2. IEEE 802.11 Wireless Local Area Network (WLAN)
Lecture 3
(Part 1)
IEEE Wireless Local Area Network (WLAN)

3. Outline 802.11 basics 802.11 Protocol stack
Network architecture
MAC protocol: DCF
MAC protocol: PCF

4. Basics

5. IEEE 802.11: basics Basic standard first ratified in 1997
Primary (original) function:
Wireless replacement for Ethernet
Frequency band: Unlicensed band
2.4 GHz: 11 channels, 3 of which are non-overlapping
5 GHz: 12 non-overlapping channels
Basic data rate
1, 2, 5.5, 11, . . ., 54Mbps
Range
Indoor: 20 ~ 25 meters
Outdoor: 50~100 meters
Specify protocols for MAC and PHY layers only
has defined many new amendments
PHY: b, a, g, …
MAC: e, i, …
Application
Enterprise and campus networking
Ad hoc networking
Public access in “hot spots”
Home networking

6. IEEE 802.11: basics 802.11 802.11b 802.11a 802.11g 802.11n Standard
Freq
Supported
Rate
Number of
Channels
Published
Market
Introduction
802.11
2.4GHz
1, 2 11
1997
N/A
802.11b
1, 2, 5.5, 11
1999
802.11a
5GHz
6, 9, 12, 18, 24, 36, 48, 54 12
2002
802.11g
1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54
2003
802.11n
2.4/5GHz
100Mbps above MAC SAP
Expected
2007
Pre-N
2005
802.11
802.11b
802.11a
802.11g
802.11n

7. Protocol Stack

8. Protocol Stacks
IEEE

9. Protocol Stacks: More Details
Data Plane
Physical Layer (PHY)
Physical Medium Dependent (PMD)
Modulation
Coding
Physical Layer Convergence Protocol (PLCP)
Clear channel assessment signal (carrier sense) (CCA)
Medium Access Control (MAC)
Access mechanisms
Fragmentation
Encryption
clear channel assessment (CCA) function: That logical function in the physical layer (PHY) that determines the current state of use of the wireless medium (WM). Either “busy” or “idle”.

10. Protocol Stacks: More Details
Management Plane
PHY Management
Channel selection, PHY-MIB
MAC Management
Power management
Determine the characteristics of the available BSS
Synchronization
Authentication
Association

11. Network Architecture

12. Network Architecture Two configurations
Infrastructure basic service set
Independent basic service set: ad hoc mode

13. Service Set Basic Service Set (BSS) Extended Service Set (ESS)
Basic building block of an IEEE WLAN
A set of stations controlled by a single coordination function
Extended Service Set (ESS)
Wireless network of arbitrary size and complexity, consists of a set of BSSs interconnected by DS.
All stations of the ESS appear to be in a single MAC layer
APs communicate with each other to forward traffic through wired or wireless network
Station mobility within an ESS is invisible to the higher layers

14. Distributed Coordination Function (DCF)
MAC Protocol
Distributed Coordination Function (DCF)

15. MAC Protocol: Overview
DCF: Distributed Coordination Function
Mandatory
Widely deployed
Contention-based random access
Using CSMA/CA to access the channel
Collision avoidance via binary exponential back-off mechanism
ACK packet for acknowledgements
For best-effort traffic
PCF: Point Coordination Function
Optional
Almost never used, if ever implemented at all.
Contention-free
Access point polls stations according to a list
For delay-sensitive application
DCF
PCF
MAC
Physical Layer (PHY)

16. MAC Protocol: Overview
Superframe structure
Consists of both contention period and contention-free period
These two periods are of variable length. Their duration depends on the traffic load at the AP and at the mobile hosts.
DCF is used in contention period
PCF is used in contention-free period
PCF has higher access priority than DCF
Each superframe is started by a Beacon
Each contention-free period is terminated by a CF-End message transmitted by the access point (AP).
PCF
Superframe
Contention Free Period (CFP)
Contention
Period (CP) CP
CFP repetition interval
Beacon
CFP

