IEEE 802.11 Architecture and Services

Sayed Chhattan Shah
Department of Information Communications Engineering
Hankuk University of Foreign Studies Korea
www.mgclab.com
IEEE 802.11 Architecture and Services

 In 1990, IEEE 802 Committee formed a new
working group, IEEE 802.11, specifically devoted
to wireless LANs, with a charter to develop a
MAC protocol and physical medium specification

Key IEEE 802.11 Standards
Standard Scope
IEEE
802.11a
Physical layer: 5-GHz OFDM at rates from 6 to 54 Mbps
IEEE
802.11b
Physical layer: 2.4-GHz DSSS at 5.5 and 11 Mbps
IEEE
802.11c
Bridge operation at 802.11 MAC layer
IEEE
802.11d
Physical layer: Extend operation of 802.11 WLANs to new
regulatory domains (countries)
IEEE
802.11e
MAC: Enhance to improve quality of service and enhance
security mechanisms
IEEE
802.11g
Physical layer: Extend 802.11b to data rates >20 Mbps
IEEE
802.11i
MAC: Enhance security and authentication mechanisms
IEEE
802.11n
Physical/MAC: Enhancements to enable higher throughput
IEEE
802.11T
Recommended practice for the evaluation of 802.11
wireless performance
IEEE
802.11ac
Physical/MAC: Enhancements to support 0.5–1 Gbps in 5-GHz
band
IEEE
802.11ad
Physical/MAC: Enhancements to support ≥ 1 Gbps in the 60-
GHz band

Wi-Fi Alliance
 There is always a concern whether products from different vendors
will successfully interoperate
 Wireless Ethernet Compatibility Alliance (WECA)
 Industry consortium formed in 1999
 Renamed the Wi-Fi Alliance
 Created a test suite to certify interoperability for 802.11 products

Basic service
set (BSS)
STA2
STA3
STA = station
STA4
Basic
Service Set
Extended
service set (ESS)
Figure 13.4 IEEE 802.11 Architecture
STA6
STA7
IEEE 802.x LAN
STA1
Access
point
(AP)
STA5
Access
point
(AP)
portal
Distribution System (DS)

 Basic service set (BSS) consists of some number of stations executing
the same MAC protocol and competing for access to the same shared
wireless medium
 A BSS may be isolated or it may connect to a backbone distribution
system (DS) through an access point (AP)
 In a BSS, client stations do not communicate directly with one another
 In an IBSS the stations all communicate directly
 No AP is involved.
 An IBSS is typically an ad hoc network.

 An extended service set (ESS) consists of two or more basic service
sets interconnected by a distribution system
 To integrate the IEEE 802.11 architecture with a traditional wired LAN,
a portal is used

 802.11 Infrastructure Mode
o at least one wireless AP and one wireless client.
 802.11 Ad Hoc Mode
o wireless clients communicate directly with each other without the
use of a wireless AP
IEEE 802.11 Operating Modes

IEEE 802.11 Terminology
Access point (AP) Any entity that has station functionality and provides
access to the distribution system via the wireless
medium for associated stations
Basic service set
(BSS)
A set of stations controlled by a single coordination
function
Coordination function The logical function that determines when a station
operating within a BSS is permitted to transmit and
may be able to receive PDUs
Distribution system
(DS)
A system used to interconnect a set of BSSs and
integrated LANs to create an ESS
Extended service set
(ESS)
A set of one or more interconnected BSSs and
integrated LANs that appear as a single BSS to the LLC
layer at any station associated with one of these BSSs
Frame Synonym for MAC protocol data unit
MAC protocol data
unit (MPDU)
The unit of data exchanged between two peer MAC
entities using the services of the physical layer
MAC service data unit
(MSDU)
Information that is delivered as a unit between MAC
users
Station Any device that contains an IEEE 802.11 conformant MAC
and physical layer

IEEE 802.11 Terminology
 Each layer has Service Data Unit (SDU) as input
 Each layer makes Protocol Data Unit (PDU) as output to communicate
with the corresponding layer at the other end
 SDUs may be fragmented or aggregated to form a PDU
 PDUs have a header specific to the layer

IEEE 802.11 Services
Service Provider Used to support
Association Distribution
system
MSDU delivery
Authentication Station LAN access and
security
Deauthentication Station LAN access and
security
Dissassociation Distribution
system
MSDU delivery
Distribution Distribution
system
MSDU delivery
Integration Distribution
system
MSDU delivery
MSDU delivery Station MSDU delivery
Privacy Station LAN access and
security
Reassocation Distribution
system
MSDU delivery
IEEE 802.11 defines nine services that need to be provided by WLAN

