Wireless Local Area Networks

1
Wireless Local Area Networks
2
Wireless Local Area Networks

• The proliferation of laptop computers and other
  mobile devices (PDAs and cell phones) created an
  obvious application level demand for wireless
  local area networking.
• Companies jumped in, quickly developing
  incompatible wireless products in the 1990s.
• Industry decided to entrust standardization to
  IEEE committee that dealt with wired LANS
  namely, the IEEE 802 committee!!

3
Wireless MAC Protocol

• Outline
• design challenges for wireless MAC
• hidden/exposed stations
• flexible control for QoS support
• two design paradigms
• multiple access based
• token based
• rationale for design choices

4
Wireless Networking Environment

• A simple model
• A single shared physical channel among users
• Omni-directional antenna, limited transmission
  range
• Same transmission rate for all users
• Channel characteristics(illustrated with
  examples)
• wireless transmission is spatial and local
• sender receiver different views of the world
• relevant contention is at the receiver side
• contention may induce collisions
• contention/collision/congestion is location
  dependent
• channel access is a collective behavior from the
  fairness perspective the notion of local is
  misnomer
• Wireless MAC how to address channel access in a
  wireless environment

5
Design Goals for Wireless MAC

• Requirements for a wireless MAC protocol
• robustness
• efficiency
• fairness
• support for priority and QoS
• support for multicast

6
Hidden Station Problem

• Hidden Stations within the range of the intended
  receiver, but out of range of the transmitter
• hidden sender C

A
B
D
C
Problem A transmits to B, if C transmits (to D),
collision at B Solution hidden sender C needs to
defer (Question who tells C, A or B?)

• hidden receiver C

A
B
D
C
Problem A transmits to B, if D xmits to C, C
cannot reply. D confuses (4 cases) Solution D
needs to be notified that its receiver C is hidden
7
Exposed Station Problem

• Exposed Stations within the range of the
  intended sender, but out of range of the receiver
• exposed sender B

A
B
D
C
Problem C transmits to D, if B transmits (to A),
B cannot hear from A Solution exposed sender B
needs to defer

• exposed receiver B

A
B
D
C
Problem C transmits to D, if A xmits to B, B
cannot reply. A confuses (4 cases) Solution A
needs to be notified that its receiver B is
exposed (how can B hears A?)
8
Summary of hidden and exposed station problem

• Receivers perception of a clean/collided packet
  is critical
• Hidden/exposed senders need to defer their
  transmissions
• Hidden/exposed receivers need to notify their
  senders about their status

9
MAC Protocol

• Resolve channel contention access
• Channel access arbitration
• know who are there
• allocate the channel among multiple senders
  receivers who share the channel
• Collision avoidance
• multiple access based
• token based
• Collision resolution
• backoff based

10
Solution Space for channel contention

• Multiple access approach
• with carrier sensing
• carrier sensing provides collision information
  at the sender, NOT the receiver
• FAMA, 802.11
• without carrier sensing
• MACA, MACAW
• cons and pros robust, solves hidden/exposed
  station problem, hard to provide QoS
• Token based approach
• TDMA, DQRUMA
• cons and pros easy to provide QoS, less robust,
  hard to handle hidden/exposed stations

11
IEEE 802 Standards Working Groups
The important ones are marked with . The ones
marked with ? are hibernating. The one marked
with gave up.
12
IEEE Standards for Wireless Networks
13
IEEE 802.11 (WLAN)
14
Common Aliases of Wireless Standards
15
Categories of Wireless Networks

• Base Station all communication through an
  access point note hub topology. Other nodes
  can be fixed or mobile.
• Infrastructure Wireless base station network
  is connected to the wired Internet.
• Ad hoc Wireless wireless nodes communicate
  directly with one another.
• MANETs (Mobile Ad Hoc Networks) ad hoc nodes
  are mobile.

16
Wireless LANs

• a) Wireless networking with a base station. (b)
  Ad hoc networking.

