wlan

[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

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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)

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[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.

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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)

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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

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[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)

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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 .

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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 .

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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)

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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)

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[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.

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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.

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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

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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

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[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.

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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

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[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)

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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.

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[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)

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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.

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[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)

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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

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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.

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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.

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[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.

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[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.

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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.

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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

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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.

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[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.

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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.

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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

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wlan

Documents

analysing wlan

wlan simulation planning

source node

comparison of wlan performance

peer network source

local area network lan

simple network

local area network links