As far as I know:

(1) Latency (τ) = transmission delay + propagation delay + queuing delay + processing delay

(2) Round-trip time (RTT) is two times the latency (assuming the same latency both ways)

However, while I was reading about slot time, I found the standard 802.3 says:

The slot time shall be larger than the sum of the Physical Layer round-trip propagation time and the Media Access Layer maximum jam.

Is this round-trip propagation time the same as RTT? If so, why add "propagation"? RTT includes other times beside propagation, that would be misleading.

1 Answer 1


Context is everything.

Your definition of latency is a generic definition.

RTT is usually defined in the context of IP networking. Since a remote host is assumed not to be under your control, all the components of the delay are lumped together.

802.3 is a LAN standard, where all the devices are under your control. In this context, they are talking about the characteristics of the particular media.

In essence, different people may use different terminology to describe the same thing.

You will find that a lot of networking terms are not precisely defined as you might expect.

  • If "in this context they are talking about the characteristics of the particular media", then they are not talking about RTT. Which would be fine by me, since then RTT != rount-trip propagation time. Could it be this means round-trip propagation time = 2*propagation time? Nov 27, 2017 at 18:16
  • Or in this case, different people may describe different things using the same terminology. ;-)
    – Zac67
    Nov 27, 2017 at 18:16
  • 1
    @ClaraSánchez RTT in the context of IP (layer 3) has an abstract meaning and usually can't be calculated, only measured (on the Internet). RTT in the context of CSMA/CD (layer 1) is twice the propagation delay from one end of the collision domain to the other.
    – Zac67
    Nov 27, 2017 at 18:19
  • Btw, would latency be τ = propagation time in the context of CSMA/CD? Nov 27, 2017 at 20:09

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