Good point Dave. I find very few instances where designing for coverage alone is correct - as long as you are thinking about the future, even minimal capacity needs today should think about a capacity roadmap. The key for Meru to jump over the issue with capacity in their single layer architecture is to provide multiple layers (aka multiple channels) in the same physical area. You can optimize the time splicing only so far and then you need multi AP radios (which I consider the precursor to 802.11n). Meru is supposed to have the 4 radio AP switch available now with the 8 radio next. There is also a company called Xirrus that makes a multi radio AP (up to 16 radios - quite a BW beast). I don't know of any white papers on how Meru does the time slicing, but I don't keep up on their website. I just know that they manipulate the NAV (network allocation vector) which tells other handsets to holdoff (reserves the channel) while they can assign a voice client to transmit. They are manipulating the 802.11 protocol to provide access prioritization - very clever.
________________________________ From: Dave Molta [mailto:[EMAIL PROTECTED] Sent: Thu 11/10/2005 3:35 PM To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU Subject: Re: [WIRELESS-LAN] Wireless-only Dorms? It's fairly easy to understand how the scheduling capabilities of Meru allow it to maximize throughput and minimize latency using a single channel throughout a building, but I still wonder about the aggregate capacity when compared to a more traditional and well-implemented overlapping cell design that leverages all available spectrum. As long as your primary goal is coverage rather than capacity, this is an excellent solution, but the whole discussion of resnet wireless is more of a capacity issue and I'm guessing that low-latency roaming won't be a big issue in the short term since resnet users are more nomadic than mobile. Meru has been doing some interesting work with multi-radio AP's that should allow them to enhance overall system capacity but I don't think any of those products are available today. dm > -----Original Message----- > From: Phil Raymond [mailto:[EMAIL PROTECTED] > Sent: Thursday, November 10, 2005 10:41 AM > To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU > Subject: Re: [WIRELESS-LAN] Wireless-only Dorms? > > Interesting discussion ongoing... > > I work to remain agnostic in regards to WLAN vendors, but I > do consider Meru a leader in developing/enabling 802.11 > technologies. Frank is correct in that they use the NAV to > holdoff data clients while voice handsets gain airtime access > (even tho they don't know it). This combined with their > holistic view of the network and flat channel architecture > (enables very fast roaming) certainly has its advantages. > Until 802.11e/r becomes prevalent in handsets these > mechanisms will serve its purpose because don't forget - > 802.11 was never made to handle voice clients. But that will > change over the next 2-3 years as cellular mechanisms are > adopted into the WLAN via IEEE 802.11k/v, etc. > > -----Original Message----- > From: Frank Bulk [mailto:[EMAIL PROTECTED] > Sent: Thursday, November 10, 2005 9:18 AM > To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU > Subject: Re: [WIRELESS-LAN] Wireless-only Dorms? > > Meru does not use PCF, but does use virtual carrier sense as > their main mechanism to control access to the medium. > > Frank > > -----Original Message----- > From: Michael Griego [mailto:[EMAIL PROTECTED] > Sent: Wednesday, November 09, 2005 11:47 AM > To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU > Subject: Re: [WIRELESS-LAN] Wireless-only Dorms? > > All of the issues listed here are great examples of the > complex nature of designing an 802.11 environment with such > stringent requirements. > With only 3 channels, even if you plan very carefully and > precisely control the output power of your APs, you're going > to get channel overlap. This will further reduce your > capacity due to the inherent collisions/retransmissions. > Especially when you factor in the client devices. A client > device transmitting on a channel will force any other device > operating on the same channel that can hear it (APs included if > course) to wait on it to complete its transmission before it > can commence. > So, you have to realize that, even though 2 APs may not be > able to hear each other, a client card between them that can > hear both of them will tie up available bandwidth on BOTH APs > while it is transmitting. Further complicating matters is a > situation where two clients connected to two different APs on > the same channel can hear each other but not both APs. > In > such a circumstance, client 1 and the AP 2 (the AP client 2 is > connected) > may transmit simultaneously. When this happens the signals > will interfere with each other upon reaching client 2, > causing client 2 to be unable to decode the packet, forcing > AP 2 to retransmit the packet. > > Complicated indeed! Guaranteeing signal strengh and > bandwidth alotments is extremely difficult. And, this > totally ignores the problems inherent with outside > interference or the fact that the environment (bookshelves, > etc) change on a regular basis, possibly forcing you to > revisit