Wireless in an hour. Introduction

Transcription

1 Page 1 Wireless in an hour Introduction In this session we will take a look at wireless network technology. This session isn t intended to be an in-depth technical analysis of wireless technologies, but to provide you with an overview of the options available for wireless networks in your establishment. We will be taking a look at the use of networks in schools, how it differs from the commercial environment, and how that can affect your wireless networks performance. We will take a look at, and explain the differences between managed and unmanaged wireless networks, and some things you should consider when you are looking to implement, or change a wireless network. It isn t just wireless infrastructure we will be taking a look at. We will also look at wireless devices notebooks and how, when looking at purchasing hardware, you should consider the wireless card in the device as this can affect the performance on your wireless network.

2 Page 2 Real-world use of wireless networks Before we start looking at the technology itself, let s consider how wireless networks are used in two different environments an educational establishment and a commercial environment. Educational establishments In a school or college, users don t typically have their own computer. The school will have ICT suites containing desktop, wired computers, and notebooks that connect to the wireless network. These may be in notebook trolleys or issued by the ICT Support department. This can be defined as a one to many relationship because many users can use one computer within the establishment. Because of this one to many relationship, most user data cannot (and shouldn t) be stored on the computer. User data includes: - User Profiles - User Home Folders

3 Page 3 Networks designed for schools, like Community Connect 4, are designed so that the user can roam around the network and access all of the above no matter which computer they log on to on the network. Now, let s consider another usage element that we must consider in the educational environment logging on and logging off. In a school, users will all log on to their computer at the same time; that could be upwards of 30 computers per class, all logging on at the same time and downloading their user profile from the server. At the end of the lesson all of those users will be logging off the network, uploading the users profile data back to the server. As well as all of the above, users also do many other tasks at the same time access shared areas, browse the Internet, print, save work at the end of a lesson and so on. Commercial environments In a commercial environment, the use of computers on a network is very different. Users typically have their own computer so the need to use roaming profiles is removed. Because it is a computer used by only one person, then the user s home folder can be located on that computer. If we take RM as an example, employees can start work between 7.30am and 9.30am so there is a gradual log on timeframe where users logon to the network when they arrive at work. Also, once logged on, they do not usually log off until the end of the working day, which again is within a two-hour timeframe. Whilst printing and accessing shared areas is still carried out on the network it would be unusual for there to be the rush at the end of the day like we would see at the end of a lesson in a school. As you can see the usage of computers and networks in educational and commercial environments are very different and we need to consider our environments when planning our wireless networks.

4 Page 4 Wireless hardware When considering a wireless network, you need to be aware of the hardware that will be required as part of your network infrastructure. Wireless access points A wireless access point is a device that allows wired communication devices to connect to a wireless network using Wi-Fi. The wireless access point is connected to the network infrastructure using an Ethernet cable and then distributes the wireless signal to the clients, (computers).

5 Page 5 Examples of wireless access points include the DLink DWL-7100AP for unmanaged wireless networks, or for managed networks, an example of an access point for Trapeze networks is the Trapeze MP-522. Wireless Repeaters A wireless repeater is a device that acts as a repeater between a wireless access point and wireless devices. A typical use of a wireless repeater is to add one when your computer is too far away from any of the other wireless access points. If configured properly, it will then extend the range of the wireless network. Many wireless access points, such as the D-Link DWL-7100AP mentioned above can be configured as a wireless repeater as opposed to an access point. In reality, it isn t often we have the need to use repeaters in an educational establishment unless the wired network infrastructure does not reach the part of the school you want to have wireless network access in. Wireless Network Interface Cards A wireless network interface card (WNIC) is a network card that connects to a wireless network, unlike a regular network interface card (NIC), which connects to a wire-based network. A WNIC, just like a NIC, works on the Layer 1 and Layer 2 of the OSI Model. A WNIC is an essential component for a wireless computer. This card uses an antenna to communicate through microwaves. A WNIC in a desktop computer is usually connected using the PCI bus. Other connectivity options are USB and PCI card. Integrated WNICs are also available, (typically in Mini PCI/PCI Express Mini Card form). In fact, it is nearly impossible to purchase a notebook computer today that doesn t include a WNIC. However, there are often various configurable options as to which network card is built in to the notebook. For example, on the RM website you are able to choose which wireless network card you want in your RM Mobile One. You can see in the screenshot above, there are three options with different prices. It is important that you consider the WNIC options closely as selecting a cheaper model or brand may have an effect on its performance.

