Wireless LAN

1
Wireless LAN
2
Whats Wireless LAN?

• Communication system established through the use
  of RF technology
• Can function either as an extension to an
  existing LAN or an alternative for a wired LAN

3
General Characteristics

• The need for WLAN
• resolving the problems of Hard-wired LAN
• providing user mobility and flexibility
• enabling to extend the range of an existing wired
  LAN
• being cheaper to install than traditional wired
  LAN

4
Types of WLANs

• Radio WLAN
• DSSS
• uses signalling schemes which are based on some
  form of coding and which use a much wider
  bandwidth over the information bandwidth
• FHSS
• uses multiple frequency channel randomly (in a
  predefined pattern)

5
Types of WLANs

• Infrared WLAN
• use part of the electromagnetic spectrum just
  below visible light as a media
• based on on-off pulse modulation and detection of
  the optical signal
• techniques
• Directed (line of sight)
• Diffuse (reflective)

6
Infrastructure vs. ad-hoc networks

• Infrastructure networks
• Communications take place between the wireless
  nodes and AP, not directly between nodes.
• can use different access schemes
• collision
• wireless nodes and AP are not coordinated
• collision free
• AP controls medium access (QoS support)
• Ad-hoc networks
• medium access collision
• wireless node may transfer data (router)

7
WLAN Protocol Topologies

• Independent Topology
• supports peer-to-peer connectivity
• implemented quickly and easily
• no need for special tools or skills, no network
  administration
• in business meeting or in the setting up of
  temporary workgroups
• limited coverage area
• gt all stations must be within the airwave
  distance of other stations with which they need
  to communicate

8
Ad-hoc networks

• Independent Topology

Ad hoc network
BSS
BSS
9
WLAN Protocol Topologies

• Infrastructure topology using a AP
• transition from the wireless media to the wired
  media via an AP
• Access Point
• have network interface card connected to the
  wired LAN
• functions as a radio relay to the mobile stations
• involves the extension of an existing LAN through
  the use of a wireless AP
• all mobile stations must communicate with the AP
• within a predetermined distance, approximately
  50300m

10
Infrastructure Configuration

• Infrastructure topology using a AP

wired network
AP
AP Access Point
BSS
11
WLAN Protocol Topologies

• Infrastructure topology using multiple AP
• can extend wireless LAN access capabilities over
  a relatively large area,
• such as college campus, industrial complex
• enables mobile stations to move within a larger
  coverage area
• provide effectively notebooks and PDAs with a
  roaming capability

12
Infrastructure Configuration

• Infrastructure topology using multiple AP

Infrastructure network
BSS
AP Access Point
AP
wired network
AP
AP
BSS
BSS
13
802.11 Protocol Entities
LLC
MAC Layer Management
MAC Sublayer
MAC
Station Management
PHY Layer Management
PLCP Sublayer
PHY
PMD Sublayer
14
IEEE 802 Network
802 OUTLINE ARCHITECTURE
802.1 MANAGEMENT
802.2 LLC
802.3 MAC 802.3 PHY
802.5 MAC 802.5 PHY
802.11 MAC
802.11 FHSS PHY
802.11 DSSS PHY
802.11a OFDM PHY
802.11b HR/DSSS PHY
15
IEEE 802.11 Standard

• Standard process
• 802.11 MAC(1997)
• 802.11a PHY, 54Mbps, 5Ghz
• 802.11b PHY, 11Mbps, 2.4Ghz
• 802.11g PHY, 54Mbps, 2.4Ghz(Super set of
  802.11b PHY)
• 802.11e for QoS support
• 802.11f for IAPP(Inter-Access Point Protocol)
• 802.11h for spectrum and transmit power
  management
• 802.11i for security enhancement
• 802.11k for radio resource management enhancement
• 802.11n for higher throughput

16
802.11 - Architecture of an infrastructure
802.11 LAN
802.x LAN
BSS1
STA1
AP
Portal
ESS
AP
Extended Service Set
STA3
BSS2
STA2
802.11 LAN
17
802.11 - Architecture of an infrastructure

