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October 1, 2004

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
To Infinity... and Beyond!
News from the wireless marketplace...

Essential Wi-Fi

Introduction to Wi-Fi

It seems that you can't turn around without hearing about Wi-Fi these days. The Internet--ever on the cutting edge of technology--is exploding with information about Wi-Fi. Computer magazines have covered it for years. But this technology is so important that even non-technical publications such as Forbes and the New York Times have dedicated space to Wi-Fi. Whatever your business, whatever your title, Wi-Fi has the potential to improve the way you work (and play), and it is so pervasive that you will almost certainly cross paths with it eventually, if you haven't already. If you're completely new to Wi-Fi, this article will give you the foundation to understand this exciting technology.

"Wi-Fi" is a colloquial name for the technology more correctly known as "802.11" (pronounced "eight-oh-two-dot-eleven"). Although Wi-Fi and 802.11 are not technically exactly the same thing, they are often used interchangeably, and most of the time that won't cause a problem. Even 802.11 engineers who understand the difference between these terms also understand that they're often used interchangeably and will get what you mean if you say "Wi-Fi". Let's face it, "Wi-Fi" rolls off the tongue a lot easier than "802.11"!

802.11 describes a technology for transmitting data wirelessly. Instead of the wires that traditional LAN's, such as Ethernet, use, 802.11 uses radio waves, similar to the way that a cordless phone uses radio waves to carry your voice between the handset and the base station. In 802.11, the "base station" is called an Access Point, which is often abbreviated as "AP". The access point usually plugs into the wired LAN like any other station, then any wireless client that is within range of the access point can access the wired LAN by going through the access point. This describes the basic architecture of an 802.11 network: one or more wireless clients within range of an access point.

Typically, the access point is used to provide access to resources on the wired LAN. For example, a doctor might access a patient's medical records, stored on a server on the wired network, over the wireless network using a PDA. Alternatively, the "resource" that the wireless clients access might be an up-link to the Internet, as is seen in the "hot-spot" model. Access points can also provide connectivity between two wireless clients. For example, an access point in a conference room could allow wireless clients within the conference room to exchange information with each other.

If 802.11 is just another way of connecting computers to form a network, then why all the excitement? What does 802.11 offer that wired LAN's don't? The main advantage of wireless LAN's is convenience. From the user's perspective, network access is available wherever the user happens to be. This opens up the freedom to work wherever is convenient--in the break room, on the balcony, etc...--, not wherever there is a network connection. Users don't have to plug in to a physical jack in the wall. They can just set down their laptop or PDA within range of the access point and log onto the network. When they're done, they just pick up and leave.

Not only can 802.11 be used to provide more convenient access in areas that already have a wired LAN, but 802.11 can be used to extend network access to areas where LAN connectivity would be otherwise impossible. For example, an apartment complex might want to increase its value to its residents by providing free Internet access. Although the main office has a DSL up-link to the Internet, it would be cost-prohibitive to extend that to the apartments using a wired LAN, since running wires is very expensive. Instead, the complex can use 802.11 equipment to cheaply extend wireless access to the apartments. 802.11 excels at providing network access in large buildings, outdoor areas, or other areas where cost or other prohibitions prevent running wires.

802.11 also has advantages from the LAN administrator's side. Wireless networks are often much easier to set up than wired networks. For small networks (less than approximately three APs and twenty-five users), at least, installation and configuration of a wireless network is much easier than a wired network, especially if wires would have to be run. Adding users to a wireless LAN is simple, since there's no need to assign the user to a network port, connect the port to the network, and so on. From the administrator's perspective, wireless networks also excel when the network is intended to be temporary, such as might be set up for a traveling convention or stage show. The equipment can easily be set up and torn down without worrying about the facilities of the venue.

A final advantage of 802.11 is cost. Although wireless LAN technologies existed before 802.11, they were proprietary (one vendor's equipment couldn't talk to another vendor's equipment) and expensive. 802.11 provides a standard, cheap way of wirelessly connecting computers together.
In next months Essential Wi-Fi we'll talk about the "Components of a Wi-Fi Network", explaining the relationship between things like access points, client adapters, antennas, Power-over-Ethernet (POE), routers, NAT gateways, and hotspot access controllers.


Technology and Engineering

RSSI Measurement and dB-Milliwatts (dBm)

Most 802.11 analysis tools and vendors' client management utilities provide a representation of signal strength. Four units of measurement are used to represent RF signal strength in 802.11. These are: mW (milliwatts), dBm ("db"-milliwatts), RSSI (Received Signal Strength Indicator), and a percentage measurement. All of these measurements are related to each other, some more closely than others, and it's possible to convert from one unit to another.

The first two units to consider are the mW and the dBm (pronounced "dee-bee-em" or spoken as "dee-bee milliwatts"). Although these are not the most common units in 802.11, we discuss them first because they are the most basic. Just like a pound is a basic unit for measuring weight, a watt is a basic unit for measuring energy (and, in keeping with metric conventions, a mW is one one-thousandth of a watt). It turns out that measuring RF energy in mW units is not always convenient. This is due, in part, to the fact that signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, then you move twice as far away, the signal will have decreased by a factor of four. This relationship can be characterized as logarithmic, and one can say that "RF power drops off logarithmically."

