by January 6, 2004 0 comments



The real challenge in designing a wireless LAN is to strike a balance between its coverage and the bandwidth made available to each user. That’s because there’s a trade off between the two. If you increase the range, then the bandwidth per user is lesser and vice versa. Keep this in mind when designing your wireless network.Here are the major steps that should be followed. 

       STEP
ONE           
Determine usage
First, identify which applications you plan to run on your wireless LAN. Is it going to be mobile users accessing regular office data such as documents and presentations? Or will it be put to more heavy usage such as accessing large graphics files, streaming audio/video, conferencing, etc. The type of application will help you decide how much bandwidth you really need and consequently the technology you should choose. Lastly, identify what applications would be run by which users and where do they sit in the building.

       STEP
TWO
           
Conduct site survey
A site survey is important as it helps define your wireless network’s range and identify where to place the APs (access points). It’s done to determine which devices could interfere with the functionality of the wireless devices. For instance, cordless phones and satellite systems could interfere with the operation of a wireless LAN, so make sure that they’re kept far away. Materials like concrete absorb RF signals more than others like wood. Similarly, metals and silvered glass like mirrors can also attenuate the signal quite a bit. So such RF signal loss factors also need to be accounted for. More obstructions could reduce the throughput you might get from an AP, so place it where you get minimum obstructions. Usually, mounting it high up on the wall is a good idea as from there it would encounter minimum obstructions, and provide maximum coverage. 
You should also identify areas where you could do without APs. For instance, in a multi-storied office building, you may or may not want to provide wireless access in the staircase, just on individual floors. Even on each floor, you need to check the range of each access point. For instance, can a mobile user walk from his seat to the meeting room without getting disconnected? Building a Wi-Fi network therefore, depends upon the level of access you want to provide. Is it accessible from the outside, and if so how far? For instance, you may not want to mount it on a place from where its range extends far outside your office. 

Another precaution to take when placing access points is to prevent channel overlap. An 802.11b or g based AP offers three non-overlapping channels. Therefore when placing APs, make sure that the adjacent ones are on non-overlapping channels. This could also be true when placing them in a multistory building, where you might get channel overlap between floors.
Lastly, remember that each access point is likely to terminate into an Ethernet network. Therefore, you need to check whether the location of the AP is suitable to extend a cable to it. Also, in case you’re not using APs that support power over Ethernet, then you’ll need to ensure that they’re near a power source, because they run off a power adapter. 

       STEP
THREE           
Determine number of users per AP
The data in Step One on the applications used by users can now be used to approximately determine how many APs are

KEY DESIGN FACTORS
RF coverage: Determine the range of each AP. Is it covering the maximum possible area?
Ensure its range doesn’t extend outside your office boundaries
Average throughput per user: Depends upon the applications being used. Increase the AP density where there are more or heavy users 
RF loss: 11 and 54 Mbps are the theoretical bandwidth limits of wireless. The actual throughput would be lower

 really needed. Use more APs in an area where there will be more users. In case there are heavy bandwidth users in a particular area, you may want to use 802.11g based APs, as they offer a maximum theoretical bandwidth of 54 Mbps. 

You just have to ensure that each AP has sufficient bandwidth for each user on it. Most APs let you specify the throughput rate to associate with each user. Whatever you specify, the AP should be configured such that the user is forced to associate with another AP if his throughput drops below this rate. This way, the AP will not unnecessarily try to keep a user hooked when he moves farther away from it. In other words, each cell that you create around an AP should be optimized to provide the maximum throughput to every user. Typically, you may like to associate around 500 Kbps of bandwidth for ordinary users. For those dealing with heavy files, increase this limit. The number of users that can be connected also depends upon the efficiency of the AP itself. Like we said earlier, APs are not able to achieve the maximum theoretical bandwidth limits. The average rate that’s possible is 4 to 7 Mbps. So once you know the number of users that would connect to each AP, and you also know how much bandwidth to associate with each, you can easily calculate the number of APs required in a given location. Just multiply the average bandwidth per user with the total number of users to get the expected total bandwidth. Then work out how many APs you will need. Eg if the maximum throughput per AP is 6 Mbps, and the total bandwidth requirement is 50 Mbps in a particular area, then you would need about 8
APs. 

       STEP
FOUR           
How much coverage? 
Typically, the RF range of an AP spans a radius of 150 to 300 feet, which can be increased if needed. The range depends upon a number of things, such as the transmitted power, rating of antennas used, and losses. You can increase the range by switching to a high gain antenna. This of course requires a few calculations to be done, so you would need to consult a wireless expert for the same. 

       STEP
FIVE           
Identify equipment
This is important as it will help you determine the overall cost of your wireless implementation. So not only do you need to take into account the wireless equipment, but other equipment as well, such as software tools, cables, and even service and support. Let’s look at some of these. 

In wireless, we have already been talking about the AP, which is the base element needed for any wireless LAN setup. The other base element is the wireless card for the clients. This is available as PCI for regular desktops, PCMCIA for laptops, and even USB based cards, which can used to quickly Wi-Fi enable any machine. Here, both 802.11b and g equipment is available. You may like to go for 802.11b equipment, which can be upgraded to you’re your needs grow. Here also, there are different categories of APs available for different segments. There are AP routers for homes, which have built-in switch, print server, and broadband connectivity. There are just simple APs to extend your existing wired network, and so on. 

Besides the AP and client, there are several other things that an organization could use depending on its requirement. There are wireless print servers, and devices to wireless enable webcams, and even projectors. After that, it depends upon what all you want to convert to wireless. There are wireless adapters for printers, webcams, and even projectors. PDAs also come with Wi-Fi connectivity. 

There might be other equipment that’s required to support the Wi-Fi equipment. For instance, if you’re using wireless to extend your existing network, you would need Ethernet cables to connect to each AP. If you’re going beyond the built-in WEP security feature, then you would need more equipment depending upon how you implement security. Some companies place a VPN before the wireless network, while others place some other authentication services such as a RADIUS server. Here, you need to add in the extra cost of a VPN box or a server. Some organizations also use biotechnology equipment like finger print readers to give access to the Wi-Fi network, so they would have to account for that into the cost.
Lastly, there’s the cost of software. This could be third party software for doing a site survey, it could be software to manage your Wi-Fi setup, or the authentication software. 

       STEP
SIX           
Devise security  policy
We talked about this in Step five, but the crucial point to remember is that a Wi-Fi network is insecure by nature. So you need to devise a security policy for it. Start by determining the level of access you want to provide to your wireless users. How critical is the data that these users would be accessing? Work out your security plan based on that. We’ll not delve deep into security in this article, as given its importance, we’ve covered it in a separate
article. 

       STEP
SEVEN
        
Manageability and support
The larger your wireless network, the more difficult it gets to manage and support. That’s why you must take into account the manageability features of your wireless equipment. Can you manage all your APs from a central console? If you’re already using network management software for your existing wired network, then would it also be able to manage your wireless network? You wouldn’t want to keep configuring or monitoring each access point
separately. 

Therefore, as can be seen, designing and managing a wireless network isn’t all that simple. It requires careful planning, and constant monitoring, just like you would do on a regular wired network. 

Anil Chopra

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