Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


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Summer 2008

   
Product-Focus
Exploring the Connect802 value proposition...
Essential Wi-Fi
For those who are new to Wi-Fi networking...
Technology and Engineering
For the engineer and Wi-Fi network administrator...
Ask the Expert
Questions from our readers...
To Infinity... and Beyond!
News from the wireless marketplace...

Product Focus

We recently established a relationship with Global Mesh Technologies and in doing so, added their exciting new product: CAMMS™, Communications and Multimedia Management System. CAMMS is communications software which enables data, images, floorplans, maps, multi-user whiteboard and VoIP phone communication between wired and wireless users in a mobile, deployed scenario. It perfectly supports the “First Responder” and Public Safely sector.

About the company and the product –
Global Mesh Technologies (GMT) develops communications software and hardware solutions that enable data sharing in a variety of wireless and mobile environments. CAMMS software (Command Anywhere Media Management System) enables users to collaborate and securely share data, voice and video.

When used in conjunction with COTS mesh networking hardware, CAMMS can be used to establish and control an ad hoc, self-forming, self-healing mobile mesh network without the need for servers or any fixed infrastructure. CAMMS can also be used as a stand-alone web-based communications tool where Internet access is available.

CAMMS unique features include: mesh formation, IP camera discovery, interactive whiteboarding, file sharing, TerraViewer mapping, GPS-AVL, IM, video conferencing, Internet sharing, Web operation, VoIP phone connectivity and more. All features are accessible via a single, intuitive GUI ensuring ease of use for non-technical users.

Please contact Connect802 Sales at 925.552.0802 to learn more about CAMMS. We look forward to hearing from you!

Essential Wi-Fi

Configuring Basic 802.11n Options
Before 802.11n, each 802.11 physical layer operated in a single frequency band. 802.11b and 802.11g operate exclusively in the 2.4 GHz ISM band, while 802.11a operates only in the 5 GHz UNII band. But 802.11n is capable of operating in either of those two bands, which leads to the question: which frequency band should you choose for your 802.11n network?

Backwards compatibility is the primary reason to choose the 2.4 GHz frequency band. Only the 2.4 GHz band can provide connectivity to 802.11b/g clients and 802.11n clients at the same time, since b/g clients don’t operate in the 5 GHz frequency band. However, the performance of the 802.11n clients will be hampered when they are forced to operate in the same frequency band as 802.11b/g clients. Because 802.11b/g clients transmit at lower data rates than 802.11n clients, they take up much more airtime to transmit the same amount of data than an 802.11n client would. Imagine a fast sports-car (802.11n) stopped at a four-way intersection while a parade (802.11b/g) creeps past. When 802.11n devices operate on the same channel as 802.11b/g devices, the 802.11n devices must use so-called protection mechanisms to avoid interfering with the 802.11b/g devices. These protection mechanisms add overhead and decrease 802.11n performance. If it is desired to support 802.11n and 802.11b/g clients on the same wireless network, the best performance will probably be realized by purchasing dual-radio access points and leaving the 802.11b/g devices in the 2.4 GHz spectrum, while giving the 5 GHz spectrum to the 802.11n devices. This will allow the 802.11n devices to achieve their full performance potential.

For raw performance, the 5 GHz frequency band is the clear winner. The 5 GHz band has much less noise and interference than the cluttered 2.4 GHz band, meaning that the RF characteristics of the channels will be conducive to higher data rates, all other things being equal. The 5 GHz band also has more non-overlapping channels available--up to 24 channels, vs. the 3 channels available in the 2.4 GHz band with 802.11b/g. This means that a much higher density of access points can be supported without increased contention between access points who are on the same channel. A higher density of access points results in higher maximum user density or more bandwidth per user. Because of the increased number of available channels, the 5 GHz spectrum is much more compatible with 802.11n’s 40-MHz channels.

40-MHz channels roughly double the throughput of the WLAN compared to 20-MHz channels, all other things being equal. The maximum data rate of an 802.11n network is 289 Mbps with 20 MHz channels and 600 Mbps with 40-MHz channels! Real-world equipment is currently not capable of achieving maximum 802.11n data rates, so a more typical scenario would be 144 Mbps with 20 MHz channels vs. 300 Mbps with 40-MHz channels. Clearly, 40-MHz channels are desirable.

