Prof. Maria Papadopouli

1
Experimenting with Mobile Computing
Peer-to-Peer Systems
Spring 2003 Comp 190 290 Seminar
Lecture 6 802.11

• Prof. Maria Papadopouli
• maria_at_cs.unc.edu

2
IEEE 802.11 family

• 802.11b physical layer uses Direct Sequence
  Spread Spectrum (DSSS) or Frequency Hopping (FH),
  operates at 2.4GHz, 11Mbps bitrate
• 802.11a between 5GHz and 6GHz uses orthogonal
  frequency-division multiplexing, up to 54Mbps
  bitrate
• 802.11g operates at 2.4GHz up to 54Mbps bitrate
• All have the same architecture and use the same
  MAC protocol

3
IEEE 802.11b physical layer

• Direct Sequence Spread Spectrum (DSSS) or
  Frequency Hopping (FH)
• DSSS
• As CDMA except all mobile hosts and base stations
  use the same chipping code
• Spreads the energy in a signal over a wider
  frequency range
• FH divides the ISM band into a series of 1-MHz
  channels
• Divides hopping sequences into non-overlapping
  sets
• Any two members of a set are orthogonal hopping
  sequences

4
Code Division Multiple Access (CDMA)

• CDMA assigns a different code to each node
• Codes orthogonal to each other (i.e inner-product
  0)
• Each node uses its unique code to encode the data
  bits it sends
• Nodes can transmit simultaneously
• Multiple nodes per channel
• Their respective receivers correctly receive a
  senders encoded data bits assuming the receiver
  knows the senders code in spite of interfering
  transmissions by other nodes.

5
CDMA Example
Sender
Zi,mdicm
d01
Data bits
d1-1
Spread code
1
1
1
1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
Time slot 1
Time slot 0
1
1
1
1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
Channel output
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CDMA Example

• When no interfering senders, the receiver would
  receive the encoded bits and recover the original
  data bit, di, by computing
• di S Zi,mcm
• Interfering transmitted bit signals are additive
• di S Zi,m Cm

M
1
M
m1
M

1
M
m1
7
802.11 direct-sequence

• Uses the Barker sequence (11-bit sequence)
• It is applied to each bit in the stream by a
  modulo-2 adder
• when 1 is encoded, all the bits in the spreading
  code change
• when 0 is encoded, they stay the same

8
Frequency Hopping
Timing the hops accurately is the challenge
Frequency slot
5
User A
4
User B
3
2
1
0
Time slot
9
802.11 Media Access Protocol

• Coordinates the access and use of the shared
  radio frequency
• Carrier Sense Multiple Access protocol with
  collision avoidance (CSMA/CA)
• Physical layer monitors the energy level on the
  radio frequency to determine whether another
  station is transmitting and provides this
  carrier-sensing information to the MAC protocol
• If channel is sensed idle for time ? DIFS, a
  station can transmit
• When receiving station has correctly completely
  received a frame for which it was the addressed
  recipient, it waits a short period of time SIMS
  and then sends an ACK

10
802.11 Media Access Protocol

• If channel is sensed busy will defer its access
  until the channel is later sensed to be idle
• Once the channel is sensed to be idle for time ?
  DIFS, the station computes an additional random
  backoff time and counts down this time as the
  channel is sensed idle. When the random backoff
  timer reaches zero, the station transmits its
  frame
• Backoff process to avoid having multiple stations
  immediately begin transmission and thus collide

11
Positive acknowledgement of data transmission
Node 1
Node 2
Time
frame
ACK
802.11 allows stations to lock out contention
during atomic operation so that atomic sequences
are not interrupted by other Hosts attempting to
use the transmission medium
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Hidden node problem
From the perspective of node 1, node 3 is
hidden If node 1 and node 3 communicate
simultaneously, node 2 will be unable to make
sense of anything Node 1 and node 3 would not
have any indication of the error because the
collision was local to node2
13
Fading problem
Node 1 and 3 are placed such that their signal is
not strong enough for them to detect each
others transmissions, and yet their
transmissions are strong enough to have
interfered with each other at node 2
14
Carrier-Sensing Functions