Distribution of Messages Within a DS
Distribution service
Primary service used by
stations to exchange MAC
frames when frame must
traverse the DS to get from
a station in one BSS to a
station in another BSS
If stations are in the same
BSS, distribution service
logically goes through the
single AP of that BSS
Integration service
Enables transfer of data
between a station on an
IEEE 802.11 LAN and a
station on an integrated
IEEE 802.x LAN
Takes care of any address
translation and media
conversion logic required
for the exchange of data
Services involved with the distribution of messages within a DS

Association-Related Services
 DS requires information about stations within the ESS
that is provided by the association-related services
 Station must be associated before DS can deliver data to
or accept data from it
 3 mobility transition types
No transition
stationary or in
single BSS
BSS transition
between BSS in
same ESS
ESS transition
between BSS in
different ESS

 Association station must establish an association with an
AP within a particular BSS
 The AP can then communicate this information to other APs
within the ESS to facilitate routing and delivery of addressed
frames
 Reassociation Enables an established association to be
transferred from one AP to another, allowing a mobile
station to move from one BSS to another
 Disassociation: A notification from either a station or an
AP that an existing association is terminated
Association-Related Services

IEEE 802.11 Medium Access Control
MAC layer
covers three
functional
areas
Reliable
data
delivery
Access
control
Security

Reliable Data Delivery
 802.11 physical and MAC layers unreliable
o Noise, interference, and other propagation effects result in the loss of a
significant number of frames
o The issue can be addressed at a higher layer such as TCP
 Timers used for retransmission at higher layers are typically on the order of seconds
 More efficient to deal with errors at MAC level
 802.11 includes frame exchange protocol
o Station receiving frame returns acknowledgment (ACK) frame
o Exchange treated as atomic unit
o If no ACK within short period of time, retransmit

 To further enhance reliability, a four
frame exchange may be used
o RTS alerts all stations within range of
source that exchange is under way
o CTS alerts all stations within range of
destination
o Other stations don’t transmit to avoid
collision
o RTS and CTS exchange is a required
function of MAC but may be disabled
Source issues a Request
to Send (RTS) frame
Destination responds
with Clear to Send (CTS)
After receiving CTS,
source transmits data
Destination responds
with ACK
Reliable Data Delivery

 Two types of proposals for a MAC algorithm
o Distributed access protocol which distribute the decision to transmit over all the
nodes using a carrier sense mechanism
o Centralized access protocol which involve regulation of transmission by a
centralized decision maker
MAC Algorithm

Point
Coordination
Function (PCF)
Contention-free
service
Contention
service
Figure 13.5 IEEE 802.11 Protocol Architecture
MAC
layer
Distributed Coordination Function (DCF)
LOGICAL LINK CONTROL (LLC)
PHYSICAL LAYER
(802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.11ad)

Distributed Coordination Function (DCF)
 DCF sublayer uses
CSMA algorithm
 Does not include a
collision detection
function because it is
not practical on a
wireless network
 Includes a set of delays
that amounts as a
priority scheme
If station has frame to
send it listens to
medium
If medium is idle, station
may transmit
Else waits until current
transmission is complete

Wait for frame
to transmit
Wait IFS
Figure 13.6 IEEE 802.11 Medium Access Control Logic
No
Yes
Yes
Yes
No
No
Wait IFS
Medium
idle?
Still
idle?
Wait until current
transmission ends
Exponential backoff
while medium idle
Transmit frame
Transmit frame
Still
idle?

Priority IFS Values
SIFS
short IFS
For all
immediate
response
actions
PIFS
point coordination
function IFS
Used by the
centralized
controller in
PCF scheme
when issuing
polls
DIFS
distributed coordination
function IFS
Used as
minimum delay
for
asynchronous
frames
contending for
access

Defer access
DIFS
Immediate access
when medium is free
longer than DIFS
SIFS
PIFS
DIFS
Busy Medium Next frameBackoff window
Contention window
Slot time
Select slot using binary exponential backoff
(a) Basic Access Method
time
PCF (optional)
Contention-free
period
Variable length
(per superframe)
Busy medium PCF (optional)
DCF
Contention period
Superframe (fixed nominal length)
Superframe (fixed nominal length)
Foreshortened actual
superframe period
PCF
defers
CF-Burst;
asynchronous
traffic defers
(b) PCF Superframe Construction
Figure 13.7 IEEE 802.11 MAC Timing
During the first part
of Superframe
interval, the point
coordinator issues
polls in a round-
robin fashion to all
stations
configured for
polling. The point
coordinator then
idles for the
remainder of the
superframe,
allowing a
contention period
for asynchronous
access

