5 ms·

MIT CSAIL network design principles

115 points by asleepawake 5y ago

twoodfin 5y ago
This is the building under discussion: https://www.csail.mit.edu/about/stata-center https://www.csail.mit.edu/about/stata-center
JustAPerson 5y ago
This article doesn't provide much context on the challenges of the building's architecture. If the exterior[1][2] did not give any indication, the inside is quite difficult to navigate. Unless you're a grad student who lives and breathes this building (where most of the EECS department is located), you'd better allocate 10 minutes to finding the professor's office you intend to visit. I can't find great pics of the interior (only some floor plans[3][4]), but there's a variety of confusing stair cases, open atrium's spanning multiple floors, irregular floor plans and office layouts, and an abundance of cluttered spaces. No good flat surface goes unused. I'd never considered that it would be impossible to provide comprehensive 2.4GHz coverage because of graph coloring problems. The point about certain 5GHz channels being effectively unreliable due to radar was also an interesting insight I'd never heard before. [1]: https://files.catbox.moe/6z4ad2.jpeg https://files.catbox.moe/6z4ad2.jpeg [2]: https://upload.wikimedia.org/wikipedia/commons/1/17/Stata_Center1.jpg https://upload.wikimedia.org/wikipedia/commons/1/17/Stata_Ce... [3]: https://csdl-images.computer.org/trans/tm/2018/05/figures/liu5-2725265.gif https://csdl-images.computer.org/trans/tm/2018/05/figures/li... [4]: https://i.pinimg.com/originals/80/f9/af/80f9af1c6cdc0c170cf69d0f47648a8e.jpg https://i.pinimg.com/originals/80/f9/af/80f9af1c6cdc0c170cf6...
- zzt123 5y ago
  Most consumer mesh systems do not support those 5ghz channels that mandate DFS. I can’t think of one that does support them, although numerous non-mesh systems do support them. If your router(s) do support those channels, they can be quite nice to have in a residential setting, as people I’ve observed tend to not have equipment operating in those channels. High traffic airports can be a problem, as you and the article mention, since DFS may get triggered with great frequency. Nearby military bases in general (not just airfields) may also result in burdensome DFS triggering, although YMMV.
punnerud 5y ago
A key tool mentioned in the article is Ekahau Pro. What are good alternatives for Ekahau? As I understand this require special hardware. Most Macs support WiFi Monitor mode that should be possible to give the same functionality without extra hardware. And is only 5Ghz working on M1? So I would expect that there is some Mac software with capability of doing the same.
- anonymousDan 5y ago
  Be interested to know the answer to this question. Also would having something like a cheap SDR make a difference to what is possible?
  - punnerud 5y ago
    Most (all?) cheap SDR don’t get close to the WiFi range. RTL2832U have one of the best price/range ratios and can detect 24-1766MHz and cost under $20. LimeSDR is probably one of the better choices and cost $199 without casing as $299 with. I have been waiting for my LimeSDR close to one year now because of the shortage of chips. You don’t get the raw data (SDR) on Mac like microwave and bluetooth, but you get all WiFi related data. Also broken packages and from other senders.
    - the_only_law 5y ago
      There was a thread recently, where some people mentioned confers about the limesdr[0]. [0]https://news.ycombinator.com/item?id=28319647 https://news.ycombinator.com/item?id=28319647
- lsllc 5y ago
  There was a thread yesterday about Ubiquiti's WiFiman iOS/Android app that does this sort of stuff (although the visualization featured in the submission is Android only). https://news.ycombinator.com/item?id=28391112 https://news.ycombinator.com/item?id=28391112 I don't know how the functionality compares to Ekahau Pro, since I've never used Ekahau Pro.
- xvf22 5y ago
egberts1 5y ago
Channel 144?! So, essentially most monitoring and survey equipment skip that, got it.
dreamcompiler 5y ago
So in summary here's what we know: 1. The building's weirdly-shaped vertical spaces cause signal interference that cannot be mitigated by the 802.11 automatic mechanisms. 2. The building was not originally wired (or at least not wired properly) for ethernet. 3. The building is a Faraday cage, but not a useful one because it interferes with wifi penetration while exterior radar signals can still get in and block some wifi channels. 4. The building is hard to navigate for occasional visitors (personal experience and other comment herein). 5. The building leaks when it rains (private communications from residents). Moral: Don't let Frank Gehry design your Computer Science and Engineering building. In fact, don't let Frank Gehry design any building if humans are expected to inhabit it.
- gaze 5y ago
  the building is also full of pests lol. All of MIT is like this and held together with duct tape and dental floss. Still the most fun place to do science I've encountered.
  - piannucci 5y ago
    No kidding. My potted Schlumbergera got nibbled by a mouse on the 9th floor. :-( I'd been nursing that plant back to health for years. This wasn't a regular occurrence up that high, but still.
  - Jugurtha 5y ago
    >with duct tape and dental floss Here (Algiers, Algeria), one equivalent to this is "with newspaper and spit" or "held together with cum". I'd love to see that phrase's equivalent in other languages.
    - azinman2 5y ago
      Why cum? Seems like an odd thing to apply to a random structure… is there some other cultural reference I’m missing to understand it other than being sticky?
lucb1e 5y ago
> Ensure that at least one signal in every neighborhood is on a U-NII-1 or U-NII-3 channel. This ensures that, even in the case of a radar hit that takes out multiple U-NII-2 channels, at least one channel is available that is not subject to the Dynamic Frequency Selection rules and will not have to change channels. I don't understand this, can someone explain this in layman terms? Are they near some military base similar to how 2.4 GHz WiFis are usually near microwaves that can interfere (except radar is not contained in a Faraday cage)?
- dsr_ 5y ago
  "The Boston Logan Airport Terminal Doppler Weather Radar (TDWR) operates on 5.610 GHz, which is wireless channel 120."
  - lucb1e 5y ago
    Aha I read over that one! Thanks
wrs 5y ago
This made me smile — such an “MIT thing” to say in the middle of an IT infrastructure document: “There is no possible 3-coloring of any space below the 9th floor of Stata.”
wolverine876 5y ago
I hate wifi. It's useful to users so they like it (it's useful to me too, I admit), but it's a serious PITA to manage and to diagnose problems. Perhaps people here - especially network professionals - have tips on these age-old issues: * What statistic(s) actually correlate best with network performance? Signal strength is what everyone assumes; is that it? Signal strength variability (i.e., availability)? Ratio of endpoint to base station active radios?, etc. * How do you efficiently diagnose user reports of intermittent network failures? How do you distinguish interference issues from base station hardware issues from user hardware issues from good old user confusion and imagination? * How do you efficiently and cost-effectively track down sources of interference, especially intermittent interference? They could come from anywhere, inside and outside your facility. They could be caused by a million different things, active and passive.

