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I have a question about store and forward in the following setup-up. 

                       Store and Forward Mechanism

PC1 <-> Switch (in port) <-------------------------------------> Switch (out port) <-> PC2

       |                        switch                            |

The link speed between PC1 and Switch (in port) is 1Gbit/s.

The link speed between Switch (out port) and PC2 is 1Gbit/s.

I wonder what would be the delay to send a packet of size S from PC1 to PC2? Is it going to be (2 x S / 1Gbit) or (3 x S / 1Gbit) because of the store and forward? Does (in principle) the store and forward mechanism "operate" at least at 1Gbit/s frequency?

Thanks,

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  • It depends on the switch model. Many switches will do wire-speed switching. Most of the switching today is done in hardware (very fast ASICs). You will need to be specific on the switch model.
    – Ron Maupin ♦
    Commented Mar 31, 2016 at 14:59
  • Assuming wire speed it sounds like a total delay of 3 x S/1Gb ... Correct?!
    – user3523954
    Commented Mar 31, 2016 at 19:25
  • Nominally, wire speed means no delay, or the same delay as experienced by a wire of the same length. Since the switch doesn't use ethernet internally, it may be transferred faster from one switch port to another switch port than it could be on a point-to-point ethernet cable, with a net result of no apparent delay.
    – Ron Maupin ♦
    Commented Mar 31, 2016 at 19:29
  • Aha ... So 2 x S/1Gbit + epsilon?! And epsilon depends on S as well?!
    – user3523954
    Commented Mar 31, 2016 at 19:35
  • Wirespeed means the device can switch/route packets at the same rate that you can send over the medium. A device will naturally add a certain delay. It would be ludicrous to believe you could have the same delay as in a cable of the same length. Even with cut-through switching you need to wait for (preamble n stuff) + 6 bytes so you get the dst mac and then you need to do a lookup before you can send. While some would consider this negligible, it is measurably more than what a cable of similar length would introduce, on the order of single or tens of microseconds.
    – kll
    Commented Apr 2, 2016 at 10:53

1 Answer 1

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Internal switching delay will be down in the nanoseconds. Thus you're only looking at the time to receive the frame plus the time to transmit the frame. I.e. a hair over 2S when there's zero contention.

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  • By contention you mean that on the "out port side" there are no available buffers? OR, the switch is busy doing store and forward for the other pairs of in/out ports as well?
    – user3523954
    Commented Apr 1, 2016 at 7:41
  • Buffer space, cost-port matrix availability, pause frames, cpu load, etc.
    – Ricky
    Commented Apr 1, 2016 at 17:49

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