Implementing Wireless Infrastructure Network with ...

Proceedings of The 9th Islamic Countries Conference on Statistical Sciences 2007 ICCS-IX 12-14 Dec 2007

Implementing Wireless Infrastructure Network with Efficient Security By Engr.Imran Anwar Ujan Assistant Professor, Institute of Information Technology, University of Sindh, Jamshoro [email protected] Engr. Liaquat Ali Thebo

Assistant Professor Department of Computer Systems & Software Engineering Mehran University of Engineering and Technology, Jamshoro

Abstract The Objective of our Paper is to implement Wireless Infrastructure network at Computer System Department. The wireless infrastructure to be created is seen as complementary to the LAN infrastructures already in place. The objective is also to facilitate mobility and, in particular, the information access, exchange and sharing among the users. Some main requirements are taken into consideration. First of all, compliance with the standard IEEE 802.11b is mandatory, but opportunity to upgrade to IEEE 802.11g was required. To ensure interoperability of WLAN (Wireless Local Area Network) products, the wireless fidelity or Wi-Fi sticker (Wi-Fi Alliance) is also a requisite. In terms of compliance with security and encryption standards, the equipment should support IEEE 802.1x, and be upgradeable to support the standard WPA (Wi-Fi Protected Access) and WPA2.

1. INTRODUCTION TO WIRELESS TECHNOLOGY EVOLUTION 1.1 The Search for True Wireless Freedom Freedom is the new driving force in telecommunications. With access and speed now consumers want to connect to the wired world without wires. Service providers are, therefore, focused on wireless networking to extend the reach of existing infrastructures. Likewise, enterprises are turning to wireless deployments as a means of facilitating productivity and providing easy access for increasingly mobile workforces. [w -1 ]

1.2 Wireless Technologies There are a number of technologies commonly referred to as wireless and it important to lay the groundwork for the discussion of the various kinds of wireless.

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1.2.1 Cell Phones Using a cell phone to dial-up to the Internet, either as a stand-alone tool or as a modem for a laptop is becoming more common. However this does typically increase your cell phone related costs significantly and the performance is generally on par with a slow dial-up connection.

1.2.2 Satellite This technology offers the opportunity to connect to the Internet from any location in the US. Using the same infrastructure that provides satellite TV service, resellers are offering Internet connectivity for home, business and mobile uses. This technology is quite expensive to purchase install and subscribe to. Unless you have no other option for connecting to the Internet,

1.2.3 Infrared This wireless technology is most widely used by television remotes. It is also used by Personal Digital Assistants (PDA’s) like Palm Pilots to transfer information. Portable printers and other peripheral devices also use it. This technology is very slow and does not offer any great advantages to non profits other than some simple conveniences.

1.2.4 Bluetooth It has more capabilities than infrared, but still very local “in the room” wireless technology. [w – 2]

1.3 Wireless LANs Wireless LAN stands for Wireless Local Area Network. It is a flexible data communications system implemented to extend or substitute for, a wired LAN. Radio frequency (RF) technology is used by a wireless LAN to transmit and receive data over the air, minimizing the need for wired connections. A WLAN enables data connectivity and user mobility.

1.4 The Need of Wireless Networks Wireless allows your computer to be free from wiring to the network and you have the potential to be mobile and use the network in a 'connectivity anywhere' style of working (e.g. in the library, various concourses, during meetings etc.). This assumes of course, that your computer is portable. This flexibility can lead to a more dynamic and spontaneous mode of working, where that is important. It can also be of great value where locations are either very difficult to wire (or indeed are sensitive areas - historically or environmentally), and where wiring is not possible (for example due to escalating costs of wiring in an area where there may be asbestos).

1.5 Application of Wireless LANs Wireless LANs have enjoyed a very fast adoption rate due to the many advantages they offer to a variety of situations. In this section, we will discuss some of the most common and appropriate uses of wireless LANs.

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1.5.1 Access Role Wireless LANs are mostly deployed in an access layer role, meaning that they are used as an entry point into a wired network. Due to a lack of speed and resiliency, wireless networks are not typically implemented in Distribution or Core roles in networks. The Core layer of a network should be very fast and very stable, able to handle a tremendous amount of traffic with little difficulty and experience no down time. The Distribution layer of a network should be fast, flexible, and reliable. Wireless LANs do not typically meet these requirements for an enterprise solution. As an access layer solution, wireless LANs cannot replace wired LANs in terms of data rates (100BaseTx at 100Mbps versus IEEE 802.11a at 54Mbps). But still, WLAN has sufficient data rate to handle all bandwidth intensive application at Access layer.