115 points by asleepawake 5y ago

This is the building under discussion: https://www.csail.mit.edu/about/stata-center https://www.csail.mit.edu/about/stata-center

JustAPerson 5y ago

This article doesn't provide much context on the challenges of the building's architecture. If the exterior[1][2] did not give any indication, the inside is quite difficult to navigate. Unless you're a grad student who lives and breathes this building (where most of the EECS department is located), you'd better allocate 10 minutes to finding the professor's office you intend to visit. I can't find great pics of the interior (only some floor plans[3][4]), but there's a variety of confusing stair cases, open atrium's spanning multiple floors, irregular floor plans and office layouts, and an abundance of cluttered spaces. No good flat surface goes unused. I'd never considered that it would be impossible to provide comprehensive 2.4GHz coverage because of graph coloring problems. The point about certain 5GHz channels being effectively unreliable due to radar was also an interesting insight I'd never heard before. [1]: https://files.catbox.moe/6z4ad2.jpeg https://files.catbox.moe/6z4ad2.jpeg [2]: https://upload.wikimedia.org/wikipedia/commons/1/17/Stata_Center1.jpg https://upload.wikimedia.org/wikipedia/commons/1/17/Stata_Ce... [3]: https://csdl-images.computer.org/trans/tm/2018/05/figures/liu5-2725265.gif https://csdl-images.computer.org/trans/tm/2018/05/figures/li... [4]: https://i.pinimg.com/originals/80/f9/af/80f9af1c6cdc0c170cf69d0f47648a8e.jpg https://i.pinimg.com/originals/80/f9/af/80f9af1c6cdc0c170cf6...

zzt123 5y ago
Most consumer mesh systems do not support those 5ghz channels that mandate DFS. I can’t think of one that does support them, although numerous non-mesh systems do support them. If your router(s) do support those channels, they can be quite nice to have in a residential setting, as people I’ve observed tend to not have equipment operating in those channels. High traffic airports can be a problem, as you and the article mention, since DFS may get triggered with great frequency. Nearby military bases in general (not just airfields) may also result in burdensome DFS triggering, although YMMV.