The problem is that 40-MHz channels are twice as wide as 20-MHz channels. This means that only a single 40-MHz channel is usable in the 2.4 GHz frequency band. More 40-MHz channels than that would not fit in the band without overlapping. Even worse, if a 40-MHz 802.11n channel is used in the 2.4 GHz frequency band, it covers enough of the band that no 802.11b/g devices can use any other channels in the band. To put this in perspective, imagine that you have an 802.11b/g network with access points on channels 1, 6, and 11. Now you assign an 802.11n access point to use 40-MHz channels. It must use channel 6, which is in the middle of the band, in order to avoid spilling its signal outside the upper and lower bounds of the band. The 802.11n access point will interfere with the 802.11b/g APs on channels 1 and 11. In other words, when an 802.11n access point uses 40-MHz channels in the 2.4 GHz band, only one channel is available! This dramatically limits the potential performance of the network. In most cases, this limitation is unacceptable, and Connect802 does not recommend using 40-MHz 802.11n channels in the 2.4 GHz band. When 802.11n devices use 20-MHz channels in the 2.4 GHz band, the number of available channels is the same as with 802.11b/g, and this is the recommended configuration. Even though 20-MHz channels limit the data rate of the individual access points, they increase the aggregate throughput of the network as a whole because they allow for more simultaneously-usable channels.

Because the 5 GHz UNII band has so many more available channels, it is practical to use 40-MHz 802.11n channels there. Even though each channel is twice as wide, this only reduces the number of available channels from 24 down to 12, a more-than-acceptable number for most installations. Since 40-MHz channels approximately double the throughput of the network compared to 20-MHz channels, it is recommended to use them in the 5 GHz band.

In summary, our recommendations for configuring basic 802.11n options are as follows:

  • Configure 802.11n access points to use the 5 GHz UNII spectrum with 40-MHz channels.
  • If backwards compatibility is desired, use separate access points or dual-mode 802.11n access points to provide 802.11b/g access in the 2.4 GHz band.
  • If it is absolutely necessary, 802.11n access points can be deployed in the 2.4 GHz band, with backwards compatibility for 802.11b/g devices, but 802.11n performance will be limited in a mixed 802.11b/g/n environment.
  • 40-MHz channels should almost never be used in the 2.4 GHz band, unless there is only a single access point in the network, and no other access points are active within range of that access point—even access points from other networks.


 Technology and Engineering

802.11n increases the maximum possible data rate for an 802.11 network from 54 Mbps to 600 Mbps. In this article, we’ll examine some of the ways in which this dramatic performance improvement is achieved. It is well beyond the scope of this discussion to detail the inner workings of the 802.11n standard. Nonetheless, a reasonable working perspective can be obtained if we’re willing to “hand wave” (i.e. accept without a detailed explanation) over some of the underlying technology.

The basic behaviors and protocols of 802.11n are identical to its 802.11b/g/a predecessors. A client device listens for beacon frames from an access point to discover its presence. The client can send probe requests to confirm that an access point is configured with a specific SSID (Service Set IDentifer; the network name.) The client associates and authenticates to the access point.

There are three main areas in which 802.11n differs from 802.11b/g/a:

Internal Engineering Improvements

A number of subtle (but significant) improvements have been made to the core operation of the protocol and the structure of transmitted data. These improvements raise the data rate from the 802.11g/a 54 Mbps to roughly 75 Mbps for 802.11n. Moreover, the actual TCP/IP throughput increased from 50% to closer to 70%. This means that while an 802.11g/a connection at 54 Mbps would provide roughly 27 Mbps TCP/IP throughput, a 75 Mbps 802.11n connection will provide close to 52 Mbps TCP/IP throughput.