6 Page 6 Conducting a wireless site survey When looking to implement a wireless network, one of the single most important things you can do is carry out a wireless network survey. The ultimate goal of a wireless site survey is to determine the number and placement of access points to provide adequate signal coverage throughout your establishment. With most implementations, adequate coverage means support of a minimum data rate or throughput. When you conduct a wireless site survey you should always consider the following: Understand the wireless requirements In order to identify optimum locations for access points, you must have a good understanding of specific requirements for the network that impacts signal coverage. For example, maximum range between a wireless device and the access point decreases as data rate and resulting performance increases. Thus, you need to know the target data rates (and throughput) to correctly interpret survey results. Also, the wireless devices may have relatively low transmit power, which must be taken into consideration when using most site survey tools. Be sure to identify the technologies that the network will implement, such as g or n, and perform the survey with these technologies in mind.

7 Page 7 Get a site diagram Before getting too far with the site survey, get a map of your establishment. If none are available, prepare a drawing that depicts the location of walls, corridors, etc. If an official building diagram isn t available, there may be a building diagram in one of the corridors showing fire escapes take a digital photograph of it and use that! Walk around your establishment Walk through your establishment before performing any testing to verify the accuracy of your site diagram. This is a good time to note any potential attenuation barriers that may affect the propagation of RF signals. For example, a visual inspection will uncover obstacles to signals such as metal racks and partitions; items that plans generally don t show. Also, note possible locations for mounting access points, such as above ceiling tiles or on pillars. These actions will make the later testing efforts go much more smoothly. Assess existing network infrastructure Determine the capacity of your existing wired networks that can interface the access points. Most buildings have wired Ethernet and in some cases optical fibre networks. Check how much of the existing networks can be made available for supporting the wireless network. This will aid you later on in the deployment when defining the architecture for the wireless network. Identify coverage areas On your site map, indicate all areas where your wireless network will need to cover, such as classrooms, hallways and the staff room. Also, identifying where users will not have wireless coverage is important to avoid wasting time surveying unnecessary areas. Keep in mind that you might get by with fewer access points and lower equipment costs if you can limit the roaming areas. Determine preliminary access point locations By considering the location of wireless users and range estimations of the wireless LAN products you re using, approximate the locations of access points that will provide adequate coverage throughout the user areas. Plan for some propagation overlap among adjacent access points, but bear in mind that channel assignments for access points will need to be far enough apart to avoid inter-access point interference. Be certain to consider mounting locations, which could be vertical posts or metal supports above ceiling tiles. Recognize suitable locations for installing the access point, antenna, and data cable. Also think about different antennas when deciding where to position access points.

8 Page 8 Verify access point locations This is when the site survey testing begins. Most wireless LAN vendors provide wireless site survey software that identifies the associated access point, data rate, signal strength, and signal quality. You can load this software on a laptop and test the coverage of each preliminary access point location. Alternately, you can use a third party site survey tool available from several different companies. From the Internet you can download a piece of software called NetSurveyor which allows you to see your wireless networks and the strength of the signal. This software is free to download and use and provides lots of useful information:

9 Page 9 To download NetSurveyor then visit the following website: Install an access point at each preliminary location and monitor the site survey tool readings by walking varying distances away from the access point. There s no need to connect the access point to the distribution system because the survey tests merely ping the access point or reads the signal strength. To make the access point easy to move about the facility, you could mount it on a pole attached to a trolley. This will also allow you to check that there is a power socket where you intend to mount the access point! Take note of performance or signal readings at different points as you move to the outer bounds of the access point coverage. In a multi-floor building, perform tests on the floor above and below the access point. Keep in mind that a poor signal quality reading could indicate that RF interference is affecting the wireless LAN. Based on the results of the testing, you might need to reconsider the location of some access points and redo testing for the affected locations. Document findings Once you re satisfied that the location of access points you ve identified will provide adequate signal coverage, document your findings on the plan of your school by depicting the location of each access point. Whoever installs your wireless network will need this information. Conducting the wireless survey As you can see, conducting a wireless survey can be a long process requiring specialist software. If you do not have the time or resources to carry out your own wireless survey then we can provide this for you through our Wireless Design Consultancy Service where we will carry out the wireless survey for you. For more information on this service, please visit:

