[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
AIM: To plan and analyse the Wireless Local Area Network using
OPNET Objectives: 1) Design and Analysis of WLAN without rts/cts
technique 2) Design and Analysis of WLAN with rts/cts technique 3)
Comparison of WLAN performance with and without rts/cts 4) Design
and Analysis of WLAN Infrastructure Basic Service Set (BSS) mode 5)
Design and Analysis of WLAN Infrastructure Extended Service Set
(ESS) mode 6) Comparison of WLAN in both BSS and ESS modes
1037089
Page 1
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Introduction 1:Wireless Local Area Network (WLAN): A Local Area
Network links the computers or desktops in a building, or an office
or a campus. Wireless Local Area Network (WLAN) is a Local Area
Network (LAN) without wires. The Wireless Local Area Network (WLAN)
technology is defined by the IEEE 802.11 family of specifications.
WLAN is a communication system which is used as an extension or as
an alternative to wired LAN where it is not possible for drawing
wires to each floor of a building and to each roo m in a building.
Wireless LAN uses Radio Frequency (RF) technology to transmit and
receive the data through the air by minimizing the need of wired
connections. Thus we can say that Wireless LAN combines the data
connectivity with the user mobility. It has gained so much
popularity in a number of vertical markets which includes retail,
warehousing, manufacturing, hospitals and academia. It is widely
used because of their wide benefit which includes increased
productivity, fast and simple network, installation flexibility,
reduced cost of ow nership and scalability etc. WLAN uses the
distribution methods to inter connect two or more nodes such as
direct spread spectrum, OFDM radio, frequency hopping spread
spectrum, infrared technology and etc. WLAN can be simple or
complex. At its least even two PCs equipped with wireless adapter
cards can form a network as long as there are within each other s
range. This type of network is called peer-to-peer network.
Figure 1: peer-to-peer network (Source: Google images)
1037089
Page 2
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Using extension points and access points the range can
be increased, doubling the actual range at which the devices can
communicate with one another. A single access point can provide
service to 15-50 clients in the network.
Figure 2: WLAN using access points (Source: Google images)
Hidden node problem:Hidden nodes in a Wireless Local Area
Network refer to the nodes which are not in the range of the other
nodes or a group of nodes. Consider a physical Star Topology with
an access point with many nodes surrounding in a circular fashion;
e ach node may be within the communication range of the Access
Point, but the nodes cannot communicate with each other, because
there will not be any physical connection with each other.
1037089
Page 3
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 3: Hidden node problem (source: Google images) The Hidden
Node problem is shown in Figure 3 above. Node C and N ode A cannot
hear each other. So if node A is transmitting, node C will not know
and may tra nsmit as well. This will in turn cause collisions.
Carrier Sense Multiple Access with Collision Avoidance or CSMA/CA
is the solution to this problem . CSMA/CA will work as follows: the
station listens before it sends. This implies that if any one of
the node is transmitting, the other node will wait for a random
period and then try again. If no one is transmitting the data then
it sends a short message, which can be considered as a request
message from the source to the
destination. This message is called the Request To Send message
(RTS). This message contains the destination address and the
duration of the transmission. Other stations will come to know that
they must wait that long before they can transmit. The destination
then sends a short message which is the Clear To Send message
(CTS). This message tells the source that it can send without fear
of collisions. Each packet is acknowledged. If an acknowledgement
is not received, the MAC layer retransmits the data. This entire
sequence is called the 4-way handshake as shown in the figure 4
given below. This is the protocol that 802.11 chose for the
standard. (Source:http://www.wireless-
telecom.com/unlicesend%20tutorial.htm)
1037089
Page 4
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 4: The 4-way Hand shake (Source: Google Maps)
RTS/CTS Techniques:One of the best ways to ensure that the
packets are not inhibited by any another transmission is to reserve
the medium for the station s use. In 802.11 this can be
accomplished by the RTS/CTS (Request to send/ Clear to send)
protocol. RTS/CTS enable a source node to issue an RTS signal to an
access point requesting the exclusive opportunity to transmit. If
the access point agrees by responding with a CTS signal, the access
point temporarily suspends communication with all stations in its
range and waits for the source node to complete its transmission.
