Cellular Phone Networks

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Cellular Phone Networks

• Some slides are based on Computer Networking A
  Top Down Approach Featuring the Internet,
  3rdedition. Jim Kurose, Keith RossAddison-Wesley,
  July 2004.

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Cellular Phone Networks

• Compared to WLANs
• Longer range
• Less speed
• Higher mobility
• More for voice services (evolving!)
• Overview
• Divides the area into cells.
• Base stations in each cells.
• User have cellular phones.
• The phones talks to the base station directly in
  wireless.

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GSM

• Global System for Mobile (GSM)
• Used by more than 3 billion people
• 2nd Generation (2G), because everything is
  digital, compared to the analog mobile phones
  which is 1G
• Operates in 900MHz and 1800MHz band, or 850M and
  1900M bands in the US
• Uplink and down link both 25MHz wide. In GSM900,
  uplink band is 890-915M, downlink is 935-960M.
• A band is divided into 124 channels with 200KHz
  spacing. The data rate is about 270Kbps for a
  channel.

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GSM

• GSM continued
• The time in a channel is divided into 8 slots.
  Each slot is 577us and is allocated to a user.
• A mobile phone is allocated two channels, one for
  uplink and the other for downlink. Separated by
  45MHz.
• The uplink and downlink slot numbers are
  separated by 3 a phone never transmits and
  receives at the same time
• FDMA/TDMA (Frequency Division Multiple Access /
  Time Division Multiple Access).
• Different from In wireless LAN, in which a node
  is given the entire bandwidth for the time it
  needs to send a packet. Difference due to the
  nature of the application.
• The peak transmission power of a GSM phone is 2W
  in 900M band and 1W in 1800M band. In contrast,
  your wireless router transmits at 20dBm, which is
  0.1W. Difference due to the distance.

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GMSK modulation

• GSM uses Gaussian minimum-shift keying (GMSK).
• MSK basically replaces 1,-1 with a positive
  half sine wave or a negative half sine wave, and
  multiply it with the carrier.
• GMSK first pass the digital waveform through a
  Gaussian low pass filter.
• Reduces interferences

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Frequency Reuse

• Adjacent cells should not use the same frequency,
  because it will cause interference.
• However, non-adjacent cells may use the same
  frequency.
• Due to frequency reuse, the actual number of
  channels used in an area is larger than 124.
• Given an area, reducing the size of cells
  (reducing the transmit power) increases the
  frequency reuse, hence increasing the total
  capacity.
• Cell planning is a major issue.

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Frequency Reuse

• Cell sizes in GSM
• Large microcell 3-30km
• Small microcell 1-3km
• Microcell 0.1-1km
• Picocell 0.01-0.1km
• Nanocell 0.001-0.01km
• Typically, there are lots of small cells plus
  several large cells. The small cells carries the
  majority of the traffic. The large cells fills
  the holes.

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Capacity

• Given the number of channels in a cell, the
  maximum number of users supported is fixed.
• With GSM, if there are C channels, we can support
  8C users at most
• The number of users can be much larger than the
  maximum number of slots. That is why some time
  you see emergency calls only.
• The hope is that not all users want to make calls
  at the same time.

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The Erlang-B Formula

• The phone company wants to allocate enough
  channels such that the probability of dropping a
  call is below a threshold.
• There is an old formula, the Erlang-B Formula, to
  calculate this. (It is probably 100 years old.)
  
• Check http//wireless.per.nl/reference/chaptr04/e
  rlang/erlangb.htm

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The Erlang-B formula

• Given N channels, where each channel supports a
  user. A new call can be served if and only if
  there is an available channel otherwise it is
  dropped.
• Assume the calls arrive following a Poisson
  process, that is, the inter-arrival time between
  two calls follows exponential distribution with
  parameter . Assume the duration of a phone
  call follows the exponential distribution with
  parameter .
• What is the probability of call dropping?

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Exponential Distribution

• The p.d.f. of exponential distribution
• A unique feature. Suppose you are waiting for a
  bus. The bus is following a random schedule, and
  the inter-arrival time follows the exponential
  distribution. If you just missed a bus, the
  probability you have to wait t seconds is given
  above. But, given that you have waited t0
  seconds, what is the probability you have to wait
  another t seconds?

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Exponential Distribution

• The probability that you have to wait for t
  seconds given you have waited t0 seconds is
• We know that
• and
• So,
• That is, no matter how long you waited, you look
  like you didnt wait at all! Memoryless.

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The Erlang-B Formula

• Consider the phone system. It can be fully
  described by the number of current users, if the
  inter-arrival time of calls and the call duration
  both follows the exponential distribution.
• Because regardless of how long you have waited
  for the next arriving call or for a call to
  finish, it looks like you didnt wait for any
  time at all
• In other words, by assuming the exponential
  distribution, the system is memoryless.

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Markov Chain

• The number of current phone calls will change
  with time
• When a new call arrives
• When a call finishes
• Given that there are i ongoing calls, or, when
  the system is in state i, the next state is
  either i-1 or i1.
• Time is fine-grained and no two events happen at
  the same time.
• The system is a Markov Chain if the probability
  to go from the current state to a next state is
  only determined by the current state, but not
  previous states.
• Because the system is memoryless, it is a Markov
  chain.

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The Erlang-B formula

• Probability to go from state i to state i-1
• Event happens when a call finishes before a new
  call arrives
• Probability to go from state i to state i1
• Event happens when a new call arrives before an
  existing call finishes

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The Erlang-B formula

• The probability that none of the i existing calls
  finishes after time t is
• Therefore, the probability that the first call
  finishes at time t among the i calls is

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The Erlang-B formula

• The probability that a new call arrives after the
  first existing call finishes is
• So, at state i, with probability , the
  next state is i-1 with probability ,
  the next state is i1.