Figure 1.1 Access Role of WLAN 1.5.2 Building To Building Connectivity Suppose that there are two or more building at opposite and we have to establish network among these buildings, and at the mid of these two buildings there is heavy traffic road, authorizer will not allow us to dig the road for cable to install. In this case WLAN will be applicable. There are two types of building to building connectivity 15.2.1Point-to-Point (PTP) PTP are wireless connections between two buildings. As shown in figure 1.3. PTP connections almost always use semi-directional or highly-directional at each end of the link.

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Building-to-Building

Connectivity

Figure 1.2 Building-to-Building Connectivity

1.5.2.2 Point-to-Multipoint (PTM) Point-to-multipoint links are wireless connections between three or more buildings, typically implemented in a "hub and spoke" or star topological fashion, where one building is the central focus point of the network. This central building would have the core network, Internet connectivity, .and the server farm. Point-to-multipoint links between buildings typically use omni-directional antennas in the central "hub" building and semi-directional antennas on each of the outlying "spoke" buildings.

1.5.3 Last Mile Data Delivery Wireless Internet Service Providers (WISPs) are now taking advantage of recent advancements in wireless technology to offer last mile data delivery service to their customers. "Last mile" refers to the communication infrastructure—wired or wireless—that exists between the central office of the telecommunications company or Cable Company and the end user. [b-1]

Figure 1.3 Last Mile Service

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1.6 Benefits of Wireless LANs For the predictable future wireless technology is likely to go together rather than replace wired connectivity in enterprise environments, due to the following reasons:

1.6.1 Installation Speed and Simplicity Installing a wireless LAN system can be fast and easy and can eliminate the need to pull wires or cable through walls and ceilings, hence overcome physical limitations. Wireless networking can succeed where wired solutions are either too costly or complicated.

1.6.2 Installation Flexibility Wireless technology allows the network to go where wire cannot go.

1.6.3 Flexibility & Scalability Wireless LAN gives organizations the flexibility to move people from office to office, reorganize departments or even entire campuses almost effortlessly. The flexibility and easy installation of wireless systems make it the preferred solution for public as well as private networks. It also makes it easy to add, remove and reinstall equipment from site to site, no limitations. As network capacity grows and predicting the required capacity becomes difficult, providers prefer to deploy flexible and modular networks, and wireless systems are far more flexible than other transmission technologies.

1.6.4 Mobility Wireless LAN systems can provide users access to real-time information anywhere within the organization. This extra mobility supports productivity and service opportunities not possible with wired networks. [w -5]

1.6.5 Time savings The users are not bound to use network services only at working areas, hence save time.

1.6.6 Convenience It provides easiest way to use and setup network and internet access over a defined area. 1.6.7 Scalability Wireless LAN systems can be configured in a variety of topologies to meet the needs of specific applications and installations. Configurations are easily changed and range from peer-to- peer networks suitable for a small number of users to full infrastructure networks of thousands of users that allows roaming over a broad area.

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1.6.8 Productivity Wireless should be seen more as an extension of the wire to increase productivity for certain users or to bring the data where the cable cannot go. [w -3 ]

1.6.9 Reduced Cost-of-Ownership & Overall Cost Savings Even though we have seen a sharp decrease in the price of wireless components, it is still an expensive solution, if wire is already installed. This is more apparent when a cost/performance analysis is made. But still it has cost advantages over traditional wired networks, although the initial investment required for wireless LAN hardware can be higher than the cost of wired LAN hardware, but overall installation expenses and life-cycle costs can be significantly lower. Longterm cost benefits are greatest in dynamic environments requiring frequent moves, adds, and changes. With the simple and flexible architecture of WLAN, organizations can save network management costs related to adds, moves and changes, guaranteeing a high term Return on Investment. [w- 5]

1.6.10 Performance The wireless is now a days running at 54 Mbps, sufficient to support current and future multimedia solutions such as Voice Over IP, streaming video and bandwidth intensive Intranet applications.