RTS/CTS is not routinely used by wireless stations, but for
transmissions involving large packets (those more subject to damage
by interference), RTS/CTS can prove more efficient. On the other
hand, using RTS/CTS further decreases the overall efficiency of the
802.11 network. 2009) To overcome the problem of uncertainty in the
wireless medium, the 802.11 MAC uses an acknowledgement (ACK)
protocol. When a packet is transmitted, the sender firsts listens
for any activity on the air, and if there is none, waits a random
amount of time before doing a transmission. This methodology is
called carrier sense multiple acce ss/collision avoidance
(CSMA/CA). (Dean, fifth edition
1037089
Page 5
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] CSMA/CA can be viewed as a "listen first, talk later"
methodology. If an ACK is not received, either due to interference
or collision, then the entire process is repeated. The MAC layer
ACK protocol is independent of the higher layer protocol, whether
it is UDP or TCP. The ACK function is not the only Quality of
Service (QoS) headache for designers looking to deliver voice
services over WLAN systems. The WLAN MAC also includes a request to
send/clear to send (RTS/CTS) mechanism. When used together, RTS and
CTS decrease the chance of collision on a system by making sure
that end stations in the vicinity of the source and destination
hear the RTS and CTS respectively. RTS and CTS add robustness to
the system at the cost of adding latency to the packets that are
transmitted using this protocol. Figure 5 explains the influence of
an RTS and CTS frame exchange.
Figure 5: Hidden node problem employing RTS/CTS Technique
(Source:http://www.eetimes.com/design/communications
-design/4008952/OvercomingQoS-Security-Issues-in-VoWLAN-Designs)
1037089
Page 6
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Hidden node problem model without rts/cts:
OPNET (Optimized Network Engineering Tools) is a discrete
network simulator which contains a comprehensive development
environment supporting the modelling and performance evaluation of
communication networks and distributed systems. OPNET provides four
hierarchical editors to develop a modelled system, Network Editor,
Node Editor, Process Editor, and Parameter Editor. Performance
evaluation and trade-off analysis require large volumes of
simulation results to be interpreted and OPNET includes a tool for
graphical representation and processing of simulation output.
Simulation runs can be configured to automatically generate
animations of the modelled system at various levels. (Koziniec,
June 2002) A hidden node problem as described e arlier in the
introduction part is the situation where the nodes which are not in
the range of the other nodes or a group of nodes try to communicate
at the same time sharing the same medium and thus resulting in the
data collision and loss of data. In this report we create the
hidden node problem network using three nodes named node A, node B
and Receiver as shown in the figure shown below. The network is
imported by opening the Project which is already created and saved
in the drive named 1332_WLAN .
1037089
Page 7
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 6: Hidden node problem model using three nodes As shown
in Figure 6, two nodes and one receiver are placed representing a
campus wide WLAN. Here Receiver will represent an access point for
WLAN. A trajectory for Node A is drawn, on which it will move
according to the predefined amount of time and distance. Simulation
is set up such that Node A will move (Node A is going out of range
of Node B) along the path represented in the Figure 6 over the
course of about 1000 seconds .
1037089
Page 8
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 7: Graphical Results of WLAN Hidden Node Problem without
rts/cts
Figure 8: Wireless LAN Data Traffic Sent and Data Traffic
Received (bits/sec)
1037089
Page 9
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
The Figure 8 shows the WLAN data traffic sent and received in
bits/second during simulation time of 1000 seconds. Traffic
received statistics indicates WLAN data traffic successfully
received by the MAC from the physical layer in bits/sec. Moreover,
it includes all data traffic received regardless of the destination
of the received frames. While computing the size of the received
packets for this statistic, the physical layer and MAC headers of
the packet are also included. Traffic Sent statistic represents
WLAN data traffic transmitted by the MAC of Node in bits/sec. While
computing the size of the transmitted packets for this statistic,
the physical layer and MAC headers of the packet are also included.
Due to the defined trajectory pattern, Node A and Node B can no
longer see each other, so both nodes no longer receive traffic from
one another, as indicated by the data traffic received line in the
Figure. Since these nodes can t detect each other s transmissions,
the collision probability for their transmissions increases, which
leads to a higher number of collisions and retransmissions. As Node
A moved away between 350 and 650 second of simulation, traffic
received during the time is almost none.
