Don't delay the move to fibre

As the cost of running increasing quantities of data along ageing copper networks spirals, migrating to optical technology may be...

As the cost of running increasing quantities of data along ageing copper networks spirals, migrating to optical technology may be the most cost-effective option for network managers. Philip Hunter reports

Traditional copper-based network solutions such as 10/100 Base-T Ethernet are slowly but inexorably being squeezed between the advancing waves of fibre and wireless solutions. Fibre is invading enterprise backbones for campus-wide inter-workgroup communications and server consolidation, while wireless Lans are taking over in places where mobile users congregate.

For the rapidly growing area of storage area networks (Sans), fibre is the only show in town. Based on the Fibre Channel standard, Sans use the same fibre physical access mechanism as Gigabit Ethernet. The growth of Sans is giving fibre a leg up into corporate networks and this is persuading some IT managers that it is the only viable solution for future broadband communications - at least within the backbone.

Fibre has not yet made significant inroads into mainstream workgroups for desktop connections, but the rapid increase in CPU power and the imminent arrival of the Infiniband bus technology are leading to PCs capable of transfer rates of 1gbps - the point at which copper cabling begins to run out of steam. Some enterprises, particularly in the financial sector, are installing fibre as a matter of course to provide greater capacity to support emerging technologies, even if they do not need the bandwidth now.

Companies whose business embraces multimedia technology already need gigabit bandwidths and for other enterprises fibre is certain to receive consideration soon. Decisions on whether to migrate cannot be delayed much longer.

IT managers encountering fibre for the first time will have to dispel several myths. The first is that by cabling with fibre you are automatically future-proofing your network. The truth is that fibre optic cable has gone through almost as many iterations as copper in the journey to higher bandwidths. Manufacturing standards have improved, as has the type of fibre preferred for given applications.

According to Peter Kjeldsen, an analyst at Gartner Group, the situation with optical transmission has been clouded by the fact that the economics of fibre change as you move down from long-haul carrier trunk networks running hundreds of miles underground to campus networks within buildings. In between are metropolitan area networks (Mans), which cover up to 20 miles.

Optical transmission over long distances originally required electrical signal regeneration every few miles. This was done by using repeaters that amplified the degraded signal and then reissued it into a new set of fibres. Over a long-haul network, a large number of these repeaters were needed, adding enormously to the cost, so it was worth investing in narrower-gauge fibres, called multimode, that could sustain the signal for greater distances. Fewer repeaters were then needed, but the cable itself was more expensive, and so this was still a significant cost.

Then, about 10 years ago, a technology called DWDM (Dense Wave Division Multiplexing) entered the long-haul network arena, bringing costs down substantially.

DWDM saves on the number of fibres needed by multiplexing multiple channels along a single fibre. Each channel is encoded onto a given wavelength of light so, by carrying a number of different wavelengths, a single fibre can carry multiple channels. The current limit is about 100 channels, which is a total bandwidth per fibre of 250gbps if each channel is 2.5gbps.

In a few years it is likely that a single fibre will be able to carry 1,000 channels this way, each at 10gbps, bringing the total capacity to 10tbps. Lucent Technologies has already demonstrated transmission at 30tbps along a single fibre in its laboratories.

Additional cost savings come from the fact that DWDM allows a single repeater to be used for multiple channels, rather than needing a separate repeater for each fibre.

An alternative would be to have one large channel, rather than carrying multiple channels along a single fibre, that is, to have one 400tbps channel instead of 40 10gbps channels. This would require just one sophisticated line card at each end of the link to drive the single channel.

Whereas DWDM, while needing single repeaters for each fibre, has to have a line card at the source and destination of every channel encoded over that fibre. With a single channel solution, the repeaters are more expensive but the total line card cost is less than for an equivalent DWDM capacity.

Therefore, the economics depend on the distance being traversed, with DWDM more likely to score over the longer haul.

A third option is to have multiple fibres, each carrying a single channel at current speeds. This is cheapest in terms of the terminating line cards, but more expensive than either of the other options in terms of repeaters and cable. This solution is more likely to win at the enterprise level and, in some cases, for Mans. A further consideration is the grade of the fibre used.

Another myth is that DWDM technology will sweep all before it and eventually take over on an end-to-end basis. There is a seductiveness about DWDM's elegant configuration, whereby applications and user groups can be segregated onto their own wavelengths, that appears to make administration much simpler. But none of this matters if the economics are not right. And they are unlikely to be in the case of DWDM at the enterprise level, at least for the foreseeable future - it will be just too expensive.

Although there are uncertainties at the physical level, there is only one winner for the link-level transmission within the optical network at the enterprise level - Ethernet. Gigabit Ethernet is purely optical and is now installed in a number of enterprises. The next version of the standard for 10gbps transmission is about to be ratified, and will be implemented at the metropolitan level, and later in enterprise networks.

The significant aspect of 10 Gigabit Ethernet, apart from its greater speed, is the incorporation for the first time within Ethernet of a Wan interface. This enables enterprise or metropolitan Ethernet networks to be extended over long distances via public Sonet/SDH networks.

SDH is the physical transmission technology used in public carrier networks within Europe. Sonet is the US version. In effect, Ethernet is bridged over the Sonet/SDH network, with virtually no drop in speed, so end-to-end transmission at 10gbps becomes possible.

According to Mark Iliffe, European support manager at networking equipment supplier Foundry, 10 Gigabit Ethernet is a highly significant development for enterprises, even if they do not appreciate it yet. As soon as they run out of capacity at the single gigabit level,

10 Gigabit Ethernet will provide a more cost-effective upgrade path than the alternative of having multiple single gigabit links.

"A lot of enterprise customers today are aggregating single gigabit ports, which is alright until you run out of fibres in the ground," he said.

This comes back to the trade off between installing more fibre or upgrading to a higher speed transmission technology.

A similar trade off calculation is having to be made by many enterprises as they agonise over whether to migrate existing campus networks from copper to fibre. There comes a point when a migration to optical technology cannot be delayed any longer and even becomes the cheaper option, as the cost of pumping more data down ageing copper networks can become prohibitive.

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