17. Contention Period Protocol
DCF: CSMA/CA
Algorithm
Station which is ready to send starts sensing the channel
If the channel is free for the duration of a DCF Inter-Frame Space (DIFS), station transmits immediately.
If the channel is busy, the station has to wait for a free DIFS, then the station must additionally wait for a random backoff time (collision avoidance, multiple of slot-time) before transmission
The receiving station has to acknowledge the reception of the packet after waiting for a SIFS period, if the packet is received correctly (CRC).
If no ACK is received after a timeout period at transmitting station, it reschedules a new transmission time for the packet, with a round of backoff
When the number of retransmissions exceeds RetryLimit (e.g. 6), the packet is dropped at the transmitting station.
Assume the upper layer retransmission mechanism will take care of this dropped packet.
Carrier Sense based on Clear Channel Assessment (CCA)

18. Inter-frame spaces
IEEE has defined different inter frame spaces
SIFS (Short Inter Frame Spacing)
Highest priority, for ACK, CTS, Polling response
PIFS (PCF IFS)
PIFS = SIFS + slot time
Medium priority, for time-bounded service using PCF
DIFS (DCF IFS)
DIFS = SIFS + 2 x slot time
Lowest priority, for asynchronous data service
EIFS (Extended IFS)
SIFS + ACK_Transmission_Time + DIFS
Used after an erroneous frame reception
SIFS < PIFS < DIFS < EIFS
Values of IFS and slot time are PHY dependent.
802.11b (DSSS)
Slot time = 20us, SIFS = 10us
802.11a
Slot time = 9us, SIFS = 16us

19. Basic DCF: An Illustration
Random backoff T0
DIFS
Resume counting down
Channel
Busy
SIFS
Successful
Transmission
STA 1
Slot Time
PIFS
DIFS
Random backoff T0
Contention window from [1, 2mCWMin[AC]]
DIFS
Successful
Transmission
SIFS
Channel
Busy
STA 2
Slot Time
PIFS
Contention Window
DIFS
Randomly choose a backoff window size and decrement backoff counter as long as the medium stays idle
Defer Access

20. Basic DCF: An Illustration
Collision/Erroneous reception
Successful Transmission
Transmitting Station
Transmitting Station
SIFS
SIFS
Transmission
DIFS
Transmission
DIFS
ACK Timeout
Receiving Station
Receiving Station
ACK

21. Exponential Backoff: More Detail
To begin the backoff procedure:
Choose a random number in [0, CWMin-1] from a uniform distribution
Listen to determine if the channel is busy for each time slot
Decrement backoff time by one slot if channel is idle
Suspend backoff procedure if channel is busy in a time slot
Resume backoff when the channel becomes idle again.
When the backoff counter value becomes 0, STA starts transmission.
When experiencing collision
Choose a new random number according to uniform distribution within interval [0, min{ 2ixCWMin-1, CWMax-1}]
Repeat the steps described above for backoff.
When you hit the ceiling: retransmission time = RetryLimit (e.g. 6)
Drop the packets
Most of the time, upper layer protocol will take care of the packet drop and initiate retransmission.

22. Backoff: An Illustration
Evolution of contention window (CW)
Increases after each failure: [0, 31], [0, 63], [0, 127], [0, 255], [0, 511], [0, 1023], then gives up
Resets to 31 after each successful transmission
Channel idle
Resume backoff
Generate a random number from [0, CWMin-1] and begin backoff
Channel busy, suspend backoff counter
Channel busy, suspend backoff counter
DIFS
DIFS
DIFS
DIFS
DIFS
STA A
Success
Success
defer
Channel idle
Resume backoff
defer
STA B
Collision
defer
Channel idle
Resume backoff
Generate a random number from [0, 2CWMin-1] and begin backoff
defer
defer
STA C
Success
defer
defer
defer
STA D
Channel busy, suspend backoff counter

