Wireless LAN

1
Wireless LAN
2
Wireless LANs

• Evolution and Technology
• IEEE 802.11
• Bluetooth
• Zigbee and IEEE 802.15

3
Evolution

• Early experiences (1970-72) IBM, HP, Motorola
• Abandoned due to limited performance and
  unavailability of frequency bands
• Early challenges
• Complexity and cost
• Bandwidth
• Coverage
• Interference
• Frequency administration
• Emergence of unlicensed bands
• Release of Industrial, Scientific and Medical
  (ISM) bands in 1985
• Applications military, home and enterprise
  networks, mobile networks, teetherless access

4
Media Access

• Media in wireless networks is shared and is
  scarce access must be controlled
• Observations
• Contention is at the receiver, not at the sender
  makes the carrier sense approach inappropriate
• Unlike Ethernet, congestion is location-dependent
• The media access protocol should propagate
  congestion information explicitly rather than
  having each device learn about congestion
  independently
• Media access protocol should propagate
  synchronization information about contention
  periods, so that all devices can contend
  effectively

5
IEEE 802.11

• Standardization group formed in 1990, first
  standards completed in 1997
• IEEE 802.11 is the first WLAN standard only one
  to secure a market
• 802.11b PHY layer supports 11 Mbps using CKK
  (complementary code keying) technology
• 802.11a PHY layer supports 54 Mbps using OFDM
• Uses CSMA/CA for contention data
• Supports both infrastructure as well as ad hoc
  modes

6
Requirements

• Single MAC to support multiple PHY layers
• Mechanism to support multiple overlapping network
• Provisions to handle interference
• Mechanism to handle hidden terminals
• Privacy and access control

7
IEEE 802.11 Protocol Architecture
Logical link control
Contention-free service
Contention service
Point coordination function (PCF)
MAC layer
Distributed coordination function (DCF)
2.4-Ghz frequency-hopping spread spectrum 1Mbps
2Mbps
2.4-Ghz direct sequence spread spectrum 1Mbps
2Mbps
Infrared 1Mbps 2Mbps
5-Ghz orthogonal FDM 6, 9. 12. 18, 24, 36, 48,
54 Mbps
2.4-Ghz direct sequence spread spectrum 5.5 Mbps
11 Mbps
IEEE 802.11
IEEE 802.11a
IEEE 802.11b
8
Topology
An Extended Service Set (ESS)
Ad hoc Network
BSS
Basic Service Set (BSS)
Infrastructure Network
BSS
9
Layered Protocol Architecture

• MAC sublayer is responsible for access mechanisms
  and fragmentation/reassembly
• MAC management is responsible for roaming in
  Extended Service Set (ESS), power management,
  association/dissociation/reassociation/ process
  for registration connection management
• PHY management decides on channel tuning
• Physical Layer convergence protocol (PLCP)
  carrier sensing and forming packets
• Physical Medium Dependent (PMD) modulation and
  coding techniques for signaling
• Station management coordination of interaction
  between MAC and PHY layers

10
Low Layer Protocol Stack
LLC
Data Link Layer
MAC Management
MAC
Station Management
PLCP
PHY Management
Physical Layer
PMD
PLCP Physical Layer Convergence Protocol PMD
Physical Medium Dependent
11
PHY Layer

• When the MAC protocol data unit (MPDU) arrive at
  the PLCP layer, a header is attached that is
  designed specifically for the PMD
• The PLCP packet is then transmitted by the PMD
  according to specification of the signaling
  techniques
• IEEE 802.11 defines three PLCP packet formats
• FHSS (frequency hopping spread spectrum)
• DSSS (direct sequence spread spectrum)
• DFIR (diffused infrared)

12
FHSS

• PMD hops over 78 channels of 1 MHz each in the
  center of 2.44 GHz ISM bands
• Each BSS can select one of the three patterns of
  26 hops
• (0, 3, 6, 9, , 75)
• (1, 4, 7, 10, , 76)
• (2, 5, 8, 11, , 77)
• IEEE 802.11 specifies specific random hopping
  pattern for each of these frequency groups that
  facilitates multivendor interpretability
• Multiple Basic Service Set (BSS) can co-exist in
  the same area by up to three APs using different
  frequency groups

