Wireless LANs

1
Wireless LANs

• Wireless proliferating rapidly.
• IEEE 802.11 --gt link access standard designed
  for use in a limited geographic setting.
• Various versions 802.11a, 802.11e, 802.11g,
  802.11n.
• Physical layer evolution -- increased rates .
• As an example, 802.11n uses multiple antennas --
  can provide very high data rates.

2
Physical Properties

• Typically use 3 kinds of physical media -- two
  based on spread-spectrum and one based on IR.
• IR transmission is diffused limited range.
  (not much in use)
• Spread spectrum -- spread signal over a higher
  frequency -- provides
• reduced impact from external interference.
• more robustness to signal loss.

3
Fading

• Signal travels and reflects off objects.
• Multiple copies converge at receiver (Red copy
  and Green copy).
• Copies interfere -- may self destruct -- called
  multipath fading.
• Signal combination depends on frequency of
  transmission.

4
Spread Spectrum

• The use of larger bandwidth provides robustness
  to fading/interference.

Wiped out frequencies
5
Frequency hopped Spread Spectrum

• Transmit signal over a random sequence of
  frequencies (not really random but
  pseudo-random).
• Computed using a pseudo-random sequence
  generator.
• Receiver uses the same generator -- they can
  synchronize (same seed).

6
Direct Sequence Spread Spectrum

• Each bit translated into N random symbols
  called chips.
• Random chips generated using the pseudo-random
  number generator.
• Transmitted sequence called a n-bit chipping
  code.
• If receiver knows the chips, it can decode.
• Others cannot, they see a higher frequency signal
  -- can be filtered out as noise.

7
802.11 PHY layers

• One PHY layer uses frequency hopping over a 79.1
  MHz range.
• A second version uses a 11 bit chipping sequence.
• Both run in the 2.4 GHz band.
• Note For other than the intended receiver
  signal looks like noise.

8
Medium Access Control

• Can we use the same protocol as in the Ethernet ?
• Carrier Sensing -- Sense channel, transmit when
  channel is idle, back-off when collision occurs ?
• Not really -- why ?

9
Hidden Terminals

• B can talk to A and C but not D.
• C can talk to B and D but not A.
• A sends to B -- C cannot make out (cannot
  sense), and it sends to D.
• Collision at B (.
• A and C are hidden from each other -- hidden
  terminal problem.

10
Exposed Terminals

• On the other hand, if B is sending A, C will
  sense channel to be busy.
• Will not send to D.
• Not good either!
• C is exposed to Bs transmission.

11
The MACA scheme

• 802.11 addresses these problems by using an
  algorithm called MACA -- multiple access with
  collision avoidance.
• Also referred to as virtual carrier sensing.
• Sender sends a Request to Send or RTS to
  Receiver.
• Tells senders neighbors of intent to send.
• Receiver sends a Clear to send or CTS to
  sender.
• Tells receivers neighbors of intent to receive.

12
Example

• A sends to B.
• As RTS tells everyone in its neighborhood that
  it is sending.
• Bs CTS tells everyone in its neighborhood that
  it is receiving.
• Now C knows that B is receiving and does not
  initiate communications with D.

13
Details

• RTS indicates the time for which the sender
  wishes to hold the channel.
• Receiver echoes this duration field to the
  sender.
• Every node knows -- how long the transmission is
  for.

14
Data transfer

• Upon a successful RTS/CTS exchange, nodes
  initiate data transfer.
• Receiver sends ACK after successfully receiving
  frame.
• Exposed terminal issue left alone
• Random wait when CTS is not received
• Back-off similar to what happens with Ethernet.

15
Access Points

• While 802.11 facilitates operations in an ad
  hoc mode, typically, some of the wireless nodes
  connected to a wireline infrastructure.
• These are called access points (APs) -- some
  people also call them base-stations (more
  appropriate for cellular networks)
• Other mobile hosts connect to the Internet via
  these APs.

16
Distribution System

• APs connected via the distribution system --
  could be Ethernet or FDDI based (or anything
  else).
• Distribution system runs at Layer 2 -- not Layer
  3 (Network Layer) entity.

17
Selection of APs

• Via a process called scanning.
• When a node wants to select an AP, it sends a
  probe message.
• APs that get this, respond with a Probe-Response.
• Node selects one of the APs (strongest signal
  ?),and sends an Association Request.
• Selected AP responds with an Association
  Response.
• Active scanning -- Probes sent actively when
  mobile joins the network or moves around and out
  of coverage.
• Passive scanning -- APs send beacons -- mobiles
  hear and if they find a more attractive AP, they
  can switch.