Feature

Fiber battles copper for gigabit serial links

You can now get gigabit connectivity with minimum fuss via standard module and connector families, and you can switch between copper and fiber links without significant redesign.

By Bill Schweber, Technical Editor -- EDN, 1/6/2000

You've heard endless predictions about the new millennium, and some of these predictions will even come true. But there's one low-risk prediction that you can make: Electronic systems will increasingly require point-to-point serial links at the 1-Gbps-and-higher rate between pc boards, chassis, and subsystems. This requirement means that the demands on the physical link will increase, and, if you don't prepare properly, the physical link will become the slowest and thus the weakest link in your signal-path chain.

It's never been easy for designers to implement low-error-rate, low-cost links that require no complex circuitry and consume no excess power, especially using copper cable as the datapath. But advances in optical-fiber interfaces and connectors have made fiber a viable alternative, with benefits, price, and performance that make it competitive with copper-based links. Further, depending on your design approach, you can interchange copper and fiber with relatively few changes to the interface components. It is already well-known that fiber has better noise immunity, RFI/EMI performance, inherent electrical isolation, and potential bit rate than copper (Reference 1).

Best of all, a variety of component and interconnect standards means that you have the best of both worlds that standards can offer. On one side, a well-defined standard means you have multiple vendors offering roughly equivalent parts, each driving toward better performance and lower cost. On the other side, no dominant standards mean you have a choice in basic performance attributes and trade-offs among the top tier of standards. Therefore, you are not forced to adopt a standard that you are uncomfortable with or that is too much of a compromise.

Watch for that ubiquitous transceiver

One of the first things you notice when you look at hardware for high-speed links is the frequent use of the term "transceiver."This usage occurs because "transceiver",like a good Freudian dream, means different things to different people. At the most basic level, it's the interface circuitry that connects to the physical media, whether copper or fiber. The transceiver must generate the requisite voltages, and the receiver must sense the small voltage swings, in the case of copper, or act as an electrical-to-optical transformer (and vice versa) for optical fiber.

At a higher level, the transceiver defines a broader function that includes the connector; the line (or fiber) driver and receiver; protective circuitry for the line driver/receiver; and associated circuitry that interfaces between the driver/receiver and the rest of your system and acts as a complete electrical and mechanical assembly.

However, it takes more than just an electronics-to-media interface to make a functional link. On the transmitting side, you usually need to accept multiple parallel data streams, combine them into a single serial stream, add coding and error-correction bits, and then present them to the interface. At the receiving side, you need to perform the complementary operations and extract a low-jitter clock from the received stream (Figure 1).

Finally, there's another industry use for the term "transceiver": to describe circuitry—usually a single IC—that performs the parallel/serial and encoding process (as well as the reverse operation). This circuitry may do the clock recovery, but it does not offer the physical line-driving and -receiving functions. Note that the serial/parallel process is often called SERDES (serializer/deserializer) in this application niche.

Because these physical transceivers are operating close to the media layer of the communications link, they are relatively independent of the source-data format and protocol. As long as the transceiver supports your data rate and meets a few other requirements, you can use many of the same components in Fibre Channel, Gigabit Ethernet, asynchronous-transfer-mode (ATM), synchronous-optical network/synchronous-digital-hierarchy (SONET/SDH), and fiber-distributed data-interface (FDDI) applications.

Simplify your life with modules

Fibre Channel (1.0625-Gbps) and Gigabit Ethernet (1.25-Gbps) applications have driven vendors to develop a range of transceiver modules (Reference 2), and you can design your own fiber interface (see sidebar "Building your own transceiver circuitry"). All modules support full-duplex links; such links are mandatory in most applications and highly desirable in nearly all others for link and throughput efficiency. Because the electrical-to-optical component demands for transmit-versus-receive functions make a single fiber as a bidirectional path impractical, the solution is two parallel paths, each with its own connector.

For nearly all module types, you can get either a fiber model or an equivalent-sized copper version that performs the same function, and you can choose optical fiber or copper without redesigning your pc board. Unlike fiber links, the electrical nature of line drivers and receivers makes it possible for you to use a single copper cable in a half-duplex mode. But again, using full duplex and two parallel paths is the most common and desired approach.

The 1×9 board-mounted transceiver was among the first standard units; the name comes from the inline arrangement of pins in a single row (Figure 2). Newer, smaller designs or designs with additional attributes are supplanting this transceiver, which solders into or onto the pc board.

Similarly, the approximately 50×14×10-mm small-form-factor (SFF) module solders to the pc board. The industry's multisource agreement defines the SFF-optical-transceiver standard. The agreement calls out many attributes, such as the module footprint and most module physical dimensions, but it does not specify a connector. In this way, the standard recognizes the existence of viable competitive connectors and a dynamic application base; designers need to be able to choose connectors based on cost, performance, availability, and less tangible comfort factors.