1.6.11 Quality of Service Multimedia applications need a network that can prioritize the data flow. For now there are no QoS features embedded within the current 802.11b, g standard. Standards bodies and vendors are looking into the needs and implementation of QoS, but standardization is not expected for the immediate future where wired alternatives provide a method of prioritizing traffic with the 802.1Q standard. [w - 4]

1.7 Quality of Life Benefits In addition to productivity, there are other “soft” benefits of using WLANs. In particular, users cite “quality of life” benefits to connecting to the LAN via a wireless connection. According to WLAN users, the three most important quality of life benefits of using a wireless LAN include:

1.7.1 Reduced stress Wireless LAN users do not have to worry about trying to find a wired connection. For example, if a wireless LAN user brings the wrong presentation to a meeting, he or she will not have to worry since the correct presentation can be accessed on the server wirelessly. In this way, stress is alleviated since the user doesn’t have to worry about being “plugged in” to access the LAN.

1.7.2 Improved family life Some users are even able to spend more time at home due to WLANs. Some wireless LAN users stated that they are able to reduce the hours they spend at work due to accomplishing more by being mobile at the office. Some users are able to save up to two hours each day at the office, and go home at a more reasonable hour.

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1.7.3 Increased status For some users, WLANs provide a level of status, in terms of both a personal edge, as well as a competitive edge. Employees that are using the wireless LAN feel that they have a personal advantage over users who do not have WLAN access. This feeling of power, in the minds of the wireless LAN users, leads to feelings of having a competitive edge.

1.8 Challenges to Deploying Wireless LANs In addition to the many benefits of deploying WLANs, there are some challenges. In particular, participants cited security concerns, slow speed, and lower reliability (compared to wired LANs) as primary challenges they are currently facing with their wireless LANs.

1.8.1 Security Concerns Many organizations are concerned that wireless LANs do not provide a level of security equal to wired LANs. In fact, some people are concerned that competitors are able to steal WLAN transmissions out of the air.

1.8.2 Lower reliability If you have microwaves running all the time, you might experience some interference.

1.8.3 Slow speed The speed of WLAN is slow as compared to wired fast or gigabit Ethernet. So really there are few minor cons, the benefits far outweigh them. [ w -7]

2. IMPLEMENTING THE WALN AT CS DEPARTMENT 2.1 Planning of Wireless Network There are six basic steps involved in planning a secure wireless LAN.

2.1.1Understanding Your Wireless LAN Needs The primary and foremost step when planning a wireless LAN is to understand wireless LAN needs. I t helps you select the type of wireless LAN that satisfies your needs. Identifying the computer programs and applications that you will be running on the computers accessing the wireless LAN is of critical importance as it helps you decide whether wireless LANs satisfy your primary needs or not.

2.1.1.1 File and Application Servers Most LANs are used for sharing files among users. These files reside on the large-Capacity hard disks on computers, which can be standalone file servers or workstations, and are connected to the network.

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2.1.1.2 The Internet Access Internet access has become one of the prime uses of LANs. LANs normally share a high-speed connection among users.

2.1.1.3 Select Suitable Type of Wireless LAN Selecting the type of wireless LAN you need depends on a number of factors, including: the number of intended wireless LAN connections, the deployment scenario (for example, home use, SOHO, enterprise, or WISP), security, and the external connectivity desired.

2.1.1.4 Peer-to-Peer or Ad-Hoc Wireless LAN Peer-to-peer wireless LANs consist of two or more computers, generally without any type of server (for example, file server or mail server).

2.1.1.5 Standalone Wireless LAN Standalone wireless LANs consist of one or more computers and may include a file server and other shared network resources. These LANs are considered relatively secure as they are normally not connected to the Internet or any external network.

2.1.1.5 Wireless LAN as a Replacement for a Wired LAN Many organizations are replacing their wired LANs with wireless LANs. Creating a wireless LAN in place of a wired LAN provides mobility to the users and makes future LAN upgrades a little easier.

2.1.1.6 Wireless LAN as an Extension to a Wired LAN A good idea when deploying a wireless LAN is to first deploy it as an extension to an existing wired LAN. This means that you leave the wired LAN intact and add a wireless LAN as a supplementary network to the original LAN.

2.2 Planning the Scope of Rollout Define the rollout scope of the wireless LAN you are planning to deploy. Decide whether you want to deploy the entire wireless LAN at once or begin by first doing a pilot and then progressively roll out the actual wireless LAN.