Figure 9: WLAN Retransmission Attempts (packets)
1037089
Page 10
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Figure 9 also shows WLAN Retransmission Attempts of
Node A and Node B. This statistic includes the number of
retransmission attempts until either packet is successfully
transmitted or it is discarded as a result of reaching short or
long retry limit. It can be seen that Node A and Node B are having
many retransmission attempts during the time when Node A is out of
Node B s transmission range. It is due to the collisions of packets
sent by both Node A and Node B simultaneously. Numbers of
retransmission are between 50 and 140 for both Node A and Node B,
while both are hidden from each other.
Figure 10: Wireless LAN Delay (sec) Once the hidden terminal
problem occurs, both nodes increase their retransmission attempts
dramatically, which naturally causes more collisions. This increase
in collisions slows down the WLAN dramatically as shown in Figure
10. It shows the WLAN Delay which represents the end to end delay
of all the packets received by the wireless LAN MACs of all WLAN
nodes in the network and forwarded to the higher layer. This delay
includes medium access delay at the source MAC, reception of all
the fragments individually. It is indicated that WLAN Delay is very
high during those retransmissions, while Node A is hidden and out
of transmission range of Node B. This shows the behaviour of the
network performance degraded due to hidd en terminal problem.
1037089
Page 11
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 11: Average WLAN Load and WLAN Throughput (bits/sec) The
Figure 11 shows the throughput of the network. Throughput of the
network is defined as the total number of bits (in bits/sec)
forwarded from wireless LAN layers to higher layers in all WLAN
nodes of the network. It can be observed that the bits transmitted
by the both nodes are summed up to form the throughput of the
network indicating that the hidden node problem has no effect on
the throughput of the netw ork, this is because both the nodes are
transmitting the bits irrespective of the reception of the bits by
the receiver.
1037089
Page 12
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Hidden node problem model with rts/cts mechanism :By switching
the scenario to hidden_node_rts_cts the new network is formed by
changing RTS Threshold (bytes) to 1024 bytes and thus introducing
the rts/cts mechanism in the network. In order to reduce the amount
of collisions caused by hidden terminal problem, the IEEE 802.11
protocol can use the RTS/CTS mechanism optionally. The same
experiment is conducted, but with the RTS/CTS mechanism enabled on
Node A and Node B. This allows eliminating some, but not all of the
collisions caused by the hidden terminal problem. The analysis of
this network can be defined with the following graph as shown
below
Figure 12: Graphical Results of hidden node problem with rts/cts
mechanism
1037089
Page 13
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 13: Wireless LAN Traffic Received by node A for both
scenarios The Figure 13 shows the comparison of the WLAN data
traffic received by Node A for both scenarios. Notice that the
traffic received on Nodes A did not change since RTS/CTS is enabled
only helpful to prevent collisions from happening.
Figure 14: WLAN Data Traffic Sent by node A for both
Scenarios
1037089
Page 14
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Figure 14 shows WLAN data traffic sent by Node A. The
significant drop in collisions had a very large effect on the WLAN
sent traffic. Data traffic sent is reduced due to the less number
of retransmissions since it is not necessary to keep sending data
other nodes w ithin a range are already transmitting (e.g. the Node
B and the receiver).
Figure 15: WLAN Retransmission Attempts by node A for both
Scenarios Figure 15 also that WLAN retransmission attempts by Node
A. The retransmission attempts did indeed increase a noticeable
amount once Node A begins to move. However, the amount of
retransmission attempts is up to 8 times fewer than when RTS/CTS
was not enabled. This means that there was a significant drop in
collisions.
1037089
Page 15
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 16: WLAN Delay for both Scenarios Figure 16 shows the
WLAN Delay. Due to less number of retransmissions, the wireless LAN
delay drops drastically for the period when the nodes are hidden to
each other. It is important to note that the delay is now little
higher when Node A and B can hear each other (when they are not
hidden from each other). It is due to the overhead caused by the
RTS/CTS handshake mechanism.