The "dBm" is a logarithmic measurement of signal strength. Since it is logarithmic, just like the power of the RF signal, as the RF signal's strength changes (logarithmically), the dBm value changes linearly. To put it more generally, if you measure a quantity that changes logarithmically (RF power) with a linear unit (mW), the unit will change logarithmically, which is inconvenient. If you measure a quantity that changes logarithmically with a logarithmic unit (dBm), the unit will change linearly, which is more convenient.

dBm values can be exactly and directly converted to and from mW values. Just like miles and kilometers can be converted directly, so can mW and dBm. The formulas to convert are:

dBm = log(mW) * 10
mW = 10^(dBm/10)

We use dBm because it's much easier to say, and write, "-96dBm" than have to say "0.000 000 000 25 mW". That's a lot of zeroes! You should realize that convenience and ease-of-understanding are two fundamental reasons why the dBm metric is used for RF signal strength, rather than mW.

The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. In 802.11b, g, and a, this numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Received Signal Strength Indicator (RSSI). Notice that nothing has been said here about measurement of RF energy in dBm or mW. RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use internally by the physical and data link layers (the hardware in the card and its drivers). For example, when an adapter wants to transmit a packet it must be able to detect whether or not the channel is clear (i.e.: nobody else is transmitting). If the RSSI is below some very low threshold then the chipset decides that the channel is clear. 802.11 does not require that a particular RSSI value correspond to any particular mW value, so each vendor makes this decision on its own. This means that you probably can't compare "signal strength" values between two vendors' chipsets, because those values are based on the RSSI, and different vendors' chipsets associate different power levels with different RSSI values.

To circumvent the complexities (and potential inaccuracies) of using RSSI as a basis for reporting dBm signal strength, it is common to see signal strength represented as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value (multiplied by 100 to derive a percentage). If all vendors used that formula for converting RSSI to signal strength percentage, then percentage for signal strength would provide a reasonable cross-vendor metric for use in network analysis and site survey work. However, if vendors do not consistently use the formula above, then we once again end up in a scenario where it's impossible to compare numbers from different vendors. For example, a vendor might hypothetically use a logarithmic function to map RSSI to signal strength, which would cause the signal strength to stay at high values longer as RSSI decreased, and then to drop off very rapidly as RSSI approached zero. Frankly, we don't know the exact details, on a model-number-by-model-number basis, of how each of the many NIC manufacturers map RSSI to signal strength percentage, so its difficult to draw concrete conclusions on this matter.


To Infinity... and Beyond!

Wi-Fi wireless networking continues to converge with the cell phone system. In the not-to-distant future it's expected that seamless roaming between Wi-Fi systems and mobile cellular networks will be a reality. Here are some examples of recent news articles that support this view.

Data Rates Increase for Cell Phone Service
On September 29, 2004 Verizon Wireless announced deployment of 300 - 500 Mbit/sec data service using the cellular phone network in selected markets. In addition to over 20 airports across the country, this new service (called 3G 1xEV-DO for CDMA) is available in Atlanta, Austin, Baltimore, Kansas City KS/MO, Las Vegas, Los Angeles, Miami/Fort Lauderdale, Milwaukee, New York City, Philadelphia, Tampa, Washington DC, and West Palm Beach. Verizon expects one-third of its entire network to be EV-DO capable by the end of the year. Verizon competitor Sprint is also looking to deploy this type of service to most metropolitan markets sometime in 2005. This is another step in the gradual convergence of cellular technology and data technology like Wi-Fi.

HP Pocket PC Phone Supports Wi-Fi and Cellular
HP has recently launched one of the first "multi-mode" devices capable of working transparently on either a cellular phone network or through a Wi-Fi network. The HP6300 allows a user to connect directly to an in-building Wi-Fi network for email, web surfing, or data transfer and switch over to the cell phone network when outside the building. Although the telephone portion of the HP6300 currently works only through the cell phone network it's anticipated (by Connect802) that devices using Voice-over-WLAN in conjunction with both Wi-Fi and cellular communication are going to soon come to market.

Bill Gates Speaks at the Computer History Museum in San Jose
On October 1, 2004, Joe Bardwell, Chief Scientist and President of Connect802, and Anita Lenk, Connect802's VP of Operations and Director of Professional Services were invited to the Computer History Museum in San Jose to hear a one-on-one conversation featuring Bill Gates, Chairman and Chief Software Architect of Microsoft. During the arm chair session with John Hennessy, President of Stanford University, the topic "Building Confidence in a Connected Marketplace" was discussed. Gates discussed his vision of how technology will impact society in the years ahead. Of great interest to Connect802 was the firm belief that technology would continue to converge, with data, voice, and video moving more and more towards a common infrastructure. Of course, maintaining security and providing inter-organization authentication was presented as being a paramount concern. The example was given that banks today can allow customers to use ATM cash machines all around the world because they have established systems whereby one bank "trusts" the information coming from the other bank. In this same way, there exists a need for one computer system to have mechanisms whereby trust can be established with other systems. A case in point, the need to have a way to receive email from senders who's identity has been validated was held up as a countermeasure to today's spam crisis.