Single-unit transceivers advance on cost, performance and standards fronts
Applied as dedicated point-to-point data links over fiber-optic cable, transceivers are steadily accommodating more communications standards and handling faster data rates at decreasing prices
TOM DeBERARDINE
AMP INC.
As a well-established data communications technology implemented in widespread point-to-point information transfer applications, fiber-optic data links comprising transmitters, receivers or transceivers are delivering standards-
based performance at affordable costs. In fact, transceiver device prices have been decreasing approximately 25% during the past few years.
User demands for higher-speed applications are driving the installation of data link devices in fiber distributed data interface, or FDDI, asynchronous transfer mode, or ATM, Fibre Channel, Fast Ethernet and 100VG-AnyLAN networks. These and other network standards offer data-transfer rates of 100 megabits per second--an order of magnitude faster than first-generation Ethernet data communications networks. In addition, communications needs of users are increasing rapidly for multimedia voice, video and data communications, which call for the use of fiber-optic networks operating at rates exceeding 100 Mbits/sec.
Because a data link device is needed at each end of a fiber-optic link, the high demand for fiber-optic systems obviously drives the high demand for transceivers. The communications market for short-wavelength, 850-nanometer data link devices is well established. Market growth is, therefore, focusing on the long-wavelength, 1300-nm device market. Here, the performance gains of multimode fiber operating at long wavelengths enable an installed fiber plant to meet present and future communications requirements. For example, the bandwidth of 62.5-micron fiber increases from 160 megahertz-kilometers at 850 nm, to 500 MHz-km at 1300 nm.
Despite the widespread availability of discrete transmitters and receivers, single-unit transceivers have become the most popular data link devices. Transceivers simplify design and board layout, offer matched transmitter-to-receiver performance and reduce parts-inventory needs. Moreover, they are compatible with the duplex connector interfaces championed by FDDI and enterprise systems connection, or Escon, standards. With built-in polarization in both the transceiver interface and connector, duplex-configured connectors reduce the risk of incorrect connections.
Standards driven
To enhance usage, transceiver designs are closely tied to common standards requirements, such as ATM, FDDI, synchronous optical network, or Sonet, and Fibre Channel. In fact, within a given range of speed, the performance requirements of different applications are similar enough so that they do not significantly alter transceiver design. A transceiver design, therefore, finds use in several different applications. Of course, the single-transceiver concept gets complicated by the need for different interfaces. Whereas the SC connector dominates the transceiver design, other standard interfaces, such as the ST connector, also find widespread use.
Trends in transceiver design point toward higher speeds. For example, transceivers are being installed in 155-Mbit/sec ATM and 1-gigabit-per-second Fibre Channel networks. However, these markets have not grown as quickly as industry analysts have predicted because many users are still evaluating communications services at these higher speeds.
Some industry analysts predict that quarter- and half-speed Fibre Channel will be bypassed as users move directly to 1.064-Gbit/sec data links. Likewise, ATM applications are expected to move to 622 Mbits/sec and 1.2 Gbits/sec, especially in backbone network applications. Desktop applications are focusing on 25-, 51- and 155-Mbit/sec speeds. Another standard that holds substantial transceiver market growth potential is Sbcon, which is an industry-standard version of IBM`s Escon system. As the high-speed network market matures, Sonet and ATM speeds to OC-12, or 622 Mbits/sec, are expected to drive the design of high-speed transceivers.
The mechanical evolution of trans ceivers is toward smaller and easier-to-apply packages. For example, pin outs have been de creased from 22 to 9. In fact, several manufacturers, such as AMP Inc., Alcatel, Hewlett-Packard Co., Northern Telecom, Siemens and Sumitomo, have agreed on standardized package designs with common dimensions, pinouts and footprints. These standard devices occupy less board space and accelerate new product design.
Drop-in replacements
With this approach, transceivers are rapidly becoming "drop-in" replacements. For example, standard pinouts allow easy migration to higher-speed transceivers; that is, a 155-Mbit/sec OC-3 transceiver and an OC-12 transceiver become interchangeable via identical pinouts and circuit board footprints.
Pinout configurations of 2x11, 1x13 and 1x9 offer a "bare-bones" approach to transceiver design by providing balanced input, output and voltage-supply pins. On the other hand, an emerging 2x9 configuration can be used for laser control and clock and data recovery. These functions are presently performed external to the transceiver.
