Copper cable data links have been evolving for many years. They have transitioned from very low data rate links to links running at multiple gigabits per second. With advances in serdes /gigabit transceiver technology (serializer and de-serializer), we are likely to see optical data link implementations relegated to its traditional optical networking arenas such as SONET, optical GbE and Fibre-Channel standards compliant applications. Copper cable-based data links are likely to replace optical implementations especially for short distance (<10 meters), high-speed links within proprietary-natured applications that tend to be non-standards based.
For early gigabit link adopters, the lack of semiconductor physical layer device advances prevented copper-based gigabit data link implementation. At the time, optical was the only choice available for achieving gigabit serial link performance. Additionally, optical provided system designers with easy solutions to solve difficult problems, i.e., gigabit serial data communications. This solution came at the expense of higher implementation costs. The high cost of optical modules, fiber, test equipment and the hard-to-find expertise often made end-products more expensive to build, test and support. For these reasons, designers typically avoided optical serial implementations and used familiar methods such as wider-parallel copper buses running at slower, but more manageable line rates.
This brute-force method using numerous parallel links, however, had limitations. As the aggregate data rate moves higher, the designer must add more parallel lines in order to scale with the data rate. At some point, depending on the design, the large number of parallel links becomes unmanageable. In addition, these implementations demand valuable board space and consume vast amounts of power. Beyond a certain number of parallel links, designers also have to carefully mange the channel skew between the copper lines to maintain signal integrity. In today's low-power, low-cost and small form- factor driven markets, optical and parallel gigabit copper solutions do not meet a designer's requirements who is "pushing the envelope" in gigabit data rate applications.
The birth of Serdes
With no optimal solution available, systems designers began to demand answers from semiconductors vendors. At the time, few companies could make a solid business case for investing in the serialization/de-serialization (Serdes) and clock data recovery (CDR) technologies addressing these issues. However, with communications equipment, consumer electronics and industrial automation equipment explosion, applications requiring the serdes technology increased. Thus, semiconductor vendors could justify developing the serdes technology that would eventually foster the gigabit serdes. Even with the advent of the new gigabit serdes technology, many systems designers still questioned its "robustness" for transmitting gigabit-per-second payloads over copper media while maintaining target bit error rates (BER). Over time, as the engineering community understood the underlying concepts of CDR/PLL technology, gigabit serdes became a powerful tool.
How fast, how far?
Typically a system design engineer wants to understand what data rate can be achieved by a gigabit serdes for a given transmission length of a particular copper media type. First, it is important to select a serdes that is designed for driving copper media. Some serdes are specifically designed to drive optical modules, for example. In this case, these serdes do not have the transmitter drive-strength or the receiver-sensitivity to drive copper media. Some serdes solutions are specifically well-suited for driving copper media as well as optical modules. In addition, serdes typically support a specific data-rate range so engineers need to select a serdes that meets their desired data rate. For example, one gigabit serdes may feature a data-rate range from 1.5 Gbps to 2.5Gbps while the another supports a data-rate range from 600mbps to 1.5Gbps.
Another key factor in developing a gigabit link is the copper media selection. Usually the media type is chosen based on numerous criteria such as system cost, weight and performance among other requirements. A popular media-type used in today's gigabit copper links is Category-5 (CAT-5) twisted-pair cable. Ready availability from multiple vendors, its known performance characteristics as well as cost-effectiveness makes Cat-5 (and other Cat-5x flavors) popular with system designers. Since gigabit links are typically comprised of differential signals, twisted-pair cable media is almost always selected for multi-gigabit links. Other considerations, such as the connector selection, play an important role in defining the link as well.
How is the performance?
Often the performance and robustness of gigabit links are gauged based on a composite of several factors. Some of the most important measurements are:
- Jitter: A measure of the signal's integrity; the short-term variations of the digital signal's significant instants in time from its ideal position.
- Vod: Differential output voltage; the amplitude of a signal being driven into the channel. Typically as the signal traverses the media signal, amplitude is "lost" due to media attenuation.
- Data Eye Diagram: An oscilloscope display in which a receiver's PRBS pattern is repetitively sampled and applied to the vertical input, while the data rate triggers the horizontal sweep. Link performance information is determined by analyzing the eye pattern. An open-eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to inter-symbol interference and noise appears as closure of the eye pattern. (ATIS Definition)
- BER: The number of erroneous bits divided by the total number of bits transmitted, received or processed over some stipulated period. Examples of bit-error ratio are: (a) transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted; and (b) information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits. The BER is usually expressed as a coefficient and a power of 10; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 105 or 2.5 - 10-5. (ATIS Definition)