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Medium Attachment Unit
A functional block diagram of the MAU is shown in Figure 3-4. As shown, the
MAU has four functional areas:
• Receive Function.
Receives serial data bit streams from the coaxial
cable and sends these to the LANIC.
• Transmit Function.
Accepts serial data bit streams from the LANIC,
and transmits this data to the coaxial cable.
• Jabber Fault Detection.
Detects an abnormally long output data
stream ("jabber") from the LANIC. and inhibits transmission to the
coaxial cable.
• Collision Detection.
Detects the presence of two or more simultaneous
transmissions on the coaxial cable and informs the LANIC of this
condition.
Receiver
The coaxial receiver has a high input impedance. (The high impedance is
necessary to allow up to 100 users to connect to the coax and not load down the
cable.) The data received from the coax is then amplified and shaped to
compensate for attenuation caused by the coaxial cable. The data is then sent
through a cable driver to the LANIC via the twisted pair of the AUI cable. The
MAV
also has a DC squelch function which turns off the AVI cable driver
when no data is present on the coax as is indicated by a lack of DC component
in signals on the coaxial cable.
Since the receiver passes all data from the coaxial cable to the AVI cable. the
receiver also monitors data sent by its own LANIC, thus implementing a local
loop-back and monitoring function.
Transmitter
The transmitter accepts data from the LANIC and transmits this data to the
coaxial cable. Data is transmitted to the cable using a high-impedance current
sink. Current drive of the coaxial cable causes multiple transmitters' signals to
add without damage to the transmitters. The coaxial cable driver adheres to the
recommended value for drive level specified in the IEEE 802.3 Standard.
Jabber Fault Detection
If the LANIC should try to send data for a time longer than the longest data
packet allowed. the network throughput could be affected. (If the LANIC
transmitted continuously. no other node could access the network.) The jabber
fault detection circuitry detects any attempt by the MAU to transmit for longer
the 20 milliseconds. and prevents this from happening by turning the transmitter
Principles of Operation
3-22
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