Gigabit Ethernet installations are growing at a rapid rate. What is driving the need for this technology? And what does the future hold in store?
November 1, 2000
Gigabit Ethernet over optical fiber was adopted by IEEE two years ago, and implementation of the technology is growing rapidly in the enterprise market. Ethernet’s popularity comes from its roots in data networking and user familiarity with the technology. The ability to gracefully upgrade from slower speeds through the use of low cost technology is one of the major advantages of the Ethernet family, and Gigabit Ethernet continues that trend.
A number of LAN protocols have come and gone (witness ArcNet, Token Ring and FDDI) but Ethernet continues to prosper. What are the reasons for Ethernet’s success? Certainly, other protocols have been more robust (i.e., Token Ring and FDDI), but Ethernet has become the de facto LAN standard. Its success is due to its competence in providing a cost-effective, scalable solution for today’s network, and its ability to evolve to higher speeds as bandwidth demands grow.
But what is driving the need for Gigabit Ethernet? Increasing levels of Internet and Intranet traffic, along with the move to switched architecture and Fast Ethernet in the horizontal has made gigabit capacity a necessity in many of today’s backbones. Using the “net” means more backbone traffic; switched architecture providing dedicated bandwidth to each user adds to that demand. More demanding audio and video applications load the backbone even more. As bandwidth capacity grows, so too grows the demand.
The development of the Application Service Provider (ASP) market has also forced companies to rethink their network capacity. Supporting applications over the network, instead of at the desktop, means major savings in administrative costs, but requires a significant increase in network bandwidth. Implementing Gigabit Ethernet has proven to be an effective solution for many companies.
Gigabit Ethernet has two different fiber implementations, 1000BASE-SX and 1000BASE-LX, in addition to a copper implementation, 1000BASE-TX. 1000BASE-SX has been the most popular implementation in the LAN, as it provides a low-cost, limited distance multimode solution. However 1000BASE-LX can operate over longer distances, often making it the choice for campus installations where distances are greater. 1000BASE-TX links are used for shorter server links and horizontal workstation links over Category 5e and Category 6 cabling.
The use of optical fiber cabling for Gigabit Ethernet installations is of growing interest and concern to system designers, end users and installers. Many legacy fiber installations were not installed with the requirements of Gigabit Ethernet in place, and may require some modification/upgrade to be able to support these speeds. Sources and detectors must meet far more stringent requirements, and in turn, place greater stress on the cabling network.
Short wavelength Gigabit Ethernet, or 1000BASE-SX, operates at 850 nm over multimode fiber, and makes up 80 to 90 per cent of the installed base. Low cost vertical cavity surface emitting lasers (VCSELs), coupled with the less expensive multimode packaging, provide a cost-effective solution for most building backbones.
1000BASE-SX is designed to support 220-metre link distances using standard 160 MHz-km. 62.5-micron fiber, and can support 275-metre links using 200 MHz-km. 62.5 fiber. If a user has 500 MHz-km. 50-micron fiber, 550-metre link distances can be supported.
A number of vendors have certified laser launch characteristics of their fiber cable and can support 300 and even 600-metre link distances over enhanced 62.5 or 50 micron fiber. 300-metre link support is important; a study commissioned by IEEE as part of the Gigabit Ethernet standard showed that over 92 per cent of in-building links are * 300 metres.
The link loss requirements of 1000BASE-SX Gigabit Ethernet place a premium on low loss connections. With 200 MHz-km. 62.5-micron fiber, approximately 2.5 dB is allocated for cable and connector loss. (The TIA/EIA-568B standard allows 3.2 dB, by using the unallocated margin in the Gigabit Ethernet loss model). While the model provides a 7.5 dB total power budget, 5.0 dB is allocated to system design issues, including intersymbol interference. Since approximately 1.0 dB of loss is allocated to the cable alone in a 300-metre link, only 1.5 dB remains for the connectors. Network design flexibility is severely limited if poorly performing connectors are used.
There is much confusion in the market about enhanced multimode fiber, and how this fiber is able to support Gigabit Ethernet over extended distance links (>300m). The confusion centres around bandwidth measurement and the relevance of an overfilled bandwidth measurement to laser launch conditions.
Overfilled bandwidth measurement closely models LED launch characteristics, but does not provide a good indication of laser performance. LEDs typically project a wide beam of light across the cross-section of the fiber, “overfilling” the core. As a result of this wide beam, a large number of modes are transmitted in an overfilled launch. Lasers project a much more focused beam. Since this beam is normally focused tightly around the core centre, a much smaller number of modes are propagated, and the behaviour of each mode is more critical to the signal integrity.
This signal integrity is more important as network speeds increase, because the amount of time the receiver has to detect each bit is decreased when the bit rate/second in increased. A “wider” detected signal may not be a problem for lower bit rate signals with a large bit period, but when speeds approach gigabits/second, very little room for error exists. And when modes do not arrive at the receiver at the same time, bit errors result.
