Network Simulation Of Wireless Lan Ieee Computer Science Essay

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This paper describes basic understanding of Wireless LAN IEEE 802.11 technology. Also provides graphs for Media Access delay and Application Response time with understandings of the results using graphical representation and comparisons under three different scenarios for a specified network model using Opnet IT Guru Academic Edition.

Introduction

IEEE 802.11

IEEE 802.11 is a Wireless Local area network protocol (WLAN) and its associated technologies such as

802.11x and WPA (Wi-Fi Protected Access) allow secure high speed wireless network access and mobile access to a network infrastructure.

"The IEEE 802.11 is a standard adopted for wireless LANS (WLAN). This technology has grown from basic 802.11 to 802.11n providing users with high bandwidth in a limited area. It defines a Medium Access Control (MAC) sublayer, MAC management protocols and services and three physical layers (PHY). Physical layers define the modulation and signalling characteristics for the transmission of data. 802.11 physical layer has two radio frequency (RF) transmission methods and one infrared (IR) are defined. The RF transmission standards in the standard are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). Both architectures are defined for operation in the 2.4GHz frequency band typically occupying the 83 MHz of bandwidth from 2.400 GHz to 2.483

GHz (US and Europe). The choice between FHSS and DSSS will depend on a number of factors related

to the users application and the environment that the system will be operating. All three PHY layers have both 1 and 2 Mb/s operation." 1

This is an evolving technology for wireless local area networks (WLAs) developed by a working group of institute of Electrical and Electronics Engineers (IEEE). IEEE 802.11 use carrier sense Multiple Access and Collision Avoidance and Ethernet Protocol for path sharing. Initial modulation used in

802.11 was using phase-shift keyring (PSK). There are newer modulation methods which provide

higher data speed and reduces vulnerability to interference.

There are number of benefits that can be achieved implementing a WLAN and few of them are discussed in the following section such as to connect the networks between two buildings using physical layer is not a cost effective method in such cases a point-to-point wireless link can be setup using wireless LAN technology.

For short term communication systems a temporary network can be created using Wireless LAN

technologies such as a conventional trade shows instead of using a traditional physical cabling.

1 IEEE 802.11 Working Group, IEEE 802.11 standard for wireless LANs, 1999, Reaffirmed 2003.

Buildings with historic values cannot be setup using cables as there are certain legislations in different countries about any changes that can be made to such buildings, in this scenario wireless LAN implementation will help in achieving the goal to setup a network

At last but not least most commonly used in household where internet is connected to multiple devices or multiple computers2

System Architecture

There are two different wireless LAN system architecture

Infrastructure

Ad-hoc

In the first mode wireless networks, communication takes place between the wireless nodes and the access point but not between the wireless nodes. This is similar to Switched Ethernet or Star based

network.

BBS1

STA

802.x LAN

STA

AP

Distributed System

AP

STA

BBS2

STA

Figure shows the components of an infrastructure-based IEEE 801.11 WLAN

Infrastructure mode or wireless network nodes called as stations are connected through an access points. Stations communicate through access points.

IEEE 802.11 allows Adhoc wireless networks as they don't need any infrastructure to work.

Protocol Architecture

IEEE 802.11 fits seamlessly in to the 802.x family for wired LAN's.

2 http://www.qsl.net/n9zia/wireless/pdf/802.11.pdf

Application

Application

TCP/IP

TCP/IP

LLC

LLC

LLC

802.3 MAC

802.3 MAC

802.11 MAC

802.11 MAC

802.3 PHY

802.3 PHY

802.11 PHY

802.11 PHY

IEEE 802.11 standard covers two bottom layers PHY called Physical Layer and MAC medium access

Layer. Physical Layer consists of two sub Layers called as Physical Layer Convergence Protocol and

Physical Medium dependent.

LLC

Station management

MAC

MAC Management

PLCP

PHY management

PMD

PMD sublayer handles encoding/decoding and modulation of the signals. PLCP sublayer provides a common PHY service access point which is independent of the technology used to transmit.

Physical Layer

IEEE 802.11 protocol supports three different layers where one is based on infrared and the other two based on radio transmission. Layers based on Radio transmission specified in Industrial, scientific and Medical band at 2.4 GHz and are based on frequency hopping spectrum and direct sequence spread spectrum.

Power Management

IEEE 802.11 standard address the issue of Power saving and explains the complete mechanism which enables to go in to sleep mode for long periods of time without losing information. Idea of power saving in 802.11 is to turn off the transceiver wherever it is not needed. Two states used for this are sleep and awake. This idea is based on access point buffers all frames for stations in power save mode.

Roaming

During moving stations it needs to change access point which is called as roaming.

