The CSAIL wi-fi network has evolved reasonably moderately as wi-fi skills
has changed within the 17 years since the contemporary network used to be specified for
the building.
This doc is an strive to speak out the most up-to-date (as of 2021) develop
solutions that we observe when making changes to the wi-fi infrastructure.
Frequent solutions
To the extent that you would bring to mind, we develop changes on the premise of on-the-space surveys
performed the speak of Ekahau Respectable, an industry-traditional instrument equipment for
wi-fi network diagnosis.
Ekahau Respectable has wonderful services and products for surveying wi-fi network coverage
and visualizing the consequences; however, localization of the see equipment
is performed manually by an operator.
Thus, in uncover for a speak to be surveyed, and therefore taken into
myth for coverage planning, we ought so that you can both bodily gain admission to
it and detect it on a scheme by monotonous reckoning.
Substances of areas of work to which gain admission to is blocked by tremendous piles of junk,
outsized desks, or different obstructions can no longer be surveyed and should not ever be
in point of fact appropriate in coverage planning.
(We on the entire speak Ekahau’s visualization in a mode that can interpolate
from measured substances but the structure of Stata is such that this
infrequently affords unsuitable results.)
All of our wi-fi gain admission to substances like a 1000BASE-T wired backhaul; we create
no longer (at least no longer deliberately) operate any “repeaters” or “mesh mode”
gain admission to substances, because this greatly reduces the ability of the
network and exacerbates the already severe “hidden terminal” field
introduced on by the structure of the building.
As a end result, when smitten by when an gain admission to point can even be located,
we must always title how the backhaul connection can even be offered.
Once in a while, it goes to no longer be feasible to present wi-fi service in a
particular speak, because there’s no such thing as a CSAIL network wiring nearby.
Whereas we are able to set up wiring, right here’s a in point of fact costly proposition in Stata
(about $1,600 per speak no longer including the gain admission to point itself) and we
need to funds for this expense.
Density
Most of nowadays’s wi-fi network skills used to be designed to aid tremendous
launch-thought areas of work with finest free-field attenuation between an gain admission to point
and all of its clients.
That is emphatically no longer the topic in Stata.
Until we had been to position an gain admission to point in every speak of business, which is both
impractical and dear, most clients will most certainly be separated by a lot of
sheetrock walls from the set their nearest gain admission to substances are located.
Nook areas of work namely will steadily need to receive some indicators
by method of the masonry- and metal-clad exterior walls of the building.
This implies that we must like a greater density of gain admission to substances than
is conventional, in uncover to ship enough signal strength in enough
areas to penetrate every speak of business.
As of 2021, the CSAIL wi-fi network consists of 82 gain admission to substances:
- 1 within the Toyota lab on P2
- 1 within the basement
- 11 on the second floor
- 12 on the third floor
- 16 on the fourth floor
- 4 on D5
- 7 on G5
- 8 on G6
- 8 on G7
- 5 on G8
- 5 on G9
(The vogue of gain admission to substances naturally decreases with floor field,
therefore the ideal CSAIL-occupied floor has one of the gain admission to
substances.)
Limited service on 2.45 GHz
A well-known method back of needing so many gain admission to substances for speak of business coverage
is that the gain admission to substances intervene with every different within the more launch
areas of the building, especially within the two- and 4-fable launch areas.
That is an extremely severe field for the 2.45-GHz frequency band,
veteran by many older devices, which has both greater signal penetration
and much fewer on hand channels (finest three, as in comparison with 22
for the more moderen 5-GHz band).
The interference field on 2.45 GHz is so severe, genuinely, that we
present at most “finest effort” develop for it: there’s no such thing as a that you would bring to mind
3-coloring of any field beneath the 9th floor of Stata, due to adjacencies
between ground introduced on by the double-peak areas.
Thus, if a user has a wi-fi field and their instrument is finest succesful
of the speak of the older IEEE 802.11g traditional, or it helps 802.11n but finest
on the 2.45-GHz band, we are able to whisper the user to update their equipment.
We configure a minimum data payment of 18 Mbit/s, so devices that toughen
finest 802.11b or “traditional” 802.11 (which might per chance per chance presumably be the genuine age to count as
“retrocomputing” for the time being) can no longer join to the network the least bit: at least
802.11g is required.
(That is critical to develop the 2.45-GHz band usable the least bit, because there
is numerous broadcast traffic on the CSAIL network and declares are
transmitted at the lowest supported data payment, leaving less air time
on hand for stout-stir communications.)
Acquire considerations for five GHz
Whereas the 5 GHz frequency band does like greatly more channels
on hand, the band is split into a lot of sub-bands which like different
comely restrictions.
For the U-NII-1 and U-NII-3 bands, that are the fitting bands supported by
user-grade wi-fi gain admission to substances, there are different energy limits
for every sub-band (channels 36–48 and 157–161).
Many user devices can operate on channel 165, which overlaps the U-NII-3
and ISM bands, but our gain admission to substances can no longer, so it’s a colossal different if
you would truly like a private wi-fi network.
