The use of fibre-optics in local area networks (LANs), such as Ethernet, has increased due to the inherent advantages of using fibre. High data rates can be maintained without electromagnetic or radio frequency interference (EMI/RFI). Longer distances can be achieved over that of copper wiring. For the industrial/commercial user, fibre offers high-voltage isolation, intrinsic safety and elimination of ground loops in geographically-large installations. Ethernet will function with no difficulty over fibre-optics as long as some simple rules are followed.
In this article, industrial network specialist Contemporary Controls, represented in South Africa by Electronic Products Design, explains the various standards for Ethernet. For the purposes of the article we will not go into explanations of the operation and characteristics of fibre-optic cable. This information can be found from a number of available technical resources.
Ethernet standards
Ethernet standards are published in ISO/IEC 8802-3:2000 which is also known as IEEE Std 802.3, 2000 Edition. This is an evolving standard with information on 10, 100 and 1000 Mbps operation. This is a very complex standard and is over 1500 pages long. From the standard we will review those portions dealing with fibre optics.
FOIRL - The fibre-optic inter-repeater link (FOIRL) was the original fibre-optic specification. It was intended to link two repeaters together with a maximum of 1 km fibre-optic cable while operating at 10 Mbps. This standard has been superceded by the 10Base-FL specification.
10Base-F - The 10Base-F standard is actually a collection of fibre-optic standards for 10 Mbps operation. It consists of three separate standards - 10Base-FL, 10Base-FB and 10Base-FP. It is not sufficient to claim 10Base-F compatibility because of these three specific implementations. The -FB and -FP standards are not popular and will not be discussed.
10Base-FL - This standard is the most popular 10 Mbps fibre implementation. The standard calls for a maximum segment length of 2 km of multimode fibre-optic cable and a minimum length of 0 km. This means that the transmitter cannot create an overdrive condition. A 10Base-FL unit must be able to communicate with a FOIRL unit but be limited to 1 km. Connectors are the ST-style, and a segment consists of a pair of cables, thereby allowing for full-duplex communication. The operating wavelength of the receivers and transmitters are 850 nm allowing for the less expensive components. The minimum average transmit level is -20 dBm while the maximum is -12 dBm. The receiver must be able to distinguish a -32,5 dBm signal and not overload from a -12 dBm signal. That means that the receiver's dynamic range must be at least 20,5 dB and that the power budget must be 12,5 dB. The intention is to use 62,5/125 fibre-optic cable. If a larger core is used, more energy will be launched which could cause overdrive on short runs. Manchester encoding is used just like 10Base-T.
100Base-X - Like 10Base-F, 100Base-X is not a unique physical layer, but details the encoding for the two most popular Fast Ethernet physical layers - 100Base-TX and 100Base-FX. One physical layer is for copper and the other for fibre optics, yet the standard applies to both. Much of the 100Base-X standard comes from the FDDI standard including the 4B/5B encoding.
4B/5B - Data transfers over the medium independent interface (MII), defined for Fast Ethernet, are done with 4-bit nibbles that represent actual data. With 10Base-FL, Manchester encoding is used which guarantees a transition within every bit cell, regardless of logic state. This effectively creates a 20 Mbaud signal for a 10 Mbps data rate. If the same encoding were used for Fast Ethernet, a 200 Mbaud signal would result, making it difficult to maintain the same 2 km maximum segment length due to bandwidth restrictions. A solution is the 4B/5B code where the 4-bit nibbles being transferred over the MII are actually encoded as five-bit symbols sent over the medium. The encoding efficiency is 80% and the baud rate increases to 125 Mbaud. This is still fast but not as fast as 200 Mbaud. The 4B/5B scheme is used for both the 100Base-TX and 100Base-FX physical layers.
100Base-FX - The actual governing specification for 100Base-FX is ISO/IEC 9314-3 which describes FDDI's physical layer medium dependent (PMD). The 100Base-FX fibre-optic physical layer is very similar in performance to 10Base-FL. Maximum segment length is 2 km for both technologies. However, for 100Base-FX this is only achieved on full-duplex links. On half-duplex links the segment length cannot exceed 412 m. Either SC, MIC or ST fibre-optic connectors can be used, but SC is recommended. Multimode fibre-optic cable (62,5/125) is what is normally used; however, larger cores can be substituted. Minimum transmitter power is -20 dBm and maximum receiver sensitivity is -31 dBm. The signalling on fibre optics is NRZI (non-return to zero inverted) since there is no concern for EMI on fibre-optic links.
With 100Base-TX, 1300 nm technology is used and since communication between 850 nm devices does not exist, there is no support for the Fast Ethernet auto-negotiation scheme. For 100 Mbps operation, the fibre-optic cable must have a minimum bandwidth of 500 MHz-km. This does not necessarily require a cable change since the same fibre-optic cable used at 10 Mbps (160 MHz-km at 850 nm) will have the necessary bandwidth at 1300 nm. Therefore, the 2 km maximum segment length can be maintained.
It is interesting to note that both 10Base-FL and 100Base-FX only specify multimode cable. The use of single-mode cable is vendor-specific. Therefore, it is best to match the same vendor equipment at each end of the single-mode link and observe maximum segment lengths. Distances of 15 km are common but full-duplex operation is a necessity.
100Base-SX - Recently, the 100Base-SX standard was released as a low-cost upgrade in performance from 10Base-FL systems. It is basically the 100Base-TX standard, but uses 850 nm devices and ST connectors. Segment lengths are limited to 300 m, but auto-negotiation of data rates is possible with other 100Base-SX compatible devices.
Conclusion
Robust Ethernet networks can be designed using fibre-optics supporting the popular 10 Mbps and 100 Mbps data rates. By using full-duplex communications, high-speed reliable communication can occur over large distances in a LAN environment.
For more information contact Jaap Willemse, Electronic Products Design, 012 665 9700, [email protected]
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