Station scans the environment by either using passive scanning by listening to signals or using passive scanning by sending probes in to the medium

Station selects best acces point by sending re-association request

Once the reassociation response is successful access point has answered station can now participate or failure and next the station continues to scan

Access point accepts reassociation request and it signals the new station to the distribution system and the distribution

Productivity

ï‚· This technology is helpful in any environment where there is a necessary for communication through wireless channel where there is a movement of devices with in a specific area network.

ï‚· Productivity of an individual will increase with wireless channel as the user can take the device to different location within an office building or specified network covered local area to work seamlessly with other devices and with other communication world.

ï‚· Now a days this is used with in many retail sectors where wireless devices are distributed to

perform different tasks to get the job done.

ï‚· Another benefit is that with no wireless infrastructure available wireless computers can form its own ad hoc networks to communicate with each other.

IEEE 802.11 standard has initialized the development of LAN's and WAN's but the technology has gone far from the basic LAN and WAN's with the implementation of 802.11a/b/g and n technologies. Most commonly used 802.11 series is the n in the present world. 3

OPNET

OPNET is one of the most commonly used network development software introduced in 1986 by an MIT graduate. Opnet allows a network engineer to design and study communication networks, protocols, devices and application. This modeller is used by most large organization during the R&D process.

This software's Object oriented modelling approach and its GUI helps relatively better method and easy means of developing models comparing with the actual world network devices, hardware and protocols. This modeller supports all major network technologies and types allowing the engineer to design and test various network scenarios with reasonable certainty of the output results.

Opnet can be used in major application areas such as

ï‚· Wireless and Satellite communication protocols

ï‚· Microwave and Fibre-optic based Network Management

ï‚· Network planning includes both LAN and WAN and analysis of performance issues even before actual implementation

ï‚· Development and management of Protocol

ï‚· Evaluation of routers, Switches and other connecting devices using Routing Algorithm

Some of the outstanding features of the Opnet makes it a comprehensive model which are as follows

ï‚· Approach using Hierarchical network models4

3 http://www.qsl.net/n9zia/wireless/pdf/802.11.pdf

ï‚· Object oriented modelling

ï‚· Different scenarios can be simulated at the same time and compared

ï‚· All the results and patterns can be imported into the modelling software

ï‚· Built in graphing tools help us to analyse different results

This software allows us to model network topologies with nested sub-networking approach by allowing nodes and protocols to be designed as classes making the best use of all features with Object oriented design. This also allows designing the behaviour of individual objects at the process level helping to interconnect them to form devices at the Node level. This allows interconnecting devices using links to form networks at the Network Level. This also helps us in organizing multiple network scenarios to compare designs from LAN's or nodes.

This network simulation software provides the ability to modify network parameters and view the effect of the changes. This is a very good process during the learning stage as establishing a physical network is an impossible task for study.

OPNET IT GURU Simulation Results

Using OPNET IT guru network simulation software we will be analysing given network to view results for Media Access delay and Application Response time with changes to the number of workstations for the given network.

Description about the Used Network Model

4 Cathy Zhang ,Ricky Chau ,Wenqi Sun . (2009). Wi-Fi NETWORK SIMULATION OPNET . ENSC 427

COMMUNICATION NETWORKS

Scenario provided has 4 Office building linked to a Wireless distribution system and each linked to its own Router connection each acting as a gateway with all together 4 routers and one for each building. Each building consists of two work stations where the office building 1 and 2 has Access point and server but office building 3 and 4 only has an Access point

In Office Building 1 & 3 all stations are configured for 1Mbps data speed. Access point is setup for Office building 1 so that it only operates for small packets of Data and large packets are dropped and with a DATA rate of 1Mbps. It can only accept a maximum queue size of 10 and response time limit is set to 5 seconds. Server configuration's WLAN parameters are set to default and also workstation parameters are set with default values. In Office building 2 Access point functionality is disabled and Data rate is set to 11 Mbps. All the routers in the Wireless distribution system are interconnected to share the connections required during transfer of data when requested.

Profile config node is setup so that to create user profiles and these profiles can be specified on different nodes in the network to generate application layer traffic. Traffic patterns can be specified in profile config and profile attributes can be set.

Application config can be used for three specifications

ï‚· ACE tier information

ï‚· Application Specification

ï‚· Voice Encoder Schemes

Application Definition: File Transfer, Email, File Print, Video Conferencing, Telnet Session, Voice over

IP call, Web browsing

Voice Encoder Schemes: All

Scenario 1

Run the simulator with simulation time set to 8 Hours

Media Access Delay(Sec) - Time it takes to successfully deliver a frame

Figure: Media Access Delay (Sec)

From the above graph it displays that the media access delays runs around 3ms for the time span of

8 hours. There is no major delay in Media access in the first 3 hours where the first break is at around after 3 hours after which every 1 hour approximately. This is very good result where Media Access delays should be minimum as the real-time applications maximum tolerable delay after which the data will be useless. Media Access delay with the minimum work stations (2) goes as low as 2ms and a maximum of 6ms

In the average result initial point starts from just above 3ms and then stays at a constant rate of just around 3ms which is less for a network and this has to be minimum to achieve best results.

