properties of contention-based media access

johnifanx98

Which options are properties of contention-based media access for a shared media? (Choose three.)

a) non-deterministic
b) less overhead
c) one station transmits at a time
d) collisions exist
e) devices must wait their turn
f) token passing

the correct answer is a,b,d.

I don't get why c) or e) is incorrect?

and why b) is correct?

mapletune

I've never come across this term "contention based", only studied CSMA/CA CSMA/CD. personally, don't like this question too much =p

not sure about the answers even, sorry can't help you =(

MrXpert

I would say that the answers given in the question are wrong

johnifanx98

MrXpert wrote: »

I would say that the answers given in the question are wrong

maybe. My answer is c) d) e).

Reixy

Contention based and shared media is legacy Ethernet, when the use of hubs was the best way in those environments first there were a lot of collisions so d is correct. And also Ethernet is a non deterministic technology, that means a) is correct. If I have to choose the other one I'd find it throug discard process the c) and e) can't be cause with hubs collisions occur when two devices transmits at the same time that's why the use of CSMA/CD, and obviously token passing is not a contention based technology, so it's just let me b) as the third one. So I think a, b and d are the correct ones.
Hope it helps!!

MrXpert

Contention based will refer to anything that shares the same electrical bus or the same airspace. Both CSMA/CD and CSMA/CA for wireless will apply to contention based. remember hubs create one big collision domain and as a result you have a half duplex environment which results in only one node being able to transmit at a time. Each node has to listen to the wire before sending a frame. Same applies to wireless medium.

The best answers in my opinion would be c,d,e.

TehToG

Contention Media Access is used for cable connections is it not? You'll have a 50mb pipe to the ISP for 50 customers say, and each customer could get 10mb max, but not all at the same time.

Your first clue is Media Access (Meaning layer 2 or below ). At those layers you either operate using CSMA/CD or CSMA/CA. You either detect collisions and avoid them but they still exist. Hence answer D

Secondly you're sharing a bus to save on putting a bigger (and more expensive) bus in. Hence answer B

Finally you have multiple users using a shared bus. Someone at number 1 can be on facebook whereas someone at number 10 can be on xbox live. Therefore you can't determine the need for the bus. Hence answer A




With some shared-bus schemas you may have to operate in half duplex but not all Hence why C and E is wrong
And finally only Token-ring uses token passing Hence why F is wrong


When given these multiple choice questions you usually get 3 answers that are ALWAYS true and the rest atleast one thats true SOME of the time. It's a fair question and the answers are right.

MrXpert

Contention based also refers to wireless.

c) one station transmits at a time. This is what we have with hubs which relies on half duplex. i.e one device can send or receive at a time.In 802.11, one device can send at a time but we do not notice the delays as its in milliseconds.
d) collisions exist. Yes collisons exist in contention based environments. We know from our CCENT studies that hubs create one big collision domain whereas switches rely on microsegmentation which means CSMA/CD is not even a consideration.
e) devices must wait their turn. In wireless, a device must have a RTS/RTC. In CSMA/CD each device listens to the wire, specifically electrical signals. If the wire is free they sent. If the wire is busy they don't sent. if they send at same time, a collision occurs, jam signal and random backoff timers are executed.


a. non deterministic=meaningless red herring answer imo
B. less over head. Given that there is a lot more listening and waiting involved with contention based there will be more overhead. if contention based=less overhead, then why do we prefer using switches compared to hubs? Imagine what happens when two or more PCs send frames at same time? frames are dropped, listening starts all over again.That is the problem with contention based because they are all fighting for the same bandwidth or shared electrical bus.
f. token passing. Doesn't use CSMA/CD or CA. Only one node can send at a time because it is dependent on who has the token. There is no contention no fighting for a shared bandwidth.

TehToG

Yea but you miss the point. It asks vaguely about media contention based. I.e. shared bus/airspace. Just because a subset of contention based connections (wireless in this case) has different rules does not mean you can apply them to the superset.

I have a broadband connection with 40mb down and 15mb up. I can send and receive at the same time over a simple ATM circuit, I share this link with my neighbours to my ISP. We share a fibre which operates in full duplex. Not ALL contention based media is restricted to half duplex. Devices only wait their turn in half duplex.

How is non deterministic a red herring. If I have 10 people with 5mb max on a 48mb connection I'll never know how the media will be used. If I have a dedicated MPLS link of 2MB for off-site backups I can determine the time and load of the line.

Overhead = Cost. Nothing else. Not processing time, not quality of service, Cost.


edit: Any time someone says something like 'processing overhead' they're really using overhead as a synonym for cost. They have to specify.

Reixy

Answers c and e sound like controlled method and not like contention based.
In the controlled method used by the MAC sublayer Network devices take turns, in sequence, to access the medium, so they wait for turns. And Also Ithat contention based methods have less overhead.

MrXpert

I think we'll agree to disagree.

At the CCENT/CCNA level you'll only be dealing with hubs, switches and basic wireless in terms of understanding contention based media. Extrapolating that to include ATM, MPLS, fiber is beyond the scope of the exam.

TehToG

I'm using those examples as they create the strongest case but they aren't needed to make the case. The question asks "Which options are properties of contention-based media access for a shared media?" but it's really looking to know is that you know differentiates Connection-based media access from the alternatives.