40 MHz Channels

802.11b/g/a transmits data in 20 MHz wide channels. Some manufacturers offer channel bonding options to allow the use of two separate channels for the transmission of a single data stream. Channel bonding (sometimes called turbo mode) is vendor-proprietary. 802.11n allows the use of a true 40 MHz wide channel (as opposed to two separate, independent channels used in parallel.) The data rate in a 40 MHz channel is twice that of a 20 MHz channel. This option would allow the data rate for 802.11n to rise from 75 Mbps to 150 Mbps.

Multiple-Input / Multiple-Output (MIMO)

By an ingenious combination of RF engineering and mathematics 802.11n allows the parallel transmission of more than one bit stream in the same frequency band through the use of multiple transmit and multiple receive radios and antennas in the same access point. This is called multiple-input/multiple output or MIMO (pronounced “my-moe”.) Up to four radios can be used (i.e. with up to four separate antennas.) A common configuration is to find that the access point uses three radios but the client device only uses two. This is referred to as a “2 X 3 MIMO system.” A 2 X 3 MIMO system is limited to two separate bit streams. Environmental characteristics determine whether or not one or more bit streams can actually be turned on. If two streams are available then the bit rate would rise from 150 Mbps to 300 Mbps (based on a 40 MHz channel.) Three streams would yield 450 Mbps and four streams would result in the maximum 802.11n 600 Mbps data rate (with which the TCP/IP throughput would be roughly 420 Mbps.) At the time of this writing, most real-world equipment is only capable of supporting up to two streams. A few devices can support three streams. No known devices can currently support four streams, although they will surely be released in the future.

Getting to 75 Mbps: The Internal Engineering Improvements

You’ve been lied to. We’ll, maybe not an outright lie but a great misrepresentation of reality nonetheless. The lie is that you can get 600 Mbps with 802.11n. You can’t. To even suggest that you can actually achieve connectivity rates of 300 Mbps in a practical, commercial environment verges on falsehood. Perhaps someday in the future these fantasy data rates will enter the realm of practical reality – but not today. What you can reasonably expect from 802.11n is something close to a consistent 75 Mbps data rate (with proper RF signal coverage and an appropriately low level of environmental noise) with data rates up to 150 Mbps in the typical best case. This initial increase in speed, up from 802.11g/a 54 Mbps, and the reduction in overhead yielding a 70% TCP/IP throughput rate (up from 50%) is the result of the following internal engineering improvements.

  • In both 802.11b/g/a and 802.11n, each data frame begins with a special sequence of bits called the preamble. The preamble sequence has been optimized in 802.11n
  • Frames are made up of bits and transmitted bits are represented by electromagnetic symbols. A guard interval of time is inserted between successive symbols so they’re clearly differentiated. 802.11n allows the use of a guard interval that is half as long as 802.11a or 802.11g.
  • A short delay must follow the transmission of a frame so that stations can recognize when no one is transmitting and so the start of the subsequent frame is clearly defined. 802.11n includes a mode whereby a shorter interframe gap time can be used.
  • Each data frame must be acknowledged by the transmission of a special ACK frame so the transmitting station knows that the frame has been successfully received. 802.11b/g/a requires each individual frame to receive its own ACK. 802.11n provides for frame aggregation whereby multiple frames can receive a single ACK for an entire sequence.
  • A number of mathematically-based error correction mechanisms are used to minimize the probability of data corruption. The error correction mechanisms in 802.11n have been optimized to significantly reduce inherent bit overhead.
  • Data is transmitted using a set of adjacent, narrowband sub-carriers in a mechanism called Orthogonal Frequency Division Multiplexing (OFDM.) Some of the OFDM sub-carriers must be dedicated to the process of transmitting and receiving the signal, leaving the rest available for carrying data. 802.11n introduces less overhead in the OFDM symbol allowing it to carry more data.
  • Guard bands are small frequency gaps that are introduced to separate adjacent signals and keep them from interfering with each other. Frames are preceded with a special synchronizing and initializing bit stream called the preamble. 802.11n uses narrower guard bands and has optimized the preamble.

