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Maximum number of laptops for an access point?

Posted on 2004-08-30
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A client is contracting with a company that has an operations room with a pit.  

The company's users are clamoring for wireless connections for the 15 laptops that will be working in the pit.  

My client has recommended against the wireless connections due to performance concerns.  He has asked me for more information to either backup his claim or go forward with the request.

I am concerned about performance AND security of the installation.  My gut feel is that for high demand (low response time and/or high bandwidth) and for information security, hardwired is still the way to go in this situation.  

The wired connections will be 100Mb ethernet for each machine with an ethernet switch.  Information security if concerned with physical security of the site.

With a wireless installation I am concerned about performance.  The laptops would be competing with each other for access to the access point or points.  I am not clear if any of the 802.11 protocols can handle 15 or more machines in a relatively small space and provide performance on par with wired 10 or 100 Mb UTP.  My worst case situation is where all 15 attempt to access time critical information at or nearly at the smae time and collide to dead on the wireless access.

Add to the above I am not sure if there are other wireless networks in use.  If they exist they could cause additional performance problems.

Given the maturing state of wireless, I am concerned about making the wireless network secure from snooping outside the room and/or the building.  Are the new wireless protocols due out soon (this fall?) mature enough to make the traffic secure from casual snooping?  (A serious effort will succeed  even if the setup is hardwired.)

No selection has been made of 802.11a/b/g equipment.  Assume new equipment will be purchased.

Laptop equipment is unknown, but I would expect new or recent models.

Specific application is unknown, thus performance requirements are fuzzy.  Given the demands being made by the users, I expect them to be very picky and unaccepting of any perceptions of poor performance.

Questions:

- Is there a rule of thumb for the number of accessing nodes (laptops) to an access point for the different wireless protocols (a/b/g)?
- How does thi number vary with bandwidth requirements, particularly high bandwidth demand?
- Are the current wireless protocols enough to prevent a casual snoop attempt?  Are the new protocols significantly better?
- Are there any performance impacts to the security protocols?

I can see I've built a case for recommending the hardwired enviroment, but I am willing to be pursuaded to the wireless setup.
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Question by:jgt10
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kronostm earned 500 total points
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Well ... jgt10 ... ethernet cable will ALWAYS be faster and will provide more safety.  However, 802.11x protocols aren't that bad either.
Let's work with your questions:

 There is indeed a limit number of clients that can connect to a certain AP. This usually is recommended as 25, but I personally don't recommend more than 10/AP. If there's no problem with you purchasing an extra AP, you can link two APs in  a switch and this problem will be solved as you stated you'll have 15 clients . However, the bandwidth issue stays. While ethernet cable allows you 100Mb transfer, 802.11b allows you only 11Mb and a&g allow 54Mb.  Now, this transfer rate isn't the actual maximum trasnfer rate. The "useful for data transfer" badwidth will vary (depends on manufacturer) from 70% to 85% of the maximum the protocol allows. Over that, you should consider that a client that doesn't have such strong signal will autonegotiate it's connection speed to 5.5Mb, 2Mb or even 1Mb (for an 802.11b example) . And over all those, 7 clients simultaneously connected will share this bandwidth , so it will be only 1/7 from that maximum for each. As you said, some other wireless networks might interfere with your signal, but being inside it's pretty much an issue not to concern with. One thing you should concern however is that between the AP and cleint(s) not to have more than one wall. 2.4Ghz frequency is attenuated by walls/obstacles.
      Now, regarding security, there are two different cases. First (and the thing to concern about) is that your signal not to get out of the building. If your phisical setup doesn't include different floors and rooms, then you won't need an extra antenna to the AP(s) and in this case you're pretty much safe as the signal won't be received by external possible attackers. Now, even if this signal is received, there are ways to protect. MAC address filtering on AP is something that can be bypassed by a fake MAC (this means a card in promiscuos mode should listen, extract a valid MAC address to connect with, rewrite it's own MAC with a known allowed one and then to launch an DoS attack (god knows how while he's still not connected) to grant unworkable the machine that normally has the MAC address he's trying to fake. Besides that there is WEP encryption of the signal ... it can be decrypted indeed, but it means lot of work and a possible attacker should be very determined in order to struggle so much.  And in the end there is a Radius server that can be installed for wireless security, that I personally don't know how to hack.

Well ... If I'll write more I guess I'll be fired, hope that's enough info here for you.
If anything else just ask.

regards
Kronos
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Expert Comment

by:daiz
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hi all, this will be my first port here so i hope i can help.

I have had quite a bit of very turbulant experience with wireless networking. I'll start by directly answering your questions:

- Is there a rule of thumb for the number of accessing nodes (laptops) to an access point for the different wireless protocols (a/b/g)?
       - The rule of thumb for coverage-oriented WLANs is roughly 25-30 users per AP. For Capacity-oriented (what you want), its 10-15  per AP

- How does this number vary with bandwidth requirements, particularly high bandwidth demand?
     - coverage-oriented WLANs are typically suited to bursty, low packet rate appliations that require low bandwidth (1-2mbps in low coverage areas). Capacity-oriented, deployd correctly will give you low-latency, high packet throughput with a more dense client population.

- Are the current wireless protocols enough to prevent a casual snoop attempt?  Are the new protocols significantly better?
     -I assume here you mean authentication and encryption protocols? WEP is POS. It sounds like you may work for a financial/trading institution and I would not even think about delpoying WEP in the environment. WPA is a much more robust and scalable solution that can provide static, key based authentication or you could hook it up to a radius server and have some very good access protection on the WLAN. WPA also patches the gaping holes that WEP left and if you read into it, you will see that it would be extremely difficult and time consuming for even the most determined of hackers to gain access to the WLAN, not to mention any data on it.

- Are there any performance impacts to the security protocols?
     All encyption based security protocols have overhaed that affect performance. I don't know what it is for WPA

Another concern would be that other equipment near the offices could affect the signal since 802.11b/g runs in the ISM band like some cordless phones, medical equipment etc. You may wish to look at 802.11a equipment which will also have the added bonus in terms of security of not as much range but i'm told, better signal integrity.


By principal, wireless networking is like a hub; broadcasting all data to all devices over a shared medium. No way to get around that. What I would suggest what you do is purchase 2-3 APs and cards all of the same brand (to take advantage of any speed boost technology they have. Then, if all users will be in the same room at the same time and not need wireless anywhere else, configure every 1/2 or 1/3 of the devices to use each different AP so they can't hunt the site for better signal strength. oh, also configure each AP to use a different channel, as far apart from each other but not on the fringes of the frequency range.

For testing, configure at least one device on each AP at the same time and then hammer the network. A good and very simple test is to transfer a large amount of very small files between devices and also a common server as this always seems to be when the WLAN likes to fall over for me. After you test successfully, you may wish to start playing with the adaptor settings such as the maximum frame size and fragmentation threshhold. If all the users access the same type of data, it may speed things up a bit to find the average size of that data then set the cards to around that.

Hope that helps
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