Introduction 2: Infrastructure Basic Service Set (BSS):A service
set is a group of devices (access points, routers, client stations,
etc) sharing the same name (SSID service sets: 1) Independent Basic
Service Set 2) Basic Service Set 3) Extended Service Set The Basic
Service Set (BSS) is also called as infrastructure basic service
set. Service Set Identifier) and technology. There are three types
of
1037089
Page 16
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] A BSS uses access point, which acts as the connection
point to the infrastructure or forms the infrastructure itself,
which is why it is called an infrastructure BSS. There is no direct
communication between the nodes in this mode. The communication
between the nodes is done through the access point in a hub
-and-spoke fashion. (Source: CWTS Certified Wireless Technology
Specialist Study Guide (Exam PW0-070) By Tom Carpenter) When we
connect a physical path cable to an Ethernet network, we are
connected to the network. Since cables aren t used in wireless
networks, something else is needed. In a wireless network, both
Independent Basic Service Set and Infrastructure Basic Service Set,
the concept of association is analogous to plugging in the patch
cable. Association means the wireless client requests and is then
granted permission to join the service set. This is important
because any given area can have multiple access points and the
client must know with which access point to communicate. This is
determined by the SSID configured on the client and the access
point.
Figure 17: Infrastructure Basic Service Set showing the hidden
node problem (Source:
http://www.wildpackets.com/resources/compendium/wireless_lan/wlan_packets)
1037089
Page 17
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
WLAN Architecture:Wireless networks have some fundamental
characteristics which make them significantly different from
traditional wired LANs. In wired LANs, an address is equivalent to
a physical location. This is implicitly assumed in the design of
wired LANs. In IEEE Std 802.11, the addressable unit is a station
(STA). The STA is a message destination, but not (in general) a
fixed location. The PHYs used in IEEE Std 802.11 are fundamentally
different from wired media. The limitations on the wireless LANs
limits their coverage area to geographical distances may be built
from basic coverage building blocks. One of the requirements of
IEEE Std 802.11 is to handle mobile as well as portable STAs. A
portable STA is one that is moved from location to location, but
that is only used while at a fixed location. Mobile STAs actually
access the LAN while in motion. Components of the IEEE 802.11
architecture : The IEEE 802.11 architecture consists of several
components that interact to provide a WLAN that supports STA
mobility transparently to upper layers. The basic service set (BSS)
is the basic building block of an IEEE 802.11 LAN The independent
BSS (IBSS) as an ad hoc network : This mode of operation is
possible when IEEE 802.11 STAs are able to communicate directly.
Because this type of IEEE 802.11 LAN is often formed without pre
-planning, for only as long as the LAN is needed, this type of
operation is often referred to as an ad hoc network. Distribution
system (DS): Instead of existing independently, a BSS may also form
a component of an extended form of network that is built with
multiple BSSs. The architectural component used to interconnect
BSSs is the DS. Extended service set (ESS): The DS and BSSs allow
IEEE Std 802.11 to create a wireless network of arbitrary size and
complexity. IEEE Std 802.11 refers to this type of network as the
ESS network. An ESS is the union of the BSSs connected by a DS. The
ESS does not include the DS.
1037089
Page 18
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] An access point (AP) is any entity that has STA
functionality and enables access to the DS, via the WM for
associated STAs. (Source: IEEE Std 802.11 -2007) Wireless local
area networks (WLANs) are often implemented as an overlay to the
wired LAN. There are two distinct WLAN architectures. They are
lightweight and autonomous, each having varied impact on the wired
network infrastructure . The two main architectures used in the
WLAN environment differ in the extent that the wireless access
point (WAP) has autonomy over access, security, and operation.
Lightweight WAPs, which form part of a centralized WLAN
architecture, have limited functionality, with most of the wireless
intelligence residing at a central controlling device (i.e., the
WLAN controller). By contrast, an autonomous architecture uses
distributed WAPs that usually do not require a wireless controller.
To differentiate between a lightweight and an autonomous WLAN
architecture requires an understanding of the role and hierarchy of
devices in a network. For instance, in the network world, there is
a widely accepted hierarchical model that identifies network
devices by classifying them into one of three layers. In an
autonomous architecture, a wireless controller is not required. The
autonomous WAPs support all necessary switching, security, and
advanced networking functions necessary to route wireless traffic.
