Re: [Server-devel] Technical questions
Am trying to reflash an XO but it quits after few lines with an error like this Net 516.6s 98KiB/s CRC mismatch for block #330: want 6ebdc7ac got 15f49d1b OLPC C2,256 MiB memory installed,S/N SHC837006DD OpenFirmware CL1 Q2E41 Q2E Use poer button to power off. I need your help. Rgds Eustace Amah ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
On Wed, Jan 27, 2010 at 6:14 AM, Eustace Amah ea...@slb.com wrote: Am trying to reflash an XO but it quits after few lines with an error like this Net 516.6s 98KiB/s CRC mismatch for block #330: want 6ebdc7ac got 15f49d1b Sounds like the image you downloaded is corrupt. Or maybe the USB stick is bad. cheers, m -- martin.langh...@gmail.com mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
On Thu, Jan 21, 2010 at 3:45 AM, James Cameron qu...@laptop.org wrote: There are rules of thumb, and there is engineering. I have two thumbs and no engine in me, so I prefer the former. Is there any simple 5-step guide to make an _initial_ antenna placement frequency selection for APs? It will of course not be perfect. I have several books on wifi but I always wish for a simple approach. Something that starts with 1 - Get a floorplan of the building 2 - Test (or guess if you haven't been there) if the construction material for all/most walls blocks/reflects wifi radio signals. Same with floor/ceiling for multi-storey buildings. (It is always possible that some walls are different than ohers and block the signal - that's something to explore once on-site) 3 - Define how many users you can connect to an AP -- some APs have configured hard limits in the software they ship. If you have to guess, assume ~40 as the upper limit. A - If the walls block wifi A1 - Are your classrooms more than N users (N being the limit defined in step 3)? You can place 2 APs per classroom - in channels 1 and 11. Plus some APs in the common (presumably open) areas (use strategy B1 for open areas). B - If the walls do not block wifi B1 - Draw circles on the floorplan with the expected radius of reach of the AP, so that you got the areas you care about well covered. Write down in pencil a 1, 6 or 11 in each circle. The circles can overlap, but only if their frequencies are different. Of course AP placement will also depend on where you can place an AP so that it is safe for the kids around, protected from the environment, secure, and with power and ethernet wiring. cheers m -- martin.langh...@gmail.com mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
One of the problem that am experiencing now is that I configured the 15 APs with same ESSID, different channels(1,6,11) shared among them and located all around the school. Remember that the APs are configured with different subnet from the lease of DHCP server but when more than 3 XO tries to connect it knocks everyone out immediately. The APs are PoE enabled. The models are Cisco Small Business Model WAP4410N. I have increased the power to the APs. Any solution to this? -Original Message- From: Martin Langhoff [mailto:martin.langh...@gmail.com] Sent: Thursday, January 21, 2010 1:09 PM To: James Cameron; aklei...@sonic.net; Reuben K. Caron; taiwo.alabi; Eustace Amah; server-devel@lists.laptop.org; Emeka Lewis Nwankwo; kene.ijezie Subject: Re: [Server-devel] Technical questions On Thu, Jan 21, 2010 at 12:28 PM, Martin Langhoff martin.langh...@gmail.com wrote: 3 - Define how many users you can connect to an AP -- some APs have configured hard limits in the software they ship. If you have to guess, assume ~40 as the upper limit. Let's reword that as: assume 30, which is where most APs start having problems. If you are using OpenWRT, you can probably support 40. If you have tested your AP, then you'll know for real. In general, it is very hard to go over 40 active nodes per channel; this is mainly due to limitations of sharing the commons of the radio spectrum. High end APs may be able to operate different antennas in different channels -- so one AP acts as if it were 3, working on 1, 6 and 11. cheers, m -- martin.langh...@gmail.com mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
On Thu, Jan 21, 2010 at 4:18 PM, Eustace Amah ea...@slb.com wrote: One of the problem that am experiencing now is that I configured the 15 APs with same ESSID, different channels(1,6,11) shared among them and located all around the school. Excellent. Remember that the APs are configured with different subnet from the lease of DHCP not really. They are within a larger subnet -- the DHCP server knows not to assign certain IP addresses. But do not tell the AP that it is within that subnet, just tell it the IP address it has. Make sure you are not running the AP as a router. ... server but when more than 3 XO tries to connect it knocks everyone out immediately. Weird - time to complain to Cisco... The APs are PoE enabled. The models are Cisco Small Business Model WAP4410N. I have increased the power to the APs. That's a good initial guess. cheers, m -- martin.langh...@gmail.com mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
