Wireless%20Networks:%20Physical%20and%20Link%20Layers

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Wireless Networks Physical and Link Layers

• Wired
• Typically point-to-point connections
• Interference effects are not significant
• Not power constrained

• Wireless
• Typically broadcasted
• Interference not only from other hosts, but other
  devices/physical world phenomenon
• Power constrained

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Adhoc vs. Wireless LANs

• Adhoc
• Peer-to-peer networking
• Short range (10s of meters)
• One device in multiple networks
• Little supervision/management

• Wireless LANs
• Substitute for wired LANs
• Longer range (100s of meters)
• Typically connected to a wired backplane
• Device typically in only 1 network.
• Needs management

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Bluetooth A Case Study for Adhoc Networks

• From
• J. Haartsen, The Bluetooth Radio System. IEEE
  Personal Communications, Feb 200.

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(No Transcript)
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Overview

• Wireless personal area adhoc networking
• Uses 2.45 GHz spectrum (open to public)
• Several miniature networks (called Piconets) can
  co-exist.
• A host can reside in multiple piconets
• Each piconet channel has 1 master and up to 7
  slaves
• Unreliable (and shared) medium
• Limited power

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Multiplexing the bandwidth

• If you do not reserve the slots when someone
  should transmit, then there would be a lot of
  collisions/contention.
• How do you allocate the slots to different hosts?
• In the 2.45 GHz range, we are allowed 2400-2483
  MHz, and we need to find out what frequency to
  use at each instance of time.
• Bluetooth uses 79 frequencies at 1 MHz spacing.

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The Multiplexing Problem
frequency
(how to divide resource among multiple channels?)
time
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Frequency-Division Multiplexing
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Time Division Multiplexing
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Frequency-Time Division Multiplexing
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• Bluetooth uses frequency-time multiplexing
  (frequency hopping)
• You do not want to perform multiplexing
  statically (since you do not know what hosts are
  present, and who will transmit)
• Dynamically determine multiplexing.
• However, if everything is dynamic then we need an
  extensive protocol to figure our who transmits
  when
• Bluetooth uses a frequency hopping pattern
  wherein the identity of the master is used to
  determine the (sequence of) frequencies that
  should be used at each time slot.

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Frequency Hopping

• Use a well defined hopping pattern sequence for
  each piconet.

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Hop Selection Logic
Master identity chooses sequence Clock chooses
index (phase) in sequence Offset established at
connection time
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Connection Establishment

• How do units find each other and establish
  connections?
• No common control unit!
• A unit wakes up to listen (scan) for its id for
  around 10 ms.
• Wake up hop sequence is 32 hops (cyclic) and
  unique for each device.
• The burden is on paging unit to ensure the
  appropriate unit is woken up.

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• It knows id of dest, and its wakeup 32 hop
  (unique) sequence
• It transmits the dest id repeatedly at different
  frequencies in the sequence every 1.25 ms
  (2625us)
• It transmits two dest id codes and listens twice
  for a response.

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Polling Device

• In 10ms (sleep period) 16 frequencies visited
  (half sequence)
• If polling device does not receive response after
  the sleep period, will repeat on the hop
  carriers of the remaining 16 in the sequence.
• Maximum delay is thus twice the sleeping period
• When dest. receives page, it returns back a msg
  with its identity.
• Paging unit then sends the dest its identity and
  clock, and the two now establish a piconet (pager
  becomes master, and dest a slave).

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Medium Access

• A piconet channel is defined by id and system
  clock of Master
• All other units are slaves
• When a piconet is established, slaves add offset
  to their native clocks to sync with Master
• Different channels have different Masters (and
  different hopping patterns)
• Wired solutions for media access control (e.g.
  CSMA) do not suffice.

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The Hidden Terminal Problem
B
A
C

• A sends to B, C cannot receive A
• C wants to send to B
• If use CSMA/CD
• C senses a free medium, thus C sends to A
• Collision at B, but A cannot detect the collision
• Therefore, A is hidden for C

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The Exposed Terminal Problem
B
A
C
D

• B sends to A, C wants to send to D
• If use CSMA/CD
• C senses an in-use medium, thus C waits
• But A is outside the radio range of C, therefore
  waiting is not necessary
• Therefore, C is exposed to B

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• Master completely controls access control, making
  access contention free.
• Time slots are alternately used for Master and
  Slave transmissions.
• The master decides for each slave-gtmaster slot
  which slave should get it.
• Only the slave addressed in the preceding
  master-gtslave slot is allowed to transmit in this
  slave-gtmaster slot.
• If the master has no information to send, it has
  to poll the slave explicitly with a short poll
  packet.

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Master-controlled Media Access
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Packet Structure(in bits)
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Acks/Retransmissions
A bit in header is used to indicate whether
previous packet was received correctly (or if a
re-transmission is needed)