802.u distance 100m,802.3z distance upto 1000km

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Jun 19th, 2010
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Hi  every body.


I was studying about the distance limitation in ethernet. My question was what  limits the distance in ethernet technology.  One link says   for example,  in 802.3 ethernet,  host must be able to detect collision with in first 64 byte of ethernet frame  that   in turn determines the 100 m distance.   But how about fast ethernet   where  hosts are connected switch port and  there is no possibility of  collision, but the distance limitation remains the same  100m, the question  is why?


Thanks and enjoy your weekend.

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Giuseppe Larosa Sat, 06/19/2010 - 09:36
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Hello Sarah,


the limitation does not come from keeping the ability to detect collisions, at least not only from that.


You can model the host to switch port as a point to point link, but it uses multiple wires (important point also this).

So it becomes a multi wire waveguide.


Signal attenuation, differential delay on wires play a role too.


Ethernet is baseband and uses pulses. To transmit a bit a Manchester code is used it is not simply an on/off modulation.


The difference between bit 0 and bit 1 is associated to signal transitions during the symbol interval.


Actually 4 bits are sent inside 5 bits sequences using an additional bit to  detect errors. (4B/5B)


So on wire the speed is 125 Mbps not 100 Mbps.


when receiver fails to detect a symbol 4 bits are potentially errored in one single event.


noise and inter-symbol interference put a limit to the distance can be reached without receiving and regenerating the signal.


802.3z is fiber based and made of two unidirectional light waveguides = 2 fibers


802.3z is not baseband but an optical carrier identified by its wavelength is modulated by a Non return to zero modulation

8 bits are sent together into a 10 bit symbol


All this combined with the use of single mode fiber ( no dispersion of power over multiple modes) contributes to have a much greater reach.


you can read this tutorial


http://en.wikipedia.org/wiki/Manchester_code


http://en.wikipedia.org/wiki/4B5B


a more electronic point of view:


http://www.citap.com/documents/tcp-ip/tcpip021.htm


Little disgression:


once we connected printers to the parallel port and the cable used for this connection could not be very long for the same reason: a true serial communication can be faster then parallel communication. This was something that surprised me when I was still studying.


Hope to help

Giuseppe

sarahr202 Sat, 06/19/2010 - 14:13
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Thanks Giuseppe for such a wonderful,detailed answer.  I devote this weekend studying those links you forwarded.

  Enjoy your weekend.

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