punnerud 5y ago

A key tool mentioned in the article is Ekahau Pro. What are good alternatives for Ekahau? As I understand this require special hardware. Most Macs support WiFi Monitor mode that should be possible to give the same functionality without extra hardware. And is only 5Ghz working on M1? So I would expect that there is some Mac software with capability of doing the same.

anonymousDan 5y ago
Be interested to know the answer to this question. Also would having something like a cheap SDR make a difference to what is possible?
- punnerud 5y ago
  Most (all?) cheap SDR don’t get close to the WiFi range. RTL2832U have one of the best price/range ratios and can detect 24-1766MHz and cost under $20. LimeSDR is probably one of the better choices and cost $199 without casing as $299 with. I have been waiting for my LimeSDR close to one year now because of the shortage of chips. You don’t get the raw data (SDR) on Mac like microwave and bluetooth, but you get all WiFi related data. Also broken packages and from other senders.
  - the_only_law 5y ago
    There was a thread recently, where some people mentioned confers about the limesdr[0]. [0]https://news.ycombinator.com/item?id=28319647 https://news.ycombinator.com/item?id=28319647
lsllc 5y ago
There was a thread yesterday about Ubiquiti's WiFiman iOS/Android app that does this sort of stuff (although the visualization featured in the submission is Android only). https://news.ycombinator.com/item?id=28391112 https://news.ycombinator.com/item?id=28391112 I don't know how the functionality compares to Ekahau Pro, since I've never used Ekahau Pro.
xvf22 5y ago

egberts1 5y ago

Channel 144?! So, essentially most monitoring and survey equipment skip that, got it.

dreamcompiler 5y ago

So in summary here's what we know: 1. The building's weirdly-shaped vertical spaces cause signal interference that cannot be mitigated by the 802.11 automatic mechanisms. 2. The building was not originally wired (or at least not wired properly) for ethernet. 3. The building is a Faraday cage, but not a useful one because it interferes with wifi penetration while exterior radar signals can still get in and block some wifi channels. 4. The building is hard to navigate for occasional visitors (personal experience and other comment herein). 5. The building leaks when it rains (private communications from residents). Moral: Don't let Frank Gehry design your Computer Science and Engineering building. In fact, don't let Frank Gehry design any building if humans are expected to inhabit it.

gaze 5y ago
the building is also full of pests lol. All of MIT is like this and held together with duct tape and dental floss. Still the most fun place to do science I've encountered.
- piannucci 5y ago
  No kidding. My potted Schlumbergera got nibbled by a mouse on the 9th floor. :-( I'd been nursing that plant back to health for years. This wasn't a regular occurrence up that high, but still.
- Jugurtha 5y ago
  >with duct tape and dental floss Here (Algiers, Algeria), one equivalent to this is "with newspaper and spit" or "held together with cum". I'd love to see that phrase's equivalent in other languages.
  - azinman2 5y ago
    Why cum? Seems like an odd thing to apply to a random structure… is there some other cultural reference I’m missing to understand it other than being sticky?

lucb1e 5y ago

> Ensure that at least one signal in every neighborhood is on a U-NII-1 or U-NII-3 channel. This ensures that, even in the case of a radar hit that takes out multiple U-NII-2 channels, at least one channel is available that is not subject to the Dynamic Frequency Selection rules and will not have to change channels. I don't understand this, can someone explain this in layman terms? Are they near some military base similar to how 2.4 GHz WiFis are usually near microwaves that can interfere (except radar is not contained in a Faraday cage)?

dsr_ 5y ago
"The Boston Logan Airport Terminal Doppler Weather Radar (TDWR) operates on 5.610 GHz, which is wireless channel 120."
- lucb1e 5y ago
  Aha I read over that one! Thanks

wrs 5y ago

This made me smile — such an “MIT thing” to say in the middle of an IT infrastructure document: “There is no possible 3-coloring of any space below the 9th floor of Stata.”

wolverine876 5y ago

I hate wifi. It's useful to users so they like it (it's useful to me too, I admit), but it's a serious PITA to manage and to diagnose problems. Perhaps people here - especially network professionals - have tips on these age-old issues: * What statistic(s) actually correlate best with network performance? Signal strength is what everyone assumes; is that it? Signal strength variability (i.e., availability)? Ratio of endpoint to base station active radios?, etc. * How do you efficiently diagnose user reports of intermittent network failures? How do you distinguish interference issues from base station hardware issues from user hardware issues from good old user confusion and imagination? * How do you efficiently and cost-effectively track down sources of interference, especially intermittent interference? They could come from anywhere, inside and outside your facility. They could be caused by a million different things, active and passive.