Wireless Networks Thread, WiFi Networks - Youtube type Channel / Videos ? in Technical; @ neilmac
As requested -
What exactly is OFDM and what does it do ?
spatial streams, what are those ...
10th October 2013, 02:57 PM #1
WiFi Networks - Youtube type Channel / Videos ?
As requested -
What exactly is OFDM and what does it do ?
spatial streams, what are those and when in wireless you refer to an antennae, is that one wire either on the router or wifi access point or the wireless card or does an antennae require 2 cables ie black and white or black and grey or white and grey etc ?
With ref to spatial streams and wifi cards when reading the tech specs I see numbers which am not sure what they represent ie 2x2:3 or 3x3:3 or whatever combination they use ( what do each of those numbers represent or mean ?
Also is there any plan for you to make videos with regards to wireless , features etc etc and posting these to a youtube channel or what exactly ?
Would be good to know as much as possible from basics all the way to advanced features and anything that will help
10th October 2013, 03:02 PM #2
I am just about to jump on a plane, so will answer this at the next opportunity.
This is not easy to answer as each point will require a bit of background, but give me some time and I will get to it.
I have thought about making videos, and may well do if there is a common question and a lot of people want it answered.
Give me a little time as I am now on the road until 21 Oct.
Thanks to neilmac from:
mac_shinobi (10th October 2013)
10th October 2013, 03:25 PM #3
no problem - I will most likely forget about this thread until you reply near the end of October, just wanted to make sure I posted the question(s) before I totally forgot.
10th October 2013, 03:37 PM #4
Here's a quick taster:
1997 - 802.11: Uses Direct Sequence Spread Spectrum (DSSS) with Binary Phase Shift Key (BPSK) and Quartery Phase Shift Key (QPSK) for data rates of 1 and 2 Mbps, with Barker coed for error correction.
1999 - 802.11b : Uses High Rate Direct Sequence Spread Spectrum (HR/DSSS) QPSK with CCK instead of Barker for rates of 5.5 and 11 Mbps.
1999 - 802.11a: Uses Orthogonal Division Frequency Multiplexing in 5GHz for rates from 6 Mbps with BPSK, through QPSK, 16 QAM and 64 QAM for rates of 54 Mbps.
2003 - 802.11g: Uses OFDM as above but in 2.4 Ghz, with protection mechanisms for legacy clients and is called ERP/OFDM (Extended Rate Physical)
2007 - 802.11n uses a refined OFDM, called High Throughput with different Modulation Coding Schemes that are too funky to explain here, but I will try and catch up.
The challenge when we troubleshoot networks is to pinpoint exactly what data rate was used to transmit a frame and why.
It's a complex subject, introduced in the CWNA class and taught at advanced level in the CWAP class. (For disclosure, I am a CWNP trainer)
The introduction of 802.11ac is an advanced form of OFDM HT, and you need to good understanding of n to grasp ac.
I will try and post more info to help people understand, though the admins may one to give guidance on this as it may not be appropriate to do it here.
10th October 2013, 04:54 PM #5
What does OFDM stand for / mean and what is it ?
10th October 2013, 11:11 PM #6
OFDM stands for Orthogonal Frequency Division Multiplexing.
Modulation is the means to manipulate a wave form to produce meaning.
DSSS (see first explanation) and OFDM are just different methods of manipulating waves so that the receiver can deduce a series of 1's and 0's.
OFDM is basically better than DSSS.
In OFDM, a 20 MHz channel is split into 52 sub carriers. An OFDM transmission comes in short bursts (called SYMBOLS). Of the 52 sub carriers, 4 sub carriers are termed Pilot Waves, with a calibration transmission that doesn't carry data. The other 48 sub-carriers will carry a transmission that has a phase shift and an amplitude shift when compared with a pilot sub-carrier. The amount of phase and amplitude shift will allow the Physical layer the means to deduce a binary code unique to that phase/amplitude shift combination. The better the signal to noise ratio, the more options there are for phase/amplitude differences, so the higher amount of bits coded per sub carrier. a Symbol is sent with all 48 sub carriers active. There is a pause and then the next symbol is sent until the whole frame has been sent. The Physical layer assembles all of this together, and after error correction will assemble the whole frame. If it passes Frame Check Sequence it gets passed up to layer 2, the MAC layer for further processing.
HT (802.11n) is still OFDM, but uses 56 sub carriers in 20 Mhz, 114 in 40 Mhz, and has a higher error correction rate of 5/6 as opposed to 3/4 for normal OFDM.
11th November 2013, 05:36 PM #7
@neilmac - Just a nudge / reminder as now the 11th of November
11th November 2013, 06:00 PM #8
What was the question again ?
11th November 2013, 06:05 PM #9
Originally Posted by neilmac
Originally Posted by mac_shinobi
As per the above to answer the above questions and also if you are / have / will do any videos to cover the above or other similar topics etc , a youtube channel if you will ?
11th November 2013, 06:27 PM #10
OK, quick answer -
OFDM (Orthogonal Frequency Division Multiplexing) is the modulation method used in 802.11a, 802.11g, 802.11n and 802.11ac. OFDM uses distinct sub carriers within a channel, each subcarrier will transmit a wave that has an amplitude and phase difference when compared to a pilot wave. The difference denotes a bit value, the meaning of which is derived from the specific coding and error rate.
It's an advanced topic,covered in depth in the advanced CWNP classes (CWAP/CWDP).
You don't need to know how OFDM is technically performed, more important is to understand why a specific modulation rate is in use when you would expect a different modulation.
The antenna is the means by which a radio creates an energy wave in the air. Antennas can be internal or external. Antennas are used to shape the energy of the wave. The CWNA class has a whole section dedicated to antenna theory, and it's important to understand the characteristics of the antenna pattern when planning a network.
When it comes to spatial streams, the first number usually denotes transmit radios or antennas, and the second, receive radios or antennas. So, a 2 x 3 would be transmit on 2 simultaneously and receive on 3, for example.
I thought about making videos but decided against it.
For a living I teach WiFi classes and help end users understand wifi planning and deployment. If I were to produce videos, it would really be to help promote my work.
With all due respect to everyone here, I don't think there is any interest in training videos. Most people are happy just to get by without professional level consultancy, which is fine. Education has pretty tight budgets and the decision makers usually wear many hats and have lots of conflicting responsibilities, usually being knowledgeable on WiFi is not a high priority.
If you personally really want to learn the best place to start is by studying, and possible taking a class. I recommend the CWNP Certified Wireless Technology Specialist (CWTS) track, it's a great course for beginners.
I am happy instead to step back and just help out when people would like to understand a technical point a bit better.
Thanks to neilmac from:
mac_shinobi (11th November 2013)
11th November 2013, 06:31 PM #11
What exactly is a spatial stream ?
Is it one connection when dealing with MIMO as far as being able to communicate on different segments / channels but keeping the data linked together so that you can transfer data faster because you are using multiple connections to send / receive the data at the same time ?
11th November 2013, 06:35 PM #12
MIMO was introduced with 802.11n.
A spatial stream is a transmission from a single antenna, 2 spatial streams = 2 transmissions.
So true MIMO means you have to have 2 antennas/radios at the transmitter and receiver.
Many client devices only support 1 spatial stream, and in 2.4 GHZ there are no 40MHz channels, so the BEST data rate available is 72 Mbps, which is a long way from the 300 mbps advertised.
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