Application Response Time

Figure: Page response time

Since the first scenario is a simple network Page response time and it is minimum with the original network as there are very few systems which trying to access the network. So application response time will be very low in this case. Average response time starts with initial page load time of just above 6ms and then drops to just above 5ms making it with an average value of 6ms which is a very good response time with the number of workstations and the whole network.

Scenario 2

This scenario is run by increasing the number of workstations to 10 and run the simulation setting the time to 8 hours

Media Access Delay (Sec)

Above graphs show the result for Media Access delay after increasing the number of work stations to 10 in each office building. Rusult with the mean values has not varied much but with only

variations starting with value just close to 4ms instead of 3ms in the previous and fluctuating between 2.5ms to 4.1ms.

Average values for the Media access delay with 10 workstations has almost resulted with same values ranging around 3ms but just 0.1ms higher than the prevoius. This provides that increasing the number of systems had very minimal effect on the Media access delay.

Application Response Time

Page response time values have not changed much compared to 2 work stations where as in the prervious it has resulted under 6ms in certain situations and some interval of time but with 10 work stations average value has always been above 6ms. This shows that on increasing the number of workstations page resposne time increases which is obvious due to high load being added to the network access. As the number of systems increased Latency increases due to sharing of bandwidth among different systems.

Scenario 3

By Increasing the number of workstations to 30

Note: As I was using this simulation for number of times for learning purpose unfortunately I have received the following information which it run for time 5hrs 13min 41 sec

You are using IT Guru Academic Edition

and have exceeded the limit on the number of simulation events. Your simulation has been stopped and the results to this

point have been saved.

-----

Simulation Completed - Collating Results.

Events: Total (50000423), Average Speed (631446 events/sec.) Time: Elapsed (1 min. 19 sec.), Simulated (5 hr. 13 min. 41 sec.) Simulation Log: 4 entries

So the following results are incomplete

Media Acces Delay

Results above show that the Media access delay has changed with increase in number of work stations to 30. Although this cannot be compared at the moment due to the results run only for less than 8 hours. Initial Media access delay value increased to a value of 6.1ms with considerabl drop to about 3.2ms and has its consistance value around 3ms. Considerable load on to the network leads to high Media access delay values which for a network depending on values of work stations and kind of data dependent being accessed through work stations.

Application Response Time

Page response time certainly increases due to increase in the number of workstations but a considerable drop in the average values because of stable access from all the workstations. This also depends on content being access and the connection of the internet during the process of running the simulation. Values are considerably affected in the initial stage with as high as 1s and dropping to 0.8s in the average value of the page response time.

Scenario 4

Media Access Delay Comparision between graphs with 2, 10 and 30 workstations for the given network analysis

The above graph displays average of all the simulations run with 2, 10 and 30 workstations in each buldling. First has an initial value of 3ms and raises to just above then reducing with a mean value of

2.5ms. With 10 workstations value drops to a certain low form initial high media access delay and

stabilizes at 3ms. Initial value for 30 workstations has an higher value of nearly 6ms but considerable drop due to constant run of the system just stablizing at 3ms.

Comparing gives analysis that eventhough there is initial increase in Media access delay but average value stabilizes at 3ms which is a good even after increase in the number of work stations.

Application Response time Comparision between graphs with 2, 10 and 30 workstations for the given network analysis

Application response time increases with the number of work stations increased to 30. With the initial result has only been to a maximum of 7ms and dropped with an average to 6ms and when compared to 10 workstations there is no much difference in the values but a vast difference can be noticed in the scenario with 30 workstations where the initial response time hits 9ms and stabilizes between 6 and 7ms. This clearly shows that increse in the number of workstations increases the resposne time.

Scenario 5

Retaining the number of wireless hosts to 30, make one change to the attribute of the Wireless

Access point in each building to improve the Media Access Delay and Application response time

In this case to improve Media access delay first we increase the RTS thershold from Zero to 1024 for access point in each building. Comparing Media access delay with result from RTS thershold Zero it displays with an initial value of 5.5ms for RTS Thershold 1024 where it was just above 7ms with RTS Thershold Zero. As the number of workstations increased in the network the WLAN throughput will reduce and thus can be solved by using the RTS mechanism. This can also be improved using Fragmentation by increasing the value to 1024.

Application Response Time

Using the same values above Page response time has considreably fallen from value 0.10 to between

0.09 to 0.10 where RTS therschold value is equal to 1024.

Conclusion

Opnet IT software lets us understand networks and its behavoiur using model software by viewing the results nearly to live enviornment and thus provides the best information for live scenarios. We have also learnt how the performance changes with different attributes changes for a model and its behaviour of network componets

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