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11 Page 11 Wireless network types When looking at wireless networks there are two main types to consider: Managed or Unmanaged networks. Managed networks Managed networks allow many benefits for large, wireless networks. With a managed network, specialist hardware is required which allows the wireless network to be centrally managed. The recommended managed network solution for schools is based around Trapeze technology. This technology intelligently manages your wireless network infrastructure, and consists of the following hardware and software: - Ringmaster: RingMaster provides centralised wireless security, Wi-Fi system management, wireless network reporting and system performance monitoring. This is a full featured suite of management and control applications that enables management and maintenance of the entire wireless network infrastructure from one or more control consoles. RingMaster goes beyond centralised Wi-Fi network control by encompassing security policy distribution, auto-configuration, and

12 Page 12 wireless LAN switch management into a single system, history reporting tool, and automatic fault detection system. RingMaster is a complete tool allowing you to perform tasks such as wireless network deployment planning. For a video on how you can use RingMaster for optimum wireless network planning, watch the following YouTube video: - Mobility Exchange: Control of Mobility Points is handled by Mobility Exchange switches. Multiple MPs are Ethernet wired back to the local MX and the MXs are controlled by RingMaster. In this way there is a fully distributed intelligence throughout the entire wireless network infrastructure and there is no single point of failure. - Mobility Points: These are the wireless access points provided by Trapeze. Mobility Points have the antenna integrated in to them. However, rather than being independent, mobility points are managed centrally from the Mobility Exchange switch and the management software, RingMaster.

13 Page 13 Unmanaged networks Unmanaged networks have no control over which wireless devices connect to it. What this means in reality is that you might have planned your wireless network by carrying out a wireless survey and decided that each room should have two access points per room. Let s say for example there are twenty wireless notebooks in the room with the two access points. We can never guarantee that they will be split equally between the two access points, 10 on each. What you are likely to find is that they will be divided unevenly between the two access points and poor performance will be seen. The a/b/g standards make no provision for clients to load balance between APs or for client association to be passed between APs. Clients will always attempt to connect to the AP with the right SSID that has the strongest radio signal and have no way of knowing how many other clients are already associated with that AP. To make matters worse, the strongest AP may often not be the one they are nearest to, so the network designer cannot even rely on physical location of APs to help the situation. Some APs have a load balancing setup screen. This initially looks attractive but in most cases this is only a load limiting function that imposes an upper limit on the number of concurrent connections that the AP will accept. It does not cause the AP to communicate with other APs with the same SSID and enable them to accept a new client connection instead. Let's assume that a new client is turned on in a room of three APS all with the same SSID and that AP1 provides the strongest wireless signal but is configured for load balancing as described above and has already reached its maximum allowed associations. The client will repeatedly try to associate with AP1 but fail. It will only associate with AP2 or AP3 once they supply a stronger signal, and this is unlikely to happen if the client is static. Hence the new client fails to associate with any AP and the user fails to get a network connection. Some clients may successfully associate with an AP providing the required SSID but offering the second strongest signal. However, this behaviour is not part of the a/b/g standards and therefore cannot be relied upon. Whilst client wireless cards from some manufacturers may well do this, most do not.

14 Page 14 Based on research that we have carried out, RM recommends unmanaged wireless networks are suitable where: - You will be implementing a wireless network with ten or less access points. - The a standard is used. This is because, as we have seen, the range of this frequency is less than a and as such there is less likelihood of interference between the access points.

15 Page 15 Wireless bands In 1997, the Institute of Electrical and Electronics Engineers (IEEE) created the first WLAN standard. They called it after the name of the group formed to oversee its development. Unfortunately, only supported a maximum network bandwidth of 2 Mbps - too slow for most applications. For this reason, ordinary wireless products were never very popular and are no longer manufactured. The full IEEE timeline can be seen by the following the link below: b The IEEE expanded on the original standard in July 1999, creating the b specification b supports bandwidth up to 11 Mbps, comparable to traditional Ethernet b uses the same radio signalling frequency (2.4 GHz) as the original standard. Vendors often prefer using these frequencies to lower their production costs. Being unregulated, b equipment can incur

16 Page 16 interference from other devices that operate in the same 2.4Ghz range, including microwaves and cordless phones. However, by installing b wireless access points a reasonable distance from other appliances, interference can easily be avoided a While b was in development, IEEE created a second extension to the original standard called a. Because b gained in popularity much faster than a, it is a common misconception that a was created after b. In fact, a was created at the same time. Due to its higher cost, a is usually found on business networks whereas b better serves the home market a supports bandwidth up to 54 Mbps and signals in a regulated frequency spectrum around 5 GHz. This higher frequency compared to b shortens the range of a networks. The higher frequency also means a signals have more difficulty penetrating walls and other obstructions. Because a and b utilise different frequencies, the two technologies are incompatible with each other. Most networking hardware manufacturers offer equipment, which is both a/b compatible, but these products merely implement the two standards side by side (each connected devices must use one or the other). You may think that using a isn t the best solution because of the limited distance the bandwidth supports. In a school environment however this can be an advantage, especially when you have classrooms next to each other, each with their own access points. Because of the limited range you are reducing the likelihood of wireless devices connecting to an access point in an adjacent room g In 2002 and 2003, WLAN products supporting a newer standard called g emerged on the market g attempts to combine the best of both a and b g supports bandwidth up to 54 Mbps, and it uses the 2.4 GHz frequency for greater range g is backwards compatible with b, meaning that g access points will work with b wireless network adapters and vice versa.