Hi Martin, Thanks for the prompt reply. I need to know what to configure on t he eth1?The client XO needs to get their IP Lease from eth1 and connect to the internet while eth0 will serve as the outbound connections. Also I have about 15 Access points installed in the school. Must I have them on channel 1,6 and 11 or can I leave them on Auto? Should I configure the Access points to lease their IP from the DHCP server or it should be fixed? Rgds Eustace Amah From: martin.langhoff.o...@gmail.com [mailto:martin.langhoff.o...@gmail.com] On Behalf Of Martin Langhoff Sent: Tuesday, January 19, 2010 9:40 PM To: Reuben K. Caron Cc: Eustace Amah; caron.reuben; Emeka Lewis Nwankwo; taiwo.alabi; kene.ijezie Subject: Re: Technical questions Hi Eustace, I will try to answer your questions -- if you think you will need more help, please do post your questions to server-devel@lists.laptop.orgmailto:server-devel@lists.laptop.org -- I will set the list so that your messages come through, and we'll help you there. First - the ISP info sounds reasonable, you will need to put those IP addresses netmasks in the eth0 configuration -- see notes here http://wiki.laptop.org/go/XS_Techniques_and_Configuration#Setting_a_static_IP_address_for_eth0 If you want to use OpenDNS, you will have to _ignore_ the ISPs own DNS servers. They may prevent you from doing that - so my recommendation is: get things started using the ISPs DNS servers. Once it's all going (so you know you're configuring things right), try to switch to the OpenDNS servers -- if it doesn't work... well... it's probably that the ISP is blocking it. In some cases, it makes sense for an ISP to block this traffic (there some malicious DNS servers out there). Consider asking them to make an exception for OpenDNS -- which is a well known non-malicious DNS service. Under linux - and specifically because this is a server that in itself provides DNS services -- you have to set the DNS servers in a special config file. See the notes here http://wiki.laptop.org/go/XS_Techniques_and_Configuration#Use_OpenDNS On Tue, Jan 19, 2010 at 9:04 PM, Reuben K. Caron reu...@laptop.orgmailto:reu...@laptop.org wrote: I wish to use the following IPaddr as the server IPss for the two separate sites 204.14.46.10 and 204.14.46.14respectively. I don't understand this part. Are those the IP addresses the ISP is telling you to use? The Servers has two network interface card each eth0 and eth1. Eth0 will serve for the external communications while Eth1 will serve for the internal communication for the DHCP server. The DHCP lease will be private IP address like 172.0.0.1 and so on. This will be loop of IPss released to the XO to either connect to each other or to the internet. So I guess that the netmodem IP will server as the Gateway address. Correct me if am wrong. The XS is preconfigured to run the network in the 178.x segment. Let it do its thing as it is a very specific setup. Can you help me with the list of files that I need to edit and what are the steps. Yes, you need to read the Installation guide, and then pick and choose what parts of the techniques and configuration page you want to use / apply. http://wiki.laptop.org/go/XS_Installing_Software http://wiki.laptop.org/go/XS_Techniques_and_Configuration cheers, m -- mar...@laptop.orgmailto:mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
On Wed, Jan 20, 2010 at 10:29 AM, Eustace Amah ea...@slb.com wrote: Should I configure the Access points to lease their IP from the DHCP server or it should be fixed? Good question -- configure them to have a fixed address in the 172.18.126.0/24 range. That range is explicitly set aside for anything you want to set with fixed addresses. cheers, m -- martin.langh...@gmail.com mar...@laptop.org -- School Server Architect - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
Thank you, James, very much for breaking down the issues and warning of the pitfalls. I for one would be interested in knowing if you can comment about specific hardware (hopefully things currently available) that might serve as examples of some of the things you've discussed: i.e.: access points, improved antenna systems, larger omni directional antennas Thanks again. Alex On Wed, Jan 20, 2010 at 05:43:31AM -0500, Reuben K. Caron wrote: And by non-overlapping ranges he means you can ONLY USE channels 1, 6, and 11, spaced out so that two APs with channel 1 do not overlap. For collective interest ... the air is a commons, divided by frequencies. The radios in the access points and the laptops switch rapidly between transmitting and receiving, and they do this using a timing beacon provided by the access point. They share in time. Access points on separate channels (frequencies) do not hear each other directly, and so they are unable to collaborate well on sharing the commons. Often one receiver