• Station (STA)
• terminal with access mechanisms to the wireless
  medium and radio contact to the access point
• Basic Service Set (BSS)
• group of stations using the same radio frequency
• Access Point
• station integrated into the wireless LAN and the
  distribution system
• Portal
• bridge to other (wired) networks
• Distribution System
• interconnection network to form one logical
  network (EES Extended Service Set) based on
  several BSS

18
802.11 - Architecture of an ad-hoc network

• Direct communication within a limited range
• Station (STA)terminal with access mechanisms to
  the wireless medium
• Independent Basic Service Set (IBSS)group of
  stations using the same radio frequency

802.11 LAN
STA1
STA3
IBSS
STA2
19
IEEE standard 802.11
mobile terminal
infrastructure network
access point
Fixed terminal
Application
Application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.3 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.3 PHY
20
802.11 - MAC layer - DFWMAC

• Traffic services
• Asynchronous Data Service (mandatory)
• exchange of data packets based on best-effort
• support of broadcast and multicast
• Time-Bounded Service (optional)
• implemented using PCF (Point Coordination
  Function)

Contention-Free
General
PCF
DCF
lt MAC Extent gt
21
802.11 - MAC layer - DFWMAC

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

22
DCF (Distributed Coordination Function)

• Distributed
• Contention-based channel access
• Based CSMA/CA
• Asynchronous data transport
• IBSS, Infrastructure BSS
• FIFO transmission queue
• Backoff

23
802.11 - MAC layer

• Priorities
• Defined through different inter frame spaces
• No guaranteed, hard priorities
• SIFS (Short Inter Frame Spacing)
• highest priority, for ACK, CTS, polling response
• 10usec (DSSS)
• PIFS (PCF IFS)
• medium priority, for time-bounded service using
  PCF
• SIFS 1time slot (20 usec)
• DIFS (DCF, Distributed Coordination Function IFS)
• lowest priority, for asynchronous data service
• SIFS 2 time slots

24
802.11 - CSMA/CA access method
contention window (randomized back-offmechanism)
DIFS
DIFS
Next frame
Medium busy
t
slot time
direct access if medium is free ? DIFS
25
CSMA/CA Explained
Free access when medium is free longer than DIFS
DIFS
Contention Window
PIFS
DIFS
SIFS
Next Window
Backoff-Window
Busy Medium
Slot time
Defer Access
Select Slot and Decrement Backoff as long as
medium is idle
26
CSMA/CA Explained

• Reduce collision probability where mostly needed
• Stations are waiting for medium to become free
• Select Random Backoff after a Defer, resolving
  contention to avoid collisions
• Efficient Backoff algorithm stable at high loads
• Exponential Backoff window increases for
  retransmissions
• Backoff timer elapses only when medium is idle
• Implement different fixed priority levels
• To allow immediate responses and PCF coexistence

27
CSMA/CA ACK protocol
DIFS
DATA
Sender
SIFS
ACK
Receiver
DIFS
Next MPDU
Other stations
t
contention
Backoff after Defer
Defer Access
28
CSMA/CA ACK protocol

• Defer access based on Carrier Sense
• CCA from PHY and Virtual Carrier Sense state
• Direct access when medium is sensed free longer
  then DIFS, otherwise defer and backoff
• Receiver of directed frames to return an ACK
  immediately when CRC correct
• When no ACK received then retransmit frame after
  a random backoff (up to maximum limit)

29
Contention Window

• CW(Contention Window)
• Contention window or backoff window
• Station selects random slot (probability same)
  0,CW, (Cwmin 7, Cwmax255)

30
802.11 - CSMA/CA access method (ACK)

• Sending unicast packets
• Station has to wait for DIFS 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
• But the sender has to wait again and compete for
  the access right.