• IEEE 802.11 to avoid collisions CSMA/CD
• Physical carrier-sensing
• Expensive to build hardware for RF-based media
• Transceivers can transmit and receive
  simultaneously only if they incorporate expensive
  electronics
• Hidden nodes problem
• Fading problem
• Virtual carrier-sensing
• Collision avoidance stations delay transmission
  until the medium becomes idle

Undetectable collisions
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RTS/CTS clearing
(1) RTS
Node 2
Node 1
Node3
Node 1
(3) Frame
RTS
(2) CTS
Time
(4) ACK
CTS
frame
Node 2
ACK
RTS reserving the radio link for
transmission RTS, CTS Silence any station that
hear them
16
Using the NAV for virtual carrier sensing
(eg 4-8KB)
(e.g.10ms)
Contention Window
Access to medium deferred
NAV is carried in the headers of CTS RTS
17
Backoff with DCF

• Contention window or backoff window follows the
  DIFS
• Window is divided in time slots
• Slot length is medium-dependent
• Window length limited and medium-dependent
• Hosts pick a random slot and wait for that slot
  before attempting to access the medium
• All slots are equally likely selections
• Host that picks the first slot (earlier number)
  wins
• Each time the retry counter increases, the
  contention window moves to the next greatest
  power of two

18
Contention window size
The contention window is reset to its minimum
size when frames are transmitted successfully, or
the associated retry counter is reached and the
frame is discarded
19
Fragmentation burst
RTS
Frame 0
Frame1
Sender
SIFS
SIFS
SIFS
SIFS
ACK0
ACK1
CTS
Receiver
SIFS
SIFS
NAV (RTS)
DIFS
Fragment 0
NAV
NAV(CTS)
ACK 0
Access to medium deferred
Backoff slots
Each fragment sets the NAV to hold the medium
until the end of the ACK of the next frame
20
Fragmentation

• When single fragments are lost, only the lost
  fragment must be retransmitted
• Change the fragmentation threshold to tune
  network behavior
• Higher fragmentation thresholds
• less overhead, but
• higher cost for lost /damaged frames (more data
  must be discarded retransmitted)
• Lower fragmentation thresholds
• higher overhead, but
• increased robustness in face of hostile conditions

21
Broadcast traffic

• Frames destined for group addresses cannot be
  fragmented
• They are not acknowledged
• There is no facility built into the MAC for
  retransmitting broadcast or multicast frames

22
Contention-based access using Distributed
Coordination Function (DCF)

• If the medium has been idlegt DIFS, transmission
  can begin immediately
• If the previous frame was received without
  errors, the medium must be free for at least the
  DIFS
• If the previous transmission contained errors,
  the medium must be free for the amount EIFS
• If the medium is busy, the station must wait for
  the channel to become idle (access deferral). If
  access is deferred, the station waits for the
  medium to become idle for the DIFS and prepares
  for the exponential backoff procedure

23
Basic service set (BSS)
BSSID 48-bit identifier that distinguish it from
other BSSs in the network Filtering link-layer
broadcasts from physically overlapping network
MH
Independent BSSs (ad hoc mode)
MH
Access Point AP
AP
Infrastructure BSS
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Discovering a BSS
Node 1
BSSID 1
At the end of the process Scan report
with Beacon interval (e.g., 0.1sec) Listen
interval (for power-saving) Timing parameters for
synchronizing the host with AP
BSSID 2
BSSID 3
Passive Active Scanning
25
Passive Scanning

• Stations moves to each channel on the channel
  list and waits for Beacon frames
• Any Beacons received are buffered to extract
  information about the BSS that sent them
• Saves battery because it does not require
  transmitting

26
Active Scanning

• Move to each channel and wait for an indication
  of an incoming frame or for the Probe Delay timer
  to expire
• If an incoming frame is detected, the channel is
  in use can be probed.
• Gain access to the medium using the basic DCF
  access procedure and send a Probe Request frame
• Wait for the minimum channel time, to elapse
• If the medium was never busy, there is no
  network, go to the next channel
• If the medium was busy, wait until the maximum
  time and process any Probe Response frames

