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I understand that one of the reasons is that ue to the effects of attenuation the collision detection mechanism is not effective beyond 2500 metres (1.5 miles). Segments cannot sense signals beyond that distance. They might, therefore, not be aware that a computer at the far end of a large network is transmitting. Should more than one computer transmit data onto the network at the same time a data collision will take place that will corrupt the data. What am I missing? Could you please explain the concept with clarity, may be with an example?

Thanks!

Rowana Ravenclaw
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  • What specific wireless networking standard are you talking about? You ask mostly as if this is a generic question, but then give a specific distance limitation, which implies a specific standard. Also, different wireless networking systems use different schemes to work out problems like this, so a single answer unbounded by a specific technology standard is likely to be wrong for other standards. – Warren Young Nov 07 '16 at 00:44

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I think you are confusing two seperate issues here.

For twisted pair and coax Ethernet the length of an individual segment is limited by signal integrity. This limitation has nothing to do with CSMA/CD.

The total size of a collision domain (and the individual segment length for half-duplex fiber systems) is limited by timing issues.

For correct CSMA/CD it is important that a collision is "seen" either everywhere or nowhere. If the receiver sees it as a collision but the sender doesn't then you get lost frames. If the sender sees it as a collision but the receiver receives it successfully you get duplicate frames.

To ensure that everyone sees a collision the minium packet length must be more than twice the propagation delay from one end of the collision domain to the other.

Finally note that with full duplex Ethernet collisions are simply not possible, so CSMA/CD is not needed or used. This allows full duplex fiber Ethernet links to operate over very long distances.

Thanks.. Could you please elaborate on this statement - 'To ensure that everyone sees a collision the minium packet length must be more than twice the propagation delay from one end of the collision domain to the other'?

Consider two hosts, A and B at opposite ends of the network.

A starts to send a packet. The data starts moving through the network towards B.

Just before the first data from A arrives B also starts transmitting. B quickly detects a collision.

Bs transmission starts moving (probablly but not nessacerally in a garbelled in in a garabled form) through the network towards A.

If A is still transmitting when the transmission from B arrives then everyone sees the collision. However if A has stopped transmitting then as far as it is concerned it sent it's data successfully. To avoid this situation the transmission time for the smalled allowed packet must be more than twice the delay from one if the collision domain to the other (including delays inside equipment).

How about explaining what exactly is meant by a length being more than a delay.

Technically I should probablly have said "packet transmission time". Of course for a fixed transmission rate transmission time and packet length (including any header/trailers) have a direct relationship.

plugwash
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  • Thanks.. Could you please elaborate on this statement - 'To ensure that everyone sees a collision the minium packet length must be more than twice the propagation delay from one end of the collision domain to the other'? – Rowana Ravenclaw Nov 06 '16 at 02:15
  • This looks like a well-written answer. Rowana: I don't see the notable ambiguity. I suggest trying harder to be more specific about your question. What needs clarification? – TOOGAM Nov 06 '16 at 04:12
  • @TOOGAM well, he writes " minium packet length must be more than twice the propagation delay" How about explaining what exactly is meant by a length being more than a delay. And he doesn't or doesn't explicitly or clearly, define propagation delay in his answer – barlop Nov 06 '16 at 23:49
  • been a while but this may explain something about propagation delay http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html – barlop Nov 06 '16 at 23:53
  • @RowanaRavenclaw i've just watched this video "Why there is a minimum frame size in CSMA/CD Protocol : Simple explanation" by How To https://www.youtube.com/watch?v=Zftv3U5_fwI And it says that a station should monitor for a collision, and it only tries to while it is transmitting. So, it a packet is too small then it won't be monitoring for long enough. If I tried saying more then i'd probably be wrong(if i'm not already, as it has been a while). – barlop Nov 07 '16 at 00:04
  • @RowanaRavenclaw From that video I got somewhat of an impression (most likely wrong), that perhaps it seems like a big snake like thing that is still being sent even while it reaches destination and even if the destination sends a jam signal the jam signal has to reach it while it is still being sent. So maybe that's where the idea of twice the propagation delay comes in.. but I don't know, it was ages ago since I looked into it. – barlop Nov 07 '16 at 00:05
  • The OP is talking about wireless networking, not wired. CSMA/CD doesn't apply. You want to be talking about [CSMA/CA](https://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidance) and [802.11 RTS/CTS](https://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS) instead. – Warren Young Nov 07 '16 at 00:41
  • hrmm the OP used the wireless networking tag but there was no mention of wireless in the post and the post explicitly asked about CSMA/CD – plugwash Nov 07 '16 at 01:09