By contrast, in lightweight WLAN architectures, hardware consists
of reducedfunctionality WAPs that operate together with a
centralized wireless controller. The controller resides deeper in
the LAN, at the distribution or possibly the core layer. The WAPs
do not function independently of the wireless controller.
(Source:
http://www.cablinginstall.com/index/display/article-display/256716/articles/cablinginstallation-maintenance/volume-14/issue-6/features/wireless/choosing-the-right-wlanarchitecture.html)
1037089
Page 19
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 18: Autonomous architecture model
Figure 19: Lightweight Architecture Model
Extended Service Set (ESS):To cover a larger area, multiple
access points are deployed. This arrangement is called as ESS. ESS
can be defined as two or more BSSs that share the same network name
or SSID and are connected to the same distribution system. The
distribution system may be wired or wireless, but it is shared by
all access points p articipating in an ESS. The concept of the ESS
allows users to roam around (physically) on the network and still
connect to the same network with the same name.
1037089
Page 20
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Each access point is assigned a different channel
wherever possible to minimize interference. If a channel must be
reused, it is best to assign the reused channel to the access
points that are the least likely to interfere with one another.
When users roam between cells or BSSs, their mobile device will
find and attempt to connect with the access point with the clearest
signal and the least amount of network traffic. This way, a roaming
unit can transition seamlessly from one access point in the system
to another, without losing network connectivity. An ESS introduces
the possibility of forwardin g traffic from one radio cell (the
range covered by a single access point) to another over the wired
network. This combination of access points and the wired network
connecting them is referred to as the Distribution System (DS).
Messages sent from a wireless device in one BSS to a device in a
different BSS by way of the wired network are said to be sent by
way of the distribution system or DS. (Source:
http://www.wildpackets.com/resources/compendium/wireless_lan/wlan_packets)
Figure 20: Extended Service Set supporting roaming between the
cells
(Source:http://www.wildpackets.com/resources/compendium/wireless_lan/wlan_packets)
1037089
Page 21
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Infrastructure Basic Service Set (BSS) Model:
Infrastructure Basic Service Set is the service set which uses
at least one access point as the connection point for the inter
communication of nodes in the network; there is no direct
communication between the nodes in this mode. In the infrastructure
mode, the wireless network consists of at least one AP (access
point) connected to the wired infrastructure. All the wireless
stations are connected to the AP. An AP controls encryption on the
network and also can route the wireless traffic to a wired network
(same as a router). It can be assumed an AP as the base station
used in cellular networks. An Infrastructure BSS wireless LAN
network that spans multiple floors on a building is designed using
an OPNET network model as shown in the figure. In this task the
Infrastructure BSS network is designed with 9 work stations, one
server, one switch and one access point. This is obtained by
switching the scenario to Infrastructure_BSS . These 9 work
stations are distributed with 3 work stations on each floor with an
access point in the second floor as s hown in the figure given
below
1037089
Page 22
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 21: Infrastructure Basic Service Set The traffic is
configured by editing nodes Application attributes. The row value
is changed to 1 and profile name as Wlan-engineer. The data rate is
changed to 1 Mbps. The following Global statistics (Email, FTP,
HTTP, Remote Login and Wireless LAN) are observed in the following
figure after simulating the network for the duration of 10
minutes.
1037089
Page 23
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 22: Graphical Results of Infrastructure_BSS Performance
The above figure describes the following terms Email Download
Response Time (sec): describes the time elapsed between sending the
request for emails and receiving emails from email server in the
network. This time includes signalling delay for the connection
setup. FTP Upload Response Time (sec): represents the time elapsed
between sending a file and receiving the response. The response
time for responses sent from any server to an FTP application is
included in this statistic . HTTP Page response time (sec):
specifies the time required to retrieve the entire page with all
the contained in line objects. In addition, the HTTP object
response time (sec) specifies the response time for each in lined
object from the HTML page.
