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 fiber optics.

FOIRL

The Fiber Optic Inter-Repeater Link (FOIRL) was the original fiber optic specification. It was intended to link two repeaters together with a maximum of 1 km fiber optic cable while operating at 10 Mbps. This standard has been superseded by the 10BASE-FL specification.

10BASE-F

The 10BASE-F standard is actually a collection of fiber 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 fiber implementation. The standard calls for a maximum segment length of 2 km of multimode fiber 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 fiber optic cable. If a larger core is used, more energy will be launched which cold 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 fiber 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 fiber optic physical layer is very similar in performance to 10BASE-FL. Maximum segment length is 2km 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 fiber optic connectors can be used, but SC is recommended. Multimode fiber 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 signaling on fiber optics is NRZI (non-return to zero inverted) since there is no concern for EMI on fiber 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 fiber optic cable must have a minimum bandwidth of 500 Mhz-km. This does not necessarily require a cable change since the same fiber 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 utilizes 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 fiber optics supporting the popular 10 Mbps and 100 Mbps data rates By utilizing full-duplex communications, high-speed reliable communication can occur over large distances in a LAN environment.

References

Ethernet—The Definite Guide, Charles E. Spurgeon, 2000, O'Reilly & Associates, Inc.

International Standard ISO/IEC 8802-3 ANSI/IEEE STD. 802.3, 2000 Edition, The Institute of Electrical and Electronic Engineers, Inc.

International Standard ISO/IEC 9314-3 Information Processing Systems—Fiber Distributed Data Interface FDDI)— Part 3: Physical Layer Medium Dependent (PMD), 1990

Industrial Fiber Optic Networks, John C. Huber, Instrument Society of America, 1995