17
The 802.11 Protocol Stack
Part of the 802.11 protocol stack.
18
IEEE standard 802.11
fixed terminal
mobile terminal
server
infrastructure network
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.11 PHY
19
Wireless Physical Layer

• Physical layer conforms to OSI (five options)
• 1997 802.11 infrared, FHSS, DHSS
• 1999 802.11a OFDM and 802.11b HR-DSSS
• 2001 802.11g OFDM
• 802.11 Infrared
• Two capacities 1 Mbps or 2 Mbps.
• Range is 10 to 20 meters and cannot penetrate
  walls.
• Does not work outdoors.
• 802.11 FHSS (Frequence Hopping Spread Spectrum)
• The main issue is multipath fading.
• 79 non-overlapping channels, each 1 Mhz wide at
  low end of 2.4 GHz ISM band.
• Same pseudo-random number generator used by all
  stations.
• Dwell time min. time on channel before hopping
  (400msec).

20
Wireless Physical Layer

• 802.11 DSSS (Direct Sequence Spread Spectrum)
• Spreads signal over entire spectrum using
  pseudo-random sequence (similar to CDMA).
• Each bit transmitted using an 11 chips Barker
  sequence, PSK at 1Mbaud.
• 1 or 2 Mbps.
• 802.11a OFDM (Orthogonal Frequency Divisional
  Multiplexing)
• Compatible with European HiperLan2.
• 54Mbps in wider 5.5 GHz band ? transmission range
  is limited.
• Uses 52 FDM channels (48 for data 4 for
  synchronization).
• Encoding is complex ( PSM up to 18 Mbps and QAM
  above this capacity).
• E.g., at 54Mbps 216 data bits encoded into into
  288-bit symbols.
• More difficulty penetrating walls.

21
Wireless Physical Layer

• 802.11b HR-DSSS (High Rate Direct Sequence Spread
  Spectrum)
• 11a and 11b shows a split in the standards
  committee.
• 11b approved and hit the market before 11a.
• Up to 11 Mbps in 2.4 GHz band using 11 million
  chips/sec.
• Note in this bandwidth all these protocols have
  to deal with interference from microwave ovens,
  cordless phones and garage door openers.
• Range is 7 times greater than 11a.
• 11b and 11a are incompatible!!

22
Wireless Physical Layer

• 802.11g OFDM(Orthogonal Frequency Division
  Multiplexing)
• An attempt to combine the best of both 802.11a
  and 802.11b.
• Supports bandwidths up to 54 Mbps.
• Uses 2.4 GHz frequency for greater range.
• Is backward compatible with 802.11b.

23
802.11 - MAC layer

• Access methods
• MAC-DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off
  mechanism
• minimum distance between consecutive packets
• ACK packet for acknowledgements (not for
  broadcasts)
• MAC-DCF w/ RTS/CTS (optional)
• Distributed Foundation Wireless MAC
• avoids hidden terminal problem
• MAC- PCF (optional)
• access point polls terminals according to a list

24
Distribute Coordination Function (DCF)

• Uses CSMA/ CA (CSMA with Collision Avoidance).
• Uses both physical and virtual carrier sensing.
• Two methods are supported
• based on MACAW(Multiple Access with Collision
  Avoidance for Wireless) with virtual carrier
  sensing.
• 1-persistent physical carrier sensing.

25
Virtual Channel Sensing in CSMA/CA

• virtual implies source station sets duration
  field in data frame or in Ready-to-Send (RTS) and
  Clear-to-Send (CTS) frames.
• Stations then adjust their NAV (Network
  Allocation Vector) accordingly!

26
1-Persistent Physical Carrier Sensing

• Station senses the channel when it wants to send.
• If idle, station transmits.
• Station does not sense channel while
  transmitting.
• If the channel is busy, station defers until idle
  and then transmits.
• Upon collision, wait a random time using binary
  exponential backoff.