23. Hidden Terminal Problem
A and C are two STAs far away from each other.
A sends to B, C cannot hear A
C wants to send to B
If use CSMA/CA:
C senses a “free” medium, thus C sends to A
Collision at B, but A cannot detect collision
Therefore, A is “hidden” for C B A C

24. Exposed Terminal Problem
B sends to A, C wants to send to D
If use CSMA/CA
C senses an “in-use” medium, thus C waits
But A is outside the radio range of C, therefore waiting is not necessary
Therefore, C is “exposed” to B B A C D

25. Solution: DCF with RTS/CTS
Request To Send/Clear To Send (RTS/CTS)
Address the hidden terminal problem
But can’t tackle exposed terminal problem
Detailed algorithm
When a station wants to send a packet, it first sends a RTS.
The receiving station responds with a CTS.
Stations that can hear the RTS or the CTS then mark that the medium will be busy for the duration of the request (indicated by Duration ID in the RTS and CTS).
Stations will adjust their Network Allocation Vector (NAV)
Time that must elapse before a station can sense channel for idle status
This is called virtual carrier sensing.
RTS/CTS are smaller than long packets that can collide

26. RTS/CTS: Illustration
Usually RTS/CTS mechanism is disabled in the product.
When should it be enabled?
When data frame is long enough to justify the overhead caused by the exchange of RTS/CTS messages.
RTS threshold is adjustable in the WLAN card.
NAV
RTS
STA 1
New random backoff
Random backoff
DIFS
SIFS
STA 3
DATA
STA 2
STA 4
CTS
ACK
Defer Access
Backoff after defer

27. Broadcast/Multicast
If a packet should be received by all the neighbors, it is a broadcast packet.
If a packet should be received by some of the neighbors, it is a multicast packet.
For broadcast/multicast, we can not use RTS/CTS mechanism.
Why?
Broadcast
Multicast

28. MAC Layer Frame Format MAC Header 2 2 6 6 6 2 6 0 ~ 2312 4 Bits 2 2 4
Control
Duration/ID
Address 1 2 3
Sequence 4
Body
FCS 2 2 6 6 6 2 6
0 ~ 2312 4
Bytes
Protocol
Version
Type
Sub-type
ToDS
FromDS
More Frag
Retry
Power Mngmt
More Data
WEP
Order
Bits 2 2 4 1 1 1 1 1 1 1 1

29. MAC Layer Frame Format
“Address 1” field always holds the receiver address of the intended receiver.
“Address 2” field always holds the address of the station that is transmitting the frame.
Scenario
To DS
From DS
Addr 1
Addr 2
Addr 3
Addr 4
IBSS (Ad Hoc network) DA SA
BSSID
N/A
Infrastructure
network
AP to STA 1
STA to AP
AP to AP RA TA

30. Homework
Why do we need 4 MAC address fields, instead of only 2?

31. MAC Layer Frame Format Protocol version: 0x00 for current version
Type + Subtype: identify the function of the frame.
Data
Control
Management
More Frag: if “0x01”, the station has more fragment(s) to send
Retry: “0x01” implies that this is a retransmission
Power Management:
“0x01” Power saving mode
“0x00” Active mode
More data:
“0x01” indicates to a station in power saving mode that more MSDUs are buffered at the AP for that station.
WEP: “0x01” implies that WEP is used in the frame body

32. Data Frame Format Data frame format ACK frame format
The “receiver address” field in the ACK is directly copied from the “Address 2” field of the corresponding received data frame
Frame
Control
Duration/ID
Address 1 2 3
Sequence 4
Body
FCS
Frame
Control
Duration/ID
Receiver
Address (RA)
FCS

33. RTS/CTS Frame Format RTS CTS
RA: Address of the STA which is the intended immediate recipient of the frame.
TA: Address of the STA which is transmitting the frame.
Duration: The time value, in microseconds, required to transmit the pending frame, plus 1 CTS frame, plus 1 ACK frame, plus 3 SIFS intervals.
CTS
Identical to the frame format of ACK
RA: copied from TA field of corresponding RTS
Duration: The duration field in corresponding RTS, minus 1 CTS time, minus 1 SIF interval.
Frame
Control
Duration/ID
Receiver
Address (RA)
FCS
Transmitter
Address (TA)
Frame
Control
Duration/ID
Receiver
Address (RA)
FCS