It’s these engineering improvements (which are included in the current 802.11n Draft 2.0 document) that are going to absolutely bring you up from 54 Mbps. It’s important to note that each improvement provides a small increase in performance. Also, not all improvements can be put into practice in all environments. For example, the reduced interframe gap is only available when no legacy 802.11b/g/a devices are present. A pure 802.11n environment is called a Greenfield deployment, implying that 802.11b/g/a is disallowed by specific configuration.

Can You Use 40 MHz Channels?

Yes; and you can mix 20 MHz and 40 MHz channels in the same environment – but watch out! First of all, forget 40 MHz channels in the 2.4 GHz frequency band. Across the 802.11b/g 2.4 GHz band there’s only enough bandwidth for a single, non-overlapping 40 MHz channel. So, if you’re in a residential environment with only one access point you have a chance to use the single 40 MHz channel in the 2.4 GHz band. If you’re in a corporate enterprise, school, or other larger space you’ll need to move to 5.8 GHz where you can actually implement multiple 40 MHz channels.

Another consideration is the fact that if you mix 40 MHz and 20 MHz channels you’re going to add some overhead for protection. Protection is the mechanism by which, prior to transmitting, a station takes steps to prevent other stations from trying to transmit at the same time. The desired transmission bandwidth is “protected” against conflicting transmissions. When some users are operating with 20 MHz channels then any 40 MHz transmitter must undertake the protection process on both of the adjacent 20 MHz channels that it wants to use as a single 40 MHz channel. It’s a small amount of additional overhead but it’s there nonetheless.

Finally, the default configuration for 802.11n is to use 20 MHz channels. Unless an IT department is in charge of the wireless clients in the environment there’s little hope of assuring that everyone is configured to actually use a 40 MHz channel.

Be absolutely sure that you quiz your hardware equipment manufacturer or vendor to find out the nuances of 40 MHz channel use for a particular brand of equipment.

If you do implement 40 MHz channels then you can expect your throughput to roughly double – from 75 Mbps to 150 Mbps. Tests have confirmed that this environment produces TCP/IP throughput consistent with, and in some cases slightly better than, the 70% expectation. You’re looking at between 105 Mbps and 125 Mbps.

The Potential for Multiple, Simultaneous Bit Streams

Multiple-Input / Multiple Output (MIMO) is typically the first thing you hear about 802.11n. It should be the last thing you consider with regard to throughput and capacity. MIMO gives you the “icing on the cake”, not the cake.

As previously discussed, the multiple antennas in a MIMO system create separate spatial streams of bits between a single transmit antenna and a single receive antenna. Two streams means twice the throughput, three streams means three times the throughput, and four streams means four times the throughput – it’s just that simple.

The exact way that streams work is well beyond the scope of this article but, suffice it to say, it’s like Arthur C. Clarke said in 1961, “Any sufficiently advanced technology is indistinguishable from magic.”

What you need to know about streams is that there is a very high probability that they can’t be established in a line-of-sight environment. That means that if your client wireless device is in sight of the access point you won’t get multiple streams. This flies in the face of conventional Wi-Fi design where we put an access point inside the conference room to support 20 people sitting around the conference table. With the access point in the conference room the people in the room will not be able to establish spatial streams to the access point 12 feet away on the ceiling.

Spatial streams need multipath reflections in order to work. That means that you’ll have a much higher probability of establishing multiple spatial streams to an access point in the next room, or far away in a large auditorium. It’s really counterintuitive – but it’s true.

The bottom line is that while you can expect a performance increase to roughly 75 Mbps from 802.11n’s basic engineering improvements, and you have the option of doubling the throughput using a 40 MHz channel, you have no way to guarantee that a user in a particular location will absolutely be able to establish multiple spatial streams. If you can get multiple streams then you’ve gotten the “icing on the cake” – but the “cake” caps out at roughly 75 Mbps.

Remember, too that with 20 MHz channels, even if you can establish two spatial streams your actual TCP/IP throughput is going to be between 105 and 125 Mbps – not 150 Mbps.

