5-4-3 rule

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The 5-4-3 rule or repeater rule is a term from network technology . It states that in an Ethernet network (10 Mbit / s) with shared access in which several electrical / optical segments ( 10BASE2 , 10BASE5 , 10BASE-T , 10BASE-F ) are connected in a tree topology , the path between any two Stations may run through a maximum of five segments with four repeaters . At least three of these segments must be "link segments" with physical point-to-point connections (FOIRL, 10BASE-FL or 10BASE-T), a maximum of two may be "mixed segments" with a physical bus connection (10BASE5, 10BASE2 or 10BASE-FP). Because two mixed segments can be used and the rule affects a collision domain, it is also called the 5-4-3-2-1 rule .

Segments without active devices are called link segments or inter-repeater lines . Their only task is to connect two repeaters to each other in order to bridge larger ranges. The left segments must use full duplex media (e.g. 10BASE-FL or 10BASE-T) so that collisions can be detected immediately. (Full duplex media use a dedicated transmission channel for each direction - the Ethernet transmission itself, on the other hand, always takes place half duplex with repeaters.)

The 5-4-3 rule is a rule of thumb that should guarantee stable operation even under unfavorable conditions. If the conditions are exactly known, this rule can be deviated from under certain circumstances.


In a classic Ethernet, collisions can occur due to half-duplex operation . These should be avoided as far as possible, but must be reliably detected in any case ( CSMA / CD ).

The transit time of a signal, and thus the maximum permitted size of a collision domain , can be determined by adding up the transit times per segment and the delay of the coupling elements. It must be below the maximum permitted round trip delay . Since this calculation can be relatively complex, a simpler repeater method was developed with the router rule, with which the runtime restriction can be adhered to.

If the maximum size of the collision domain and thus the maximum duration of a round trip delay is exceeded, late collisions will occur which can no longer be absorbed by CSMA / CD, and packet loss will result.

It should be noted that the 5-4-3 rule only applies to the scope of a single collision domain. If the collision domain is divided by using a bridge / multibridge ( switch ), the counting starts again.

Repeaters, hubs and switches

Network scheme when using a repeater: connection of two segments via a repeater

If two hubs are connected to one another via an uplink with 10BASE-T , they count as two half repeaters or as one repeater. A new segment is created when another hub is connected to a port (of the repeater) for end devices ( no uplink).

If a bridge or switch is used instead of a hub , the 5-4-3 rule does not apply. The scope of the rule also ends at a switch port. A switch connects several different collision domains. It also enables full duplex operation in which collisions can no longer occur.

Fast Ethernet

With Fast Ethernet (100 Mbit / s), only two Class II repeaters can be cascaded using the maximum segment length (max. 92 bit latency / delay each ). No further repeater may be connected to a Class I repeater (max. 140 bit latency).

Gigabit Ethernet

The operation of a single repeater per segment was originally defined for Gigabit Ethernet . However, such devices were no longer built and the IEEE 802.3 specifications have not been updated since 2011 .


  • Christian Baun: Compact computer networks. 3rd edition, Springer Verlag, Berlin / Heidelberg 2015, ISBN 978-3-662-46931-6 .
  • Gerd Küveler , Dietrich Schwoch: Computer science for engineers and natural scientists . Volume 2: PC and microcomputer technology - computer networks . 5th edition. Springer-Verlag, 2007, ISBN 978-3-8348-9191-4 .
  • Bruce Hartpence: Network Basics Practical Course . 1st edition. O'Reilly Verlag, Cologne 2011, ISBN 978-3-86899-151-2 .

Web links

Individual evidence

  1. IEEE 802.3 13.3 Transmission System Model 1
  2. 5-4-3-2-1 Repeater rule . Retrieved July 30, 2017.
  3. IEEE 802.3 1.4.255 link segment
  4. a b IEEE 802.3 13. System considerations for multisegment 10 Mb / s baseband networks
  5. IEEE 802.3 13.4 Transmission System Model 2
  6. IEEE 802.3 13.5 Full duplex topology limitations
  7. IEEE 802.3 29. System considerations for multisegment 100BASE-T networks
  8. IEEE 802.3 41. Repeater for 1000 Mb / s baseband networks