2.3 Performing Site Survey You must perform a site survey for each physical location where you intend to install a wireless LAN. The site survey includes careful consideration of the geographic

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2.3.1 Locating the Best Spots for Wireless Access Points The best spots for installing wireless LAN access points are the areas that allow the least obstructed signal transmission and are the closest to the wireless LAN users.

2.4 Setting Up Requirements and Expectations It is important to establish practical expectations while setting requirements for the LAN. When setting requirements, you should make sure that they are practical and possible given the technologies you are planning to use.

2.5 Estimating the Required Wireless LAN Hardware and Software Estimate the required hardware and software that you will need to deploy your wireless LAN successfully.

2.6 Evaluating the Feasibility of WLANs and the ROI Deploying a wireless LAN might not be the best idea for all scenarios. You should carefully examine the outcome of your planning. In addition to feasibility, you might also want to estimate the return on investment (ROI) of deploying a wireless LAN. The calculation of ROI should include the costs associated with deployment and usage and the amount of money to be saved over a reasonable period of time. A positive ROI will be a good sign for wireless LAN deployment, whereas a negative ROI could become an inhibiting factor. [b- 2].

2.7 Wireless LAN Planning for Computer System Department Following are some of our assumptions about the computer system department.: Computer system department has x

8 labs

x

8 teachers offices

x

a chairman office

x

35 employees and

x

Approximately 500 hundred students.

2.7.1 Computer system department Wireless LAN Needs Requirement

Purpose

Wireless LAN connectivity

To provide staff with mobile computing devices the freedom to move about and enhance productivity.

Internet access

Each user must have Internet access to interconnect.

VPN Gateways

To provide connectivity to remote users. Table 2.1 WLAN Needs for CS Dept 1842


2.7.2 Planning the Rollout We have take computer system department as rollout for implementation of wireless LAN in university.

2.7.3 Site Survey We perform a site survey by carrying an AP with and use laptop computer, equipped with a wireless LAN adapter and monitoring software, to figure out the dead spots and the best locations for the APs.

Lab/office/classroom Number of users

Computing devices

Need Wireless LAN

LAB # 1 - 8

30 each

Desktop computers

Yes

Teacher’s Office (8)

1 each

Desktop/Laptop

Yes

Classroom

Case dependent

Laptop

Yes

Admin Office

4- 6

Desktop/laptop

Yes

Table 2.2 Site Survey report of CS Dept

2.7.4 Setting Up Requirements and Expectations The following are our minimum requirements and expectations: x

The proposed LAN will not be a complete wireless solution. It will be a hybrid LAN consisting of both wired and wireless LAN-based technologies.

2.7.5 Estimating the Required LAN Hardware and Software Estimated LAN Hardware and Software for Computer system wireless LAN, along with the cost required are:

Equipment

Description

REQUIREMENTS

QUANTITY

TOTAL PRICE

Wireless LAN Access

At least 54 Mbps. 40

Adapter

wireless LAN

(PCMCIA / PCI).

Access Point

Provide

54

wireless

recommended

Mbps 5

Rs.1,00,000/=

Rs.30,000/=

connectivity

Table 2.3: Hardware & Software requirement for CS dept

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2.7.6 Evaluating the Feasibility of Wireless LANs and Estimating Return on Investment (ROI) We assess that the following demonstrates the feasibility of using wireless LANs at computer system department. Enhanced mobility Fewer cables involved. Enhanced productivity.

2.9 Implementing the Wireless network We still have a lot of work to do in considering network design, equipment selection, installation and setup time, and eventually performance tweaking. After you can design, install, and set up a wireless network, you need to know a bit more about the various equipment and configuration options—from access points to antennas, cabling to client software. [w - 6]

2.9.1 Network design The first step in our process was deciding that we wanted to overlay our wired network with wireless. We feel that it would be unwise to depend totally on wireless. Moreover, with the hybrid network we are able to connect to and use all resources of wired network as well. However, there are some places on campus where only wireless exists because of the difficult logistics at certain locations for installing wired connections.

2.9.2 Equipment selection Our next step was finding a vendor. We have been fortunate because we found a vendor who is virtually unlimited in scope of work, which is Linksys. Hence we have bought a Wireless Broadband Router of Linksys. The wireless LAN card may be of any vendor, because all those are nearly identical and compatible with every other vendor.

