Forum

Hello all,

What is the maximum aggregate capacity for an 802.11 DSSS system in a co-located environment?
Please explain how you get the answer.

According to me, since the max data rate using 802.11 is 2 Mbps and there can be 3 non-overlapping channels, the answer should be 6 Mbps, but it seems this is wrong.

Hi BlueIce:

Sounds ok as far as it goes. You probably want to add that IEEE 802.11 throughput if seldom more than half to the data rate regardless of the PHY and modulation. So in this case you end up with about 3 Mbps aggregate throughput.

I hope this helps. Thanks. /criss

OK. After confirmation, it seems that the answer is 6 Mbps. Sorry about that.

Now, another similar question:
Using IEEE compliant 802.11b wireless LAN systems, what is the maximum data transmission rate that can be achieved in any given area without network contention?

A. 11 Mbps
B. 22 Mbps
C. 33 Mbps
D. 54 Mbps

Hi BlueIce:

If all the equipment uses HR/DSSS PHYs then the maximum data rate per channel is 11 Mbps. Assuming three non-interfering channels in the FCC's 2.4 GHz ISM band the maximum aggregate data rate is 33 Mbps.

One might argue that ERP PHYs comply with the 802.11b requirements for HR/DSSS and should be considered applicable to your question, but 162 Mbps is not offered amongst the answer choices.

Am I missing what you are getting at?

I hope this helps. Thanks. /criss

That was my answer as well. This question comes from some sample tests I found online and I'm beginning to doubt the accuracy of the answers given in these tests.

The given answer for this question is 11 Mbps.

Criss, I just noticed that the question does not mention a co-location environment. Does "without network contention" mean "not co-located"?
Maybe the given answer is correct...

Hi BlueIce:

I think the answer is correct for the way the author intended the question to be understood.

The question boils down to: "What is the maximum HR/DSSS data transmission rate?" All the rest of the question is distractors. All the offered answers are valid ERP data rates.

I hope this helps. Thanks. /criss

By the way, I've seen "ERP-OFDM" mentioned in the study guide. What does ERP stand for?

PS: I've sent you a private message. Did you receive it?

Thanks for your help.

Hi BlueIce:

The IEEE 802.11g amendment primarily added a clause 19 to the IEEE 802.11 standard. Clause 19 defines a distinct physical layer or PHY. It is named Enhanced Rate Physical (ERP) and is backwardly compatible with HR/DSSS and DSSS found in clauses 18 and 15 respectively.

The ERP PHY has five groups of modulations. They are ERP-DSSS, ERP-CCK, ERP-OFDM, ERP-PBCC, and DSSS-OFDM. Each modulation within a group corresponds to a certain combination of preamble, header, payload, and data rate.

I prefer answering posts rather than email. The IEEE standards say precious little about Inter Symbol Interference (ISI) and delay spread. The IEEE 802.11 Handbook has several pages. You should have a copy of that book and of my review of it posted on this forum.

I hope this helps. Thanks. /criss

OK. Here is the question I sent to you via PM:
Which of these will be the most susceptible to multipath interference?
A. 1 Mbps DSSS
B. 5 Mbps FHSS
C. 11 Mbps DSSS
D. 1 Mbps FHSS

Thanks for your help.

Hi BlueIce:

The modulation that represents data with the physically smallest RF symbols will be the most challenged by multipath induced ISI. This equates roughly to the higher data rate modulations. However the technology to distinguish ever smaller variations in RF symbols is constanly improving. Thus an older technology with a lower data rate may have more difficulty with multipath than a newer technology with a higher data rate.

But IEEE 802.11 FHSS only comes in 1 and 2 Mbps modulations. That leaves answer C as the clear winner.

I hope this helps. Thanks. /criss