation is also called as call
  delivery. In this operation, the pointer chain is
  traced from the HLR through many VLRs. After the
  find operation, HLR points now directly to the
  destination VLR.

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AUTHENTICATION SCHEMES

• Authentication schemes / protocols are necessary
  for
• Identification and validation of user
• Voice privacy issues
• Signaling message privacy
• Security issues

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TERMINOLOGIES

• MIN Mobile Identification Number It is a North
  American Numbering Plan. It serves as the
  Mobile Telephone Number and it is known to the
  user. It is programmed into the MS at the time of
  purchase.
• SSD Shared Secret Data It is a secret key for
  encryption. It is shared between MS and the
  Authentication Center (AUC) that normally resides
  in the HLR.
• ESN Electronic Serial Number It is created
  during manufacture of mobile equipment . It is a
  32 bit serial code of which 3 are manufacturers
  code and the rest of the bits form a unique MS
  identity number.

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AUTHENTICATION SCHEMES

• Authentication schemes can be of two types.
• They are
• WITHOUT SHARING SCHEME (WS)
• SSD shared secret data is shared only
  between the AUC (Authentication Center).

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Registration in WS Scheme

• (i) MS detects it is a new location area based
  on signals from the base station (BS).
• (ii) MS executes CAVE (Count and Voice
  Encryption Algorithm) using SSD and it produces a
  Authentication Result (AUTHR).
• (iii) MS requests registration and it supplies
  AUTHR, ESN, MIN, a random number, COUNT value (it
  is an account of most significant events like
  number of calls received made etc). This history
  count is also maintained by the AUC.

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WS REGISTRATION SCHEME

• (iii) The Authentication is forwarded in various
  steps via the BS MS VLR HLR, and finally to
  the AUC (Authentication Center).
• (iv) The AUC retrieves its copy of the SSD
  associated with the MS (MIN) from its database.
  It also executes the CAVE algorithm and produces
  and produces a new AUTHR Authentication Result.
• (v) If the new Authentication Result (AUTHR new)
  matches with the Authentication Result sent
  (AUTHR sent) then AUC sends a positive response
  for the registration

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WS Registration Scheme
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CALL ORIGINATION Authentication

• Vpmask Voice Privacy Mask. It is necessary for
  the encryption of voice transmitted over the air
  interface between MS and BS.
• Smekey It is the signaling message encryption
  key.
• Once the MS has been authenticated the serving
  MSC/BS system will initiate the Registration
  Notification to determine the called partys
  location and will finally route the call to
  called party.

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Call Origination Authentication
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CALL TERMINATION

• Call Termination occurs when the MS receives a
  call.
• Call Termination Procedure
• Routes trunk to the destination MSC.
• MSC pages the MS.
• MS responds, by executing CAVE and sends AUTHR,
  COUNT, ESN, MIN, RAND for authentication purpose.
• The standard authentication process occurs and
  after the authentication a voice channel is
  established for the call.

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SHARING SCHEME

• SSD shared with the visited system as well.
  Since the visited system has the SSD, it can
  authenticate MS at call origination and call
  delivery.
• Reduces the message flow and call set up time.
• During the registration process since old VLR
  has the latest value of COUNT the AUC sends a
  COUNT request for the verification and this adds
  to network traffic. But after this initial
  registration the new VLR is fully equipped for
  authentication for call delivery or origination.

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Sharing Scheme Registration
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Sharing Scheme Call Origination
In sharing scheme VLR can directly authenticate
the MS during call origination. Note that for a
user with High Call Frequency sharing scheme is
better. For a user with high mobility WS scheme
is better.
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GENERATIONS OF WIRELESS TECHNOLOGIES

• First Generation Systems AMPS
• Advanced Mobile Phone Service which uses FDMA
  for assignment of voice channels to radio
  frequencies.
• Second Generation Systems GSM
• Global System for Mobile which uses a
  combination of TDMA and FDMA.
• 1st and 2nd generation systems designed primarily
• for speech and low bit data services.

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Third Generation Wireless Systems

• Better system capacity
• High speed suitable for wireless internet access
  and multimedia services.
• Uses new network technologies like ATM
  backbones (Asynchronous Transfer Mode) network
  management.
• Uses cdma2000 / wideband come for multiple
  access of air interface.

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HANDOFF MANAGEMENT

• Three issues for handoff management
• Handoff Detection
• Channel Assignment
• Radio Link Transfer

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Metrics for Handoff Detection

• Word Error Indicator (WEI)
• Received Signal Strength Indication (RSSI)
  (dB)
• Quality Indicator (QI) SNR or S/I

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STRATEGIES FOR HANDOFF DETECTION

• Mobile Controlled Handoff (MCHO)
• Network Controlled Handoff (NCHO)
• Mobile Assisted Handoff (MAHO)

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MCHO

• MS Quality maintenance processing

yes
MeasurementProcess
Select new channel or time slot
no
Execute ALT or TST
Link Quality Acceptable?
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NCHO

• BS supervise quality of current connections by
  making RSSI measurements
• MSC commands surrounding BSs to make measurements
  of these links occasionally.
• Based on these measurements MSC makes decision
  when and where to effect the handoff.

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MAHO

• Handoff process is more decentralized.
• MS makes measurements from surrounding BSs and
  reports measurements back to old BS.
• Network decides when to effect the handoff.

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COMPARISONS

• In MCHO the handoff is fastest compared to NCHO
  and MAHO.
• In MAHO and NCHO network signaling is required to
  inform the MS about the handoff decision made by
  the network is transmitted on the failing link.
• May give rise to Forced Terminations.

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HANDOFF FAILURES

• Reasons
• No channel available on selected BS
• Handoff denied by network for lack of resources.
• Target link failure during handoff
• Network takes long time for handoff set up after
  initiation.

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NETWORK SIMULATOR

• ns is an object oriented simulator, written in
  C, with an OTcl interpreter as a front-end

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GETTING STARTED WITH NS

• Now we are going to write a 'template' that can
  be used for all of the first Tcl scripts. We can
  write Tcl scripts in any text editor.
• First of all, we need to create a simulator
  object. This is done with the command
• set ns new Simulator
• Now we open a file for writing that is going to
  be used for the nam trace data. (Network Animator
  trace data).
• set nf open out.nam w
• ns namtrace-all nf

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NS2 Introduction

• The first line opens the file 'out.nam' for
  writing and gives it the file handle 'nf'. In the
  second line we tell the simulator object that we
  created above to write all simulation data that
  is going to be relevant for nam into this file.
• The next step is to add a 'finish' procedure that
  closes the trace file and starts nam.
• proc finish
• global ns nf
• ns flush-trace
• close nf
• exec nam out.nam
• exit 0

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NS2

• The next line tells the simulator object to
  execute the 'finish' procedure after 5.0 seconds
  of simulation time.
• ns at 5.0 "finish
• ns provides you with a very simple way to
  schedule events with the 'at' command.
• The last line finally starts the simulation.
• ns run