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Sephiroth
Alessandro Volta
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UNDERSTANDING KEY ASPECTS OF WiFi

UNDERSTANDING KEY ASPECTS OF WiFi

 WiFi is a somewhat notorious, though essential commodity. In many cases, when its behaviour does not meet the user’s perception of the claims made by the ISP (e.g. Virgin Media), the blame is quickly heaped on the ISP’s router (e.g. Superhub 2/2ac, Hub 3.0).

Disregarding the obsolete Superhub 1, the Virgin Media hubs are constructed and certified to the WiFi Alliance standard.  This means, that apart from faulty devices, they broadcast WiFi signals as powerfully as any other router operating to the same standard (e.g. ‘N’, ‘AC’).

With that out of the way, we can look at what affects WiFi signal strength.  First, we need to consider how signal strength is measured (dBm). Then we can consider at what signal strength things can be done on the Internet over WiFi.

SIGNAL STRENGTH EXPLAINED

Fairly obviously, signal strength (power) must be ultimately measured in Watts. Given that a toaster, for example, consumes 2kw (kiloWatts), WiFi power levels need to be harmless and are prescribed in each country by regulation.

In the UK, the maximum permitted transmit power expressed in mW (milliWatts) is:

2.4 GHz band                                100 mW

5 GHz band ch 36 - 64                200 mw      (these are designated as ‘indoors’ channels)

5 GHz band ch 104-140              1000 mw   

Commonly available signal strength measurement apps (e.g. inSSIDer) express power on the Bel logarithmic scale.  In the WiFi case, the measurement unit is dBm (deciBel relative to the milliWatt). 

A key number to note is that 0.0001 mW = -40 dBm. 

In scientific notation 1.00E-04 mW = -40 dBm.

Conversely, 1.0 mW = 0 dBm.

dBm is not a linear scale; it is logarithmic.  Without going into the mathematics, this is illustrated by the following simple table:

3 dB gain

+3 dB

Double signal strength

3 dB loss

-3 dB

Half signal strength

10 dB gain

+10 dB

10x more signal strength

10 dB loss

-10 dB

1/10 of signal strength

 

Below is a table generated in Excel that takes us closer to understanding what measurement apps such as inSSIDer report.

dBm

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

-3

0

W

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

5.01E-04

1.00E-03

mW

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

5.01E-01

1.00E+00

 

Next, we’ll consider at what signal strength things can be done on the Internet over WiFi.

 

SIGNIFICANCE OF TRANSMIT POWER LEVELS

Most users of inSSIDer or similar analysis tools will have seen the dBm readings.  They will also know that as distance increases from the hub, or as walls intervene, the WiFi power falls (to a larger negative value).

So, how far can you go and what can you do with your signal level? We’ll deal with “how far you can go” in the next section.

Below is a table showing the signal/power levels and what you can expect to do in those circumstances.

 

Signal strength

What you can do

Required for

-25 dBm

The highest power level that you are likely to see right up close to the hub.

Anything

-30 dBm

Maybe 2m from the hub in the same room direct line of sight.

Anything

-40 dBm

In the same typical home room either at furthest separation or with an intervening non-water absorbant or metal object.

VOIP; Video conference; video streaming; real time programs

-50 dBm

In a room directly above the hub, or in an adjacent room with non-water absorbant walls.

VOIP; Video conference; video streaming; real time programs

-67 dBm

Minimum power level for response time sensitive applications.

VOIP; Video conference; video streaming; real time programs

-70 dBm

Minimum power level for reliable packet delivery.

Email; browsing

-80 dBm

Minimum power level for browsing connectivity but with risk of packet loss.

Not much

-90 dBm

The noise level will drown any packet data.

Nothing

 

 

AROUND THE HOUSE

The table below describes how common/household materials can affect the WiFi power level.  Acknowledgement is made to physics.stackexchange.com.

 

Material

2.4GHz attenuation

5GHz attenuation

Interior Drywall/plasterboard

3-4 dB

3-5 dB

Wooden hollow door

3-4 dB

6-7 dB

Brick /Concrete/Breeze Block Wall

6-18 dB

10-30 dB

Glass/Window

2-3 dB

6-8 dB

Double-pane coated glass

13 dB

20 dB

Steel door

13-19 dB

25-32 dB

Ceiling/Floor combination

6-8 dB

6-10 dB

 

Now let’s examine a theoretical house. In the next diagram, A is the equivalent of a concrete wall; B is the ceiling/floor made of plasterboard & wood; C is a plasterboard wall.

 WiFi House Attenuation.png

The diagram is reasonably self-explanatory (acknowledgement to zen.co.uk).  However, there are points of note that should make sense to some frustrated home users.

  1. Room 1 signal strength in the 2.4GHz band, if the hub is not placed behind a TV, is unlikely to fall below -45 dBm.
  2. Room 2 signal strength in the 2.4GHz band at device (a) in the furthest position away from Room 1 is likely not to be better than -70 dBm depending on the exact wall material (5GHz band -75 dBm). If device (a) is moved close to wall A, signal strength of -65 dBm could be expected (5GHz band -70 dBm). In the 5GHz band,
  3. Room 3 explains exactly what this document is really about. If the wireless signal in the 2.4GHz band to device (b) has to pass through wall A (as well as floor B), then WiFi will be barely usable if at all.  The wireless signal to device (c) in Room3, not having to pass through concrete wall A, would have perfectly usable WiFi.
  4. Room 4 signal strength in the 2.4GHz band will be high in the -50 to -55 dBm range (5GHz band -55 to -60 dBm).

 

CONCLUSION

Without knowing the foregoing information, the WiFi user might well be tempted to blame the hub for their WiFi woes. The signal strength at the WiFi client is the most important driver for satisfactory WiFi. 

 END.

 

Seph - ( DEFROCKED - My advice is at your risk)