your ever-so-finely-tuned RF plan. Interestingly > enough, all these issues are also extremely relevant if > you're interested in looking to deploy any sort of VoIP/WiFi (VoFi). > > I'd suggest that, if you're truly interested in providing > coverage/bandwidth that takes a lot of these issues into > account, you might want to take a look at the Meru Virtual AP > architecture. The controllers in these systems keep track of > every 802.11 device each AP can here and employ a pretty darn > impressive scheduling algorithm for getting the most out of > the available channel capacity. Not only that, but they > actually control when clients are allowed to transmit, > further removing unknowns from the RF use equations and > improving channel usage and capacity. I believe they do this > using the PCF, or Point Coordination Function, in the 802.11 > spec... I've not seen any other wireless switch system that > makes use of it near to the level that the Meru system does. > It's pretty cool. We're in the process of deploying Meru as > our second generation wireless overlay here at UTD, mainly to > decrease the need for complex channel planning, individual AP > configuration, and to support a future VoFi implementation. > > --Mike > > > Phil Raymond wrote: > > If someone forced me to assign a rule of thumb at this high > level, I > > would assign a conservative data rate of 1 Mbps to each > student as a > > requirement. For an 802.11g ONLY network running at the > highest data > > rate (aka strongest signal) using enterprise class AP's > (data thruput > > does vary between AP vendors, be careful here), you should > expect to > > get 15-20 Mbps of upper layer thruput per AP. That would > yield 15-20 > > students per AP. For 802.11a, this will probably hold. For 802.11g, > > due to the limit of 3 channels, you will get an overall > reduction in > > capacity due to shared bandwidth between AP's in a densely > deployed AP > > > environment. > > > > Also, this assumes that you design the network for the > highest signal > > strength - a very important point. In most instances this won't be > > possible due to the environment. Thus I would reduce the available > > bandwidth by 33% and say that 10Mbps is available. > > > > Hence I would go with the low end of 10Mbps available per AP. > > > > To take this to a lower level of analysis, I would want to > know what > > applications the students would be running. Perhaps you use the > > analogy of a low end DSL connection that provides 768Kbps > downlink and > > > 128kbps uplink. Then you stick with the 1 Mbps/student and > assume it > > supports most if not all applications they will use. You might also > > consider a swag at peak operating times (evenings) and > assume ~50% of > > the available students are online (simple queuing theory > assumption). > > Then you could say that a single AP would cover minimally > 20 students. > > > There is my rule of thumb at this high level. I would consider it > > conservative if you design the network properly. > > > > In a typical dorm with a lot of walls (and bookcases...), you will > > probably find that your coverage requirements and capacity > > requirements will be in alignment (and thus balanced). What > I mean by > > that is that you will find that in order to provide a good > signal in a > > > dorm environment you will need to place a denser AP > deployment (due to > > > the thick walls, etc.). This means that as a consequence > your capacity > > > will also be increased due to the denser deployment. > > > > Other factors not considered here are the use of client cards. > > Performance between different manufacturers (you get what > you pay for) > > > will vary. Some cards will be noisy and interfere, others will have > > higher SNR requirements, etc. > > > > Hope this helps and not confuses - as I said, it is not a trivial > > subject. > > > > -----Original Message----- > > From: Larry Press [mailto:[EMAIL PROTECTED] > > Sent: Wednesday, November 09, 2005 9:51 AM > > To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU > > Subject: Re: [WIRELESS-LAN] Wireless-only Dorms? > > > > Phil Raymond wrote: > > > > > >> The initial design needs to consider coverage AND capacity. > >> > > > > Phil (and others), > > > > Have you got a rule of thumb for the number of students per > G access > > point in a college dorm? > > > > Larry Press > > > > ********** > > Participation and subscription information for this EDUCAUSE > > Constituent Group discussion list can be found at > http://www.educause.edu/groups/. > > > > ********** > > Participation and subscription information for this EDUCAUSE > Constituent > Group discussion list can be found at http://www.educause.edu/groups/. > > > > ********** > Participation and subscription information for this EDUCAUSE > Constituent Group discussion list can be found at > http://www.educause.edu/groups/. > > ********** > Participation and subscription information for this EDUCAUSE > Constituent Group discussion list can be found at > http://www.educause.edu/groups/. > > ********** > Participation and subscription information for this EDUCAUSE > Constituent Group discussion list can be found at > http://www.educause.edu/groups/. > ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/.
