FL MAU - Fiber link circuitry
To communicate over a fiber optic link, at least the following three components are necessary:
- Optical transmitter and its driver
- Optical cable
- Optical receiver with signal shaping circuits
Optical transmitter and receiver, together with appropriate circuitry, are usually a part of a MAU (Media Attachment Unit) module. Both external (see the picture) modules connected via AUI, as well as internal modules designed to fit into a hub or switch, are available. Transmitter usually uses a LED, receiver usually uses a PIN diode. Both receiving and transmitting diodes come in a special package that incorporates an appropriate optical fiber connector. Several types of connectors are used. Two most common types are on the pictures of MAUs. The one on the left uses a ST connector, fastened in a similar way as BNC connectors (bayonet joint). The other on the right uses a square SC connector, fastened using a moving part of the connector exterior. SC connectors can be fixed into pairs with a special clamp. (The picture shows SC connectors inserted into a protective rubber cap that protects the sensitive optic from dirt.)
Optical fiber cable is usually made of glass and contains one or more pairs of fibers. Each fiber has several coats of protective insulation. Unless special methods are employed, one fiber transfers data in one direction only. Therefore, like in TP cables, two fibers are necessary for a full Ethernet connection, one to transmit and the other to receive. Currently, fibers of three different diameters are mainly in use: 9 μm / 125 μm (SM), 50 μm / 125 μm, 62.5 μm / 125 μm (MM). What do these values mean? Optic fiber consists of two layers with different refraction indexes: the inner "core" and the outer "cladding". Communication is carried through the core, cladding creates a barrier that keeps the light beam inside the core. In addition, a fiber can be either of multimode (MM) or singlemode (SM) type; this indicates the number of modes that the fiber can carry. SM cables have much better parameters (bandwidth, distortion, attenuation) so they are primarily used for long distances. MM cables are used primarily in LANs, for distances up to several kilometers. The main advantage of MM cables is lower price of elements and technology needed to connect them. Each cable needs to be connected at least to a receiver and a transmitter. The core of a SM cable is only 8 to 9 μm thick, requiring high-precision connectors and other components. For a 62.5 / 125 μm multimode cable, it is not critical whether a hole for the fiber is 126 or 128 μm wide.
Usually, visible wavelength of 850 nm (red) is used for 10Mbps communication. For 100Mbps, wavelength of 1300 nm is used, due to better characteristics of fibers at this wavelength.
Main component of a fiber link MAU is usually HFBR-4663 or ML4668. Structure of this device is very similar to its counterpart for TP cables:
- At the transmitter, signal coming from AUI is amplified and leads to the output driver, which is essentially a controlled current source. To minimize noise, the driver consists of a constant (but changeable, usually 50 mA) current source that is switched between Vcc and the LED output. When no transmission takes place, noise gate tells the output driver to transmit IDLE signal. This function is similar to link test of TP circuits; however, it is simpler and consists of a 1 MHz signal. This signal allows the other station to monitor link quality and adjust receiver gain.
- At the receiver, signal passes from the optical receiver output through the input amplifier with adjustable gain to the noise gate, which suppresses signals with frequency less than 2.5 MHz, including the IDLE signal. Then, the signal goes into AUI drivers. The multiplexor before output AUI drivers is used to create local loopback that routes data from AUI(TX) directly to AUI(RX). According to the schematic, this function is on by default, meaning that transmitted data are returned through the receiver output. However, if a collision is detected, loopback is turned off and receiver outputs actual data. The entire circuit therefore behaves as if a coaxial-cable link is used instead of the optical one.
- Except for these two, let's say "signal shapers", the circuit contains a collision detector. Collision is detected simply by the fact that whenever someone transmits, others listen. If the TPEX circuit detects simultaneous activity in both the receiving and transmitting part, it signals collision on the CD AUI signal.
HP Fiber Optic Technical Training Manual (65 pg.) [local copy]
HP AN1038 - Complete Fiber-Optic Solutions for IEEE 802.3 FOIRL, 10Base-FB, and 10Base-FL (16 pg.) [local copy]
HP HFBR-0400 series - Low Cost, Miniature Fiber Optic Components with ST, SMA, SC and FC Ports (29 pg.) [local copy]
HP HFBR-4663 Single Chip 10BASE-FL Transceiver (16 pg.) [local copy]
ML4668 Low Power Single Chip 10BASE-FL Transceiver (17 pg.) [local copy]
ML4664/ML4669 10BASE-FL to 10BASE-T Converter (17 pg.) [local copy]
Panduit - ST and SC Fibre Optic Connectors (1 pg.) [local copy]
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