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The Erlang-B formula

• Suppose you know that the probability that there
  is no existing call is p0. What is the
  probability that there is one ongoing call?
• Note that in equilibrium, the number of 0 to 1
  transitions is the same as 1 to 0 transitions.

0
1
2
N
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The Erlang-B formula

• You can apply similar arguments to get
• where .
• Then, considering that all probabilities summing
  up at 1, you will get pi for all 0ltiltN

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The Erlang-B formula

• Finally, the probability of blocking is the
  probability that a call arrives when the system
  is in state N.
• Because Poission traffic is raondom, it is simply
  pN.

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http//elm.eeng.dcu.ie/kaszubow/Biography/Lecture
5.pdf
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Management of GSM

• Base Transceiver Station (BTS)
• Has 1 to 16 transceivers
• Base Station Controller (BSC)
• Controls hundreds of BTS
• Mobile Switching Center (MSC)
• Typical MSC supports up to 100,000 mobiles and
  5000 simultaneous calls
• MSC are connected with each other.
• Gateway MSC connects the GSM system to external
  networks, e.g. PSTN.
• Each MSC controls at least one Base Station
  System (BSS)
• Authentication Center (AUC)
• Operations and maintenance center (OMC)

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GSM

• Visitors Location Register (VLR) Each MSC
  connects to a VLR. The VLR is a data base with
  the information about cellphones temporarily
  located in the area served by particular MSC.
• Home Location Register (HLR) database of all
  cellphones permanently registered in the system.
  Stores
• Current location of the phone.
• All parameters needed by the system to establish
  a connection with the user.
• The address of the VLR currently associated with
  the phone
• Encryption keys for data transmission and user
  authentication

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GSM Call Flow (Simplified)

1. User enters the phone number and presses the
   send button.
2. To set up the phone call, the phone needs to send
   information to the MSC. The phone sends Radio
   Resource Channel Request to the associated BSS.
   The phone then waits to hear from the BSS at the
   Access Grant Channel (AGCH). The request is sent
   on the Random Access Channel (RACH) according to
   ALOHA.
3. The BSS allocates a Traffic Channel (TCH),
   including the frequency and time slot, and
   broadcast it in the AGCH. It also contains
   information about time and frequency corrections.
4. The phone applies the corrections and tune to the
   assigned TCH.
5. MSC checks whether the phone is authenticated.
6. The BSS enables ciphering with the phone. At this
   step the connection has been set up between the
   phone and MSC. The BSS just forwards the message.
7. The phone sends a connection set up request to
   the MAC with the called phone number. The MSC
   connects to the PSTN and allocates the voice
   communication channel between the BSS.
8. Make the conversation.
9. User presses the end button. The MSC releases
   the voice channel with the BSS. The MSC informs
   the PTSN about the call release and the PTSN will
   inform the call has been released on its end. The
   phone then releases the TCH.

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GSM indirect routing to mobile
home network
correspondent
Public switched telephone network
mobile user
visited network
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GSM handoff with common MSC

• Handoff goal route call via new base station
  (without interruption)
• reasons for handoff
• stronger signal to/from new BSS (continuing
  connectivity, less battery drain)
• load balance free up channel in current BSS
• GSM doesnt mandate why to perform handoff
  (policy), only how (mechanism)
• handoff initiated by old BSS

new routing
old routing
old BSS
new BSS
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GSM handoff with common MSC

• 1. old BSS informs MSC of impending handoff,
  provides list of 1 new BSSs
• 2. MSC sets up path (allocates resources) to new
  BSS
• 3. new BSS allocates radio channel for use by
  mobile
• 4. new BSS signals MSC, old BSS ready
• 5. old BSS tells mobile perform handoff to new
  BSS
• 6. mobile, new BSS signal to activate new channel
• 7. mobile signals via new BSS to MSC handoff
  complete. MSC reroutes call
• 8 MSC-old-BSS resources released

old BSS
new BSS
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GSM handoff between MSCs

• anchor MSC first MSC visited during call
• call remains routed through anchor MSC
• new MSCs add on to end of MSC chain as mobile
  moves to new MSC
• IS-41 allows optional path minimization step to
  shorten multi-MSC chain

correspondent
anchor MSC
PSTN
(a) before handoff
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GSM handoff between MSCs

• anchor MSC first MSC visited during cal
• call remains routed through anchor MSC
• new MSCs add on to end of MSC chain as mobile
  moves to new MSC
• IS-41 allows optional path minimization step to
  shorten multi-MSC chain

correspondent
anchor MSC
PSTN
(b) after handoff
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General Packet Radio Service (GPRS)

• General Packet Radio Service
• Supports data service.
• GSM with GPRS is often called 2.5G, because it is
  providing a moderate level of data service
• Supports IP, PPP (Point to point protocol). The
  mobile device can be used as a modem.
• Different coding scheme can be used, CS-1 to
  CS-4, with different over head. Per time slot,
  the data rate is 8, 12, 14.4, 20.2 kbps.

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GRPS

• Multiple Access
• Users are assigned frequency channels and time
  slots.
• Packets are constant length, determined by the
  GSM slot.
• Downlink first come first served
• Uplink Slotted ALOHA for reserving, dynamic TDMA
  for data transmission

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GSM SIM
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GSM Security
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Reading

• http//www.eventhelix.com/realtimemantra/Telecom/G
  SM_Originating_Call_Flow.pdf