17 Page n The newest IEEE standard in the Wi-Fi category is n. It was designed to improve on g in the amount of bandwidth supported by utilising multiple wireless signals and antennas (called Multiple Input, Multiple Output, or, MIMO technology) instead of one. An additional technique employed by n involves increasing the channel bandwidth. As in a/b/g networking, each device uses a preset Wi-Fi channel on which to transmit. Each n channel will use a larger frequency range than these earlier standards, also increasing data throughput. This standard has recently been ratified, however n wireless devices, including access points and wireless network cards, have been around for a while n connections should support data rates of over 100 Mbps.

18 Page n also offers somewhat better range over earlier Wi-Fi standards due to its increased signal intensity n equipment is backward compatible with g equipment. Which is the best wireless band to use in your establishment? As you can see, we have several bands available to us and deciding which one to implement across our wireless network is a decision we need to make. I have explained the different bands available to us but deciding which is the best to use in our establishment is not necessarily a case of go for the newest and quickest. There are several things we need to consider before deciding which frequency to use across our wireless network. Wireless devices We need to consider all of the wireless devices on our network. We need to ensure that we select a compatible frequency that they all can use. There is no point deploying an n wireless network if the notebooks we use on the network don t have compatible network cards built-in, or there would be additional expense by purchasing additional wireless network cards for our mobile devices. Additionally, if you deploy wireless N access points, you don t get the performance you would expect if you have wireless devices which are

19 Page a or b devices as this prevents the n access point to run at the maximum throughput. Risk of interference Later in this session we will look at what can cause interference on your wireless network. Generally speaking though, b, which operates in the 2.4GHz range, is more susceptible to interference than those that operate in the 5GHz range. We will look further on in this session how to overcome the limitation of the 2.4GHz range later. However, choosing a or n, (if your devices support it) would be a better option.

20 Page 20 SSIDs The SSID (Service Set IDentifier) is a token that identifies an (Wi-Fi) network. The SSID is a key that is configured by the network administrator for the wireless network. The SSID must be used to connect to an wireless network. However, the SSID can be discovered by scanning the available wireless networks. By default, the SSID is part of the packet header for every packet sent over the WLAN. SSID access points continuously broadcast radio signals that are received by the client computers if enabled. Based on the automatic or manual configuration, the client can connect to the access point. A SSID is generally 32 bit long, but when displayed to the user, it is projected into a human readable ASCII format. Multiple access points can share the same SSID if they are for the same wireless network. The SSID is configured within the AP itself through its configuration utility in the case of unmanaged wireless networks, or through the central configuration utility in the case of a managed wireless network.

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22 Page 22 Wireless channels Wireless networks can operate on different channels. This allows us to help keep our wireless network uncongested and reduces the risk of many devices using the limited bandwidth wireless networks give us. Consider this example: Every year I go on holiday to the LeMans 24 hour race in France. Usually there are about ten of us go and we have sets of walkie-talkies, like the ones above. These walkie-talkies have different channels. All walkie-talkies must be set to the same channel in order to be able to communicate with each other.

23 Page 23 The problem comes when other people, with their same walkie-talkies have theirs set to the same channel as you then start to hear their conversations and it becomes increasingly difficult for us to communicate. Switching to a different channel may help to reduce this interference but slowly the other channels fill up. Direct communication between an client (computer or other wireless device) and a wireless access point occurs over a common channel frequency. You set the channel in the access point, and the wireless network card in the device automatically tunes to the frequency of the access point having the strongest signal. The wireless network card then continues with association and communications with the chosen access point. To support roaming, the wireless network card will periodically scan all access points and reassociates with the access point having the strongest signal (if the current access point signal amplitude is below a specific threshold). As a design rule, access points within range of each other should be set to channel frequencies with minimal signal overlap. Otherwise, you ll find that roaming doesn t work well, and performance will degrade because of interference between access points. Within the 2.4GHz band, (802.11b/g/n) the following channels are available: Channel Frequency You can see that these different channels all operate within 2.4GHz.

