10. October 2009 by admin.

So, how can I …

How can I get customers to use/play with WiFi Hotspot Sharing System, some interesting stuff about WiFi…

I just came across this:

http://www.cepro.com/article/how_to_debug_wifi_and_optimize_wireless_in_the_home/

* First off… Some interesting things about the new 802.11n (as in November.)

Don’t Leap to 802.11n

I’m sure you’ve heard a lot about 802.11n, the newer WiFi technology. Unfortunately, 802.11n, in my opinion, is not ready to be professionally installed as a wireless infrastructure for your customer.

Its major problems right now are:

It’s still in draft form and will be until mid-2009, the major reason that no commercial WAP utilizes 802.11n. Interoperability is still a problem.

It has so many mode variations — 576 to be exact — it’s just not possible to predict performance between any two devices.

in the presence of competing ‘g’ networks (even in a neighbor’s home) it will “fall back” to g speeds

It is subject to many of the interference problems of ‘g’ .

It can seriously screw-up existing ‘g’ networks

Something that most people forget is that to utilize its potential higher bandwidths (over about 60 Mbps) requires it connect to a 1000Base-T wired network.

(Maybe they have fixed some of those problems by now?)

* But here is what initially caught my eye. Many of the Hotspot Sharing Systems (often called “External WiFi”) that I deploy on marine / yachts … have a Signal Strength reading in dBm to tell you how strong a WiFi Hotspot’s signal is. I tell my customers that the lower negative numbers are the stronger one. The lower the negative number the stronger the signal is. But how high of a negative number is too high to be useable?

This chart was at the above link.

FIGURE 1. 802.11G Data Rate vs. Path Lost

Figure 1. Data rate vs. received signal strength for two popular WAPs. Both exhibit a common “cliff” effect where data rate drops off quickly.

According to this chart, it looks like getting past -80 dBm is rapidly approaching the point where Internet Speed might suffer. Keep in mind that it’s a shared medium (Hotspots are shared by others…) and the typical Internet link supplying a hotspot is 12 Megabits per second or slower. Many are 2 Megabits per second or slower.

So, you might be able to operate down into -90 dBm and still get 1 or 2 Mb/s of Internet Speed. Assuming your system can hold that signal and there are no other factors affecting your resulting Internet Speed. (I have seen that even with good signal in some very crowded marinas … things can can get really ugly with a lot of WiFi users.)

Anyway I thought it was interesting. If any of my Marine WiFi Hotspot System users test this out in their travels - I would be interested in hearing about your experiences - especially using this chart as a basis. So just how weak of a Hotspot Signal could you lock onto and have useable Internet?

Thanks!

—

Alan Spicer

Alan Spicer Telecom / Alan Spicer Marine Telecom

http://www.marinetelecom.net - http://www.wifiyacht.net

+1 954-683-3426

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10. October 2009 by admin.

* Alan Spicer’s Note: I found this today while looking for a graphical representation of WiFi Channel Frequencies showing the overlapping and non-overlapping channels. I had an occassion on a marine vessel (motor yacht) today to use the knowledge of what channels can be used in WiFi, in not only 2.4 Ghz but also 5 Ghz, by access points (or access point / routers) that are (or could be?) within each others signal pattern (you could call it their “footprint” as the do with Satellite Signals reaching the Earth.) Anyway I found the article to be not only interesting but refreshing in the way the author describes OFDM (Orthogonal Frequency Division Multiplex) as an analogy (analogous to…) a Rythm Guitar Player and a Lead Guitar Player [OFDM and FDM.] I am obviously not the author of the article, I only wish I was. I am now a fan of the author and his web site, as well as the web site that I found it on.

http://www.cwnp.com/community/articles/understanding_ofdm_-_part_1.html

and…

http://www.wirelesstrainingsolutions.com/

Written by Rick Murphy    
Wednesday, 23 September 2009 
The 48-string Guitar

(I placed this picture just for visual effect because of the title and analogy discussion in the article. But I did download the http://www.guitar-chords-laboratory.com/ Guitar Chords Laboratory and it’s a pretty cool program, if you have an interest in playing the guitar at all. It is able to show chords and actually play them…)

Like the sidewalk under your feet, always solid, never doubted, Orthogonal Frequency Division Multiplexing (OFDM) is the rock that supports current and near-term wireless technologies, including 802.11a, 802.11g, 802.11n, WiMAX, and LTE. As a wireless professional, you’ll be working with OFDM-based technologies for the foreseeable future. An understanding of OFDM will give you an edge in designing and maintaining the networks under your care.