SIFS
 Any station using SIFS to determine transmission opportunity has
the highest priority
 SIFS is used in the following circumstances:
o Acknowledgment (ACK)
 Station responds with an ACK frame after waiting only for a
SIFS gap
 Provides for efficient collision recovery
o Clear to Send (CTS)
 Station ensures data frame gets through by issuing RTS

Point Coordination Function (PCF)
 Point coordination function (PCF) resides in a point coordinator also known as
Access Point , to coordinate the communication within the network
 The AP waits for PIFS duration rather than DIFS duration to grasp the channel
 Channel access in PCF mode is centralized
o Access to the medium is restricted by the point coordinator
o Associated stations can transmit data only when they are allowed to do so
by the point coordinator
 Due to the priority of PCF over DCF, stations that only use DCF might
not gain access to the medium
 To prevent this, a repetition interval has been designed to cover both
Contention free or PCF & Contention Based or DCF traffic

PCF Operation
 Reserving the medium during the contention-free period
 The polling list
o Stations get on the polling list when they associate with the AP
o Polls any associated stations on a polling list for data transmissions
o Each CF-Poll is a license to transmit one frame
o Multiple frames can be transmitted only if the access point sends
multiple poll requests

Frame Control
Figure 13.8 IEEE 802.11 MAC Frame Format
2
Duration/ID2
Address 16
Sequence Control2
QoS Control2
High Throughput Control4
Frame Check Sequence (FCS)4
Always present
0—7951
Address 46
Address 26
Address 3
MAC
header
6
octets
Present only in
certain frame
types and subtypes

Control Frames
• The purpose is to request that the AP transmit a frame that has been
buffered for this station while the station was in power saving mode
Power Save-Poll (PS-Poll)
• First frame in four-way frame exchange
Request to Send (RTS)
• Second frame in four-way exchange
Clear to Send (CTS)
• Acknowledges correct receipt
Acknowledgment (ACK)
• Announces end of contention-free period that is part of PCF
Contention-Free (CF)-end
• Acknowledges CF-end to end contention-free period and release stations
from associated restrictions
CF-End + CF-Ack:
Assist in the reliable delivery of data frames

Control Frames
Duration field in RTS frame

Control Frames
The receiver of a CTS frame is the transmitter of the previous RTS frame, so the MAC
copies the transmitter address of the RTS frame into the receiver address of the CTS frame

Data Frames
 Data frames carry higher-level protocol data in the frame body
 Eight data frame subtypes
o Organized in two groups
 First four carry upper-level data
 Remaining do not carry any user data
o Data
 Simplest data frame, it may be used in both a contention or contention-free period
o Data + CF-Ack
 Carries data and acknowledges previously received data during contention-free period
o Data + CF-Poll
 Used by point coordinator to deliver data and also to request that the mobile station send a data
frame that it may have buffered
o Data + CF-Ack + CF-Poll
 Combines Data + CF-Ack and Data + CF-Poll

Management Frames
Used to manage
communications between
stations and APs Management of associations
• Request, response,
reassociation, dissociation,
and authentication

Management Frames
 Beacon
o announce the existence of a network
o transmitted at regular intervals to allow mobile stations to find and identify
a network, as well as match parameters for joining the network
 Probe Request
o Mobile stations use Probe Request frames to scan an area for existing
802.11 networks
o Include SSID and the rates supported by the mobile station
o Stations that receive Probe Requests use the information to determine
whether the mobile station can join the network
 Probe Response

Management Frames
 Disassociation and Deauthentication
 Association Request
 Authentication

IEEE 802.11 Physical Layer Standards
Standard 802.11a 802.11b 802.11g 802.11n 802.11ac 802.11ad
Year
introduced
1999 1999 2003 2000 2012 2014
Maximum data
transfer
speed
54 Mbps 11 Mbps 54 Mbps
65 to
600 Mbps
78 Mbps
to 3.2
Gbps
6.76 Gbps
Frequency
band
5 GHz 2.4 GHz 2.4 GHz
2.4 or 5
GHz
5 GHz 60 GHz
Channel
bandwidth
20 MHz 20 MHz 20 MHz
20, 40
MHz
40, 80,
160 MHz
2160 MHz
Highest order
modulation
64 QAM 11 CCK 64 QAM 64 QAM 256 QAM 64 QAM
Spectrum
usage
DSSS OFDM
DSSS,
OFDM
OFDM SC-OFDM SC, OFDM
Antenna
configuration
1´1 SISO 1´1 SISO 1´1 SISO
Up to 4´4
MIMO
Up to 8´8
MIMO, MU-
MIMO
1´1 SISO