may be trying to receive from a distant transmitter while a nearby transmitter on a separate channel transmits. A radio receiver is less sensitive if there is a nearby transmitter. This is called desensitisation, and it manifests as higher noise or lower signal level in the receiver. If the noise is high enough, or the signal low enough, then the packet is not decoded and a retry must happen. Retries pollute time commons. The amount of desensitisation depends on the difference in frequency; and falls off more sharply the greater the difference. For typical access point hardware, the desensitisation caused by five channel spaces is sufficiently low to provide reasonable range; hence 1 vs 6 vs 11. Slightly more range can be had by reducing the number of access points and improving the antenna systems. To cover a larger open area you may gain greater overall performance by restricting the number of access points to 3, using a larger omnidirectional antenna and increasing their height above ground. To cover a larger closed area you may gain performance by using directional panel antennas that face all in the same direction ... creating a series of cells. Above all you should test performance before and after making a change; with all radios fully active and operating. Theory is helpful, but reflections from metal surfaces in a built environment can mock theory badly. -- James Cameron http://quozl.linux.org.au/ ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
Re: [Server-devel] Technical questions
On Wed, Jan 20, 2010 at 06:10:34PM -0800, aklei...@sonic.net wrote: I for one would be interested in knowing if you can comment about specific hardware (hopefully things currently available) that might serve as examples of some of the things you've discussed: i.e.: access points, improved antenna systems, larger omni directional antennas No, not really. It's not a problem domain that can be trivially addressed by specific hardware. Success depends on many factors, not just the hardware. Also the hardware available to me in Australia is no doubt quite different to the hardware available to others. There are rules of thumb, and there is engineering. A radiofrequency engineer (I'm an amateur radio operator) ... can determine by theory and measurement the link budget [1] between an access point and a laptop in varying positions, and then assess the performance of a system as a whole; access points and laptops. There are two main problems: received power too low, or too much noise. If the received power is too low for the sensitivity of the receiver, things can then be done to correct it; such as increasing sensitivity of the laptop radio, changing position of laptop, orientation of laptop antennas, obstacles in line of sight, obstacles in fresnel zone [2], orientation of access point antennas, position of access point antennas, and increasing the transmitted power. a. increasing sensitivity of the laptop radio; is not an option for you unless you are changing which model of laptop is being used, b. changing position of laptop; is very obvious, and kids know how to do that quite quickly, ... in some places it doesn't work, so they avoid those places, c. orientation of laptop antennas; usually these should be vertical to the ground, parallel with the access point antennas, d. obstacles in line of sight; if the access point antenna can be seen by eyesight then this is a good start, e. obstacles in fresnel zone; usually only relevant for long distances, see the theory article, ... it is why windows don't always work for signal propagation, f. orientation of access point antennas; again normally these should be vertical, g. position of access point antennas; the higher they are, the greater coverage (and intereference) they will have, ... on the other hand the longer the cable is between the access point and the antenna, the more loss is felt, h. increasing the transmitter power; within the constraints of the country regulations and the limits of the access point. There are environmental considerations that also affect a link budget ... extreme humidity or rainfall can increase signal loss, direct sunlight can increase noise, and other radio systems outside the site can increase noise. On the other hand, if received power is adequate and the problem being experienced is noise from other radios in the other access points or laptops, then some of the above can be *reversed* to reduce the problem. I think I mentioned that in detail in my previous mail. There's no simple answer, sorry. But if your school looks very much like another school, and their hardware choices worked well, then chances are the same choices will give a similar result. References: 1. http://en.wikipedia.org/wiki/Link_Budget ... the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system 2. http://en.wikipedia.org/wiki/Fresnel_zone -- James Cameron http://quozl.linux.org.au/ ___ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
