Investigation of the IEEE 802'11 Medium Access Control MAC Sublayer Functions

1
Investigation of the IEEE 802.11 Medium Access
Control (MAC) Sublayer Functions

• B. P. Crow, I. Widjaja, J. G. Kim, and P. Sakai
• University of Arizona
• 2004. 9. 9
• Kisuk Kweon

2
Contents

• Introduction
• Architecture
• MAC sublayer
• Distributed Coordination Function
• Point Coordination Function
• Simulation
• Conclusions

3
Introduction

• IEEE 802.11
• Wireless Local Area Network (WLAN) standard
• Physical layer (PHY) and Medium Access Control
  (MAC) sublayer
• Mandatory support
• 1 Mbps
• Asynchronous data transfer
• Optional support
• 2 Mbps
• Distributed time bounded services (DTBS)
• MAC sublayer
• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)

4
Architecture

• Independent Basic Service Set (IBSS)
• Ad Hoc Network
• Infrastructure Network
• Established using APs
• Extended Service Set

5
MAC Sublayer

• MACs responsibility
• The channel allocation
• Protocol data unit (PDU) addressing
• Frame formatting
• Error checking
• Fragmentation and reassembly
• Channel allocation
• Contention period (CP)
• Contention free period (CFP)

6
MAC Sublayer

• Three different types of frames
• Management frame
• Station association and disassociation with AP
• Timing and synchronization
• Authentication and deauthentication
• Control frame
• To end contention-free period
• Handshaking during the contention period
• ACK during CP
• Data frame
• Data frame (in both CFP and CP)
• Combined with polling and ACK during CFP

7
Distributed Coordination Function

• DCFs responsibility
• Support asynchronous data transfer
• Support contention services
• Carrier Sense Multiple Access with Collision
  Avoidance (CSMA/CA)
• Carrier sensing
• Physical carrier sensing
• Virtual carrier sensing
• By sending medium reservation through RTS and CTS
  frames
• Duration field in these frames
• An NAV (Network Allocation Vector)

8
Distributed Coordination Function

• Priority Access to the wireless medium
• Three Inter-Frame Space (IFS) intervals
• Short IFS (SIFS) the highest priority
• ACK, CTS, data frame, and to response to a poll
  from the PCF
• Point Coordination Function-IFS (PIFS) 2nd
  highest
• Distributed Coordination Function-IFS (DIFS)
  lowest
• RTS/CTS 3 options
• Never use RTS/CTS
• Always use RTS/CTS
• Use RTS/CTS when the MPDU exceeds the
  RTS_Threshold

9
Distributed Coordination Function

• hidden terminal problem
• RTS-CTS exchange
• RTS request to send
• CTS consent to send
• problem high overhead for short frames

D
B
C
A
RTS
D
A
B
C
CTS
CTS
D
A
B
C
data
D
B
C
A
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Distributed Coordination Function

• Before transmitting asynchronous MPDUs, a STA
  shall use the CS function to determine the medium
  state.
• If busy, the Station will
• defer a DIFS gap
• then generate a random backoff period for an
  additional deferral time
• to resolve contention.

11
Distributed Coordination Function
12
Distributed Coordination Function
13
Point Coordination Function

• PCF
• Optional capability
• Contention-free service
• Performed by the Access Point (AP)
• CFP repetition interval
• Contention free and contention-based traffic
• Initiated by a Beacon frame
• Polling
• The method by which polling tables are maintained
  and the polling sequence is determined is left to
  the implementor

14
Point Coordination Function
15
Point Coordination Function

• The PC first waits for a PIFS period.
• PC sends a data frame (CF-Down) with the CF-Poll
  Subtype bit 1, to
• the next station on the polling list.
• When a STA is polled, if there is a data frame
  (CF-Up) in its queue, the frame is sent after
  SIFS with CF-Poll bit 1.
• Then after another SIFS, the CF polls the next
  STA.
• To end the CF period, a CF-End frame is sent.

16
Simulation Model

• Markov chain for the burst error model
• Default attribute values for Ad hoc and
  infrastructure network

17
Simulation Results Ad Hoc Network
18
Simulation Results Infrastructure Network
19
Conclusions

• The efficiency delivered by the DCF is reasonably
  high
• MSDU length is longer than 500 octects
• Fragmentation_Threshold is set to 500-1000
  octects
• RTS_Threshold is set to 250-500 octecs
• Medium is relatively clean (BER better than 10-6)
• Real-time service can be transported by the PCF
• Compromised performance for data and voice
  traffic is achieved when the voice payload length
  is between 100 to 400 octets long