What is GPON, and why it replaced ADSL for home broadband

GPON (Gigabit-capable Passive Optical Network) is a fibre access network that brings internet and telephone to homes over one shared fibre. An optical line terminal (OLT) at the exchange sends light down a single fibre, passive splitters in the field divide it between up to 64 homes, and an optical network terminal (ONT) at each home turns it back into Ethernet, Wi-Fi and a phone line. Nothing between the OLT and the ONT needs power.

In the areas I have worked, GPON replaced copper telephone lines and ADSL broadband. ADSL gave customers up to 2 Mbit/s. GPON packages run from 10 to 100 Mbit/s, most customers are on about 50 Mbit/s, and content from cache servers and BDIX loads at up to 450 Mbit/s. The same fibre also carries the customer's telephone line.

Three parts: OLT, splitter and ONT#

Splitters are often used in stages. On the links I work on, the light passes through two 1:8 splitters in series, so one OLT port serves up to 64 ONTs.

One fibre carries both directions#

The two directions use different wavelengths, so they do not interfere on the same fibre (ITU-T G.984.2).

• Downstream (OLT to homes): the OLT sends one stream, and the splitters copy it to every ONT. Each ONT keeps only the data addressed to it and drops the rest. Downstream data is encrypted, so one customer cannot read another's traffic (ITU-T G.984.3).
• Upstream (homes to OLT): all ONTs share the same fibre back, so they cannot send at once. The OLT gives each ONT short time slots, and each ONT sends only in its own slot (time-division multiple access, TDMA).

The numbers that matter#

The 2.488 Gbit/s downstream is shared by every ONT on the port. If 64 customers on 50 Mbit/s packages all download at full speed at once, they ask for 3.2 Gbit/s, more than the port can carry, and each gets about 39 Mbit/s (2488 ÷ 64). In practice not everyone is busy at the same moment, so packages hold most of the day and slow down at peak time.

What GPON does better than copper#

No power between the exchange and the home#

Splitters are glass, not electronics. There is nothing in the street to power, cool or back up with batteries, so a power cut in the area does not take the street equipment down.

Much higher speeds for customers#

On the same routes, ADSL over copper gave customers up to 2 Mbit/s. GPON packages run 10–100 Mbit/s, about 50 Mbit/s for most customers.

Content served from inside the country is faster still, up to 450 Mbit/s. That covers two kinds of traffic: popular content kept on cache servers in the operator's own network, and traffic exchanged with other Bangladeshi networks through BDIX, the Bangladesh Internet Exchange. Neither has to cross the international link, so it is not limited by the package's internet speed.

Fewer faults#

In my experience, GPON lines give fewer faults than the copper lines they replaced. Fibre carries light, not current, so it is not affected by moisture in joints or electrical interference the way copper pairs are.

Internet and telephone in one package#

The ONT has a telephone port as well as Ethernet and Wi-Fi. One fibre and one box at the customer's home carry both services, which makes it simple to sell internet and phone as a bundle.

Every ONT can be checked from the exchange#

The OLT shows each ONT's status: online or offline, the optical power it receives, and its alarms. The ONT also acts as the customer's home gateway, and its settings can be checked and changed remotely (ITU-T G.988). Many complaints can be diagnosed, and some fixed, before anyone goes to site.

Where GPON falls short#

One cut takes down many customers#

Everyone behind a splitter shares the fibre above it. When road work cuts a feeder fibre, every ONT behind that point loses service at once: with a 1:64 split, up to 64 customers on one OLT port.

Bandwidth is shared#

As the numbers above show, all ONTs on a port share 2.488 Gbit/s down and 1.244 Gbit/s up. When many customers stream or download at the same time, speeds drop.

The ONT needs power at the customer's end#

The network itself needs no power in the street, but the ONT at the home does. During load shedding, an ONT without backup goes offline, and the telephone on its phone port goes silent with it. A copper telephone kept working in a power cut because the exchange powered the line.

GPON and ADSL side by side#