802.11a and 802.11g are the “tried and true” members of the 802.11 family of wireless local area network (WLAN) products. Both 802.11a and 802.11g (802.11a/g) make use of OFDM. The manner in which these two technologies exploit the features of OFDM is relatively uncomplicated, which makes them good choices with which to begin our exploration of OFDM. Once we have a good working understanding of OFDM and are able to visualize its operations it will be much easier to understand the changes that 802.11n encompasses.
 
802.11n is the most recent addition to the 802.11 family of WLAN technologies. In development since 2002, it has now officially been ratified by IEEE ballot and is published for use by the networking community. Because of its advanced features and higher communications rates, we can expect a gold rush of activity surrounding this new amendment. 802.11n has already proven that it can provide much higher data transfer rates than were possible using earlier forms of wireless technologies. The higher speeds of 802.11n are realized thanks to innovations such as Multiple Input Multiple Output (MIMO), wider channel bandwidth options (40 MHz), and improvements to existing mechanisms such as OFDM. One specific improvement to OFDM is the way in which 802.11n takes advantage of previously unused (by 802.11a/g) subcarriers. To appreciate these improvements, it’s necessary to explore what subcarriers are in the first place and how they were used previously, with 802.11a/g.

WiMAX and LTE are cutting-edge, broadband wireless technologies that hold great promise for delivering high-speed connectivity within wireless metropolitan area networks (WMANs). These are important “next gen” technologies and the way they use OFDM is a bit complex. For that reason we’ll postpone our examination of WiMAX’s and LTE’s use of OFDM until later in this commentary.

Before there was Orthogonal Frequency Division Multiplexing (OFDM), there was Frequency Division Multiplexing (FDM), a reliable but slower wireless signaling method. These two multiplexing techniques will be compared, against two common forms of rendition used in guitar performances, namely lead and rhythm guitar. The reason to do that is to give a recognizable analogy of something invisible and alien (OFDM) to something familiar and pleasant (music). The first comparison makes the assertion that FDM is comparable to lead guitar, while the second is of OFDM to the chords produced by a rhythm guitarist.
 
For the first example, we’ll compare the use of FDM, with a single-carrier, to the performance of a lead guitarist fingering the notes of a riff. Each string is sounded clearly and separately as the guitarist presses briefly behind the frets, while plucking the strings with a pick. This action continues in a swiftly moving serial progression to the end of the score.
 
That’s similar to the way in which FDM single-carrier* signals are transmitted. In FDM, an input data stream arrives at the transmitter radio chain to be modulated serially, onto a single-carrier channel by means of some form of phase modulation (BPSK, QPSK, 16-QAM, or 64-QAM). Each modulation event induces a clear and separate pulse, called a “modulation symbol”, which represents a select portion of the bits that make up the input data stream.
 
Just as the sound from the guitar travels through the air to reach a listener’s ears, FDM induced signals also propagate over the air interface where they likely encounter a receiving antenna. With music, the listener’s ears pass the guitar sound onto auditory nerves, which deliver the sounds to the brain, where the sounds may be interpreted pleasantly (or not). In wireless, the receiving antenna, directs the FDM signal to the receiver’s radio chain where processor chips decompose the modulation symbols back into the original information bits and send them to the higher network layers for interpretation.
 
In the second example, a rhythm guitarist presses down on several strings at once, while strumming the pick across all of the strings. This action produces a “chord” which is a composite sound made up of multiple individual notes played together. Each note still maintains its individuality, but what’s heard by the listener is a melodic, composite sound containing all of the individual notes superpositioned as one.
 
