Ethernet hubs and repeaters operate at the Physical Layer of the OSI
Reference model and are defined by IEEE 802.3c/d. They are used to connect
together one or more Ethernet cable segments of any media type. If an Ethernet
segment were allowed to exceed the maximum length or the maximum number of
attached systems to the segment, the signal quality would deteriorate. (If
unchecked this would ultimately lead to errors in the data.)
Hubs and repeaters may be used between a pair of segments (see below) to
provide signal amplification and regeneration to restore a good signal
level before sending it from one cable segment to another. By allowing two or
more LAN segments to be connected, they allow the network to span a larger
distance. They also provide electrical isolation from failures in the cable
or attached systems, protecting equipment on other LAN segments from the
effect of the fault.
A very important fact about hubs and repeaters is that they allow users to
share an Ethernet LAN. A network of repeaters and hubs is therefore called a
"Shared Ethernet" or a "Collision Domain". The various systems
sharing the Ethernet all compete for access using the CSMA/CD access protocol.
This means that only one system is allowed to proceed with a transmission of a
frame within a Collision Domain at any one time. Each system has to share a
proportion of the available network bandwidth.
Regenerating Preambles and Repeating Runt Frames
The DPLL in the transceiver can not always lock on to the start of a frame's
preamble. Usually this takes a number of bits to completely synchonise with the
Manchester Encoded data. An important job of a repeater is therefore to
reconstruct the preamble. For example, consider a frame sent by X (above). This
frame will be received by the computer Y and the repeater. In each case, the
first part of the preamble will be lost, since the transceiver will not have
acquired lock on to the Manchester Encoded clock signal. If the shortened
preamble were to be sent on to computer Z, it may be insufficient to allow Z to
acquire a lock before the start of the frame. The repeater must therefore
reconstruct the full preamble before the frame is sent out of the repeater.
An Ethernet LAN may transport parts of a frame (e.g. as a result of a
collision), any under-sized frame is known as a Runt. A repeater must extend any
very short bit sequences (less than 96 bits) to ensure that they are seen by all
computers on the LAN, this ensures proper operation.
Note that since a repeater is responsible for changing the bit timing, and
may introduce additional bits, as above, it can not guarantee that the
Inter-Frame Gap (IFG) generated by the sender is preserved in all cases. It
should however not reduce this below 47 bit times.
Collisions and Repeaters
If a repeater sees a collision on a cable segment, the repeater detects this
(in the
normal way), and then generates a JAM signal to all connected
output ports. This ensures that every computer connected to the LAN is aware of
the collision, and does not try to transmit during the collision period.
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