Wi-Fi Data Rates and Coverage

Wi-Fi data rates, coverage and interference are all interlinked topics. It's a simple rule in Wi-Fi that you can't "have it all". If you use high power, you have big cells and typically more interference. Lower power means more APs are needed. Let's take a look at these interlinked concepts.

Wi-Fi Speed, Data Rates and Coverage

When we design and assess Wi-Fi networks, we refer to performance in terms of network speed.

For each individual transmission, the sender selects the highest data rate it considers “safe” based on the known information about the environment (SNR history) and the signal strength (last RSSI received).

• A data rate is measured in Mbps (megabits per second)

• Different generations of Wi-Fi have different minimum and maximum data rates and different steps (rates) between them

  • 802.11b = 1 - 11 Mbps 802.11a/g = 6 - 54 Mbps

  • 802.11n = 7 - 600 Mbps 802.11ac = 7 - 6,933 Mbps

  • 802.11ax = 8 - 9,608 Mbps

• With the 802.11n/ac/ax protocols, we think of the data rates in terms of MCS rates, which range from 0 (lowest) to 32 (highest rate).

We determine the coverage of an access point by documenting the SNR and RSSI values when connected to that AP. 

A fast wireless network has the following key characteristics:

• Transmissions are sent at high data rates

• SNR is high (above 25)

• RSSI is well above -67 dBm

ASSOCIATED BLOGS:

• Troubleshooting with DNA Center
• Wi-Fi Concepts
• Wi-Fi Terminology
• Copper vs Fibre

Utilisation and Interference

Utilisation is a key measure of how “busy” the wireless spectrum is in a particular environment. The higher the utilisation, the more retries, collisions and other errors will naturally occur. We normally count both data traffic and admin traffic as part of assessing utilisation, but we don’t include interference.

Utilisation tracks how close to maximum capacity a network is operating at. We normally rely upon the network to report on utilisation levels, but it’s also possible to collect this in real time from an on-site survey.

As a general rule, 2.4 GHz networks with utilisation above 40-50% are considered “congested” , while 5 GHz networks should not exceed 20-40% to be considered congested. 

The threshold is higher for 2.4GHz, because these signals naturally go farther, the admin traffic overhead is much higher.

Interference is closely related to utilisation (as high utilisation can cause interference), but we track it separately.

A transmission is considered interference (or noise) when it gets in the way of a good signal. It may collide, or simply raise the noise floor, which reduces the SNR (see signal strength) and lowers the maximum transmission speed.

ASSOCIATED BLOGS:

• Wi-Fi Predictive Designs
• Wi-Fi Assurance Review

Wi-Fi Channels

Digital transmissions use the radio frequency spectrum, which is divided up into logical channels for ease of use and reference.

Wi-Fi channels are 20 MHz wide, but can be bonded in groups of 2, 4, or 8 channels together, using 40, 80 or 160 MHz of spectrum for a single transmission respectively.

Importantly, a 2.4 GHz channel number represents 5 MHz of spectrum, whereas a 5 GHz channel number represents 20 MHz of spectrum. Channel 11 (on 2.4 GHz) is different to channel 36 (on 5 GHz)

There are a few key channel descriptions you should recognise:

• Primary Channel = this channel is always used for administrative transmissions (e.g. Beacons). It is not always the first or lowest channel, but can be any channel in a bonded group.

• Bonded Channel = when multiple channels are used for a single transmission

• Channel Overlap = when two transmissions can use the same part of the spectrum, even if their primary channels differ

ASSOCIATED BLOGS:

• Wi-Fi Grades
• Wi-Fi Concepts
• Wi-Fi Surveys
• Wi-Fi and the Cinema Story

Signal Strength and RSSI

A signal is an intended transmission from a source to a receiver. Signal Strength refers to how strong that transmission is received by a receiver.

RSSI (Received Signal Strength Indicator) is the measurement of signal strength from the receiver’s perspective. Most (but not all!) vendors report RSSI as a dBm value.

There are many parallels between sound and radio signals, including some of the characteristics of attenuation and measuring approaches.

We often use the following terms when we are discussing signal strength:

• Attenuation = the passive reduction in signal strength between transmitter and receiver

• Bleed = Signals that extend beyond the range of desirable coverage (e.g. between floors, or beyond external walls)

• dBi= stands for dB(isotropic). This is the forward gain of the antenna, measured in dBi. Forward gain is measured against a hypothetical isotropic antenna. Essentially this is antenna gain.

• dBm = a way of calculating the strength of signals and noise. Decibels are a logarithmic measure of RF power.

• EIRP = the legal allowable limit on how much energy can be transmitted

• mW = Milli-Watts are another way of representing RF power - this time on a standard, non-logarithmic scale.

• Power Level = a vendor-specific measure of signal strength, often in a range of 1 - 8

• SNR = Signal-to-Noise ratio = the difference between transmission strength and active sources of RF (see interference)

ASSOCIATED BLOGS:

• Cisco Hyperlocation
• RF Power Tuning