Although most transceivers are manufactured to standard specifications, such as ATM or FDDI, devices typically include a design margin between the optical output and the receiver sensitivity specifications. This margin provides a guard band in the data link power budget.
Consider the FDDI physical-layer medium dependent specification, for example. The specification requires a minimum of -20-decibel relative to milliwatt optical output from the transmitter and a -31-dBm sensitivity in the receiver. If a manufacturer guarantees a -19 dBm minimum optical output and a -33.5-dBm receiver sensitivity, the power budget is increased by 3.5 dB. Measured values typically add 2 dB to the budget. This guard band allows for device and measurement errors, and provides designers with an extra margin of device performance confidence.
Another area where transceivers have been improved is operational stability. Thermal compensation circuits have been added to transceivers, for example, to maintain a consistent optical output over a range of temperatures. Maintaining low distortion in the receiver is also important to avoid data errors. Receivers are typically designed for a 50% duty cycle; that is, they are able to accommodate an equal number of high and low pulses to maintain their data decision threshold in the middle of the pulse height. Departures from a 50% duty cycle could cause the threshold to move upward or downward, which would result in data-dependent jitter and false readings.
Although data transmission coding techniques, such as 4B/5B, 8B/10B, and 27-1 pseudorandom binary sequence, or PRBS, are designed to scramble bit patterns to achieve a 50% duty cycle in a nonreturn to zero data stream, not all specifications achieve the intended pattern. For example, the 4B/5B pattern in an FDDI application yields a duty cycle that varies between 40% and 60%. As the duty cycle varies in an AC coupled receiver, transceiver sensitivity and pulsewidth distortion both degrade unless the device is designed to compensate for pulse variations. Devices are, therefore, being designed to provide distortion-free, constant operation under both temperature and duty-cycle variations.
Once used almost exclusively for telecommunications, singlemode transceiver are increasingly being used in network and data communications backbones. For interbuilding links and other backbone applications, singlemode transceivers combine wide bandwidth and long transmission distance to facilitate high-speed links in campus-wide applications. Singlemode and multimode transceivers are expected to exist side by side in many network and data communications applications.
Decreasing costs
Smaller, simpler packaging standards promote reduced transceiver prices, both in component and application costs. Transceiver manufacturers are also reducing costs and increasing performance within their links. Technologies like multichip module packaging allow multiple chips--light-emitting diodes, or LEDs, detectors and drivers--to be encapsulated on a printed circuit card. This packaging method not only benefits high-speed operation by eliminating the capacitive and inductive effects of discrete packages, but also fits more functions into a smaller area.
Equally important are the methods of simplifying transceiver design and assembly. Consider the method of producing the previous generation of transceivers. The LED was mounted in a traditional TO-can package, which typically contained a focusing mechanism, such as a lens or a microbead. The TO-can package was, in turn, placed into a device mount on the transceiver board. The alignment of the light source with the transceiver`s output port has proved to be labor-intensive.
In multimode devices, a new method employs a molded lens placed over an LED mounted directly to the transceiver card. The lens directs and focuses the light beam. Design advantages include fewer parts, elimination of the TO can and device mount, and simpler active alignment during manufacture. These advantages reduce costs without lowering performance.
Additional technologies are being developed in hopes of driving transceiver costs even lower. These efforts are exploiting semiconductor techniques such as photolithography to produce optical elements that can be inexpensively mass-produced. Another approach, waveguide technology, might permit easier and more-precise control of light beams for directing and focusing optical energy. Holograms, another technology, can perhaps provide optical focusing and collimating in a low-cost, easily manufactured format. Already being implemented are vertical-cavity lasers or surface-emitting devices, which are supplying improved performance, easier alignment, and low-drive currents that generate high optical outputs at low-power consumption.
Light source variations
Short-wavelength lasers for compact disk players have received attention because of their installation in Fibre Channel applications. Operating in the 790-nm range, these devices hold the promise of low cost and high-volume manufacturing. The drawback, however, is that the device`s optical characteristics are ill-suited to the low-loss windows of optical fibers. The result translates into short transmission distances.
The data link market is, therefore, undecided whether CD-laser-based transceivers will survive. As 1300-nm long-wavelength transceivers continue to mature, costs are expected to drop continuously, which will make these transceivers more attractive to the customer. u
Tom DeBerardine is product manager of optoelectronic devices at AMP Inc. in Somerville, NJ.