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Recommended Wireless Setup Primary School

Posted on 2008-11-11
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Hi Experts,

We have been appraoched to configure a wireless setup for a Primary School with 15-45 users using the Wireless at one time. Here is how they will be using it.

15-20 Users in each class room.

Class Room A = 15-20 Users
Class Room B = 15-20 Users
Class Room C = 15-20 Users

They would like the ability to be able to give all the people in each class room Wireless Access to the school network for things such as file sharing, printing, and internet. They also sometimes create movie files and share them on the network.

In the past we have used devices such as the wonderful D-Link DWL-3200AP which is a very robust Access Point and where looking at putting one of these in each classroom. We have never used them in an envrironment such as this and can see a problem with 15-20 Users accessing the same access point.

Has anyone in the past worked with school wireless networks and come across any real heavy bandwidth problems with Wireless.

The laptops will hopefully have a minimum 54MB connection into the network but as there 15-20 Users in the one room it would not be sufficient to have them all using the same access point as the speed when opening files and printing over the network will be incredibly slow.

Looking forward to hearing your 'Expert' thoughts on the subject.
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Question by:logicsolutions
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Expert Comment

by:Darr247
ID: 22943164
15-20 users at once on each of 3 APs should not be a problem... the bottleneck will almost-certainly be your internet connection when they're sharing that.

While 802.11g 'says' it's a 54Mbps connection, true throughput is closer to 30Mbps because of frame overhead and timing. Same with 802.11a.

Still, with 15 users that should provide ~2Mbps per user, assuming everyone has traffic at once... even 20 at once would provide bandwidth equivalent to a T1 line for each client. You would need a DS-3 (aka T3) line on the internet feed to provide even 1Mbps to 45 clients at once.

1Mbps is sufficient to stream most video content available online; 768Kbps is what most home DSL lines provide (for comparison, most home cable internet connections max out at 4-6Mbps).

By the way, you probably would have gotten a faster response by giving this question a value higher than 50 points. Since premium members get unlimited points to ask questions with, I'm not sure why anyone value their queries at less than the maximum (500 points). My filters block questions under 100 points to weed out the 'non-technical' zones (which can offer a maximum of 50 points), so I didn't see it until the 'neglected question' robot sent me an alert when it reached 12-hours with no response.
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by:logicsolutions
ID: 22943956
Thanks Darr,

Didn't know the points thing. Increased as recommended.

How would they go copying files over the network. Would one person copying a large file bottleneck the others. The other concern I also have is printing to a network printer. Will it really bomb out if a couple print at the same time.

Many thanks Again.

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Expert Comment

by:Darr247
ID: 22945903
> Would one person copying a large file bottleneck the others.

It shouldn't. It should split local bandwidth equally... if 2 people on separate APs are transferring a file between them, it should transfer at an effective rate of about 30Mbps; if those 2 people are connected to the same AP, it would transfer at an effective rate of 15Mbps. i.e. a single AP can talk to only one client at a time, so it should split the bandwidth between them. If it's talking to 15 clients simultaneously, all should be slowed down about equally (to ~2Mbps each). Rather than speaking strictly in megabits per second, real world transfer times should be e.g. a 1mebibyte file (1,048,576 bytes) should take about 4 seconds to transfer @ 2Mbps, so for general surfing (like, research lookups on google.com), you should not notice any slowdown at 2Mbps... and it would have to be quite a large file for the transfer to last long enough for anyone to notice. With the full 30Mbps available, it would take about 3 minutes to transfer a CD full of files; limited to 2Mbps, that extrapolates out to almost 50 minutes (of course it would be much faster to just walk the CD to the other computer in either of those situations).


> Will it really bomb out if a couple print at the same time.

Even at 2Mbps they'll transfer faster than most printers can print... to free up resources, use identical printers and check the Enable Printer Pooling box on the Ports tab in the Printer Properties (in Printers and Faxes), then check all ports those printers are installed on in that one Properties box... when one is busy it will send to the next one that's free (that works best if all the pooled printers are in a common area, of course... else you'll have a hard time finding where your document was sent), and ensure they have the maximum amount of memory installed (most inexpensive printers cannot be upgraded like that... which is one of the reasons they're cheaper).

Depending on building construction and layout of the rooms, it may also be possible to use more than one AP per room without interfering with each other to increase local bandwidth. e.g.

(This probably won't format very well here, but if you copy and paste it to Notepad it might look better.)

__________________________________________________________
|AP1                           |    AP11                     |   AP6                         |
|                                 |                                |                                 |
|         SSID                |         SSID                |         SSID                 |
|            ROOM1          |            ROOM2          |            ROOM3           |
|                                 |                                |                                 |
|                         AP6  |                         AP1  |                         AP11 |
----------------------------------------------------------------------------------
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by:logicsolutions
ID: 22957263
The above configuration Darr as drawn is exactly how the rooms are panned out. There are three rooms next to each other. Sometimes the people using the notebooks will alternate between rooms. Is it possible to have all 6 AP's using the same SSID so as to appear as one wireless network connection. Or do we need to have seperate SSID for each class room?
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Darr247 earned 1000 total points
ID: 22957406
It's possible, but not necessary to all use the same SSID.
If you set one AP on channel 6 in room 3 (as shown at the top of my crude ASCII art), how strong is its signal in room 1? (again, room 1 is referenced according to the drawing above.)
If it's a weak-enough signal, that configuration (above) should work...  if it's still strong enough to get a good connection in room 1, then a better configuration might look like

__________________________________________________________
|AP1                           |    AP11                     |   AP6                        |
|                                 |                                |                                 |
|         SSID                |         SSID                |         SSID                |
|            ROOM            |            ROOM           |            ROOM             |
|                                 |                                |                                 |
|                         AP6  |                         AP1  |                         AP11 |
----------------------------------------------------------------------------------

i.e. all using the same SSID.
Then, same channel interference should be mitigated by the 802.11 cooperation... when one is ready to send it checks to see if the band is open... if it isn't, it backs off a 'random' amount of time (in milliseconds) before checking again. If they're on different SSIDs in that situation the radios cannot really tell there's traffic - so they start sending anyway and cause collisions or noise dropouts... but if the same-channel radios are far-enough apart then they won't cause collisions on different SSIDs.

That's assuming 3 APs can't keep up all on the same SSID with the channels set up 1, 11, 6 (or 6, 1, 11 or 11, 1, 6), which shouldn't be a problem unless you have an Internet connection faster than 45Mbps.  Like so:

_________________________________________________
|AP1                         |    AP11                  |   AP6                      |
|                               |                              |                               |
|         SSID              |         SSID              |         SSID              |
|            ROOM         |            ROOM         |            ROOM         |
|                               |                              |                               |
-----------------------------------------------------------------------------

But even using different SSIDs in each room you would just setup a profile for each SSID... that should need to be done only once... then the laptop should roam to the stronger signal when moving to the other rooms.
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by:logicsolutions
ID: 22957641
Thanks Darr. What would happen if you put 6 AP's all using the same SSID and Channel. Is that not recommended!
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Expert Comment

by:Darr247
ID: 22958126
> What would happen if you put 6 AP's all using the same SSID and Channel. Is that not recommended!

You would get about 1/3 the amount of bandwidth as having just 3 APs on channels 1, 6 and 11.

I really think you could get by using just a single D-Link DWL-3200AP in each room...  but if you want to be prepared, go ahead and pull enough cable to mount 2 APs in each room, so if you decide you're just not getting enough speed (consistently), then adding another AP to each room will be easy. It should cost very-little more (as long as it's all done at once) to pull 2 cables to each room rather than 1.
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