24 Page 24 In the above diagram you can see how the channels overlap slightly. When configuring the wireless access point channels you should make sure that there is enough distance, frequency speaking, between the channels you select. So, in rooms next to each other you wouldn t select channels 1,2,3 and so on for the access points. It would make sense to select 1, 6 and 11, and then back to channel 1, and so on. Within the 5GHz band, (802.11a/n) the following channels are available: Channel Frequency In the diagram below, just like the other frequency bands we can see that the channels overlap so we wouldn t have access points within range of each other using adjacent channels. We should employ a channel deployment as 36, 48, 40 and so on. In the above diagram we can see many more channels available than in the configuration screen for an Apple wireless access point below:

25 Page 25 The reason for this is certain channels are only available in certain countries. In terms of throughput, there's not much of an issue with wireless access points set to the same channel if the load on the network is light. Nevertheless, keep in mind that having the access points set to the same channel will sometimes confuse the wireless network cards, which leads to roaming issues. In addition, setting access points near each other to the same channel will degrade capacity, and performance will be significantly lower as traffic on the network increases. As a result, it s best to assign non-overlapping channels to nearby access points.

26 Page 26 The Four-Colour Problem The Four Colour Problem is a mathematical curiosity dating back to 1852, when Francis Guthrie, trying to colour in a map of counties of England, noticed that four colours sufficed to ensure that adjacent regions (i.e. those sharing a common boundary segment, not just a point) received different colours. It is not a great leap to substitute the pattern of adjacent cells of a wireless LAN in a building for the counties map, and the available frequency bands for the colourist s paints. So, mathematics seems to be telling us that whatever the pattern of adjacent wireless cells on a plane surface, they can always be configured in a way that completely avoids channel interference, using only the four non-overlapping channels and avoiding adjacent cells using the

27 Page 27 same channel. (In fact, in typical situations, even three channels can be enough). Consider this example of a school with three classrooms that all have wireless access points installed in them: Classroom 1 Classroom 2 Classroom 3 If all three wireless access points were assigned to the same channel then there is the distinct possibility that you would find wireless devices in Classroom 1 attaching to the wireless access point in Classroom2. By having the access points configured to different channels, this risk is reduced significantly.

28 Page 28 Wireless security Another important consideration for your wireless network is to implement security so that only authorised wireless devices can connect to the network. The most common way to secure the wireless network is to use Wireless Protected Access, or WPA, (although this has by-and-large been replaced by WPA2). Wi-Fi Protected Access (WPA and WPA2) is a certification program developed by the Wi-Fi Alliance to indicate compliance with the security protocol created by the Wi-Fi Alliance to secure wireless computer networks. The Alliance defined the protocol in response to several serious weaknesses researchers had found in the previous system, WEP (Wired Equivalent Privacy). The WPA protocol implements the majority of the IEEE i standard. The Wi- Fi Alliance intended WPA as an intermediate measure to take the place of WEP pending the preparation of another IEEE standard i. Specifically, the Temporal Key Integrity Protocol (TKIP) was brought into WPA. TKIP could be implemented on pre-wpa wireless network interface cards that began shipping as far back as 1999 through firmware upgrades. Because the

29 Page 29 changes required fewer modifications on the client than on the wireless access points, most pre-2003 APs could not be upgraded to support WPA with TKIP. The later WPA2 certification mark indicates compliance with the full IEEE i standard. This advanced protocol will not work with some older network cards and it is important that if you are implementing WPA2 security that all of your devices will work with this mode of security. WPA2 Personal WPA2 is most commonly used in Pre-shared key mode, (PSK). PSK s can be defined as a shared-secret. All the devices on the network know the PSK and data is encrypted as it leaves the connected device and arrives at the wireless access point, and vice-versa. This also means that before a device can connect to your network it needs to know the key. It is important that your WPA2 is strong as, although this is secure, it is not unbreakable as recently the WPA2 algorithm was broken. There are many WPA2 key generators available on the Internet. One good one I have used previously for my home network can be used by following the URL below: One of the many benefits of building computers on to a Community Connect 4 network is the fact that computers containing wireless network cards can have their WPA2 personal security configured during the computer build process. Settings for the wireless networks used in your establishment can be added to a build template, and during the build process the network will be configured so that, once built, the computer can join the wireless network without any additional configuration by the network support team. To configure the wireless security settings in a computer build template: WPA2 Enterprise Use of WPA in the enterprise mode requires an authentication server such as RADIUS (Remote Authentication Dial-In User Service). The 802.1x/Extensible Authentication Protocol (EAP) manages user authentication and optional key distribution x/EAP is a general protocol and is not specific to wireless networks. It merely provides a framework for the presentation of user credentials, which may be in whatever form has been deployed. EAP authentication requires several steps, during which the access point (authenticator) acts as the intermediary between the unauthenticated client device (supplicant) and the authentication server.