27
802.11 - MAC layer (cont)

• Priorities
• defined through different inter frame spaces
• no guaranteed, hard priorities
• SIFS (Short Inter Frame Spacing)
• highest priority, for ACK, CTS, polling response
• PIFS (PCF IFS)
• medium priority, for time-bounded service using
  PCF
• DIFS (DCF, Distributed Coordination Function IFS)
• lowest priority, for asynchronous data service

DIFS
DIFS
PIFS
SIFS
medium busy
next frame
contention
t
Access (after CWmin) if medium is free ? DIFS
28
802.11 - CSMA/CA basic access method
contention window (randomized back-offmechanism)
DIFS
DIFS
medium busy
next frame
t
direct access if medium is free ? DIFS
slot time

• station ready to send starts sensing the medium
  (Carrier Sense based on CCA, Clear Channel
  Assessment)
• if the medium is free for the duration of an
  Inter-Frame Space (IFS), the station can start
  sending after CWmin (IFS depends on packet type)
• if the medium is busy, the station has to wait
  for a free IFS, then the station must
  additionally wait a random back-off time
  (collision avoidance, multiple of slot-time)
• if another station occupies the medium during the
  back-off time of the station, the back-off timer
  stops (fairness)

29
802.11 - CSMA/CA (cont)

• Sending unicast packets
• station has to wait for DIFS (and CWmin) before
  sending data
• receivers acknowledge at once (after waiting for
  SIFS) if the packet was received correctly (CRC)
• automatic retransmission of data packets in case
  of transmission errors

DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other stations
t
waiting time
contention
30
IEEE 802.11 MAC Protocol

• CSMA Version of the Protocol
• sense channel idle for DISF sec (Distributed
  Inter Frame Space)
• transmit frame (no Collision Detection)
• receiver returns ACK after SIFS (Short
  Inter Frame Space)
• if channel sensed busy gt binary backoff
• NAV Network Allocation Vector (min time of
  deferral)

31
802.11 - CSMA/CA with RTS/CTS

• Sending unicast packets
• station can send RTS with reservation parameter
  after waiting for DIFS (reservation declares
  amount of time the data packet needs the medium)
• acknowledgement via CTS after SIFS by receiver
  (if ready to receive)
• sender can now send data at once, acknowledgement
  via ACK
• other stations store medium reservations
  distributed via RTS and CTS

DIFS
data
RTS
sender
SIFS
SIFS
SIFS
ACK
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
contention
32
Collision Avoidance

• RTS freezes stations near the transmitter
• CTS freezes stations within range of receiver
  (but possibly hidden from transmitter) this
  prevents collisions by hidden station during data
  transfer
• RTS and CTS are very short collisions during
  data phase are thus very unlikely (similar effect
  as Collision Detection)
• Note IEEE 802.11 allows CSMA, CSMA/CA and
  polling from AP

33
Fragmentation in 802.11

• High wireless error rates ? long packets have
  less probability of being successfully
  transmitted.
• Solution MAC layer fragmentation with
  stop-and-wait protocol on the fragments.

34
Point Coordinated Function (PCF)

• PCF uses a base station to poll other stations to
  see if they have frames to send.
• No collisions occur.
• Base station sends beacon frame periodically.
• Base station can tell another station to sleep to
  save on batteries and base stations holds frames
  for sleeping station.

35
MAC-PCF (Point Coordination Function) like
polling
t0
t1
SuperFrame
medium busy
PIFS
SIFS
SIFS
D1
D2
point coordinator
SIFS
SIFS
U1
U2
wireless stations
stations NAV
NAV
36
MAC-PCF (cont)
t2
t3
t4
PIFS
SIFS
D3
D4
CFend
point coordinator
SIFS
U4
wireless stations
stations NAV
NAV
t
contention free period
contention period
37
DCF and PCF Co-Existence

• Distributed and centralized control can co-exist
  using InterFrame Spacing.
• SIFS (Short IFS) is the time waited between
  packets in an ongoing dialog (RTS,CTS,data, ACK,
  next frame)
• PIFS (PCF IFS) when no SIFS response, base
  station can issue beacon or poll.
• DIFS (DCF IFS) when no PIFS, any station can
  attempt to acquire the channel.
• EIFS (Extended IFS) lowest priority interval
  used to report bad or unknown frame.