13
DSSS

• DSSS communicates using non-overlapping pulses at
  11 Mcps
• The ISM band at 2.4 GHz is divided into 11
  overlapping channels spaced at 5 MHz
• A PHY layer management sublayer of AP covering a
  BSS can select one of the choices
• Because of wider bandwidth, DSSS provides a
  better coverage and a more stable signal

14
Carrier Sense Multiple Access (CSMA
appropriateness?)

• Carrier sense provides information about
  potential collision at the sender, but not at the
  receiver
• Since the receiver and sender are not co-located,
  carrier sense does not provide adequate
  information for collision avoidance
  interference at the sender does not imply
  interference at the receiver

15
Carrier Sensing

• Carrier sensing in IEEE 802.11 is performed
  physically or virtually
• PHY sensing is through the clear channel
  assignment (CCA) signal produced by PLCP
• CCA is generated by sensing detected bits or by
  checking the RSS
• Virtual carrier sensing is done based on a
  network allocation vector (NAV) more later

16
MAC Layer

• MAC Sublayer
• Defines the access mechanisms and packet formats
• MAC Management
• Defines roaming support in the ESS, power
  management and security

17
MAC Sublayer

• Reliable data delivery
• Access mechanisms
• Contention-based
• CSMA/CA
• Contention-free
• RTS/CTS
• Point Coordination Function (PCF)

18
Reliable Data Delivery

• High degree of unreliability and large timers for
  retransmissions used in higher layers motivates
  to deal with errors at the MAC layer
• Each transmission is followed by an ACK as an
  atomic unit. Retransmission is done if the ACK is
  not received
• RTS/CTS exchange

19
Hidden Terminal Problem
A is transmitting a packet to B
Node X finds that the medium is free, and
transmits a packet
No carrier ?OK to transmit
20
Exposed Terminal Problem
A is transmitting a packet to B
X can not transmit to Y, even though it will not
interfere at B
Presence of carrier ? holds off transmission
21
Busy Tone
B is receiving a packet from A
X not OK to transmit
X OK to transmit

• Receiver transmits busy tone when receiving data
• All nodes hearing busy tone keep silent
• Requires a separate channel for busy tone

22
RTS/CTS dialog
Defer
Any node that hears this RTS will defer medium
access.
23
RTS/CTS Dialog
Defer
Defer
RTS
Any node that hears this CTS will defer medium
access.
24
RTS/CTS Dialog
Defer
Defer
Data
ACK
25
Access Control

• Distributed Coordination Function (DCF)
• Point Coordinated Function (PCF) Centralized

26
Distributed Coordination Function (DCF)

• DCF sublayer makes use of a simple CSMA algorithm
• Collision detection (CD) is not included because
  of its impracticability in wireless networks
• DCF includes a set of delays called interframe
  space (IFS) to provision priority

27
IEEE 802.11 Medium Access Control Logic
Wait for frame to transmit
Medium idle?
No
Yes
Wait IFS
Still idle?
Wait until current transmission ends
No
Yes
Wait IFS
Transmit frame
Still idle?
No
Yes
Exponential backoff while medium idle
Transmit frame
28
IEEE 802.11 DCF

• Uses RTS-CTS exchange to avoid hidden terminal
  problem
• Any node overhearing a CTS cannot transmit for
  the duration of the transfer
• Any node receiving the RTS cannot transmit for
  the duration of the transfer
• To prevent collision with ACK when it arrives at
  the sender
• Uses ACK to achieve reliability

29
IEEE 802.11 DCF

• CSMA/CA
• Contention-based random access
• Collision detection not possible while a node is
  transmitting
• Carrier sense in 802.11
• Physical carrier sense
• Virtual carrier sense using Network Allocation
  Vector (NAV)
• NAV is updated based on overheard RTS/CTS
  packets, each of which specified duration of a
  pending Data/Ack transmission
• Collision avoidance
• Nodes stay silent when carrier sensed busy
  (physical/virtual)
• Backoff intervals used to reduce collision
  probability

30
Backoff Interval

• When the channel is busy, choose a back-off
  interval in the range 0,cw
• cw is contention window
• Count down the back-off interval when medium is
  idle
• Count-down is suspended if medium becomes busy
• When back-off interval reaches 0, transmit RTS

31
Dynamic Contention Window

• Binary Exponential Back-off in 802.11 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)
• When a node successfully completes a data
  transfer, it restores cw to cwmin