1037089
Page 24
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Remote Login Response Time (sec): is defined as the
time elapsed between sending a request and receiving the response
packet. Measured from the time a client application sends a request
to the server to the time it receives a response packet. Every
response packet sent from a server to a remote login application is
included in this statistic. Wireless LAN Data Dropped (Buffer
Overflow) (bits/sec): is defined as the total size of higher layer
data packets (in bits/sec) dropped by all the WLAN MACs in the
network due to: a) Full higher layer data buffer, or b) The size of
the higher layer packet, which is greater than the maximum allowed
data size defined in the IEEE 802.11 standard. Wireless LAN Media
Access Delay: represents the global statistic for the total of
queuing and contention delays of the data, m anagement, delayed
Block-ACK and Block-ACK Request frames transmitted by all WLAN MACs
in the network. For each frame, this delay is calculated as the
duration from the time when it is inserted into the transmission
queue, which is arrival time for higher layer data packets and
creation time for all other frames types, until the time when the
frame is sent to the physical layer for the first time. Hence, it
also includes the period for the successful RTS/CTS exchange, if
this exchange is used prior to the transmission of that frame.
Similarly, it may also include multiple numbers of back off
periods, if the MAC is 802.11e -capable and the initial
transmission of the frame is delayed due to one or more internal
collisions. Wireless LAN Throughput (bits/sec): defines the total
number of bits (in bits/sec) forwarded from wireless LAN layers to
higher layers in all WLAN nodes of the network. Total Traffic Sent
(bits/sec): represents WLAN data traffic transmitted by the MAC in
bits/sec. While computing the size of the transmitted packets for
this statistic, the physical layer and MAC headers of the packet
are also included. This statistic also includes Data-Null, CF-Ack,
CF-Poll and CF-Poll+CF-Ack frames, which are specified as data
frames in the IEEE 802.11 standard, sent during the contention free
periods, if PCF operation was enabled for the BSS of this MAC.
1037089
Page 25
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] Total Traffic Received (bits/sec): represents WLAN
data traffic successfully received by the MAC from the physical
layer in bits/sec. This statistic includes all data traffic
received regardless of the destination of the received frames.
While computing the size of the received packets for this
statistic, the physical layer and MAC headers of the packet are
also included. This statistic also includes Data-Null, CF-Ack,
CF-Poll and CF-Poll+CF-Ack frames, which are specified as data
frames in the IEEE 802.11 standard, received during the contention
free periods, if PCF operation was enabled for the BSS of this MAC.
(Source: OPNET Network Simulation Software, Version: 14.5) By
observing the graphs in the above figure and by understanding the
definitions of the parameters it can be concluded that application
response time for the Infrastructure_BSS is high. And it can be
seen that FTP Upload Response time is very hi gh compared to Email
Response Time, HTTP Page Response Time and Remote Login Response
Time. The application data received is lower than the application
load, this occurs due to dropped data as the buffer get congested
and full. The Wireless LAN throughput remains at high unaffected by
the mode of transmission as it just involves the total number of
packets transmitted from the Wireless LAN to the higher layers in
all WLAN nodes of the network. Wireless LAN access is very high and
it can be seen in the graph that the average delay goes to nearly
0.80 seconds and remains there for so long. As the buffers get
congested and full, Wireless LANs drop packets and thus the network
reaches the saturation position. Since all the clients are needed
to communicate with the other client using only one access point,
this result in the channel overlapping and resulting in collision
of the data leading to high data dropping, increase in
retransmission attempts, less received data and large delay in WLAN
which can be observe d clearly in the graphs.
1037089
Page 26
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Infrastructure Extended Service Set (ESS) Model:
To overcome the problem of channel overlapping two more extra
access points are introduced in the network with a single access po
int in each floor of a building as shown in the figure shown below.
Deployment of additional access points increases the WLAN capacity.
Distributing the wireless clients on different floors among the
access points reduces the contention for each shared medium. The
Infrastructure Extended Service Set (ESS) can be defined as two or
more BSSs that share the same network name or SSID and are
connected to the same distribution system. The distribution system
may be wired or wireless, but it is shared by all access points
participating in an ESS. In this task an ESS is designed using 9
work stations, one server, one switch and 3 access points. Each
access point is connected to the same switch.