38
Interframe Spacing in 802.11.
39
CSMA/CA Protocol congestion control
40
Congestion AvoidanceIEEE 802.1 DCF

• Before transmitting a packet, randomly choose a
  backoff interval in the range 0,cw
• cw is the contention window
• Count down the backoff interval when medium is
  idle
• Count-down is suspended if medium becomes busy
• When backoff interval reaches 0, transmit packet
  (or RTS)

41
DCF Example
Let cw 31
B1 and B2 are backoff intervals at nodes 1 and 2
42
Congestion Avoidance

• The time spent counting down backoff intervals
  contributes to MAC overhead
• Choosing a large cw leads to large backoff
  intervals and can result in larger overhead
• Choosing a small cw leads to a larger number of
  collisions (more likely that two nodes count
  down to 0 simultaneously)

43
Congestion Control

• Since the number of nodes attempting to transmit
  simultaneously may change with time, some
  mechanism to manage congestion is needed
• IEEE 802.11 DCF Congestion control achieved by
  dynamically adjusting the contention window cw

44
Binary Exponential Backoff in DCF

• When a node fails to receive CTS in response to
  its RTS, it increases the contention window
• cw is doubled (up to an upper bound typically 5
  times)
• When a node successfully completes a data
  transfer, it restores cw to CWmin

45
MILD Algorithm in MACAW Bharghavan94Sigcomm

• When a node fails to receive CTS in response to
  its RTS, it multiplies cw by 1.5
• Less aggressive than 802.11, which multiplies by
  2
• When a node successfully completes a transfer, it
  reduces cw by 1
• More conservative than 802.11, where cw is
  restored to Cwmin
• 802.11 reduces cw much faster than it increases
  it
• MACAW cw reduction slower than the increase
• Exponential Increase Linear Decrease
• MACAW can avoid wild oscillations of cw when
  congestion is high

46
CSMA/CA Protocol fairness
47
Fairness Issue

• Many definitions of fairness plausible
• Simplest definition All nodes should receive
  equal bandwidth

A
B
Two flows
C
D
48
Fairness Issue

• Assume that initially, A and B both choose a
  backoff interval in range 0,31 but their RTSs
  collide
• Nodes A and B then choose from range 0,63
• Node A chooses 4 slots and B choose 60 slots
• After A transmits a packet, it next chooses from
  range 0,31
• It is possible that A may transmit several
  packets before B transmits its first packet

A
B
Two flows
C
D
49
Fairness Issue

• Observation unfairness occurs when one node has
  backed off much more than some other node

A
B
Two flows
C
D
50
MACAW Solution for Fairness

• When a node transmits a packet, it appends its
  current cw value to the packet
• All nodes hearing that cw value use it for their
  future transmission attempts
• The effect is to reset all competing nodes to the
  same ground rule

51
Weighted Fair Queueing

• Assign a weight to each node
• Goal bandwidth used by each node should be
  proportional to the weight assigned to the node

52
Distributed Fair Scheduling (DFS)
Vaidya00Mobicom

• A fully distributed algorithm for achieving
  weighted fair queueing
• Key idea if sender A has weight 1 and sender B
  has weight 2, they split the bandwidth 1 to 2
• Choose backoff intervals proportional to
• (packet size / weight)
• DFS attempts to mimic the centralized
  Self-Clocked Fair Queueing algorithm Golestani
• Works well on a LAN

53
Distributed Fair Scheduling (DFS)
B1 5
Collision !
B2 5
B1 15 (DFS actually picks a random value
with mean 15) B2 5 (DFS picks a
value with mean 5)
Weight of node 1 1 Weight of node 2 3 Assume
equal packet size