30 Page 30 The initial exchange in this series occurs when the supplicant attempts to connect to an access point. Until authentication is complete, the access point will only allow EAP packets. The access point replies to the connection attempt with a packet containing an identity request. The access point forwards the supplicant s identifier to the authentication server, which checks an internal or external database for details about the supplicant. Based on this information, the authentication server sends back an authentication challenge (via the access point) appropriate to the supplicant s presumed identity. The supplicant returns a packet containing the response, which is verified by the authenticating server. The authenticating server then issues an accept or reject message to the access point. If the supplicant is accepted, the access point transitions the port to an active state and allows other traffic. If the supplicant is rejected, the connection is dropped and may optionally be logged. EAP may also be configured to implement dynamic key management if desired. If so, once a client is accepted for access, the authentication server sends the master TKIP key to both the client device and the access point. The client device and the access point then acknowledge each other and install the TKIP key. One useful feature of this protocol is the fact that both clients and access points are authenticated, which prevents the hijacking of clients by rogue or unauthorized access points. The beauty of EAP is that it is an open-ended specification. It details the methods for the presentation and transfer of authentication credentials, without requiring any particular one. EAP may be tailored to the environment in which it is to be used with a fine degree of control, as it is possible to deploy multiple authentication methods, and then allow either the access point or client specify which protocol should be used to govern the process. This allows different clients to connect with different levels of trust. The unconstrained nature of the protocol allows support for future improved methods as they are developed. If you are considering implementing WPA Enterprise in your establishment then you must ensure that your APs are compatible with this security method. Secondly, you will need to deploy an 802.1X Authentication Server. There are several options available for this step:

31 Page 31 - FreeRADIUS: An open source RADIUS server that is available for all major operating systems. - Microsoft Windows 2003 Internet Authentication Service: This is the Microsoft implementation of a Remote Authentication Dial-in User Service (RADIUS) server and proxy in Windows Server Lots of documentation for FreeRADIUS can be downloaded from the following website: More information on Windows 2003 Internet Authentication Service can be found here: Also, you must make sure that your hardware will support WPA2 Enterprise. If your hardware does support it then you will be able to configure the WPA2 Enterprise details, as the below example shows:

32 Page 32 Wireless interference Interference is still plaguing many wireless LAN deployments. Many establishments have gotten by without any issues, but in some schools have installations that don't operate as well as planned. The perils of interfering signals from external RF sources are often the culprit. As a result, it's important that you're fully aware of RF interference impact and how to avoid these. As a basis for understanding the impact of RF interference in wireless LANs, let's quickly review how devices (wireless network cards and access points) access the medium: Each device only transmits packets when there is no other device transmitting. If another device happens to be sending a packet, the other stations will wait until the medium is free. The actual protocol is somewhat more complex, but this gives you enough of the basic concepts. Wireless interference involves the presence of unwanted, interfering RF signals that disrupt normal system operations. Because of the medium access protocol, an interfering RF signal of sufficient amplitude and frequency can appear as a bogus station transmitting a packet. This causes legitimate stations to wait for indefinite periods of time until the interfering signal goes away. To make matters worse, an interfering signal generally doesn't abide by the protocols, so the interfering signal may start abruptly while a legitimate

33 Page station is in the process of transmitting a packet. If this occurs, the destination will receive the packet with errors and not reply to the source station with an acknowledgement. In return, the source station will attempt retransmitting the packet, adding overhead on the network. Of course this all leads to delays and unhappy users. In some causes, will attempt to continue operation in the presence of RF interference by automatically switching to a lower data rate, which slows the use of wireless applications. The worst case, which is fairly uncommon, is that the stations will hold off until the interfering signal goes completely away, which could be minutes, hours, or days. Sources of RF interference that may cause issues As I mentioned earlier, networks operate on two different frequencies 2.4GHz and 5GHz. For 2.4 GHz wireless LANs, there are several sources of interfering signals, including: - Microwave ovens - Wireless phones, (not mobile phones, landline phones) - Bluetooth enabled devices - Other wireless LANs - Electrical devices such as power lines - Large metal cabinets (for instance fume cupboards in science labs) - Excessive metal pipe work The most damaging of these are 2.4 GHz wireless phones that people are starting to use in some establishments. If one of these phones is in use within the same room as an b wireless LAN, then expect poor wireless LAN performance. Microwave ovens operating within 10 feet or so of an access point or radioequipped user will generally just cause b performance to drop. Bluetooth enabled devices, such as laptops and PDAs, will also cause performance degradations if operating in close proximately to stations, especially if the station is relatively far (i.e., low signal levels) from the computer that it's communicating with. Other wireless LANs that may be operating can cause interference unless you coordinate the selection of b channels.