In an effort to shed some light on this issue, TIA FO2.2 recently completed work on extended distance support for Gigabit Ethernet over 62.5 micron fiber. The goal of the TIA study was to set guidelines that would support 1000BASE-SX over link distances of 500 metres, using 200 MHz-km. 62.5-micron multimode fiber. This work determined that both the launch characteristics of the laser and the performance of the fiber in the launch region had a significant effect on the laser bandwidth of the fiber. After extensive study of multiple transceiver and 62.5 micron fiber combinations, it was found that by restricting the mode power distribution of the laser source and measuring fiber within that launch area, transmission over the 500-metre link distance could be assured.
FO2.2 defined new measurement methods for fiber bandwidth using restricted modal launch and set requirements for transceiver power distribution. Restricted modal launch defines laser launch requirements for new high-performance 1000BASE-SX transceivers. No more than 25 per cent of the power may be within a 4.5-micron radius from the core centre, but 75 per cent of the power must be within a 15-micron radius.
The ability to support these extended distance links will not be achieved using legacy equipment or installed fiber. New multimode fiber that has been measured using the new restricted modal launch procedure will be required, along with new electronics that have transceivers manufactured to meet the new launch requirements. At this time, very few transceiver manufacturers have expressed an interest in producing these new transceivers. The IEEE length distribution study, which shows that less than eight per cent of in-building links are longer than 300 metres, indicates that the market size is not large and could be served using other, currently available options.
Long wavelength Gigabit Ethernet, or 1000BASE-LX, operates at 1300 nm and can run over singlemode or multimode fiber. 1000BASE-LX can support 550-metre link distances over 62.5 or 50 micron multimode fiber, or 5.0 kilometre link distances over singlemode fiber. However, LX electronics are more expensive, and a special mode conditioning patch cord is required to guarantee that the system will operate over multimode fiber.
Singlemode electronics are more expe
nsive to produce because of alignment requirements and the laser technology. Aligning a laser source and detector with an 8-10 micron singlemode core, instead of a 50 or 62.5 micron multimode core, is much more difficult — and expensive. VCSELs used in short wavelength Gigabit Ethernet electronics are less costly than the Fabry-Perot lasers used in long wavelength (1000BASE-LX) equipment.
The mode conditioning patch cord moves the laser launch away from the centre of the multimode core, where irregularities in the refractive index profile can degrade the signal. This is necessary in some multimode fibers where defects at the centre of the core make light transmission unpredictable. These centre defects, or “centre dips”, are a result of the manufacturing process, and occur in fiber manufactured with both inside vapour deposition (MCVD) and outside vapour deposition (OVD). Since low-speed technologies used LED sources with an overfilled launch, this defect at the centre of the core did not affect transmission of the light signal. However, when using high bit rate singlemode lasers that launch into the centre of the multimode core, these defects are fatal, and create bit errors.
Fiber manufacturers now realize that these centre defects are undesirable, and control their process to better control the index profile at the core centre. However, there is a considerable amount of installed fiber that has this defect. To utilize this fiber, the IEEE Gigabit Ethernet standard sanctioned the use of a mode conditioning patch cord. This special patch cord offsets the singlemode launch away from the core centre to a position where the index profile of the core is uniform, and the transmitted light behaves predictably. The patch cord accomplishes this mode conditioning by splicing an offset singlemode fiber to a multimode fiber on the transmit side. While this is effective in “conditioning” the launch, it is not an easy thing to do and is expensive. When these cords are required, the cost of a 1000BASE-LX solution is significantly more expensive than 1000BASE-SX.
1000BASE-LX is an ideal solution for a typical campus environment. With its longer distance support, especially using singlemode fiber, LX is ideal. The IEEE 802.3z link distance survey showed that over 80 per cent of campus links are more than 500 metres — ideal for 1000BASE-LX.
Gigabit Ethernet installations are growing at a rapid rate. While many cabling issues have been resolved within the standards committees, work continues to ensure that end users have a robust infrastructure that will support this demanding technology.
In fact, even as end users move to adopt the latest standards based Ethernet technology, work is underway to develop a 10 Gigabit Ethernet standard. This will be the first Ethernet standard that will not only be a LAN standard, but will also encompass WAN requirements, allowing users to connect across the service provider network using native LAN technology.
Expected to be adopted in 2002, infrastructure and electronics providers are already developing solutions to support 10-gigabit speeds. Next generation multimode fiber supporting 300-metre link distances, using low cost 850 nm serial technology, appears to provide the lowest cost short distance links. A number of other technologies have been proposed for longer distance links.
Many challenges remain before 10 Gigabit Ethernet becomes a reality, but cabling professionals must remain up to date with the latest technology so they can inform clients of all available options.
John Kamino is a Business Manager for Lucent Technologies – Optical Components Group. He was previously the fiber offer manager for Lucent’s structured cabling business, and has held positions in sales and engineering. He has a BS in Chemical Engineering from the University of Nebraska-Lincoln and an MBA from Mercer University.