INTRODUCTION
Multi-segment Ethernet networks can be constructed by using repeaters and hubs.
A segment is defined as a length of cable consisting of one or more cable sections
and associated connectors with each end terminating in its characteristic impedance.
For example with 10BASE5, the segment represents the complete end-to-end length of
thick coaxial cable even though several medium attachment units (MAUs) are clamped
onto the cable. The maximum length of a 10BASE5 segment is 500 m and this would
represent the network diameter of the Ethernet network if no repeaters were used.
However, Ethernet can be expanded to a larger network diameter by using repeaters
as long as the network diameter does not exceed the collision domain of Ethernet.
We will limit discussions
to 10 Mbps, shared Ethernet. With shared Ethernet, all nodes participate in media
arbitration and must reside within one collision domain. Another characteristic
of shared Ethernet is that communication is half-duplex. Although all
nodes can send and receive, there cannot be any simultaneous sending or
receiving. This would result in collisions and it is this detection of
collisions that is used to arbitrate media access. Repeaters must not
interfere with this arbitration method by favoring one node over another. REPEATER REQUIREMENTS The requirements for
repeaters are stated in IEEE 802.3. The standard uses the term "repeater set" which
consists of a repeater with two or more attached MAUs. These MAUs may also have an
AUI cable connecting the repeater to its attached MAU, but with modern repeaters this
is not usually the case. We will use the terms repeater and repeater set interchangeably.
A repeater is usually viewed as a two-port device, whereas a repeating hub
has more than two ports. Their operation is the same. A valid signal on
one port is retransmitted to all other ports. Regardless whether we are using
DIX V2.0 or IEEE 802.3 frame format, the expansion issues are the same.
Adding a repeater should be transparent to the network by not causing
any disruption of Ethernet's basic operation or impacting media arbitration.
Repeaters are commonly viewed as devices that restore the amplitude of the
signal to correct the effects of cable attenuation. However, Ethernet repeaters
are required to do more. Repeaters must do the following:
As a signal propagates down a cable, it suffers loss of signal strength and symbol
symmetry. Jitter is also introduced due to effects identified as inter-symbol
interference. These effects must not accumulate through the use of repeaters. Repeaters
must restore the integrity of the signals; this includes retiming.
The preamble of an Ethernet frame consists of 64 bits, but due to transceiver startup delays it is possible that not all bits are present. The repeater must count the bits in
the incoming preamble and insert bits if any are missing. This means that the repeater
must have a first-in-first out (FIFO) buffer in order to accomplish this. All regenerated
frames will have the proper 64-bit preamble. Preamble regeneration should not be confused
with packet store and forwarding. According to the standard, repeaters are not allowed to
store and forward. Bridges and routers provide this functionality, not repeaters.
Ethernet relies upon collision sensing as it arbitrates access to the cable. Repeaters
must reinforce the detection of a collision by asserting the same collision signal on all
ports. It does this by sending out a 32-bit jam signal. If the collision was sensed during
the 64-bit preamble, the preamble is still repeated but a 32-bit jam signal is appended so
that all ports see a minimum of 96 bits for proper collision detection by devices
connected to the ports. This is called fragment extension.
Restore the amplitude of the signal
Restore the symmetry of the signal
Retime the signal
Rebuild the preamble
Enforce collisions on all segments
Extend fragments