31
RTS / CTS Concept
A
RTS
DATA
(1) RTS
B
(2) CTS
CTS
ACK
(3) DATA
NAV (RTS overhearing)
C
NAV (CTS overhearing)
D
(4) ACK
32
802.11 - DFWMAC

• Sending unicast packets
• Station can send RTS with reservation parameter
  (duration) after waiting for DIFS (reservation
  determines 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
• NAV (Net Allocation Vector)

33
Hidden Node Problem
Using an RTS-CTS handshake to resolve hidden node
problems
A
B
C

• A transmits request (RTS) to B
• B replies that the channel is Clear (CTS). Both A
  C overhear the broadcast
• A sends its data to B. C is blocked from
  transmitting.
• B acknowledges the data transfer

(1) RTS
(2) CTS
(3) DATA
(4) ACK
34
Exposed Node Problem

• B is transmitting to A
• C overhears this, and is blocked
• P wants to transmit to Q,
• but is being blocked by B

B
C
A
D
(1) RTS
DATA
B
RTS
(2) CTS
A
(3) DATA
CTS
ACK
NAV (RTS overhearing)
C
NAV (CTS overhearing)
(4) ACK
others
RTS_overhear
CTS_overhear
35
802.11 - DFWMAC
DIFS
RTS
DATA
sender
SIFS
SIFS
SIFS
CTS
ACK
receiver
NAV(RTS)
other stations
NAV(CTS)
t
36
Fragmentation
frag1
RTS
frag2
Sender
DIFS
ACK1
CTS
ACK2
Receiver
SIFS
SIFS
SIFS
SIFS
SIFS
NAV (RTS)
NAV (CTS)
DIFS
NAV (frag1)
data
Other stations
NAV (ACK1)
t
contention
37
Optional PCF
Contention Free Service
Contention Service
MAC
PCF Optional
DCF (CSMA/CA)
PHY
38
Optional PCF

• Contention Free Service uses Point Coordination
  Function (PCF) on a DCF Foundation
• PCF can provide lower transfer delay variations
  to support Time Bounded Services
• Async Data, Voice or mixed implementations
  possible
• Point Coordinator resides in AP
• Coexistence between Contention and optional
  Contention Free does not burden the implementation

39
PCF(Point Coordination Function)

• Point Coordination Function
• centrally controlled multiple access mechanism
• uses a poll and response protocol to eliminate
  the possibility of contention for the medium
• point coordinator (PC) is always located in an AP
• supports time bounded services

40
Contention Free operation
PCF Defers for Busy Medium
CFP repetition interval
CFP repetition interval
Contention Free Period
Contention Period DCF
PCF (Optional)
CF-Burst
Busy medium
Variable Length
Reset NAV
PCF Defers
NAV
41
Contention Free operation

• Alternating Contention Free and Contention
  operation under PCF control
• NAV prevents Contention traffic until reset by
  the last PCF transfer
• So variable length Contention Free period per
  interval
• Both PCF and DCF defer to each other causing PCF
  Burst start variations

42
PCF Burst
CFP repetition interval
Contention Free Burst
PIFS
Contention Period
D1
D2
D3
D4
Busy medium
U1
U2
U4
No Up
CF_End
Dx AP-Frame
Ux Station-Frame
SIFS
Reset NAV
NAV
Min Contention Period
43
PCF Burst

• CF-Burst by Polling bit in CF-Down frame.
• Immediate response by Station on a CF_Poll.
• Stations to maintain NAV to protect CF-traffic.
• Responses can be variable length.
• Reset NAV by last (CF_End) frame from AP.
• "ACK Previous Frame" bit in Header.