For the final bands (collectively known as U-NII-2, channels
52–64 and 100–144), federal
legislation designates wi-fi networks as a secondary speak, with the principle
speak being radar systems; gain admission to substances operating on this band need to implement
“dynamic frequency different”, repeatedly monitoring for any establish of a
safe radar signal.
If an gain admission to point detects a radar signal, it need to straight (interior
milliseconds) quit operating on its configured channel and switch
to an unrestricted channel within the U-NII-1 or U-NII-3 band.
The certification tests for this are sufficiently costly and hard
that manufacturers in most cases steal no longer to gain certification for
user-grade equipment, which is why low-end gain admission to substances most continuously
create no longer toughen operation on U-NII-2 channels.
The Boston Logan Airport Terminal Doppler Weather Radar (TDWR)
operates on 5.610 GHz, which is wi-fi channel 120.
CSAIL’s wi-fi equipment can no longer speak channels 120 or 124 due to FCC
restrictions.
(MIT campus wi-fi does speak both channels, I’m no longer positive by what
comely authority — campus wi-fi makes speak of a undeniable hardware vendor who
might per chance per chance like received different certifications.)
In addition, channel 144 overlaps the boundary between the U-NII-2C and
U-NII-3 bands; while our gain admission to substances toughen this channel, none of
our monitoring and see equipment helps it, so we create no longer speak it.
Bandwidth
Within the 5-GHz band, the 802.11ac (“Wi-Fi 5”) and 802.11ax (“Wi-Fi 6”)
standards enable for bandwidths of 20 MHz (single-channel), 40 MHz (two
channels), 80 MHz (four channels), or 160 MHz (eight channels).
Many user devices acquire default with 80 MHz bandwidth configured;
in some cases (e.g., Apple devices) this might per chance possibly per chance no longer be configured the least bit.
Consequently of the high density of gain admission to substances, both CSAIL and MIT networks
in Stata speak 20-MHz bandwidth completely; private gain admission to substances as effectively
need to be both configurable, and in put collectively configured, to speak 20 MHz — there
merely are no longer enough channels on hand for speak of any different
bandwidth.
This does imply that clients can no longer gain anything shut to the stir
marketed by user-grade wi-fi equipment (which is supposed
for speak in a unruffled single-family dwelling with out a different nearby gain admission to substances).
With 20 MHz bandwidth, one of the stir of an older 802.11n consumer,
or an 802.11ac consumer with a single “spatial circulation” like a cell phone,
is ready 75 Mbit/s.
A more most up-to-date 802.11ac Wave 2 or 802.11ax consumer with three spatial streams
(like a conventional contemporary pc pc) can construct about 225 Mbit/s in put collectively.
(There is one well-known exception: the Bliel convention room (32-G601)
is located removed from any CSAIL or MIT wi-fi network, and has finest a single
gain admission to point.
We configure that gain admission to illustrate speak 80-MHz bandwidth since it
can no longer intervene with different customers.)
Targets for network planning
When planning whether or no longer so that you can add or relocate an gain admission to point, or whether or no longer
to reconfigure radio parameters, we aid the following goals
in mind:
- Steer obvious of co-channel interference. As eminent above, the structure
of Stata manner that in loads of areas, two gain admission to substances might per chance per chance every like
clients in a zone the set their indicators overlap, but no longer be in a field to hear
every different — ensuing in a
so-known as “hidden terminal” field, whereby both gain admission to substances
strive to transmit to different clients at the same time, but a consumer within the
overlap speak can’t decode the signal intended for it as a result of
interference.
We strive to the ideal extent that you would bring to mind to develop positive
that no consumer can hear two gain admission to substances transmitting on the same channel.
This effectively requires us to speak all 20 of the usable channels in
the 5-GHz band (and, as eminent above, is fully very no longer going within the
2.45-GHz band). - Present at least two usable indicators in every speak. Between outages
introduced on by radar detection, hardware failures, firmware updates, different
upkeep, and simple ability limitations, this might per chance possibly per chance infrequently be the case
that the closest gain admission to illustrate a given speak is possibly no longer in a field to
service all clients.
We strive to develop positive there’s a second gain admission to point
interior vary of each and every speak of business, though as a result of geometry of Stata this
isn’t persistently feasible.
When inspecting see results and smitten by
gain admission to-point placement, we gaze at the received signal strength from the second-
and third-strongest gain admission to substances at every speak. - Be obvious that that at least one signal in every neighborhood is on a
U-NII-1 or U-NII-3 channel.
This ensures that, even within the case of a radar hit that takes out a lot of
U-NII-2 channels, at least one channel is on hand that’s no longer field
to the Dynamic Frequency Selection solutions and should not ever like to commerce channels. - Steer obvious of adjacent-channel interference. To the extent that you would bring to mind, we strive to
aid some distance from having the strongest and second-strongest indicators in any speak
be on straight adjacent channels (i.e., channel numbers 4 aside). This
might per chance be very no longer going for the 2.45-GHz band, so we finest like in mind this advise when
selecting channel assignments for the 5-GHz band.
