The following is from Network World Fusion, & addresses the Q's relating to PC Weeks 'disinformation' that surfaced a few weeks back...
Yes, there is an issue with distance limitations and Gigabit Ethernet.
No, it does not mean the end of the world for this somewhat promising
technology. The fact is, while ATM vendors everywhere have been jumping up and down all month pointing to limitations of Gigabit Ethernet over multi-mode fiber, the market and standard are moving along very nicely. And while the final spec may not allow multi-mode fiber to be the ultimate extended campus backbone technology, it will do little to slow deployment of this technology. Here's why:
The Issue: Recent lab tests have discovered a bonus feature of
transmitting high-speed signals along a multi-mode fiber: DMD (or
Differential Mode Delay for the acronysm-impared). DMD occurs when
you attempt to transmit a laser signal down a multi-mode fiber for an
extended distance. Concern has been raised recently that DMD might
prevent Gigabit Ethernet from reaching extended distances. Just what is an extended distance? Well, that depends upon the source. Based on the panic-stricken people that called me this past week, that extended distance ranged anywere from 50 to about 200 meters.
According to IEEE, however, an extended distance for Gigabit Ethernet is between 260 and 550 meters (roughly 850 to 1750 feet for the
metric-impared), as detailed in the specification. These distances vary depending upon the diameter of the multi-mode fiber (62.5 micron has a shorter distance than 50 micron) and the type of wavelength used to power the fiber (short wavelength has a shorter distance than long wavelength).
The Reality: This is the non-issue of the week. At a recent IEEE 802
Plenary meeting, the 802.3z Gigabit Ethernet Task Force addressed this
DMD concern. Members agreed to some slight changes to the
specification that would allow controlled launch of the 1000Base-LX and SX (long wavelength and short wavelength) specs on schedule - all within the distance limitations agreed upon previously (260 - 550 meters). The consensus is that most vendors will not have to do very much to their existing products to meet the spec. By the way, single-mode fiber is not subject to the DMD limitation. But that is not the issue. Let's assume that Gigabit Ethernet does get limited
to a total distance of 200 meters over multi-mode fiber. Will Gigabit
Ethernet crash and burn? Nope.
First off, Gigabit Ethernet will find a nice market as a server interconnect technology - which doesn't need to go more than a few feet. In even the worst server farm scenarios, a single shot of 100 meters is going to be the extreme.
Secondly, Gigabit Ethernet as a backbone technology for a single building doesn't need to go more than a hundred meters. This is where you'll see the highest demand for the technology - the building
backbone and departmental switch markets are ripe for the new wave of Gigabit/Router products now emerging in the market.
Thirdly, you're not likely to see much demand for Gigabit Ethernet as a campus backbone. Users we have spoken to seem to be budgeting the least amount of money in this area for Gigabit Ethernet (compared to the server/workgroup/department areas).Thus, the financial impact of not being able to go hundreds or even thousands of meters will be just like the impact on volume shipments - negligable. For those users that do have extended campus backbone requirements (i.e., above a few
hundred meters), single-mode fiber is a very likely - and viable - alternative. True, it may be difficult for some users who have already packed their conduit chock-full of multi-mode. But nobody ever
said that implementing Gigabit Ethernet on an extended campus was guaranteed right up there with life, liberty and the pursuit of happiness.
Interestingly, we have spoken to some large and high-profile users who have have recently decided to scale back their ATM plans and implement a greater level of Gigabit Ethernet. Are they concerned about distance limitations? Only to a point. While they plan on less ATM in the
workgroup/department segment (where Gigabit Ethernet will make its headway), the extended campus plans still call for ATM as the core switching fabric at this time (partly to support multiple media across the net - i.e., data, PBX, etc...) via a combination of ATM switches and Gigabit Ethernet with ATM uplinks.
Even still, asking Gigabit Ethernet users to deploy single-mode fiber for extended campus communications is a lot like asking an NSX or Lambourghini owner to spend $2k every 5,000 miles for new tires - it's no big deal. If you want the speed, you've got to be prepared to pay where the rubber hits the road. So in review: Gigabit spec still on target ... Distances allowable are acceptable ... No need for panic! |