1037089
Page 27
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 23: Infrastructure Extended Service Set Simulation is
performed in the duration of 10 minutes with changing the BSS
Identifier value to the respective floor value. Comparison of both
the modes; that are Infrastructure Basic Service Set and
Infrastructure Extended Service Set is performed by importing
the
1332_infrastructure_ess
into the scenario
Infrastructure_ESS . This results in the
following graph as shown in the figure
1037089
Page 28
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Figure 24: Graphs showing the Performance Comparison of BSS and
ESS From the figure 24 it is clear that average FTP Traffic
Received (bytes/sec) is increased to a great extent in the Wireless
LAN Infrastructure Extended Service Set mode compared to
Infrastructure Basic Service Set and it can also be observed that
the data received a bit earlier in ESS mode. From the figure 24 it
is observed that FTP Upload Response Time is faster (smaller value)
for the network in ESS mode compared to FTP Upload Response Time of
the network in the BSS mode. From the figure 24 it is understood
that average WLAN delay is lowered for the network in the case of
ESS mode compared to the network in the BSS mode. It can also be
seen that WLAN Data Dropped (bits/sec) is almost zero for the ESS
mode network while for the BSS mode network the Data Dropped
(bits/sec) linearly increases with the time as shown in the
figure.
1037089
Page 29
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET] From the figure 24 it is understood that the average
WLAN throughput (bits/sec) is much improved in the ESS mode network
while compared to BSS mode network.
It can be concluded that deployment of additional access points
increased the W LAN capacity. Distributing the wireless clients on
different floors among the access points reduced the contention for
each shar ed medium. WLAN packet drops can be observed and
significant reduction in it is found as positive effect. Moreover,
application throughput is increased significantly. It can also be
observed that average WLAN delay is significantly lowered due to
lowered less contention. Also application end to end delay was
lowered despite higher throughput. It can be concluded from this
task that increasing the number of access points is a useful
alternative when the data rate cannot be increased.
1037089
Page 30
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
Conclusion:
From the task 1 it can be concluded that with the introduction
of rts/cts (request to send/clear to send) mechanism hidden node
problem overcomes the problem where the network faces without
rts/cts mechanism. RTS/CTS handshake mechanism helped to improve
the WLAN performance when nodes are hidden from each other. The
WLAN outgoing traffic stayed at a constant rate, even though Node A
was in motion and moving away (hidden from Node B). When RTS/CTS is
disabled, network performance dropped significantly. When RTS/CTS
is enabled, LAN delay stayed around 30ms, outgoing traffic remained
stable, and collisions were kept to a minimum. From the task 2 it
can be concluded that with the introduction of extra two access
points the performance of WLAN is increased. When a s ingle access
point (BSS) is used the following problems are seen; a much delay
in WLAN, less received traffic, much dropped data. These problems
are occurred due to channel overlapping and usage of a single
access point. These problems are overcome with the introduction of
two more extra access points (ESS) and thus having an access
allowance to the individual three clients in the respective floor.
This reduces the channel overlapping and in turn results good
performance. By distributing the wireless clients on different
floors among the access points reduced the contention for each
shared medium. Moreover, application throughput is increased
significantly. It can also be observed that average WLAN delay is
significantly lowered due to lowered less contention. Also
application end to end delay was lowered despite higher throughput.
It can be concluded from this tas k that increasing the number of
access points is a useful alternative when the data rate cannot be
increased.
1037089
Page 31
[WLAN SIMULATION PLANNING AND ANALYSING WLAN January 27, 2011
USING OPNET]
References:
1) http://www.wireless-telecom.com/unlicesend%20tutorial.ht m 2)
http://www.eetimes.com/design/communications-design/4008952/Overcoming
QoS-Security-Issues-in-VoWLAN-Designs 3) CWTS Certified Wireless
Technology Specialist Study Guide (Exam PW0-070) By Tom Carpenter
4)
http://www.wildpackets.com/resources/compendium/wireless_lan/wlan_packets
5) IEEE Std 802.11 -2007 6)
http://www.cablinginstall.com/index/display/article
display/256716/articles/cabling
installation-maintenance/volume-14/issue-6/features/wireless/choosing-the-rightwlan-architecture.html
7) Koziniec, M. W. (June 2002). Using OPNET to Enhance Student
Learning. Perth, Australia 8) OPNET Network Simulation Software,
Version: 14.5 9) Dean, T. (fifth edition 2009). Network + Guide to
networks. Boston
1037089
Page 32