The SFF module comes in both 10- and 20-pin versions. The first 10 pins provide basic functions (Figure 3); the 10 additional pins support extra functions, such as photoconductor bias, clock recovery, laser-bias monitor, and laser-power monitor. A typical optical SFF is the Methode Electronics MLC-25-4-X-TL, which operates from 3.3V (Figure 4). You can order the MLC-25-4-X-TL in versions for 850-nm multimode fiber as well as 5-, 10-, or 20-km, 1300-nm, single-mode fiber.

Another choice for modules is the Gigabit Interface Converter (GBIC), which is approximately twice the width of the SFF design (Figure 5). Unlike the SFF, the GBIC module is a removable, plug-in unit, and you can hot-swap GBIC modules, important considerations in many designs that can outweigh the GBIC's lower density. The GBIC module blind-mates with its receptacle, using an entirely front-access operation in which you need not reach around behind the module to lock it down. The combination of hot-swapping and blind-plugging means that you can replace defective modules (or evaluate suspect ones) and switch your media between copper and fiber after you install the design into its system. A module such as the Cielo GBE-1250ELX can push your potential serial distance: It uses a 1550-nm, distributed-feedback laser as the light source and reaches to 100 km in a full-duplex GBIC module (Figure 6).

Connectors are critical

Any time you're designing for the gigabit-per-second realm, the connector is an integral part of the electrical and mechanical design—not just an afterthought. The fact that there is no one optical fiber that eclipses all others complicates the connector situation. Depending on your system, you may need to support thinner single-mode fibers, which are required for longer distance links; thicker multimode fibers, which are acceptable for shorter links; and lower cost, higher loss plastic fibers.

The established SC connector is still quite popular, and it is standard on the GBIC module. Some vendors claim that few new designs are using this connector because it is larger than newer connectors and supports only half as many ports along a given linear card edge. Contradicting this view, other vendors are announcing new SC-based modules. You should base your decision on performance needs, anticipated channel density, and availability. If you suspect that you will need a higher density connector in your next-generation design, you can choose one when your requirements are firm.

The three connectors you can use with the SFF module that appear to have the most market design-in acceptance are the LC, the MT-RJ, and the VF-45; key industry connector and electronics vendors back each one. In some cases, a module vendor is committed to backing only one connector type; in other cases, the vendor is supplying similar modules but with the connector you specify.

Bell Labs/Lucent Technologies developed the duplex-LC receptacle, a device that supports both single-mode and multimode fiber. The receptacle resembles the common telephony RJ-45 jack and fits the same cutout and housing. The cable has a small ferrule that aids alignment and provides handling protection. The LC proponents include IBM Microelectronics; Methode Electronics; MRV Communications; Sumitomo Electric Lightwave; and Molex, which recently made a joint announcement that it will provide SFF components using this connector.

Hewlett-Packard (now Agilent Technologies) worked with supporters such as Amp, Siecor, Fujikura, and USConec to develop the MT-RJ system. They based this system on the precision-molded plastic ferrule that Japan's NTT Labs invented about 10 years ago. The form factor is slightly smaller than the RJ-45 telephone modular jack; therefore, it fits into the same panels as an adapter. The MT-RJ and RJ-45 systems also have similar press-to-release latching mechanism. Like the LC, it supports both single-mode and multimode fibers of various diameters. When you use it with a multimode fiber, the connector has an insertion loss of less than 0.2 dB, which is much less than the 0.75-dB maximum insertion loss that the TIA/EIA-568A specification calls out.

Finally, 3M developed the VF-45 design as part of its Volition Cabling System. The VF-45 does not use a ferrule but instead has an injection-molded V groove to hold the fiber in the connector and align it to the active components of the transceiver. This feature eliminates some of the cost associated with the tighter tolerance components of the other connector families. Currently, the VF-45 system supports only multimode fibers. Vendors that are providing VF-based transceivers include Infineon Technologies, Honeywell, and Sumitomo.

Which connector is the best? The answer is the same as for most design questions: It depends. Each connector offers trade-offs in cost, performance, upgradability, multiple sources, and the fibers it supports, as well as data rates and bit-error rates (BERs). For example, the LC proponents claim that the wider pitch between the duplex fiber ports of the LC receptacle means that the transceiver has fewer potential EMI and crosstalk problems, translating to lower cost transceiver-module designs and lower BER.

Shape up and ship out your data

Because the link is serial but your data is often parallel, you need to perform the appropriate transform at each end of the link. The fiber or copper transceiver is transparent to the data bit- format and protocol. Therefore, some SERDESs add any needed sync bits, coding, and error-correction bits, and others assume you already have the data prepared. Carefully read data sheets, because parameters such as jitter are critical at high data rates (see sidebar "Jitter can cause bugs").

