Chapter 34. Repeaters
Repeaters are physical hardware devices: they have a primary function to regenerate the electrical signal (shown below):
Reshaping the waveform
Amplifying the waveform
Retiming the signal
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 two physically close buildings together (when they are too far apart to just extend the segment). They can be used to connect floors of a building that would normally surpass the maximum allowable segment length. Note: for large extensions, as in the above example, two Repeaters are required. For shorter extensions, only one 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 can run into problems with the transfer rate (1 Mbps vs. 10 Mbps) when you connect 1Base5 to 10BaseT. A repeater cannot connect Token Ring to Ethernet because the Physical Layer is different for each network topology.
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 of which side they are on). This means that any network traffic on Floor 1 will also appear on Floor 5, and vice versa.
Nodes A & B could be furiously be exchanging files; this network traffic would also appear on Floor 1. Repeaters don't provide 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 that is being used. Repeaters will also add a propagation delay to the signal that is being repeated. Check that rules, such as the 5-4-3 Rule for IEEE 802.3, are not broken. For instance, check--for XNS Ethernet--that a maximum of only 2 Repeaters are located between any 2 nodes.
You are allowed to parallel Segments using multi port repeaters. Multi port 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 (e.g. by using two repeaters).
Fiber Optic Repeaters join 2 segments together with a fiber optic link. The Transfer rate is not changed through the fiber. The advantages are 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 fiber optic repeaters are required (one at each end of the fiber).
Fiber Optic Repeater
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