Connect802 Corporation

Later today this weeks Tweet stream will be posted for your review to the Connect802 blog at: http://t.co/16CzPQ8a

A transmitter selects a data rate (hence a modulation type) to minimize the number of corrupted packets transmitted.

Earlier this year we talked about BPSK modulation (simple, hard to corrupt) and QAM256 modulation (complex, easily corrupted).

Low data rate signals are less easily corrupted because bit representation is less complex and hence, easier to recover from the air.

Low data rate signals have “longer range” because they’re less easily corrupted in the air compared to high data rate signals.

With all else equal high-data-rate signals are easily corrupted and have short range. Low-data-rate signals are robust and have longer range

There is an inverse relationship between data rate and the signal strength and signal to noise ratio required to receive a signal.

802.11 includes a function called Dynamic Rate Shifting so a client device can choose from various modulation and encoding methods for Tx

We’ve discussed that the unbalanced power effect may result in data rate asymmetry–different upstream and downstream data rates

When your AP has a higher Tx power than your clients you must pay close attention to the RSSI requirements avoid problems

The edges of a 27 dBm coverage cell can’t be used by a 13 dBm client (unless the AP had a 14 dB better receive sensitivity — Not Likely.)

High-powered APs may be as much as 14 dB stronger than a typical client (27 dBm vs 13 dBm). They’re usually 3-4 dB more sensitive, if that! 

It’s much easier to increase transmit power than it is to improve receive sensitivity so – watch out for UPE with “high power” APs

Ideally (but not practically) for each dB of increased power relative to the client an AP should have a dB of increased receive sensitivity

If a high-powered AP lacks enhanced receive sensitivity, it won’t be able to receive data from clients at the edges of its coverage range

A high-powered AP will be able to achieve maximum downstream bit-rate and a very large cover cell in most situations

Because of the UPE, buying a high-powered AP won’t improve functional range unless the AP also has improved receive sensitivity

You may see high-powered (200 mW and up) APs advertised for improving range – do they really increase range or is it marketing hype?

You have to do the math to see how much “making up” of client deficiencies can be done.

On the upstream side, an AP’s increased sensitivity will make up for a client’s lack of transmit power.

On the downstream side, a stronger AP’s increased transmit power will make up for a client’s lack of sensitivity

You can “make up” for decreased transmit power (in your design) if the AP has a greater receive sensitivity.

You compare both the Tx power of APs and clients as well as the receive sensitivity of the devices

You need to consider the power-versus-sensitivity factor when you’re designing a wireless network.

It’s worth repeating: an AP with “X” dB more Tx power that is also “X” dB more sensitive than a client does not create a UPE problem.

Consider an AP that has 6 dB more Tx power than its client device. If that AP is also 6 dB more sensitive, no UPE will exist.

A difference in an AP and a client’s receive sensitivity can help or hurt the Unbalanced Power Effect.

In the example given, even though both devices transmit at the same output power, a 4 dB Unbalanced Power Effect exists

We’re discussing an example where a 54 Mbps connection is achieved by an AP @-72 dBm and a client @-68 dBm (the client is less sensitive)

To catch up or review the discussion, last week’s Tweet stream has been posted on the Connect802 blog: http://t.co/16CzPQ8a

Later today this weeks Tweet stream will be posted for your review to the Connect802 blog at: http://t.co/16CzPQ8a

It’s going to be next week before we discuss this example and we’ll review the Tx and RSSI assumptions then.

In the example (AP=54Mbps@-72 dBm) we’ll assume the client requires -68 dBm to achieve 54 Mbps (it’s a little less sensitive).

Imagine an AP and client both transmit at the same power. In this example we’ll assume that the AP achieves 54 Mbps 802.11g at -72 dBm.

“Your” deficit in Tx power benefits from “my” increased receiver sensitivity – and vice versa.

When considering the Unbalanced Power Effect, it’s important to understand that differences in receive sensitivity also matter

When the RSSI for both ends is between about -89 to about -72 dBm, data rate asymmetry will result if client and AP have different Tx power

When RSSI goes below about -89 to -94 dBm, a typical 802.11 device can’t achieve even 1 Mbps data rate

Once the RSSI for either end of a link goes below about -89 to -94 dBm, the link will drop

Once the RSSI for both ends of a link exceeds about -72 dBm, both ends can typically achieve their highest data rate; UPE doesn’t matter

Depending on the difference in transmit power between the AP and the client, the degree of asymmetry that UPE causes will vary

To assess the Unbalanced Power Effect you must consider receiver sensitivity in addition to received signal strength.

As long as the client can achieve at least 1 Mbps data rate, the link will be up, but asymmetric. The AP will achieve a higher data rate

The link drops when the less capable transmitter can no longer reach across the link with a sufficient RSSI.

1 Mbps is the lowest possible data rate in 802.11. If the previously-described link gets any weaker, the link will drop

In the example given previously, due to UPE, the AP achieves a downstream rate of 48 Mbps. The client achieves 1 Mbps upstream.

Because 802.11 includes adaptive modulation, UPE can result in a difference in upstream vs downstream data rate instead of a dropped link

When UPE is present, we think, “The client can hear the AP, but the AP can’t hear the client.”

We typically think of the Unbalanced Power Effect as resulting in no link at all when it is present.

At an RSSI of -75 dBm, a typical client might be able to receive a data rate of 48 Mbps. Meanwhile, the client can only transmit at 1 Mbps

Because the AP is 19 dB stronger than the client, the client will see a -94 + 19 = -75 dBm RSSI from the AP (when the AP sees client’s -94)

If the transmit power of the AP and the client was the same (in our example), both of them would see an min RSSI of -94 dBm from each other

When there’s a difference in Tx power between client and AP the weaker of the two limits the range of a connection.

If an AP is 19 dBm stronger than a client then (obviously) the client is 19 dB weaker than the AP

In the present example we’ve got an AP that is 19 dBm stronger than the client (20 dBm – 1 dBm = 19 dBm)