IEEE 802.11b
 Extension of 802.11 DSSS scheme
o Data rates of 5.5 and 11 Mbps
o Complementary Code Keying (CCK) modulation gives higher
data rate with same bandwidth and chipping rate

IEEE 802.11a
 Makes use of the frequency
band called Universal
Networking Information
Infrastructure (UNNI)
o UNNI-1 band (5.15 to 5.25
GHz) for indoor use
o UNNI-2 band (5.25 to
5.35GHz) for indoor or outdoor
o UNNI-3 band (5.725 to 5.825
GHz) for outdoor
 Advantages over IEEE
802.11b and g
 IEEE 802.11a
o Utilizes more available
bandwidth
o Provides much higher data
rates
o Uses a relatively uncluttered
frequency spectrum (5 GHz)

IEEE 802.11g
 Higher-speed extension to 802.11b
 Operates in 2.4GHz band
 Compatible with 802.11b devices
 Combines physical layer encoding techniques used in 802.11 and
802.11b to provide service at a variety of data rates
o ERP-OFDM for 6, 9, 12, 18, 24, 36, 48, 54Mbps rates
o ERP-PBCC for 22 and 33Mbps rates

IEEE 802.11n
 Has enhancements in three general areas:
o Multiple-input-multiple-output (MIMO) antenna architecture
 Most important enhancement
 A device can transmit multiple spatial streams at once
• only directed to a single address
o Radio transmission scheme
 Increased capacity
o MAC enhancements
 Most significant change is to aggregate multiple MAC frames
into a single block for transmission

MSDU1
MAC
header
PHY
header
MSDU1
MAC
header
PHY
header
MSDU2
MAC
header
PHY
headerACKPHY
header
SIFSor
backoff
MSDU2
SIFS
ACKPHY
header
SIFS
MSDU3
MAC
header
PHY
header MSDU4
MAC
header
PHY
headerACKPHY
header
SIFSor
backoff
SIFS
ACKPHY
header
SIFS
Block
ACK
PHY
header
SIFS
Block
ACK
PHY
header
SIFS
ACKPHY
header
SIFS
x
xRetransmitted due to single bit error
MPDU subframe MPDU subframe
MPDU subframe
(a) No aggregation
(c) A-MPDU aggregation
(b) A-MSDU aggregation
(d) A-MPDU of A-MSDU aggregation
MPDU subframe
MPDU delimiter
MPDU subframe MPDU subframe
MAC
header
PHY
header MSDU1
A-MSDU
subframe
MSDU2
A-MSDU
subframe
MAC
header
PHY
header MSDU1
A-MSDU
subframe
MSDU2
A-MSDU
subframe
MAC
header MSDU3
A-MSDU
subframe
MSDU4
A-MSDU
subframe
MSDU3
A-MSDU
subframe
MSDU4
A-MSDU
subframe
MSDU2
MAC
header MSDU3
MAC
header MSDU4
MAC
header x
x
x
A-tMSDU delimiter
Figure 13.11 Forms of Aggregation

IEEE 802.11ac
 Includes the option of multiuser MIMO (MU-MIMO)
o On the downlink the transmitter is able to use its antenna
resources to transmit multiple frames to different stations, all at
the same time and over the same frequency spectrum
o Each antenna of a MU-MIMO AP can simultaneously
communicate with a different single-antenna device, such as a
smart phone or tablet
 Requires that every 802.11ac transmission be sent as an A-MPDU
aggregate

IEEE 802.11ad
 A version of 802.11 operating in the 60-GHz frequency band
o Offers the potential for much wider channel bandwidth than the 5-
GHz band
o Few devices operate in the 60-GHz which means communications
would experience less interference than in the other bands used by
802.11
 Undesirable propagation characteristics:
o Losses are much higher in this range than in the ranges used for
traditional microwave systems
o Multipath losses can be quite high
o Millimeter-wave signals generally don’t penetrate solid objects

802.11ac and 802.11ad Differences
802.11ac
 Supports a MIMO antenna
configuration
802.11ad
 Is designed for single-antenna
operation
 Has a huge channel bandwidth
of 2160 MHz

IEEE 802.11 Architecture and Services

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