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Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

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12.2.5 10Broad36<br />

12.2.6 1Base5<br />

Ethernet <strong>and</strong> TCP/IP networks 323<br />

as specified in the 802.3 st<strong>and</strong>ard was limited to a 1 km fiber link between<br />

two repeaters, with a maximum length of 2.5 km if there are 5 segments in the<br />

link. Note that this is a link between two repeaters in a network, <strong>and</strong> cannot<br />

have any nodes connected to it<br />

• 10BaseFP<br />

A star topology network based on the use of a passive fiber optic star coupler.<br />

Up to 33 ports are available per star, <strong>and</strong> each segment has a maximum length<br />

of 500 m. The passive hub is completely immune to external noise <strong>and</strong> is an<br />

excellent choice for noisy industrial environments<br />

• 10BaseFB<br />

A fiber backbone link segment in which data is transmitted synchronously.<br />

It is designed only for connecting repeaters, <strong>and</strong> for repeaters to use this<br />

st<strong>and</strong>ard, they must include a built in transceiver. This reduces the time taken<br />

to transfer a frame across the repeater hub. The maximum link length is 2 km,<br />

although up to 15 repeaters can be cascaded, giving great flexibility in<br />

network design<br />

This architecture, whilst included in the 802.3 st<strong>and</strong>ard, is no longer installed as a new<br />

system. This is a broadb<strong>and</strong> version of Ethernet, <strong>and</strong> uses a 75 ohm coaxial cable for<br />

transmission. Each transceiver transmits on one frequency <strong>and</strong> receives on a separate one.<br />

The Tx/Rx streams require a 14 MHz b<strong>and</strong>width <strong>and</strong> an additional 4 MHz is required for<br />

collision detection <strong>and</strong> reporting. The total b<strong>and</strong>width requirement is thus 36 MHz. The<br />

cable is limited to 1800 meters because each signal must traverse the cable twice, so the<br />

worst case distance is 3600 m. It is this figure that gives the system its nomenclature.<br />

This architecture, whilst included in the 802.3 st<strong>and</strong>ard, is no longer installed as a<br />

new system. It is hub based <strong>and</strong> uses UTP as a transmission medium over a 500 meter<br />

maximum length. However, signaling is 1 Mbps, <strong>and</strong> this means special provision must be<br />

made if it is to be incorporated in a 10 Mbps network. It has been superseded by 10BaseT.<br />

12.3 Signaling methods<br />

Ethernet signals are encoded using the Manchester encoding scheme. This method allows<br />

a clock to be extracted at the receiver end to synchronize the transmission/reception process.<br />

The encoding is performed by an exclusive or between a 20 MHz clock signal <strong>and</strong><br />

the data stream. In the resulting signal, a 0 is represented by a high to low change at the<br />

center of the bit cell, whilst a 1 is represented by a low to high change at the center of the<br />

bit cell. There may or may not be transitions at the beginning of a cell as well, but these<br />

are ignored at the receiver. The transitions in every cell allow the clock to be extracted, <strong>and</strong><br />

synchronized with the transmitter.

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