Examining current developments in fiber standards might make things clearer for the end user.As I was deciding how to present the latest developments in the world of fiber standards, I put myself in t...
March 1, 2000
Examining current developments in fiber standards might make things clearer for the end user.
As I was deciding how to present the latest developments in the world of fiber standards, I put myself in the position of an end user who has to make sense of all the available options.
Should I go with a distributed or centralized fiber topology? What type of fiber do I install today? Will the cabling meet my needs looking forward 10 years from today? The aim of this column is to help answer these questions, by providing information on current developments in the standards.
The standard topology in the TIA/EIA 568-A standard is a distributed topology, with optical fiber used predominantly for backbone cabling and Category 5 or higher performance copper used for horizontal distribution. If fiber is provided to the desktop, it can be used instead of, or in addition to, copper. In making this decision, the end user should be aware that there is a huge imbedded base of networking equipment running over copper that still needs to be supported. This tends to skew the decision to make provision for copper in the horizontal.
One way to provide a fiber to the desk solution is to use a centralized fiber topology according to TIA/EIA bulletin TSB-72. In this approach, the optical fiber cable extends from the work location through the telecommunications room (formerly called a telecommunications closet) and is terminated in a centralized location in the building that houses all the data networking equipment. The maximum distance supported is 300 meters, which can accommodate up to 92 per cent of existing buildings.
The centralized fiber approach lends itself well to a collapsed backbone architecture and requires an efficient system for managing a lot of fibers. The decision to go with this approach is an economic one, but one should take into account the building design, cabling and electronics.
CHOOSING OPTICAL FIBER
The big question is what type of optical fiber to deploy for backbone or horizontal cabling. Local area networking in the future will require the capability of 10 Gb/s data rates in the backbone and at least 1 Gb/s to the desktop. The maximum distances that are supported for different fiber types and applications are illustrated in the above table. Existing multimode fibers are significantly limited in reach for gigabit data rates and beyond.
It should be noted that gigabit-networking equipment using VCSEL lasers operating at 850 nm (1000BASE-SX) is more economical than networking equipment using conventional lasers operating at 1300 nm (1000BASE-LX). For this reason, the use of 50/125 m optical fiber (compared to 62.5/125 m) is preferred for new installations.
What about 10 Gb/s data rate capability? Here things get more complicated. There is a high-speed study group (HSSG) within IEEE that is looking at different 10 Gb/s Ethernet technologies. The group has a timeframe to select one or more technologies and issue a standard by 2002.
At the last meeting it was reported that the choice of technologies has been narrowed down to six options among the 18 that were studied. Four of the options are optimized for single-mode fiber operation and two of the options are optimized for multi-mode fiber operation. Depending on the choice of technology, the single-mode solutions can support distances from 2 km. to 40 km. The 10 Gb/s multi-mode solutions are targeted for a distance of 300 meters.
Let’s look at the two multi-mode solutions in more detail. The first solution is to use a 10 Gb/s serial data rate and 850 nm light sources. This solution will support a maximum distance of only 100 meters using standard multi-mode fibers (see table) and up to 300 meters using a new higher bandwidth 50/125 m multi-mode fiber. The specifications for this fiber are currently under development at TIA.
Another solution proposed by HP is to use four light streams that are multiplexed onto a single fiber at a rate of 3.125 Gbaud using 1300 nm light sources. This technology will support 10 Gb/s data rates over existing multi-mode fibers for distances up to 300 meters in the backbone, but will require more expensive electronics.
So, where does this leave the end user? The standards and technologies are evolving, and part of this evolution is the availability of better grades of multi-mode fibers that can support longer distances and higher data rates. Users should inquire about availability of these fibers and have plans to eventually migrate the network to 10 Gb/s in the backbone and 1 Gb/s to the desktop.
Ultimately, there is no black and white answer. The choice of topology and cable type will depend on current and future applications and on the relative economics involved.CS
|OPTICAL FIBER CABLING DISTANCES (in meters) FOR DIFFERENT APPLICATIONS|
|850 nm||850 nm||1300 nm|
|100 Mb/s FDDI||2000|
|155 Mb/s ATM||1000||>1000||2000|
|622 Mb/s ATM||300||>300||500|
|1Gb/s SX / LX||220 / 550||>800||5501|
|10 Gb/s SX / LX||*1002||3002||3003|
|1) mode conditioning patch cord 2) serial 850 nm 3) 4-WWDM 1300 nm|
Paul Kish is a senior Product Manager, IBDN Systems & Standards at NORDX/CDT, Pointe Claire, PQ. He is also the Chair of the TIA TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.