4.2.6 Wireless LAN Protocols

1
4.2.6 Wireless LAN Protocols

• All radio transmitters have some fixed range
• Hidden station problem Fig. 4-11a
• Exposed station problem Fig 4-11b
• MACA (Multiple access with collision avoidance)
  Fig 4.12 Sender A sends a short frame with
  length of the data frame to follow the receiver
  B replies with a CTS on receipt of CTS, A sends
  the data frame
• MACAW MACA for wireless (i) An ACK frame is
  sent by receiver for each frame received (ii)
  CSMA is used to prevent two stations from
  transmitting RTS (iii) Backoff algorithm for each
  source-destination pair (iv) Stations exchange
  information about congestion

2
Wireless LANs

• Sections 4.4-4.6

3
Wireless LANs

• 802.11 Protocol stack Fig. 4-25
• 802.11 Physical layer Infra red (TV remote
  control), FHSS (Frequency hopping spread
  spectrum)--- for random control and security,
  DSSS (Direct sequence spread spectrum), OFDM
  (Orthogonal frequency division multiplexing),
  HR-DSSS ( High-rate Direct sequence spread
  spectrum)

4
802.11 MAC Sublayer Protocol

• How is it different from Ethernet? In Ethernet, a
  station waits until the ether goes silent and
  starts transmitting. If it does not receive a
  noise burst back within the first 64 bytes, the
  frame almost assuredly been delivered correctly.
  With wireless, this does not hold.
• Hidden station problem Fig. 4-26a C is
  transmitting to B---but A does not know that B is
  busy so it too transmits leading to collision at
  B
• Exposed station problem Fig. 4-26b A is
  transmitting to D B wants to transmits to C but
  wrongly concludes that C is busy.
• Hence, 802.11 does not use Ethernet protocol

5
Two modes of 802.11 operations

• DCF (Distributed coordination function) CSMA/CA
• Method 1 When a station wants to transmit, it
  senses the channel. If it is idle, it just starts
  transmitting. It does not sense the channel
  while transmitting. If the channel is busy, the
  sender defers until the channel goes idle. In
  case of a collision, it uses binary exponential
  backoff.

6
DCF (Cont.)

• Method 2 MCAW Fig. 4-27 BC are in the range
  of A D is within the range of B but not within
  As range
• A sends RTS to B C also listens and declares a
  period of busy or NAV (Network Allocation Vector)
• B sends CTS to A this is also received by D
  which declares a period of NAV
• On receiving CTS, A sends the data
• B sends an ACK.

7
Handling unreliability through smaller fragments

• 802.11 allows frames to fragmented into smaller
  pieces, each with its own checksum
• Stop-and-wait protocol is used to send the
  individual fragments

8
PCF

• Base station polls other stations
• No collisions occur due to centralized control
• Base station broadcasts a beacon frame
  periodically
• Beacon frame has system parameters such as
  hopping sequences and dwell times (time at each
  frequency), etc. It invites stations to join.

9
802.11 Data Frame Format

• Fig. 4-30
• Frame classes Data, control, management
• Frame control
• Duration How long the channel will be busy (NAV)
• Address1, 2SRC and DST
• Address 3,4---Base stations of SRC and DST

10
Broadband Wireless

• 802.16 is the standard for broadband wireless
• Example Erecting a big antenna on top of a hill
  just outside of town and let homes receive the
  transmissions
• 802.16 receivers are wireless but not mobile
  802.11 primarily deals with individual mobile
  users with limited batter power
• 802.16 operates in the 10-66 GHz range while
  802.11 operates in the ISM band (2.4 GHz)
• Summary 802.11 is more like a mobile Ethernet
  802.16 is more like a stationary wireless cable
  television.

11
BLUETOOTH

• Short-range, low-power, inexpensive radios
• Piconetbasic unitMaster node and up to 7 slave
  nodes within a distance of 10 meters all
  communication is master-slave not slave-slave
• Piconert is a centralized TDM system, with the
  master controlling the clock and determining
  which devices get to communicate in which time
  slot---all communication is between master and
  slave only
• Multiple piconets can coexist in the same room
  they may be connected via bridge nodes forming a
  scatternet
• Up to 255 parked nodes in the net

12
Mobile IP

• Every IP address contains a network address and a
  host number---not suitable for mobile computers
• Goals for mobile IP
• Each mobile must be able use its home IP address
  anywhere
• Software changes to the fixed hosts are not
  permitted
• Changes to the router softwrae and tables are not
  permitted
• Most packets for mobile hosts should not make
  detours on the way
• No overhead should be incurred when a mobile host
  is at home
• Solution Every site that wants to allow its
  users to roam creates a home agent every site