Session 1544
Wireless LANs
15
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
1544 Wireless LANs
Overview










Introduction
IEEE 802.11 Standard
Types of 802.11 WLANs
802.11 WLAN Mobility Types
WM Access Mechanisms
WLAN Problems
Quality of Service
Simulation Models
Simulation Results & Analysis
Conclusions
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
2
1544 Wireless LANs
Introduction
A wireless LAN is one in which a mobile user can connect to a local area
network (LAN) through a wireless (radio) connection.
A standard, IEEE 802.11, specifies the technologies for WLANs.
IEEE 802.11 WLAN architecture
HL
Datalink Layer
LLC
MAC
PHY
Wireless Function
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
3
1544 Wireless LANs
The IEEE 802.11 Standard
 Original 802.11
•
•
•
•
2.4 GHz operating frequency
Data rates of 1 Mbps/2Mbps
Frequency Hopping (802.11 FHSS)
Direct Sequence (802.11 DSSS)
 Supplements of 802.11
•
•
•
802.11a – Operation up to 54 Mbps using OFDM in
the 5 GHz frequency range
802.11b – Extension of the initial DSSS 2.4 GHz band
up to 11 Mbps
802.11e – MAC layer DCF and PCF enhancements for
QoS assurance
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
4
1544 Wireless LANs
Types of 802.11 WLANs
 Ad hoc Wireless Network
This type of wireless network does not have any backbone
infrastructure and has at least two wireless stations. It is also referred
to as Independent Base Service Set (IBSS).
 Infrastructure Wireless Network
This type of wireless network consists of multiple cells interconnected
by Access Points (APs) and a Distribution System (DS) such as
Ethernet. It is also called as Extended Service Set (ESS).
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
5
1544 Wireless LANs
802.11 Mobility Types
 No transition
Refers to stations that do not move and are moving within a BSS.
(Supported)
 BSS transition
Refers to stations that move from one BSS to another BSS within the
same ESS. (Supported)
 ESS transition
Refers to stations that move from a BSS in one ESS to another BSS in a
different ESS. (Not supported)
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
6
1544 Wireless LANs
The 802.11 MAC
 Distributed Coordination Function (DCF)
•
•
•
•
•
•
Mandatory implementation
Uses CSMA/CA protocol
No service differentiation
Works for both IBSS & ESS
Operates during the Contention Period (CP)
Waits a period of DIFS interval before transmission
 Point Coordination Function (PCF)
•
•
•
•
•
Optional Implementation
Uses a Point Coordinator (PC) which resides in the AP
Works only for ESS
Operates during the Contention Free Period (CFP)
Waits a period of PIFS interval before transmission
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
7
1544 Wireless LANs
Hidden Terminal Problem
Statement
Every station in a wireless network has limited radio transmitting range.
This may lead to two stations communicating with the same receiving station
which results in a collision. The station causing the collision is termed as
“hidden” with reference to the receiving station.
Solution
The RequestToSend(RTS) & ClearToSend(CTS) mechanism is used to resolve
the hidden terminal problem.
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
8
1544 Wireless LANs
RTS/CTS Mechanism
Algorithm
•
•
•
•
•
Sender transmits RTS frame
Receiver acknowledges RTS with CTS frame
Sender receives the CTS frame and the channel is reserved
Sender sends the DAT frame
Receiver sends the ACK frame to the sender to end the 4-way handshake
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
9
1544 Wireless LANs
Quality of Service
Definition
QoS is a broad term used to describe the overall experience the end-user or
application will receive over a wireless network.
Standard parameters used for measuring QoS are
•
•
•
•
Bandwidth
Network Availability
Media Access Delay
Throughput
DCF and PCF are evaluated using Media Access Delay and Throughput
parameters.
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
10
1544 Wireless LANs
OPNET Simulation Model – DCF
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
DCF Parameters
Values
Type of Network
Ad hoc
Length of Simulation
180 seconds
Packet Size Range
1400-2000 Bytes
Interarrival time
0.1-0.2 seconds
RTS Threshold
1700 Bytes
Number of Stations
10
Physical Characteristics
Direct Sequence
Data Rate
1 Mbps
PCF Functionality
Disabled
11
1544 Wireless LANs
OPNET Simulation Model – PCF
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
PCF Parameters
Values
Type of Network
Infrastructure
Length of Simulation
180 seconds
Packet Size Range
1400-2000 Bytes
Interarrival time
0.1-0.2 seconds
RTS Threshold
1700 Bytes
Number of Stations
8
Physical Characteristics
Direct Sequence
Data Rate
1 Mbps
PCF Functionality
Enabled
CFP Beacon Multiple
1
CFP Offset
0
CFP Interval
0.01
Max Failed Polls
2
Beacon Interval
0.02 seconds
12
1544 Wireless LANs
OPNET Simulation Results – DCF
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
13
1544 Wireless LANs
OPNET Simulation Results – PCF
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
14
1544 Wireless LANs
OPNET Simulation Results –
Analysis
 DCF Mode
For this mode, the delay is low and the throughput high when the
network uses RTS/CTS mechanism. This maybe as a result of fewer
collisions caused by RTS/CTS.
 PCF Mode
For this mode, the delay and throughput for a PCF workstation are
lower than that of a DCF workstation. The lower delay may be
attributed to the fact that PCF operates under the CF mode and the
lower throughput may be attributed to the overhead involved in the
transmission of polling packets.
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
15
1544 Wireless LANs
Conclusions
 Summary
This presentation described the current status of IEEE 802.11 Wireless LANs
and evaluated the two basic wireless medium access mechanisms DCF and
PCF using OPNET Modeler. The QoS offered by the two mechanisms was
measured using Media Access Delay and Throughput metrics.
 Future Work
DCF and PCF do not provide prioritized access to the wireless medium. The
IEEE 802.11e is currently working on enhancing the 802.11 MAC with
mechanisms like Enhanced DCF (EDCF) and HCF. It will be interesting to
evaluate these mechanisms using OPNET.
Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties.
16