Using GPON for FTTD Applications

With increasing bandwidth requirements, modern-day businesses are faced with tough

decisions on what to do with the existing cabling within the building. As speeds increase

to 1 Gbps to the desktop, new Category 6 copper cables are often recommended for

better transmission characteristics. However, replacing the cables with new Category 6

cabling is a labor intensive and costly undertaking. And it’s sure that bandwidth

requirements will continue to grow and eventually exceed 1 Gbps and drive toward 10

Gbps.

The logical alternative is to install a fiber-based distribution network that can handle

speeds well beyond 1 Gbps, that is, Fiber-To-The-Desktop (FTTD). For the FTTD, you

have to choose the best optical technologies: Gigabit Passive Optical Network (GPON)

based optical technologies. This blog introduces the concept of using GPON for FTTD

applications to serve the needs of the modern-day business.

Introduction

Today’s corporate networks, government facilities and military installations are built using

two, and sometimes three, separate copper architectures: One to carry data (typically a

Category 5 copper network), a second to carry voice (typically a Category 3 copper

network), and in some cases a third where coax delivers video. New technologies such

as GPON are fully capable of supporting all of these services on a single fiber distribution

architecture. The copper deployment model creates an environment that in today’s

technology is wasteful and inefficient to maintain. GPON allows an operator to effectively

deliver all of these services with the right user experience. Its QoS and high bandwidth

capabilities are the mechanisms needed to converge voice, video and data all onto the

same fiber optic network allowing more efficient maintenance, cabling and overall

performance.

In addition to the installation of more switches and routers to address the continuous rise

in bandwidth, redundant networks and equipment requirements have led to crowded

equipment rooms, complex wiring closets and increased HVAC requirements.

Converging all of these services onto a single GPON distribution platform provides the

bandwidth needed while significantly reducing the equipment, cabling and power

required.

Already deployed by many telecommunications carriers, GPON has quickly established

itself as the worldwide standard for delivering voice, data and video. Among other

benefits, GPON provides an enormous amount of bandwidth—2.5Gbps downstream and

1.25Gbps upstream—over a single strand of glass.

The PON technology consists of an Optical Line Terminal (OLT) in the data center and a

series of Optical Network Terminals(ONTs) at or near the user’s desktop. Starting at the

OLT, voice and data are transformed from an electrical format into optical signals. This

traffic is then sent over the fiber optic network to the appropriate ONT, where it is

separated back into electrical voice and data. This architecture uses purely passive

optical components such as optical splitters between the OLT and ONT, further reducing

the chance of equipment failure.

The core underlying technology is still Ethernet, with GPON Encapsulation Mode (GEM)

as the packaging format. GEM packages the IP packets efficiently with minimum

overhead as they transit between the OLT and ONT. Each fiber can be shared by up to

64 ONTs, minimizing the amount of fiber required. Although multiple users share the

PON, robust QoS and bandwidth mechanisms ensure that the traffic is correctly

prioritized and that each user gets the required bandwidth.

Advantages of GPON

GPON offers some key advantages over Ethernet and current voice networks.

Robust Security

Fiber is inherently harder to tap into than a copper-based circuit. A fraudulent ONT

cannot be spliced onto a fiber optic network, because the GPON system identifies each

ONT based on pre-defined serial numbers and operator settings. Critics of fiber highlight

the fact that all users receive the same downstream broadcast, creating the potential for

eavesdropping. However, to counter this threat, PON employs a 128-bit advanced

encryption scheme. The scheme incorporates a two-way key exchange, making it

virtually impossible to intercept another user’s data. As well, any device used to tap into

the fiber must be able to decipher GPON GEM ports and traffic containers, which is not a

typical function in standard Ethernet devices. All of these inherent capabilities make

GPON a very secure environment for transporting sensitive data.

Lower OPEX

Two different distribution networks are used in most corporate environments: one carries

voice and the other carries data and video. In large enterprises this has led to congested

wiring closets and equipment racks with daisy-chained switches. In addition, the mass of

copper cables reduces air flow and necessitates increased cooling system requirements.

With GPON, the voice and data network can easily be collapsed into one fiber optic

infrastructure for all services. GPON technology offers tremendous economies of scale.

One GPON chassis can support up to 4608 users on a 1×64 split of fiber. Typical

deployments use a 1×32 split with 2304 users per chassis.

In most deployments, Ethernet switches are stacked in daisy-chains in an equipment

closet. This creates a concentrated point for significant heat dissipation, requiring HVAC

within the closet. With GPON systems a passive non-powered splitter is placed in the

closet and removes HVAC concerns.

In addition, overall power requirements for an optical Ethernet solution equivalent to the

GPON solution will be much higher. A typical 388 port optical Ethernet switch with

Gigabit Ethernet optical ports will have a minimum power supply of 1400 watts or 3.6

watts per port. A typical 12 port optical Ethernet switch is even less efficient at around

4.58 watts per port. A typical GPON OLT system serving 2304 users will have around 0.6

Watts per user—between nearly 6 to 7.6 times less power consumption.

Higher Bandwidth

In most deployments, Ethernet is limited to 1000Mbps shared among many users. In one

of the best case scenarios, 24 users are connected for an average of 41.7 Mbps per

user. More commonly, the switches are stacked in daisy-chains, significantly reducing

true bandwidth per user for applications such as e-mail, access to graphics/video or

databases. In GPON, the full downstream line rate of 2.5 Mbps across 32 users delivers

an average of 78 Mbps per user—a 90 percent increase in available sustained

bandwidth. When needed, an ONT can deliver up to the 1000 Mbps.

In recent years, fiber has come to the forefront of communications as a secure,

economical and scalable alternative to copper for enterprise and government

applications. In particular, GPON has been widely adopted in North America as the

leading fiber optic networking technology. It has typically been deployed for Fiber-To-

The-Home (FTTH) applications, but it can equally serve large corporate environments in

FTTD applications. For its part, FTTD can collapse the voice, data and video networks

into one and has the potential to deliver virtually unlimited bandwidth while escaping the

security constraints and OPEX of copper.

The Deployments of GPON in FTTD Applications

In a campus environment, the typical approach is to locate the OLT shelf in the main

building and then run fiber to the individual satellite buildings. In this case, the fibers

reach optical splitters placed inside the buildings. The fibers in turn fan out at the optical

splitters, reaching individual ONTs or users. To connect voice, data and video traffic back

to the network core, the OLT offers multiple 1Gbps or multiple 10Gbps uplinks, providing

bandwidth that can scale as your traffic demands rise. For a campus scenario, GPON is

especially effective because it eliminates the need for active power consuming

equipment in the closets or basements of many of the buildings.

A slightly different approach can be used for large buildings. In this environment, the OLT

shelf is located in the basement with the fibers running up the elevator shaft or riser pipe.

Optical splitters are located on each floor where the fibers fan out to individual users. The

OLT uplinks are connected directly to edge routers in the basement.

In either scenario, the GPON system can allow each PON to easily support up to 64

ONTs.

Summary

FTTD offers a wide range of cost and power saving opportunities compared to copper

and optical Ethernet networks. And using GPON for FTTD can collapse voice, data and

video networks into one while overcoming the security constraints and evolution issues

associated with copper networks. Therefore, in large institutional settings and campus

environments, an FTTD architecture using GPON makes a compelling network

alternative.