Ethernet Repeaters

Repeaters are physical hardware devices that have a primary function to regenerate the electrical signal by:

  • Reshaping the waveform
  • Amplifying the waveform
  • Retiming the signal

Repeaters can be purchased for any physical layer protocol. In this discussion, the function of Ethernet Repeaters will be examined.

Purpose of a Repeater

The purpose of a repeater is to extend the LAN Segment beyond its physical limits as defined by the Physical Layer's Standards (e.g. Ethernet is 500m for 10Base5). A LAN Segment is a logical path such as the logical bus used by all 802.3 Ethernet types. A LAN Segment is given an identification number called a Segment Number or Network Number to differentiate it from other segments.

Typically, repeaters are used to connect 2 physically close buildings together that are too far apart to just extend the segment. Can be used to connect floors of a building together that would surpass the maximum allowable segment length. Note: for large extensions as in the above example, 2 Repeaters are required. For shorter extensions, only 1 Repeater may be required.

Repeater's OSI Operating Layer

Repeaters operate at the OSI Model Physical Layer.

Repeater's Segment to Segment Characteristics

Repeaters do not "de-segment" a network. All traffic that appears on one side of the repeater appears on both sides. Repeaters handle only the electrical and physical characteristics of the signal.

Repeaters work only on the same type of Physical Layer: Ethernet to Ethernet or Token Ring to Token Ring. They can connect 10Base5 to 10BaseT because they both use the same 802.3 MAC layer.

You will run into problems connecting 1Base5 to 10BaseT because the transfer rate is different (1 Mbps vs. 10 Mbps). You would need a store and forward device to do that. A repeater cannot connect Token Ring to Ethernet because the Physical Layer is different for each.

Repeater Addressing: MAC Layer and Network Segment

The MAC Layer Address is used to identify the Network Card to the Network. The Repeater is transparent to both sides of the segment and both sides can "see" all the Mac Addresses regardless on which side they are on. This means that any network traffic on Floor 1 will appear on Floor 5 and vice versa.

Nodes A and B could be furiously exchanging files and this network traffic would also appear on Floor 1. Repeaters provide no isolation between segments, there is only one collision domain.

Because Repeaters provide no isolation between segments and the repeater is transparent to both sides of the segment, both sides of the repeater appear as 1 long segment. The Network Number or Segment Number is the same on both sides of the Repeater.

When using repeaters, make sure that the overall propagation delay does not exceed the Physical Layer Standard being used. Repeaters will add a propagation delay to the signal that is being repeated also. Check that rules such as the 5-4-3 Rule for IEEE 802.3 are not broken or for XNS Ethernet that a maximum of only 2 Repeaters are between any 2 nodes.

You are allowed to parallel Segments using multiport repeaters. Multiport repeaters have several inputs/outputs. Notice that all floors have the same Segment Number. You are not allowed to create a loop between two segments by using two repeaters.

Fibre Optic Repeaters join 2 segments together with a fibre optic link. The Transfer rate is not changed through the fibre. The advantage is noise immunity and longer distances. Segments can be joined up to 3000m apart and still be within the propagation delay specification for the Physical Layer. Two fibre optic repeaters are required: one at each end of the fibre.

Fibre Optic Repeater

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Copyright July 2013 Eugene Blanchard