xPON

PON stands for Passive Optical Network. There are many “flavors” of PON including BPON, EPON, GPON, XGS-PON, etc. First we will look at GPON, as it is the most ubiquitous type of PON in use today.

GPON

The passive in PON means that subscribers are multiplexed onto a single fiber strand based on equipment that does not use power. There exists a splitter that uses a prism to passively split the fiber light to all subscribers in the area. In GPON, you can have up to 64 separate subscribers off the same, single fiber strand. In the CO (central office), this fiber strand connects to a single port on the network equipment.

In PON, the CPE is called a ONT (Optical Network Terminal), and the “PE” device is called an OLT (Optical Line Terminal). (The OLT isn’t truly a PE, it would connect to a real PE upstream).

Below is a typical diagram for a PON architecture:

It is important to note that the prism does not filter light, as in WDM. Instead the prism simply splits and “mirrors” the downstream traffic to all ONTs, and reflects the upstream traffic from any single ONT to the OLT.

Downstream Traffic in GPON

Traffic in the downstream direction is easiest to explain. Downstream traffic is simply sent from the OLT port and broadcast to all ONTs. For example, let’s imagine there are 64 ONTs on a network. For any given unicast packet that is sent in the downstream direction from the OLT to a single ONT, 63 ONTs must discard the packet, and only 1 ONT is actually interested in receiving the packet.

This may sound extremely inefficient and insecure, but there is some complex technology used to workaround potential problems. The ONTs all register with the OLT, and in the process create a sort of encryption channel. When a packet is destined for only a single ONT, the OLT encrypts the payload in such a way that only the target ONT can decrypt it. All other ONTs, upon receiving the traffic, realize that the payload is not destined for themselves and drop the traffic. They could not decrypt it if they wanted to, anyways. This encapsulation is called GEM (GPON Encapsulation Method).

It is a little crazy to imagine that if you are a subscriber of this service, and you download a large file, literally every other ONT off your line will actually “receive” the file as well. But the other ONTs cannot decrypt the traffic. At any given time, almost all the traffic a single ONT “sees” is actually dropped, due to the broadcast nature of GPON.

Upstream Traffic in Gpon

Traffic in the upstream direction uses a different approach - TDM. Each ONT synchronizes its clock with the OLT. The OLT gives each ONT a time slot to send traffic. Without TDM, two ONTs may send traffic and collide with eachother. They both are sending at 1310nm, so if they overlap they will “garble” each others signal. By using TDM, each ONT has a time slot in which only it is allow to send traffic.

You may think, with 1.25Gbps and 64 equal time slots, how can the provider offer gigabit services to its customers? Doesn’t each subscriber only get 20Mbps upload? (20Mbps * 64 = 1.25Gbps). The answer is that, when an ONT has a lot of data to send, it communicates with the OLT and the OLT grants the ONT more time slots. This allows any single subscriber to upload a large amount of data at 1Gbps if other ONTs are not currently sending much traffic. In the diagram above, the dotted lines representing each ONT’s upstream traffic indicate that the traffic uses TDM. The time slots are flexible to allow for situations when a subscriber has more traffic to send.

Why do providers use GPON?

GPON certainly adds operational and troubleshooting complexity as compared to an “Active Ethernet” architecture. In an Active Ethernet setup, each subscriber gets its own dedicated fiber strand, and its own dedicated switchport in the CO. This optimizes bandwidth management and is simple to deploy, but much more costly.

• Active Ethernet architecture

Simply put, a provider uses GPON to save money. As a subscriber, Active Ethernet will give you better service, but the provider realizes that it can offer “good enough” service with GPON and save a tremendous amount of money.

Trenching and running fiber can be quite costly. With GPON you can simply run a single fiber strand to a neighborhood, and then connect up to 64 subscribers from the splitter point. Also, a GPON setup allows one transceiver in the CO per 64 customers, as opposed to a one-for-one transceiver for every customer.

GPON also has built in bandwidth oversubscription, because a provider will usually offer a 1G symmetrical service, but only has 2.5G/1.25G bandwidth for each OLT port. This allows you to oversubscribe at a rate of 25:1 in the downstream direction and 50:1 in the upstream direction. In the real world, subscribers use high bandwidth so infrequently that this isn’t a major issue.

Other flavors of PON

EPON is Ethernet PON, which uses Ethernet for collision avoidance. It was originally symmetrical 1Gbps, but in recent years has been updated to 10Gbps symmetrical. EPON uses the same wavelengths as GPON: 1310nm and 1470nm

XGS-PON is 10Gbps symmetrical GPON. It is compatible with GPON because it uses different wavelengths: 1577nm and 1270nm. This allows you to use both GPON and XGS-PON ONTs off the same exact splitter with a special transceiver on the OLT side. By doing this, you can easily upgrade from GPON to XGS-PON. In some cases it can simply be a transceiver swap on the ONT.

These days, many greenfield deployments look at XGS-PON over GPON. XGS-PON allows for up to 128 ONTs on a single OLT, so the cost per subscriber of a XGS-PON system and GPON system may be very similar.

Operating PON systems

GPON/EPON/XGS-PON are all open standards, but you typically by a “system” from a vendor. The system will usually include the OLT, ONT, splitter, and software for managing the OLT and ONTs. Popular vendors are Calix, Huawei, Adtran, and Alcatel Lucent. The software will include a registration system, which allows you to register new ONTs based on some identifier such as the serial number. This will take care of the OLT/ONT communication mentioned before, such as encryption and TDM negotiation.

Futher Reading/Watching

https://www.calix.com/video/webinars/pon-101--building-your-knowledge-base.html

• Great intro to PON webnar

https://www.youtube.com/watch?v=1ydIqqfjJkw&ab_channel=TheOpticalNetworker

https://www.youtube.com/watch?v=iRTAoq7y98c&ab_channel=TheOpticalNetworker

https://www.youtube.com/watch?v=DDp9-tSYpU0&ab_channel=TheOpticalNetworker

• General discussion about technologies used for broadband

https://sites.google.com/site/amitsciscozone/gpon/gpon-fundamentals