New transceiver designs ease distance limitations

New transceiver designs ease distance limitations

By MICHAEL M. FISK, MARK HEIMBUCH, and JEFF GRAHAM, MRV Communications Inc.

As transmission rates increase, transceiver designs must adapt to maintain fiber`s advantage.

In intra-building transport, multimode fiber is routinely being selected in favor of copper. Although most cost studies show copper-based systems to be slightly less expensive, the installation of multimode fiber guarantees utilization of future high-speed data protocols within the building.

Similarly, singlemode fiber is being used for interbuilding transport to facilitate the migration from Fast Ethernet to Gigabit Ethernet and higher-speed transmission. As the next generation of products is released to the marketplace, data rates as high as 2.5 Gbits/sec will be commonplace in uplink switching and network backbone applications. Specifications are currently being discussed for a "Fast Gigabit Ethernet" protocol, in which data rates will exceed 10 Gbits/sec.

Yet while it offers superior performance versus copper for high-speed applications, fiber also can encounter limitations as transmission rates increase. New transceiver designs have emerged that overcome these limitations while still providing the robust and reliable performance needed in high-bandwidth transmission.

Limiting factors

In 850-nm multimode fiber systems at data rates above 100 Mbits/sec, modal dispersion becomes the dominant limiting factor. Distance is sacrificed for each increase in bandwidth (see Figure). For Gigabit Ethernet, the maximum transmission distance over 62.5-micron multimode fiber is approximately 275 m. This maximum distance has created problems in several upgrades at facilities where Fast Ethernet was used successfully. Shifting the wavelength of the multimode transceiver to 1310 nm will increase the transmission distance to as much as 600 m at 1.25 Gbits/sec. As data rates continue to increase, however, either the introduction of wavelength-division multiplexing or the use of singlemode transceivers will be necessary for most interbuilding distances. The latter option is unquestionably more desirable on either new plants or existing plants where singlemode fiber has already been routed.

A singlemode transceiver using a 1310-nm Fabry-Perot (FP) laser can provide Gigabit Ethernet transmission up to or slightly beyond 10 km. This distance, contrary to popular wisdom, is dispersion limited. Although the effects of waveguide and material dispersion at 1310 nm are nominal, the FP laser`s center wavelength can be as low as 1300 nm and as high as 1320 nm at room temperature. Because lasers used in data-communications transceivers are uncooled, the wavelength can shift 䔯 nm over a temperature range of 0° to 50°C. The effective distance a dispersion-limited singlemode signal can transmit is determined by the equation:

L= 1/(4DlB)

where: L = the distance in kilometers

D = the absolute value of dispersion

l = the spectral width (RMS) during modulation

B = the data rate

Assuming a worst-case modulated spectral width of 6.5 nm for an FP laser at 1.25 Gbits/sec, and a worst-case dispersion of 3 psec/(nm-km) taken 20 nm from the zero-dispersion point at 1310 nm, the dispersion-limited distance of an FP laser transceiver would calculate to approximately 10 km.

By dramatically reducing either the spectral width or data rate, the distance limitations caused by dispersion can be eliminated. Because the goal is to increase distance without affecting the data rate, the reduction of spectral width becomes necessary. Reduction is achieved by replacing the FP laser with a distributed feedback (DFB) laser, which has an extremely narrow spectral width (0.01 to 0.1 nm).

With the use of DFB lasers in singlemode transceivers, which reduces the effect of waveguide and material dispersion, the limiting factor for transmission distances now becomes attenuation. Standard singlemode fiber attenuates a 1310-nm optical signal at approximately 0.4 dB/km. The total system link budget of a singlemode transceiver operating at 1.25 Gbits/sec averages about 9 dB--after the effects of time and temperature are factored in--providing transmission distances in excess of 20 km.

Since the attenuation of a 1550-nm optical signal is almost half of that at 1310 nm, transmission distances can double to 40 km by using a 1550-nm DFB laser. Increasing both the receiver sensitivity and coupling efficiency of the laser diode allows for even further transmission distances-as much as 50, even 70 km-for Gigabit Ethernet.

Transceiver packaging

Industry standardization of the "1ٻ" transceiver with the SC-duplex connector interface has enabled equipment manufacturers to upgrade from multimode transceivers to those with singlemode FP and DFB lasers without complicated redesigns. New package designs are being introduced, meeting the market`s desire for smaller and more versatile transceivers.

Small-form-factor (SFF) transceivers, based on a new breed of SFF connectors, enable optical port density equal to that of electrical ports. Providing data rates favorable to Fast Ethernet, OC-3 (155 Mbits/sec), OC-12 (622 Mbits/sec), and Gigabit Ethernet protocols, the SFF transceivers have 2ٷ and 2䁾 pinouts. Most offer a 50% reduction in size and upgraded features such as transmit disable and a choice of 5V or 3.3V operation. To date, few standards have been adopted and it is uncertain which formats will be accepted by the marketplace. Currently, only two SFF connector manufacturers can provide singlemode terminations, while only one manufacturer is delivering singlemode SFF transceivers.

Another format, the Gigabit Interface Converter (GBIC), has a special mechanical package that allows the transceiver to slide into (and out of) any GBIC front-panel port. The electrical connection is made through a 20-pin zero-insertion-force connector on the back of the transceiver, enabling the end-user to switch from multimode to FP singlemode to DFB singlemode formats in seconds. An address chip inside the unit guarantees that the equipment`s system management is informed of the change. The GBIC, though much larger than the SFF transceiver, provides ultimate modularity at a very reasonable price. u

Singlemode transceivers can compensate for distance limitations as transmission rates increase.