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Joining a BSS

• Authentication and association are required
• Mobile host (MH) initiates the process
• Once a MH has authenticated to an AP, it issues
  an Association Request
• AP process the association request
• 802.11 does not specify how to determine whether
  an association should be granted
• One common consideration is the amount of space
  required for frame buffering
• When association request is granted, AP responds
  with successful status code and the Association ID

28
Re-association Procedure
1.Reassociation Request My old AP was
MH
New AP
Old AP
(1)
(3)
MH
(2)
(4)
Access Point
AP
MH
2.Reassociation Response I am your new AP, Here
is your association ID
3.IAPP Please, send any buffered frames for
4.IAPP Sending frames
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Power-saving mode

• Hosts shut down the radio transceiver and sleep
  periodically
• During sleeping periods, APs buffer unicast
  frames for sleeping hosts
• Frames are announced by subsequent beacon frames
  (announcement traffic indication ATIM messages)
• To retrieve unicast buffered frames newly
  awakened hosts use PS-Poll frames
• Host requesting a frame with PS-Poll must stay
  awake until it is delivered

30
Buffering at AP

• 802.11 mandates AP to use aging function to
  discard buffered frames
• Mobile hosts depend on AP to buffer traffic for
  at least the listen interval specified with the
  association
• Standard forbids aging function from discarding
  frames before the listen interval has elapsed
• Vendors develop different buffer management
  policies

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Delivering multicast and broadcast frames

• Cannot be delivered using polling algorithm
• Buffering is identical to the unicast case,
  except that frames are buffered whenever any MH
  associated with the AP is sleeping
• AP indicate whether any broadcast/multicast
  frames are buffered in the beacon message (e.g.,
  first TIM bit0)
• TIM indicates when a DTIM beacon will be sent
• Buffered broadcast/multicast are transmitted
  after a DTIM beacon

32
Channel Partitioning Protocols

• Partition a broadcast channels bandwidth among
  nodes sharing the channel
• Space-division multiplexing (SDP)
• Time-division multiplexing (TDM)
• Frequency-division multiplexing (FDM)
• Code-division multiplexing (CDM)

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Examples of TDM FDM
34
TDM

• Eliminates collisions is fair
• Each node gets a dedicated transmission rate of
  R/N bps during each frame time
• Two main drawbacks
• A node is limited to
• an average rate of R/N bps
• has to wait for its turn in the transmission
  sequence
• Even when it is the only node transmitting

35
FDM

• FDM creates N smaller channels of R/N bps
• It avoids collisions is fair
• Shares all the principal disadvantages with TDM

36
CDMA

• Multiple users per channel
• Each bit being sent by the sender is encoded by
  multiplying the bit by a signal (the code) that
  changes at a much faster rate (chipping rate)
  than the original sequence of data bits
• Orthogonal spreading codes
• Nodes can transmit simultaneously

37
Simple CDMA Example
Sender
Zi,mdicm
d01
Data bits
d1-1
1
1
1
1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
Time slot 1
Time slot 0
1
1
1
1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
Channel output
Time slot 1
Time slot 0
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Regulations

• International Telecommunication Union (ITU)
  sub-organization of the UN
• Worldwide coordination of telecommunications
  activities
• ITU-R Standardization in the wireless sector
• Frequency allocation

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Forward Error Correction (FEC)

• Add redundant information to the original pkt
  stream
• Reconstruct approximations or exact versions of
  some of the lost packets
• Sends redundant encoded chunk after every n
  chunks
• Redundant chunkexclusive OR of n original chunks
• Send lower resolution data as redundant
  information

40
Physical Layer

• Conversion of a stream of bits into signal _at_
  transmitter
• Conversion of the signal to a stream of bits _at_
  receiver
• Frequency selection
• Generation of the carrier frequency
• Signal detection
• Modulation of data onto a carrier frequency and
  depending on the transmission scheme encryption