34 Page 34 Troubleshooting wireless networks Troubleshooting wireless networks can be a time consuming and difficult exercise to undertake, as often, poor wireless network performance is inconsistent. One minute the wireless network in a room can be performing fine, the next minute it is performing poorly and users have difficulty accessing resources on the network. The 5 rules of wireless. The 5 rules of wireless will help you to set up your wireless network to get the best performance from it. 1. Ensure you have the best client to AP ratio. We have already talked about how it is really important to carry out a wireless network survey. By carrying out this survey you can determine how many wireless AP s you will need to ensure you get the best connectivity for your wireless devices. RM recommends 15 devices per access point. With unmanaged wireless networks this can be hard to guarantee however with enough access points per room you can help to ensure this ratio is met. 2. Make sure you can see the access points you want to connect to.

35 Page 35 Unless you have a detailed RF site survey assessing the properties of the building, then the access point(s) you are intending to connect to need to be in line of sight. The ideal location for an access point is to be wall mounted with the aerial approx 50 centimetres from the ceiling. Multiple access points should be placed on their own wall where practical, or have at least a 5ft separation. If you have an unmanaged wireless network then there is no way to efficiently load balance the clients to access points. For Example, if you have one AP located in a room with 20 notebooks, another AP in the next classroom, the notebooks will almost with exception connect to the nearest AP irrespective of loading thus grossly overloading it. When a computer is attempting to connect to an access point it will simply use the wireless signal strength and matching SSID that, as I have mentioned earlier, could mean that a single access point becomes overloaded with clients. To facilitate the best chance of a fair contention, make sure that access points are situated equidistant from the clients. An ideal solution would be to wall mount the access point s (as described above) on the opposite sides of the room. 3. Use b whenever possible a is fundamentally unsuitable for any high density Community Connect classroom deployment due to the limited throughput. Any equipment supporting this standard should be replaced at the first opportunity g is only suitable to provide a wireless network for a low client count/low bandwidth hotspot activity. The limited number of nonoverlapping channels (3 in total, channels 1,6 and 11) means that it is very difficult to install enough access points in a small area to support a high number of clients. In addition the performance of g can be significantly degraded by legacy b clients a has a significantly greater number of non-overlapping channels (at least 8). This together with the shorter range of 5Ghz mean that it is easier to install access points in higher densities required to support clients in a school. Also, the 5Ghz radio spectrum is not prone to interference in the same way as b/g s 2.4GHz 4. Use standard wireless technology. You may see access points that have proprietary technology built in to them. Examples of this include Super G or Turbo. While this may seem to have benefits, often, these technologies need to be used with wireless network cards that have the same technology built in to them which

36 Page 36 can mean an additional cost (and risks) of upgrading then network cards in your wireless devices. 5. Do not use third-party wireless client software. Do not install the client utilities that may come with a wireless card, use only the built-in Windows Zero Configuration service. A lot of third clients interact badly with Community Connect computer security and in many instances allow students unauthorised access to the wireless connection properties or in worst cases do not work at all. Troubleshooting wireless infrastructure Check the basics Before jumping in, spending time trying to solve the wireless network issue you are seeing, it s always worth trying the basics. Firstly, if the wireless network performance is affecting a single room or access point, try rebooting it! If the access point has encountered an internal error, a simple reboot is likely to fix it. If it is a computer that is failing to connect to your wireless network, do the same thing! Access point firmware Hardware vendors regularly update the firmware of their wireless access points. These firmware updates include software fixes and improved reliability, including connections. If you are having issues then it may be there is an updated version of the firmware for your access point. For the latest firmware for RM s recommended wireless access point, the D-Link 7100, the latest firmware update can be downloaded from:

37 Page 37 Europe- GB%2FDLTechProduct&cid= &packedargs=locale= &pagename=DLinkEurope-GB%2FDLWrapper&p= Note: It is important that you always download the firmware update for the UK as different countries use different wireless frequencies so applying a firmware update for North America, for example, may make your access point inaccessible. Check for new sources of interference Using the a frequency, the likelihood is that devices such as microwaves or cordless telephones shouldn t prove an issue as these operate in the 2.4Ghz frequency and a uses 5GHz. However, it s worth checking to see if it is affecting your wireless network. If you have to use b/g, then check to see if there are any devices such as these that have been moved in to the local area. If your school is in a residential area, or close to office buildings, there could be other wireless networks that are interfering with yours. Use a network analysing tool such as Netsurveyor to view the wireless networks around you. If another wireless network is using the same channel(s) as your network then you may need to look at rotating the channels on your access points to reduce the interference. Ensure the wireless network SSID is being broadcast It is possible on most access points to stop the wireless network s SSID (Service Set IDentifier) from being broadcast, i.e. being displayed as an available wireless network. It is recommended that the SSID is always broadcast as sometimes not having this being broadcast slows down the connection between the wireless device and the wireless network. As you can see in the screenshot below for the D-Link DWL-7100 access point, SSID Broadcast is enabled.

38 Page 38 You may have read that this has a security risk as any wireless device can see the wireless network, and potentially individuals who will attempt to hack the network. However with strong security in place, such as WPA security, this shouldn t be an issue. Add more access points It may be that your wireless network is simply overloaded. Remember that unmanaged wireless access points don t have load balancing built in. It could be that adding a second access point to a room will improve the wireless network performance significantly. Remember that we recommend a maximum of 15 wireless access devices per access point so it is likely that most classrooms would benefit from at least 2 access points. Troubleshooting clients Wireless network card drivers As with access point firmware, updated drivers are regularly released by the manufacturers of the network cards in devices. Should you be experiencing connection difficulties with a batch of notebooks, then it is worthwhile checking to see if there is an updated version of the wireless network card drivers. RM regularly release Driver Updates for Community Connect that may include updated drivers for existing hardware. Alternatively, you can go directly to the hardware manufacturers website to check if there are updated drivers. For a small number of computers, you can update the drivers on each machine. However, if you have a large number, it may be less time consuming to rebuild the computers after importing the updated drivers using the RM Management Console in CC4.

39 Page 39 To reduce the amount of time it takes to rebuild the computers, through the CC4 Build environment, you can use the option to Rebuild using only cached operating system. This will rebuild the computer using the previously downloaded operating system and contact the build server to download the hardware drivers, which will include the updated wireless network card drivers. Multiple SSIDs It is advisable that wireless clients only have one SSID listed in the network connection properties. The reason behind this is that when multiple SSIDs are listed it can delay the connection to the school s wireless network. As a result, we have seen in testing, Group Policy Object processing failures, as, when these should be applied, the wireless device isn t connected to the wireless network. This may not always be possible, in the example of staff laptops that are used on home wireless networks, but SSIDs should be kept to a minimum in order to speed up the connection to the wireless network in school.

40 Page 40 Summary Wireless networks allow us to have great flexibility and truly mobile computing. It doesn t come easily however, and there is a lot to think about and plan before implementing or expanding your existing wireless network. The use of networks varies significantly between our environments and commercial ones and as such, we have much more to think about. It is strongly recommended that a wireless site survey is carried out in order for you to reliably set up your wireless network. By carrying out this survey before implementing your wireless network it will help to minimise poor wireless network performance. The IEEE has defined different bands based on the standard. In an educational environment we recommend using b as this gives us higher throughput and a shorter range which, initially, may not sound like a good thing but actually it is as it can reduce the likelihood of access point signals crossing room boundaries. It is important when setting up wireless access points that you rotate the channels they use. If this isn t set up correctly you may find that devices in one room may connect to an access point in another room and suffer from poor performance.

41 Page 41 Security is important on your wireless network. Using WPA or WPA2 provides the security that will prevent unauthorised devices connecting to your network. Remember that you can build Community Connect 4 computers with wireless settings already set up as part of the computer build process. Interference can come from many devices such as microwaves or cordless telephones. Using a reduces the likelihood of such items causing interference on your wireless network as this band uses the 5GHz radio frequency. The items mentioned in this session can typically interfere with wireless networks operating on the 2.4GHz band b. When troubleshooting your wireless network always start with the basics reboot. Try and identify a pattern when the wireless network performance suffers. Remember the 5 rules of wireless and use the tools I have mentioned in this session to fix the wireless issue.