2.9.3 Installation, setup and Performance Tweaking The next consideration was how much wireless coverage should be installed. This decision was limited by our pockets. We wanted to blanket the entire campus, but our pocket didn’t allow so, to proceed towards such a big project. So, we decided to install wireless network within our own Computer System department. Keeping in mind, the practibility of implementation of wireless network within the Computer System department, we had to see, where to install our Access Points. Accordingly, we divided the location of our Computer System department in four area coverage portions. To verify practibility of implementation, firstly we installed one Access Point on the wall of the corridor, which (Thanks ALLAH) excellently worked and covered the whole corridor. Since, there are no ventilators or windows on the corridor side in the laboratories, as such it didn’t work well in the laboratories. Hence, we installed one more Access Point on the wall of the corridor, which excellently worked and covered all the laboratories. 1844


In order to cover the teacher’s offices and the admin room, we installed one Access Point on the wall adjacent to rooms, which also worked excellently. Next, Access Point was installed at the wall outside the lecture rooms, which also worked excellently within the class halls. The final forth Access Point was installed outside the wall of laboratories, which covered a wide area being opened one. The main feature of our mini implementation uptil where pocket allowed is that, it works uptil 300 feet in open space and 100 feet in covered area. Softly, we have to arrange five Access Points at a time, to cover the whole area of Computer Systems department, but we could not do so, as our pocket did not allow.

Fig 2.1 2.9.4 Costs The largest single expense of this project was access points at Rs.6400/= The remaining costs were for two wireless NICS (for the access points) of Rs.2500/= each.

Equipment configurations 2.10 Configuring the Access Point The method for configuring the AP differs from manufacturer to manufacturer. The following are some of the basic methods for configuring the AP.

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2.10.1 Wirelessly Configuring an AP Using Vendor-Provided Software or Web Browser The software that is used to configure the AP can be a Web browser, in which case the AP acts as a Web browser and lets you configure the AP parameters, or it could be special software that a vendor provides that helps you with the AP configuration steps.

2.10.2 Configuring an AP Using a Wired LAN An AP can be connected to a wired LAN during configuration if it does not support wireless configuration or if it is to be used as a wireless LAN bridge. Just like configuring an AP wirelessly, an AP can be configured using web browser or setup software that is provided by the AP vendor.

2.10.3 Important WLAN AP Configuration Parameters SSID or Network Name The wireless LAN adapters use these SSIDs to identify the APs you want to connect with them. The SSIDs are names, generally up to 32 characters long, that are assigned to each AP.

2.10.4 Security Settings 802.11 standard supports Wireless Equivalent Privacy (WEP)-based security. WEP security allows all data communication between wireless LAN clients and/or APs in encrypted form.

2.10.5 AP operation mode settings 2.10.5 Connecting a Wireless LAN to the Internet There are many different methods by which a wireless LAN can be connected to the Internet. The following are some of the more common.

2.10.5.1 Directly connecting the access point to a broadband connection. Our linksys AP broadband router can be connected directly connected to internet via DSL or cable modem, share an Internet connection among its users.

2.10.5.2 Connecting the access point with a wired LAN that is already connected to the Internet Our AP can also be connected to the internet with a wired LAN. If an Internet connection is already available to the wired LAN. [b-2]

2.11 Configuring the wireless G – Broadband Router (WRT – 54G) To access the Web-based Utility, launch Internet Explorer or Netscape Navigator, and enter the Router’s default IP address, 192.168.1.1, in the Address field. Then press Enter. There are seven main tabs: Setup, Wireless, Security, Access Restrictions, Applications & Gaming, Administration, and Status. Additional tabs will be available after you click one of the main tabs. [m-1]

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2.11.1 The Setup Tab - Basic Setup The first screen that appears displays the Setup tab. This allows you to change theRouter's general settings. Fig 2.1 The Setup Tag in WRT 54G

Fig 2.2 – The Basic Setup of AP/WRT 2.11.2 The Wireless Tab – Basic Wireless Settings The basic settings for wireless networking are set on this screen.

Fig2.3 - The Wireless Tab 1847


2.11.3 The Wireless Tab - Wireless Security The Wireless Security settings configure the security of your wireless network. There are four wireless security mode options supported by the Router: WPA Pre-Shared Key, WPA RADIUS, RADIUS, and WEP.