44
IEEE 802.11 Standardization

• IEEE802.11 MAC Other Activities

Ad-Hoc Publicity
Ad-Hoc Regulatory
MAC Layer
802.11e / TGe MAC Enhancements -QoS
802.11 MAC
802.11f / TGf Inter-Access Point Protocol
802.11
MAC
802.11i / TGi Enhanced Security Mechanisms
45
IEEE 802.11 Standardization

• IEEE802.11 current status

46
IEEE 802.11 Standardization

• IEEE802.11 current status

47
IEEE 802.11 WG Activities

• IEEE802.11e/TG e
• enhance the 802.11 MAC to improve and manage
  Quality of Service
• provide classes of service, and enhanced security
  and authentication mechanisms
• consider efficiency enhancements in the areas of
  the DCF and PCF
• applications
• transport of voice, audio, video
• video conferencing, media stream distribution,
  enhanced security applications
• mobile and nomadic access applications

48
IEEE 802.11 WG Activities

• IEEE802.11f/TG f
• develop recommended practices for an Inter-Access
  Point Protocol (IAPP) which provides the
  necessary capabilities to achieve multi-vendor
  Access Point interoperability across a
  Distribution System supporting IEEE P802.11
  Wireless LAN Links
• This IAPP will be developed for the following
  environment(s)
• A DS consisting of IEEE 802 LAN supporting an
  IETF IP environment
• Others as deemed appropriate

49
IEEE 802.11 WG Activities

• IEEE802.11g/TG g
• to develop a higher rate PHY extension to 802.11b
• date rates more than 20Mbps
• backwards compatible and interoperable with
  existing 2.4GHz IEEE 802.11b 11Mbps standard
• uses existing MAC layer
• Extended rate OFDM mandatory mode
• using 802.11a and 802.11b
• the capability to detect either 802.11a or
  802.11b preambles automatically
• Optional modes
• CCK-OFDM
• PBCC-22, PBCC-33

50
IEEE 802.11 WG Activities

• IEEE802.11h/TG h
• enhance the current 802.11 MAC and 802.11a PHY in
  5GHz license exempt bands, enabling regulatory
  acceptance of 802.11 5GHz products.
• provide improvements in channel energy
  measurement and reporting, channel coverage to
  improve spectrum and transmit power management
• provide Dynamic Channel Selection and Transmit
  Power Control mechanisms
• Dynamic Channel Selection
• Transmit Power Control

51
IEEE 802.11 WG Activities

• IEEE802.11i/TG i
• enhance the 802.11 MAC to enhance security and
  authentication mechanisms
• Address security mechanisms beyond RC4 WEP 40-bit
  encryption, more robust against eavesdropping
• Focus on key distribution and authentication
  between Stations and Stations to APs
• Scaleable security framework to serve home
  networks, enterprise, Ad-Hoc, Public Environments
• Data traffic Voice, Data, Multi-media

52
IEEE 802.11 WG Activities

• WNG SC
• changed from 5GSG which has investigated the
  globalization and harmonization of the 5GHz band
  jointly with ETSI-BRAN, and MMAC PPDU frame
  format
• synthesize industry inputs for the Next
  Generation WLAN and recommend a global standard
  initiative
• will deliver recommended requirements and
  standards plan

53
IEEE 802.11 WG Activities

• Roadmap

1st Generation
2nd Generation
3rd Generation
4th Generation
Analog Cellular
Digital Cellular

• Outdoor
• 5.15-5.35GHz
• 25-50Mbps

• Outdoor
• 5.15-5.35GHz
• 25-50Mbps

825-890MHz Voice only
IMT-2000
Cellular PCS - 2.5G
Wireless Multimedia Internet Appliance
WCDMA/CDMA2000 Mobile 384kbps Stationary 2Mbps
Wireless Multimedia Internet Appliance
HiperLAN2 IEEE802.11a
IEEE802.11a 5G NG WLAN

• Indoor
• 60-65GHz
• 155-622Mbps

• Indoor
• 60-65GHz
• 155-622Mbps

2.4GHz 2Mbps, 11Mbps
IEEE802.11 Wireless LAN
IEEE802.11b Wireless LAN
IEEE802.11x 1Mbps

• 5GHz/24Mbps
• 5GHz/54Mbps

Wired LAN