A Good Many Networks

Shutterstock / Sparkus design
Shutterstock / Sparkus design

Corporate computer networks are called local area networks (LAN) to distinguish them from wide area networks, such as the phone system or the internet. The traditional ethernet LAN uses a standardized cabling network of twisted-pair copper or fiber cables connecting switches around a building that connect to users or wireless access points. Each cabling section is limited to 100 meters, and the speed of the network is limited by the grade of the cabling, which often required upgrades as network speeds increase.

A passive optical LAN is an alternative to the traditional LAN. Optical refers to fiber optics, of course. Passive refers to the way a passive optical LAN replaces electronic ethernet switches and structured cabling with optical couplers and single-mode optical fiber. This creates a network that is simpler and cheaper to install and operate and almost infinitely upgradeable.

A passive optical LAN uses the same technology as fiber to the home (FTTH). About 15 million Americans and hundreds of millions worldwide get their broadband internet over fiber optics to the home, mostly on networks based on the international Gigabit Passive Optical Network (GPON). Because of the extremely high volume of GPON products, the networking equipment is cheap, reliable, easy to install and readily available from many sources.

The GPON network sends optical signals downstream on one fiber, always single mode, and then splits the signal in a passive optical coupler to connect as many as 64 fibers, one for each user. Signals from the users back to the network head end go the other way on the same fiber, and the passive coupler works in reverse to combine the signals headed back to the head end. One big advantage of a GPON network is there is no need for electronics between the computer room and the user, just fiber. The telecom closet needed in traditional LANs is not required.

At the user end is either a 4-port ethernet switch or a triple play (voice, data and video) converter similar to ones used in residential FTTH networks. Downstream bandwidth is 2.5 gigabits/second and upstream is 1.25 gigabits/second. And unlike most optical networks that work over two fibers sending data in opposite directions, the GPON network sends signals both ways at different wavelengths in the same fiber. For the network users, both networks look the same with Cat 5 cabling ports for wired devices or a wireless access point for Wi-Fi connections.

The GPON network is much simpler in design and installation than a traditional LAN, which makes it much cheaper to operate. The cost savings increase with the number of users, since FTTH networks are designed for a large number of users, while traditional LANs get cumbersome when the number of users becomes large.

Another major drawback for traditional LANs is size; they are designed for one building and require switches and cabling less than 100 meters apart to connect users in each building. GPON was designed for FTTH in cities, so scaling a network to cover large areas is simple: just run single-mode fiber to wherever connections are needed up to 20 km away.

At this point, a GPON passive optical LAN already makes sense. But wait, there’s more.

The international committees behind the GPON standards knew that networks were always getting faster, so they have developed standards for several versions operating at 10 gigabits/second, or 10 GPON. All of those standards are designed to be overlaid on a PON cable plant that is already running regular GPON. So, if you have a GPON network now, you can add 10 GPON to the current cable plant and operate both networks simultaneously. That allows seamless upgrades, or having two independent networks, separating high bandwidth users from lower ones, such as those using mobile devices to connect with Wi-Fi or separating employees and guests.

Who’s using passive optical LANs now? The first users were mostly government facilities with a large number of users on the LANs where the cost savings and wide area coverage were a big selling point. Sandia National Laboratories installed one of the first big passive optical LANs with more than 10,000 users spread out over more than 250 buildings. Now schools, libraries, hospitals, hotels and many other commercial users have become passive optical LAN users.

Network users and contractors may be reluctant to try something new, especially if it involves installing single-mode fiber inside buildings. But other networks, such as wireless, distributed antenna systems and hyperscale data centers, are operating on single mode. Single-mode is a technology that contractors must learn if they want to stay current.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor

Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.

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