Round trip delay

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In computer networks, the Round Trip Delay ( RTD ) describes the time it takes for a signal to travel from sender to receiver and back again within a collision domain . It is measured in bit times .

In the case of network access methods with a common medium and the CSMA / CD method , it is necessary to detect collisions if two stations attempt to transmit via the medium at the same time. The round trip delay is set to a certain number of bit times.


For Ethernet with 10 and 100 Mbit these are:

  • 512 bit times (due to the minimum possible data frame size of 64 bytes).
  • In addition, there are 64 bit blocking times that are used for synchronization with Ethernet (7 byte preamble and 1 byte start frame delimiter).

Two participants in a collision domain are only allowed to be separated from each other by a maximum that a signal transmission between them takes less or a maximum of 576 bit times.

Influence on maximum segment length

The reason for this lies in the CSMA / CD process: a collision is only recognized as such if the sending participant is informed by means of the jam signal that a collision is taking place during the sending process. If the most distant participant in the network detects a collision when receiving the first bit, it sends back the jam signal, which must reach the original sender before it has completed sending the packet.

The delay time (also called latency time) that a repeater needs to amplify and forward a signal is called the basic delay . Based on this and the duration of a bit time in µs, the industry standard IEEE 802.3 calculates and defines the maximum length of the cable connections within a collision domain for different Ethernet transmission methods.

Since the calculation of the maximum extent of a collision domain is quite cumbersome in individual cases, an easy-to-remember rule of thumb for 10 Mbit Ethernet networks was created with the 5-4-3 rule .