34. Performance Metrics Throughput Delay Receiver Transmitter
Number of payload bits correctly received at the intended recipient within a unit time.
Excluding the PHY and MAC header.
Delay
Queueing delay.
Channel access delay.
Transmitter
from LLC
MSDU T0
Queueing
Delay
(T1 – T0)
Buffer
to LLC T3 T1
Media Access
Delay
(T2 – T1)
Defer,
Backoff,
Transmit,
Retransmit
Buffer T2
Wireless Channel
. . .
PPDU
PPDU
Throughput
[1] MSDU: MAC Service Data Unit
[2] PPDU: PHY Protocol Data Unit

35. Fragmentation Motivation Penalty
Wireless channel is lossy
Longer frame size yields higher packet error rate
Penalty
For each frame, physical and MAC layer headers much be appended to each frame.
This causes additional overhead.
So, it is a tradeoff between packet error rate and the overhead.

36. Fragmentation
The length of a fragment MSDU shall always be an even number of octets, except the last fragment.
The length of a fragment shall never exceed aFragmentationThreshold unless WEP is invoked.
aFragmentationThreshold is a MIB variable
MSDU
Fragment 1
Fragment 2
Fragment 3
MAC
Hdr
Frame
Body
CRC
MAC
Hdr
Frame
Body
CRC
MAC
Hdr
Frame
Body
CRC

37. Fragmentation: More Detail
Sequence Number
Each MSDU transmitted by a STA is assigned a Sequence Number from a single modulo 4096 counter, starting from 0 and incrementing by 1 for each MSDU
Each fragment of an MSDU (and its retransmission) contains the same assigned Sequence Number
Sequence number remains the same for all retransmissions of a data unit
Fragment Number
Indicate the number of each fragment of MSDU
Retransmission has the same fragment number
4 bits
12 bits
Frame
Control
Duration/ID
Address 1 2 3
Sequence 4
Body
FCS
Fragment Number
Sequence Number

38. Fragmentation Transmission
Fragment burst for basic access scheme
DIFS
Source
Backoff window
SIFS
SIFS
SIFS
SIFS
SIFS
Frag 0
Frag 1
Frag 2
ACK0
ACK1
ACK2
Destination
DIFS
Fragment burst for RTS/CTS scheme
NAV(RTS)
NAV(Frag 0)
NAV(Frag 1)
NAV(CTS)
NAV(ACK 0)
NAV(ACK 1)
Source
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
Frag 0
Frag 1
Frag 2
RTS
Destination
ACK0
ACK1
ACK2
CTS

39. Point Coordination Function (PCF)
MAC Protocol
Point Coordination Function (PCF)

40. PCF: Illustration Delay (due to a busy medium) CFP repetition interval
Contention Free Period (CFP) repetition interval: Superframe
NAV
NAV
Contention Free Period
Contention Period (CP)
Foreshortened CF Period CP
DCF
DCF B
PCF B
PCF
Channel Busy
STA 1 (AP)
PIFS
SIFS
SIFS
SIFS B
Data + CF-Poll
Data +
CF-Poll
CF-END
Data + CF-ACK
CF-ACK
CFP: Polling only
SIFS
SIFS CP
CP: Listen before talk

41. References
1. Raphael Rom, Moshe Sidi, “Multiple Access Protocols – Performance and Analysis”, Springer-Verlag 1989
2. Anurag Kumar, D. Manjunath, Joy Kuri, “Communication Networking – An analytical approach”, Elsevier 2004
3. Giuseppe Bianchi, “Performance Analysis of the IEEE Distributed Coordination Function”, JSAC, March 2000
4. Z. Hadzi-Velkov, B. Spasenovski, “Saturation Throughput – Delay analysis of IEEE DCF in Fading Channel”, ICC 2003