To play chords, the rhythm guitarist positions fingers on the strings in pre-defined patterns much like an OFDM radio prepares the data stream to be transmitted by performing a serial-to-parallel conversion. The guitarist strums across all of the guitar’s strings to sound the chord, while in OFDM, subcarriers (analogous to the guitar’s strings) are created through a mathematical function called Inverse Fast Fourier Transforms (IFFT). 802.11a/g allows 48 subcarriers to be individually phase modulated to represent bits from the input data stream. Once induced, the individual modulation symbols combine into a single transmission burst known as an “OFDM symbol”, which is comparable to the chord produced by a rhythm guitar. At the receiving side, the OFDM symbol is recovered through a Fast Fourier Transform (FFT) operation, the reverse of the IFFT operation, which splits the composite OFDM symbol back into its component, modulation symbols. Like the guitar chord, an OFDM symbol efficiently packs more information into each burst. The bits represented by the modulation symbols are queued and presented to a parallel-to-serial conversion step, resulting in a bit stream that can be copied to baseband and sent up the protocol stack.
You can think of the way that OFDM is used in 802.11a/g, as a rhythm guitar with 48 strings. That gives a clear, high-level, auditory and visual analogy of this use of OFDM. But to be more precise, there are really 64 subcarriers created by the 802.11a/g IFFT/FFT functions. It’s just that, only 48 of them are used to carry user data. In order to take this investigation deeper and to prepare for more complex versions of OFDM, it’s now necessary to leave our guitar analogy and talk about OFDM straight-up.

* for FDM multi-carrier, try picturing a jam session with several lead guitars
 
To be continued… Watch for “Understanding OFDM - Part 2 – Subcarriers Unstrung” –

Summary of Part 1

OFDM is used in current and soon-to-be released wireless communications technologies.
FDM can be compared to the single notes played sequentially by a lead guitarist
OFDM can be compared to the multi-note chords produced by a rhythm guitarist
Like a chord produced by a guitar, an OFDM symbol contains multiple information components
802.11a/g uses a form of OFDM which creates 64 subcarriers
Only 48 of these subcariers are used to represent bits from the input user data stream.

Rick Murphy’s Homepage - http://www.wirelesstrainingsolutions.com

(Obviously please visit cwnp.com and Rick Murphy’s Homepage - you can bet I will be.)

P.S. It’s very interesting to read about not only what happens at the Computer Networking Level, but also what happens at the Radio / Communications Level - of how this stuff works. And surely this can be beneficial to those of us that actually deploy both WiFi 802.11 and Cellular 3G (3.5G and 4G) systems for our friends and customers. 

Additional references I looked at:

http://en.wikipedia.org/wiki/Orthogonality#Communications

In mathematics, two vectors are orthogonal if they are perpendicular, i.e., they form a right angle.

Communications

In communications, multiple-access schemes are orthogonal when an ideal receiver can completely reject arbitrarily strong unwanted signals using different basis functions than the desired signal. One such scheme is TDMA, where the orthogonal basis functions are non-overlapping rectangular pulses (”time slots”).

Another scheme is orthogonal frequency-division multiplexing (OFDM), which refers to the use, by a single transmitter, of a set of frequency multiplexed signals with the exact minimum frequency spacing needed to make them orthogonal so that they do not interfere with each other. Well known examples include (a and g) versions of 802.11 Wi-Fi; Wimax; ITU-T G.hn, DVB-T, the terrestrial digital TV broadcast system used in most of the world outside North America; and DMT, the standard form of ADSL.

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

Orthogonal frequency-division multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions — for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath — without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.

OFDM Transmitter (Idealized system model)

OFDM Receiver (Idealized system model) 

—

Alan Spicer Telecom / Alan Spicer Marine Telecom

http://www.marinetelecom.net - http://www.wifiyacht.net

+1 954-683-3426

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