32
Priority-based Access Provisioning

• Using different values of inter frame space (IFS)
• SIFS (short IFS) used for immediate response
  actions
• PIFS (Point coordination function IFS) used by
  the centralized controller while issuing polls
• DIFS (Distributed coordination function IFS)
  minimum delay for asynchronous frames contending
  for access
• DIFS gt PIFS gt SIFS

33
802.11 CSMA/CA
S2
S1
R
X
Channel Busy
NAV
S2
S1
R
NAV
X
34
Point Coordination Function (PCF)

• PCF is implemented on top of DCF
• The time sensitive traffic are controlled by the
  PCF and the remaining traffic contend for access
  using CSMA/CA
• The centralized polling master (point
  coordinator) issues polls using PIFS
• The poll responses use SIFS
• The point coordinator could issue polls in a
  round robin fashion
• Seizing of the medium by the PCF is avoided by
  using superframes where the point coordinator is
  allowed to poll for a fixed duration and then
  idle for the rest of the superframe period to
  allow the asynchronous traffic to contend for the
  medium.

35
MAC Frame Format

• Frame Control (FC) Indicated type of frame,
  provides control information
• Duration/connection ID (D/I) If used as a
  duration field -indicates time (in ms) for which
  the channel will be allocated for transmission of
  a MAC frame. In some control frames, it contains
  an association, or connection identifier
• Addresses Context dependent. Types include
  source, destination, transmitting station,
  receiving station
• Sequence Control Used for fragmentation/reassembl
  y.
• Frame Body Contains an MPDU or its fragment
• Cyclic Redundancy Check (CRC) 32-bit frame check
  sequence

36
Frame Control Field
PV
Type
SubType
TO DS
FROM DS
MF
RT
PM
MD
W
O
4
2
2
1
1
1
1
1
1
1
1

• Protocol Version (PV) 802.11 version, currently
  version 0
• Type Identifies the frame as control,
  management, or data
• Subtype Identifies the function of frame
• To DS The MAC coordination sets this bit to 1 in
  a frame destined to the distribution system
• From DS The MAC coordination sets this bit to 1
  in a frame leaving the distribution system
• More Fragments (MF) Set to 1 if more fragments
  follow
• Retry (RT) Set to 1 if retransmission
• Power Management (PM) Set to 1 if transmitting
  station is in sleep mode
• More Data (MD) Indicates that a station has
  additional data to send
• Wired Equivalent Privacy (WEP) WEP implemented
• Order (O) The frames must be processed in order
  if set to 1.

37
IEEE 802.11 Management Sublayer

• Registration
• Handoff
• Power Management
• Security

38
Registration

• A management frame called beacon is transmitted
  periodically by the AP to establish the timing
  synchronization function (TSF)
• TSF contains BSS id, timestamp, traffic
  indication map (TIM), power management, and
  roaming information
• Received Signal Strength (RSS) measurements are
  done on the beacon message
• Association process by which an MS registers
  with an AP

39
Handoff

• Mobility Types
• No transition MS is static or moving within a
  BSA
• BSS transition MS moves from one BSS to another
  within the same ESS
• ESS transition MS moves from one BSS to another
  BSS which belong to a different ESS
• Reassociation service is used when an MS moves
  from one BSS to another within the same ESS

40
Handoff procedure in IEEE 802.11
8. IAPP indicates reassociation to old AP
AP2
Beacon Periodically
AP3
AP1
4. Probe Response
3. Probe Request
1. Strong Signal
5. Choose AP with strongest response
2. Weak Signal start scanning for handoff
41
Power Management

• How to power-off during idle periods?
• IEEE 802.11 buffers data at the AP, and sends the
  data when the MS is awakened
• Using TSF, all MSs are synchronized they wake
  up at the same time to listen to beacon
• With every beacon a TIM is sent that has a list
  of stations having buffered data
• An MS learns that it has buffered data by
  checking beacon and TIM

42
Security

• There are provisions for authentication and
  privacy in IEEE 802.11
• Open system authentication (default)
• Request frame sends the authentication algorithm
  id
• the response frame sends the result
• Shared key authentication
• Request frame sends the authentication frame id
  for the shared key that is shared between itself
  and the AP
• The second station sends a challenge text
• The first station sends the encrypted challenge
  as the response
• The second station sends the authentication result