A preliminary 802.11n design should be based on RF predictive CAD modeling and simulation to identify the most suitable access point locations based on RF signal coverage. Only by comparing the preliminary model to the environment during on-site throughput and multipath analysis can any assessment be made regarding the possibility of achieving multiple spatial channels. In no case will 100% of an 802.11n access point’s coverage area be able to receive multiple spatial channels; it’s always a subset area.

Ask the Expert

We've run into an issue at one of our facilities in La Courneuve, France. At this building, we have 5 Access Points on channels 10, 11, 12, 13, and 10. My question: How are channels regulated outside the United States, particularly in France? Are we prohibited from using channels 1- 9 in France?

In France, only channels 10-13 are allowed. Given the assumption of a minimum of 5 channels of separation, I would conclude that only one channel could be used at a time without interference between them. It might be worthwhile to set APs to channels 10 and 13 (only three channels of separation, but the maximum available in France) and run throughput tests to see if performance improved relative to setting them all to the same channel. If channels 10 and 13 are viable, then perhaps you could end up with a little net throughput increase relative to setting all the APs to the same channel.

For example: You have 5 APs. Assuming that all APs are within range of each other, then, putting them all on the same channel means that only one can talk at a time. Average throughput per AP should be about 4 Mbps (20 Mbps usable channel capacity divided by five APs).
Now, let’s say that you use 10 and 13, so that three APs are on 10 and two are on 13, in this pattern: 10, 13, 10, 13, 10. You’re going to lose some of your usable channel capacity to interference between the overlapping channels. The question is, “How much, and is the net effect worse or better than putting them all on the same channel?” In the U.S., the answer is pretty much always, “Put them on non-overlapping channels,” but given that France doesn’t give you but one non-overlapping channel, that might not be the case. Let’s say that your usable channel capacity is reduced by interference from 20 Mbps to 10 Mbps. Now, the APs on channel 10 get roughly 3.3 Mbps each (10 Mbps usable channel capacity divided by 3 APs) and the APs on channel 13 get roughly 5 Mbps each (10 Mbps usable channel capacity divided by 2 APs). You can see that this situation is not much better than just putting all of the APs on the same channel. On the other hand, if interference only reduced usable channel capacity to 12 Mbps, then the 10, 13, 10, 13, 10 plan would be slightly better than the 10, 10, 10, 10, 10 plan. Channel 10 would get about 4 Mbps per AP and channel 13 would get about 6 Mbps per AP, for a net improvement of 2 Mbps compared to the 10, 10, 10, 10, 10 plan. Only real-world testing can confirm whether the 10, 13, 10, 13, 10 plan would be better than 10, 10, 10, 10, 10.

I would speculate that using channels 10, 11, 12, and 13 at the same time would result in much worse performance than simply setting all APs to the same channel. At this point, the APs are so close together (in terms of their frequency) that interference is very likely, and it would probably be better to set them on the same channel and let them share a single pool of bandwidth using CSMA/CA. Remember in class when we set the APs to channels 1 and 3? One AP seemed to be cruising along at 20 Mbps, while the other AP was having dropouts and only achieving data rates of a few hundred kbps. In my experience, the situation only gets worse the closer together the channels are.


To Infinity... and Beyond!

802.11y Boosts Wi-Fi Power For Longer-Distance Links
Like all wireless technologies, the current 802.11 standards (a, b, g, and n) are full of compromises. They use unlicensed frequency bands, so they’re easy to deploy, but the output power that unlicensed devices can use, and therefore their range, is limited. The nearly-finished 802.11y standard could change all that.

802.11y operates in the 3.65 GHz frequency band. This band requires licenses, but the licenses are non-exclusive, so they should be much easier to get than licenses for exclusive bands. The licenses are available only in parts of the country that don’t have pre-existing ground-to-satellite or radar users that overlap, which excludes most of the eastern seaboard, and most major cities.

The up-side is that this band will allow for up to 50 MHz of bandwidth and up to 20 Watts EIRP from a transmitter, which is five times as much power as an omnidirectional 802.11b/g transmitter can use. The increased output power can be expected to increase range, making 802.11 more viable for outdoor point-to-point and point-to-multipoint links—at least in rural markets where the 3.65 GHz licenses will initially be made available.

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