[3 pages — 2155 words]

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:

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

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.

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