Sometimes, you need a fast serial link, but one link doesn't meet your throughput mandate, and vendors know it. For example, Vitesse Semiconductor, which has a line of ICs spanning a variety of configurations and speeds, offers the VSC7212 Gigabit Interconnect Chip. Within this $20 (5000) 3.3V IC, which supports a raw data rate of more than 2 Gbps, are 8B/10B encoders/decoders, serial/parallel/serial circuitry, and buffers that accommodate timing differences and link slowdowns. The $25 VSC7139 quad transceiver provides 1.05- to 1.36- Gbps-per-channel operation (Figure 7); you can use a common clock for the four channels, or you can use an independent clock for each channel, embedded in each data stream. Internal clock-recovery circuitry then drives the IC's data-decision function.

Applied Micro Circuits, also supplying a large series of gigabit devices, offers the S2065 quad-channel transceiver, which supports 1.3-Gbps transfer per full-duplex channel. Targeting systems that need higher availability via primary and secondary paths and hot-swapping, the IC provides two high-speed differential inputs and outputs for each receiver and transmitter channel. You can configure the $75 (100), 208-bump TBGA IC's I/O for either four 8-bit channels or one 32-bit channel.

ICs such as Texas Instruments' TLK-2201 make the serialization/deserialization a straightforward operation for copper links. The 1.25-Gbps SERDES is an all-CMOS device that includes 8B/10B encoding and decoding, differential receivers with 200-mV thresholds, and 50? line drivers. It consumes less than 300 mW from a 2.5V supply. The $7 (1000) 64-pin VQFP IC has a faster sibling, the $25 (1000) 2.5-Gbps TLK2500.

Further targeting copper links and providing four channels of operation is the $29.50 (10,000) Motorola MC92600 Warplink transceiver. In this device, each full-duplex channel operates as fast as 1 Gbps, for a total throughput of 8 Gbps. The integral line drivers drive 50 or 75? lines, and typical range is 1.5m of backplane etch or 10m of cable. The data interface for each channel of this IC is a TTL-compatible, 8- or 10-bit parallel port; word synchronization within the IC can remove as much as 40 bit-times of skew and tolerate six bit-times of drift after synchronization.

Also pushing past the basic 1-Gbps barrier are ICs from Triquint Semiconductor. The company's TQ8214 multiplexer accepts 8- or 16-bit-wide positive-ECL (PECL) data, and its outputs are OC-48/STM-16-compliant serial data at 2.488 Gbps. The $119 (1000) IC also includes a 10- to 75-mA laser-diode driver. For the other side of the link, the TQ8224 demultiplexer has both a PLL-based clock function and a data-recovery function; it can align the data with the recovered clock in several ways, so you can use the IC in standard and nonstandard applications.

or designs blasting past the 1-Gbps area by a factor of 10, Oki Semiconductor offers a full line of GaAs ICS for the OC-192 region. These devices include 16-to-1 multiplexers/demultiplexers, diode drivers, postamplifiers, AGC amplifiers, limiting amplifiers, and clock-and data-recovery devices. Be prepared for some unique designs at these rates, due to speed, power, and layout factors, and don't even think of using copper links here; this territory is for fiber only.

Maxim has also entered the high-speed multiplexer/demultiplexer arena with the $29.95 (1000) MAX3831, a four-channel, 2.5 Gbps SDH/SONET-compatible device targeting rack-to-rack, shelf-to-shelf, and similar interconnections. Within this 3.3V, 64-pin TQFP IC, which uses low voltage-differential-signaling (LVDS)-compatible parallel I/O, is a 10-bit-wide elastic buffer for 7.5 nsec of skew between any data input and the reference clock, a clock generator (2.488 GHz), plus a pattern generator for high-speed, built-in test functions.

In some applications, you need to combine both serial and parallel modes to achieve your needed throughput. Using the DS90CR483 transmitter and matching DS90CR484 receiver from National Semiconductor, you can aggregate 48 CMOS/TTL parallel inputs into eight LVDS data streams and achieve a data throughput of 5.38 Gbps. The 3.3V IC pair, with an input clock that reaches 112 MHz, has a separate, parallel clock line to simplify data recovery and reconstitution at the receiver with acceptable BER levels. It employs pre-emphasis to reduce copper-cable-loading effects, and it uses dc balance to reduce intersymbol-interference distortion.

For those applications in which you wish to use a clock-recovery circuit that is separate from the SERDES—either to achieve a specific performance level or to isolate functions for flexibility—vendors offer protocol-independent ICs. For example, the Micrel/Synergy SY87701V provides clock recovery and retiming for NRZ data from 32 to 1250 Mbps—which is suitable for SONET/SDH, ATM, and Fibre Channel situations—via a PLL dedicated to each of these functions. The 28-pin SOIC, with differential PECL and 100k ECL-compatible inputs and outputs, automatically detects and indicates when it is in phase-lock with the incoming bit stream.

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Author info

You can reach Technical Editor Bill Schweber at 1-617-558-4484, fax 1-617-558-4470 bill.schweber@cahners.com.

 

 

REFERENCE

1. Travis, Bill, "Fiber vs copper: Sometimes it's not an easy choice," EDN, Nov 21, 1996, pg 46.

2. Marsh, David, "Category 6 cable: Gigabit Ethernet over copper," EDN, Dec 9, 1999, pg 71.