From what I gathered, CSMA detects that a medium is idle by checking for the occurence of the interpacket gap (IFS, 96 bits of 0s).

I'm wondering however what happens when you send an Ethernet frame which includes an IFS as its payload over the wire. Intuitively, it should cause another sender listening on the wire to immediately start its transmission, thus causing a collision. Is that expected or am I missing something?

1 Answer 1


In ethernet, a sender must wait until no others are sending. This means that there is no traffic detected.

You are confusing the IPG with with traffic. The IPG is an idle time, and an ethernet frame will not contain idle time. The minimum IPG is a long enough time to send 96 bits on the medium, but it is idle time.

  • How does the sender detect that the medium is idle, then? Mar 20, 2016 at 22:55
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    The medium is idle if the carrier is undisturbed beyond the limits of background noise. A potential sender listens to the medium to see if there is a carrier (CS), and if there is a carrier, if there is no traffic on the carrier.
    – Ron Maupin
    Mar 20, 2016 at 22:58
  • So a signal of zeros would still have a carrier, right (I just realized that the IPG is not equivalent to transmitting zeros, but nonetheless)? Can the end of an Ethernet packet determined in the same way? Mar 20, 2016 at 23:12
  • Related: 0 bits aren't modulated as a constant 0 signal. Right? Mar 20, 2016 at 23:15
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    The carrier is a signal, and you modulate the carrier to send ones and zeros. The ethernet frame ends when no more bits are sent. Your next comment deals with encoding, and that is really a different question, which should be asked in a different question, but the encoding scheme depends on the ethernet version. This link discusses some encoding schemes.
    – Ron Maupin
    Mar 20, 2016 at 23:19

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