DSSS? CCK? DQPSK? Help!

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Hi everybody.

Making some N+ notes and I'm in the wireless bit right now. I'm getting confused about the DSSS, CCK & DQPSK modulation techniques as used by 802.11b. Is it that CCK is a part of DQPSK, which itself is part of DSSS? I don't get it! And do I even need to?

Would appreciate a little guidance. Thanks.

malik

 

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Dunno, but I passed Network+. :hhhmmm

 

To answer your last question first ( ) IMHO you don't need to know the nitty-gritty details here.

DSSS is the name of the technique of spreading the signal where the transmission is spread over a frequency range. This is opposed to FHSS (Frequency hopping SS) where there is a single carrier that hops about the frequencies.

The next part is how the bits are encoded for transmission. This is where is gets *very* complicated with fearsome maths!

DQPSK is one method of doing this, used for the slower links, such as the beacon (I believe). CCK is a different system, not related, but works well at the higher speeds.

Harry.

 

Agreed

There's no way you would be expected to know anything about this other than the ABSOLUTE basics at the N+ level. Don't waste your time focusing on stuff like this - you are not expected to know it and worrying about things like this is like not being able to see the wood for the trees - you will end up filling your head with stuff that is irrelevant for your exam.

 

Thanks much for all your replies gentlemen. You've been very helpful. I'll just worry about the difference between FHSS & DSSS for now and put CCK & DQPSK to the side.

 

malik06,

First, understand that 802.11 is the IEEE standard for wireless data networking using radio waves. The IEEE 802.11 standard has three current types:

1. 802.11a
2. 802.11b
3. 802.11g

Since the 802.11 standard uses radio waves, the physics is similar to sound waves. Assume an ad-hoc wireless "network" consisting of you and a friend. Imagine that the friend is close to you like in the same room. Since the friend is close, you can use a normal soft conversational volume and speak in full sentences as well as get a response quickly. Networkingly, that's relatively high bandwidth communications because it's occuring quickly.

Now, imagine you and your friend are in different rooms in a house. Since you don't know where your friend is in the house, you talk to your friend in a higher volume like asking "WHERE ARE YOU?" The process of speaking at the relatively higher volume has the cost of you having to stop and take a breath. The time to stop and take a breath reduces your effective bandwidth of communication.

Now, imagine you and your friend are on the opposite sides of a rugby pitch. Now both of you definitely have to increase your volume that much higher than the house situation because that's quite a far distance to be communicating from. You need to take more time to stop to take a breath because yelling across a rugby pitch needs more power than yelling from room to room in a house. That effectively further reduces your bandwidth of communication.

So, we've learned that relatively speaking the further away the two communicators are, the lower the bandwidth.

We will focus on the 802.11b type of Wireless LAN. Now, since we know that bandwidth is dependent on range, that is why the 802.11b standard has the three bandwidths of 5.5Mbps/11Mbps, 2Mbps, and 1 Mbps. Guess what? Here's the relationship...

1. IEEE 802.11b = Direct Sequence Spread Spectrum (DSSS)
   1. 5.5Mbps/11Mbps = Complimentary Code Keying (CCK)
   2. 2Mbps = Differential Quadrature Phase Shift Keying (DQPSK)
   3. 1Mbps = Differential Binary Phase Shift Keying (DBPSK)

So CCK is similar to the way that you can maintain a continous conversation in the same room, DQPSK is similar to the way that you have to raise your voice with delays in the house, and DBPSK is similar to the way you have to SHOUT with longer delays on the rugby pitch. So the methods of Keying are independent of each other because they occur at different bandwidths. However, all three keying methods are part of DSSS which is used by 802.11b.

My source even mentions "A description of each modulation is well beyond the level of this discussion."

I hope this answers your question.

Source:

1. CCDA® Exam Certification Guide (CCDA Self-Study, 640-861), 2nd Edition , pp. 86~87 - http://www.ciscopress.com/title/1587200767

 

r.h.lee,

Absolutely brilliant answer mate! The lightbulb has gone on. Thanks!

 

I passed on my first try.

And... I may be in my very first support role very soon. I've done the 1st interview with my prospective supervisor, and then the technical interview with him and HR, and it looks very good. They know I have no hands on experience, but it looks like I may very well get the job anyway. BOOYAH!

Hmm. Gotta come up with a fancy N+ cert sig for down below here now....

 

Congrats mate! Well done

As you've just passed it, and im just starting it, can i ask what materials you used?

Thanks mate

 

Congrats on the pass!

 

They are different ways of combining waveforms to transmit a signal, you don't need to know anything other than the acronyms and the standards they relate to for the exam.

Frequency shift keying is probably the most basic commonly used method, but its not used for WiFi.

http://en.wikipedia.org/wiki/Frequency-shift_keying

I looked into radio modems and amateur radio for home use in the early 1990's, unfortunately they had many problems then, low transmission distance, low bandwidth, unsecure etc.

Its the sort of stuff electronics or physics students generally study along with the Fourier transform.

and yes makes me realise my maths is bit crap too !

 

Congratulation on the pass the the upcoming work, mate.

 

Congrats!

 

Congratulations

 

Congrats

 

Thanks very much! And thanks to everyone for their congratulations. I appreciate it.

I should mention again that my grade was not brilliant. You may want to take that into consideration before reading further.

The main textbook I worked from was the Sybex book, "Network+ Study Guide, Deluxe Edition 2nd Ed". The ISBN is 0782144160. (It was the designated textbook for the live course I took.) The book is pretty heavily recommended on this site, but I have to say that I find it a bit of a disappointment. It has a fair amount of errors in it, some outdated info, and a couple of the chapters are a bit of a hodge-podge.

I also used the Syngress published "Network+, Exam N10-003, Study Guide & Practice Exams". ISBN is 1931836426. I worked with it early on to get a jump on the live course. I wish this book had been my course text. It's more precise and detailed than Sybex, IMHO.

I bought the TrainSignal videos and watched those a few times. They were informative (and surprisingly funny in places!).

The last 36 hours or so before the exam I read over the CramSession Network+ Guide.

I also spent a lot of time on Wikipedia. That's a brilliant (and free) resource.

The Number One thing I can recommend to you is to not rely completely on one source for your studies because every resource I used contains errors (except for Wikipedia, I think). When one source contradicts another, you research it online and figure it out!

 

Congrats on the pass!

 

On the contrary, Wikipedia is absolutely filled with errors. It has a lot of information because users can contribute directly to it... but it also has a lot of errors for the same reason. It might have had few errors in the sections you were looking in... but for most subjects, Wikipedia is not a citable, reliable reference source.

For the record, I do use and like Wikipedia... I just can't cite it as a reliable reference.

 

I think most people realise the pros and cons of a Wiki, ok incorrect information can easilly be posted but it can also be corrected very quickly. In my experience most of the stuff I've read has been fairly accurate.

Certification based books are often rushed to market and can therefore contain many errors, as such I wouldn't really single out wikipedia for criticism. The cert books often don't benefit from the peer review other established textbooks get because their lifespan is also often limited. Does anyone actually spend much time reading the Errata on the publishers website ? Personally I just disregard the stuff that seems wrong.

Peoples brains are generally pretty well evolved for processing information, people value information not just based on its source but how well it fits into their mental map. Neural Networks work on the same principle of giving statistical weights to pieces of information.