By: Mike Wade – VP Wireless Engineering (Hula Networks)
Welcome to the Hula Networks Wireless Minute.
This is the first of a series of informational articles relating to wireless networking. These articles will attempt to answer questions that regularly come up during the course of implementing a wireless network.
The Unbalanced Power Effect:
The term “Unbalanced Power Effect” was created by a colleague of mine, Joshua Bardwell. While the phenomenon has always existed in two way RF communication, Joshua came up with the term. The Unbalanced Power Effect describes the effect where two wireless devices of unequal transmit output power are attempting to communicate, and one cannot receive the other as well due to this difference.
IEEE 802.11 is a set of specifications for implementing wireless local area networking using certain bands, and further defined channels in the radio frequency area of the electromagnetic spectrum. There are many aspects of this specification, this article will focus on ERP (Effective Radiated Power) and RSSI (Receive Signal Strength Indicator) These can be thought of as two sides of the same coin. ERP is the amount of RF energy that is being transmitted into the air. RSSI is the amount of RF energy that is being received by an antenna. Both of these will be expressed in this article in dBm, or decibel milliwatt. dBm is an abbreviation of the power ratio in decibels [dB] of the measured power [RF energy] referenced to one milliwatt [mW].
First we need to understand that there is a minimum dBm level that will provide the best opportunity for a client device to obtain the maximum data rate. While signal strength (dBm) is not the only factor to consider when determining maximum data throughput, it is the one factor that we have the greatest ability to predict and design for. Other factors are more a product of what might be obtainable in the environment where the wireless network is installed and may or may not be available. This requires a discussion of the MCS (Modulation and Coding) index. I will have another article on this in the next newsletter. For now, let’s use -62 (neg 62 dBm) as our optimal signal strength to determine the boundary of our cell. I haven’t chosen this number arbitrarily as it happens to be the RSSI where most current WiFi devices can obtain the maximum data rate. This will be made clear in the MCS article.
Picture a circle with an AP in the center. The access point is transmitting at -20 dBm (a commonly available ERP for commercial access point radios) and the edge of the circle represents the -62 dBm boundary. This is what you would have in a vacuum, or an ideal lab setting with zero attenuators. This coverage area will not remain constant, RF energy will fluctuate as much as 5 dBm up and down in any environment and must be accounted for in the design.
In the real world there will be walls, doors, and architectural features that will attenuate, reflect, and absorb this signal in very different ways. Now we are seeing an undefined shape surrounding the access point representing the same -62 dBm boundary. The shape is created by the environment where the AP is installed.
We will add the client device transmitting at -15 dBm (again a common cellphone ERP). This will be another unidentifiable shape created by the surrounding environment. This shape will be smaller since the transmit power is lower than the AP.
Now that we understand the individual -62 dBm boundaries of these two devices we will put them both in context to demonstrate the unbalanced power effect. You can see here that the client device is within the AP’s -62 dBm boundary, but the AP is not in the client devices -62 dBm boundary. This is the unbalanced power effect.
Think of two people in a large open space, the coach has a megaphone while the rower does not. The coach gives the rower an instruction. The rower can hear the request and complies. The rower asks the coach a question. The rower cannot be heard. There is a difference in volume (ERP) between the two transmitters and the distance (attenuation) makes it impossible for the unamplified voice to be heard.
It must be noted that the unbalanced power effect does not describe an improperly designed wireless network. It is perfectly acceptable to have all client devices within a -62 dBm area created by the access point radios while the AP’s remain outside of the clients -15 dBm boundary. There is still a connection between the AP and client. Generally, more data flows downstream than upstream. Meaning a client will send very little data to request a large amount of data. For instance, it only takes a few clicks to start downloading a movie. Therefore, the upstream link can successfully be a lower data rate as long as the downstream link remains within the optimal limit.
In the earlier example where the coach and rower are trying to communicate with only one megaphone, the rower was not heard at all. This isn’t entirely true with WiFi. The boundaries depicted here show only the -62 dBm limits, signal does not stop here. RF energy will go quite far. It is a matter of how far from the transmitter it will be useful to a client device. This is generally going to be as far away as the signal can be accurately identified above the surrounding noise.
A strict design specification may require that all specified client devices must be able to maintain maximum data rate connection with access points at all times. In this case the design engineer must take the ERP of each possible client device and find the weakest one and design for that. An example of that would be a hospital that has mobile computers on carts and patient monitoring devices. The computers have an ERP of -16 while the patient monitors have an ERP of -14. The overall design must allow for a -14 cell size around all access point radios so that no client device is ever outside of this boundary. In this case there will be many more AP’s installed than if the specification called for -62 cell sizes. In fact, the AP’s will likely need to have their TX output power turned down so that the ERP matches the client devices.
That is the Unbalanced Power Effect in a nutshell. I hope this helps you in your quest to design, manage, install a great wireless network.
VP Wireless Engineering