WLAN question

[Glaanieboy]

I am currently doing a self-study of Bridge course chapter 3 from Cisco. In this chapter, they explain how WLANs work. I have a question about this part:

Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.

When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.

Does this mean that a 11mbits network is actually 5.5 mbits? And a 54 mbits actually 27 mbits? Thanks.

No, I can't ask my teacher, it's 9 pm here and I am taking the test tonight.

 

[blinden]

hope it's not too late, but, as I understand, 'overhead' to which they are refering to eating most of the bandwidth includes things such as acknolwedgements and headers of the packets and such that consume bandwidth without producing actual data.

 

[j79zlr]

I know nothing of the actual physical workings, but I believe that is what half duplex mode is for. Most cable modems should be set to half duplex to avoid packet collisions since half duplex only allows one-way transfers, but I think thats their point, wireless cannot easily detect this? I more confused, I need to sleep.

 

[Glaanieboy]

I passed the exam with a score of 94%, so I think I understand it. There was a question in the exam that went like this:

Your chief wants WLAN in the building for replacement of the current wired LAN. You know something about the actual bandwidth in a WLAN. What do you tell your chief?
a) In a WLAN about 20% of the actual bandwidth is available
b) In a WLAN about 70% of the actual bandwidth is available
c) In a WLAN about 50% of the actual bandwidth is available
d) In a WLAN you can use the full 100% of the bandwidth available

c) was the right answer

 

[blinden]

good going on the test, i hate when they make you look too far into a question and the answer is way easy to remember...

i'm going to start craming for some certs after christmas as i'm graduating in july and need to secure a job.