2.4 The wireless tag – Advanced wireless setting 2.11.4 The Wireless Tab - Advanced Wireless Settings This tab is used to set up the Router’s advanced wireless functions.

2.11.5 The Security Tab – Firewall Firewall Protection. Enable this feature to employ Stateful Packet Inspection (SPI) for more detailed review of data packets entering your network environment.

2.11.6 The Security Tab – VPN Pass through Use the settings on this tab to allow VPN tunnels using IPSec, PPTP, or L2TP protocols to pass through the Router’s firewall.

Fig 2.5 The Security Tab 1848


2.11.7 The Administration Tab – Management This section of the Administration tab allows the network’s administrator to manage specific Router functions for access and security.

Fig 2.6 The Administration Tab

2.11.8 The Status Tab – Wireless The Wireless screen on the Status Tab displays the status of your wireless network.

Fig 2.7 The Status Tab

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Configuring The Wireless Client D-Link AirPlus® G DWL-G510 Using the Configuration Utility LAN Card uses the Configuration Utility as the management software. The utility provides the user an easy interface to change any settings related to the wireless adapter. If you are using Windows XP, you can use either the Zero Configuration Utility or the D-Link Configuration Utility. [m-2]

Fig2.8 Two ways for wireless client configuration

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Figure 2.9 Wireless Client configuration utility Conclusion Today wireless LANs are rapidly being deployed without much regard to the security risks that they introduce. The currently available 802.11-standard devices include WEP protocol-based security, which is almost always disabled when a manufacturer ships the device. The situation is further weakened by the known weaknesses in the WEP protocol. Hence, besides enabling the security features of WEP, depending on the security requirements, alternate protocols and extensions to the WEP protocols must be explored to provide the desired level of security when building a secure wireless LAN. Common ways to extend a wireless LAN include the enforcement of security policies and to use enhanced encryption and authentication methods. The IEEE 802.11i task group is working on enhancements on both encryption and authentication

in

wireless

LANs. 802.1x

will

be

incorporated

into

the

802.11i

specifications. Enhancements to WEP include the temporal key integrity protocol (TKIP). TKIP provides three security improvements : fast key packet (key-hashing per packet), real message integrity checking (to prevent forgery) and dynamic key management (re-keying). The Advanced Encryption Standard (AES) is also in the draft 802.11i specifications. AES is a symmetric block cipher operating on blocks of 128-bits using three possible key sizes : 128, 192 and 256 bits. There is an “advanced subset” of 802.11i, known as WI-FI Protected Access (WPA), which is compatible with existing hardware; it includes both 802.1x and TKIP. The Internet Engineering Task Force (IETF) has produced an internet-draft document defining a new authentication protocol based on EAP called Protected EAP (PEAP). It works by wrapping the EAP protocol within TLS thus protecting EAS

message

exchanges. Any EAP authentication method running within PEAP is provided with

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protected key exchange and session resumption ( which allows fast re-authentication when a wireless client roams from one AP to another ).

References Books 1. CWNA (Certified Wireless Network Associate) of ‘Planet3 Wireless’ by : Mc–Grawhill Publications. 2. Building Secure Wireless Networks with 802.11: By Jahanzeb Khan (Principal Engineer with RSA security) and Anis Khwaja (Wireless deployment Engineer). 3. Maximum Wireless Security: By Cyrus Peikari, Seth Fogie. Sams Publishing.

Manuals 1. WRT54G Linksys Wireless Broadband Router v 3.1 2. DWL-G510 D-Link AirPlus G Wireless G PCI Adapter. 3. DWL-G122 D-Link AirPlus G Wireless USB 2.0 Adapter.

Others 1. Beyond the Hot Spot: Wireless for Profit, by Dave Park, BelAir Networks. 2. Clip Your Strings: Wireless Technologies and Nonprofits, by L. Allen Poole, IMS Consultant. 3. Networking Made Easy: from TiVo Inc. 4. Wireless Networking from Black Box Networking Services. 5. Fire Cracker, Mega speed Wireless, from Chapel Communications. 6. Wireless Implementation at Bethune-Cookman College: Prepared by John Hofmann Director of Technical Services. The center of Information Technology BethuneCookman. 7. WLANs – Improving Productivity and quality of life. Sage Research Inc.

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