PON Active Network Basics

After explaining the passive network design and topology, we are now diving into the active network components in a PON. The two main devices in a PON network are the customer facing CPE called ONT (Optical Network Terminal) and the carrier facing OLT (Optical Line Terminal).

In simplified terms the OLT is acting like a switch with PON ports to terminate multiple subscribers per PON port (determined by the split ratio, as seen before), with PON transmission magic. The ONT is a media converter from PON to Ethernet, which could also include a home router element.

We will only give a high level overview of protocols involved in a PON network and won’t go into much detail. The main objective of this article is to give a broad overview of the technologies involved, so we will stay closer to the surface. But before we dive a bit deeper, let’s have a look at the two most widespread standards for PON, GPON (ITU G.984) and XGSPON (ITU G.987). Alternative names for XGSPON can be XG-PON1/2 or NG-PON1/2, to make things a bit more confusing.

PON technologies and specs

The main characteristica of GPON and XGSPON, as we will be considering them here, are as follows:

	GPON	XGSPON
Wavelength Downstream	1480nm-1500nm	1575nm-1580nm
Wavelength Upstream	1290nm-1330nm	1260nm-1280nm
Max. Linerate Downstream	2.488 Gbit/s	9.953 Gbit/s
Max. Linerate Upstream	1.244 Gbit/s	9.953 Gbit/s

As we can see, GPON and XGSPON are using different wavelength ranges, so they can co-exist on the same fiber. This makes migration from one protocol to the other more convenient, since you don’t have to upgrade all ONTs at the same port at once. Also we note that XGSPON offers symmetrical bandwidth on up- and downstream, thus finally eliminating the technical justification for unbalanced up-/downstream products.

Those two technologies are currently widely available and can be deployed. The next evolution of PON networks is not determined yet, 25G PON and 50G PON are being developed and pushed by different parties, but there is no clear winner as of now. Expectations are that most deployed PON networks will migrate from GPON to XGSPON first, and newstarters will go with XGSPON. This means that any new PON technology is at least a few years away from being widely deployed.

Spotlight Control Protocol

Now we are looking a little bit under the hood, and discuss some elements of active PON networks. Since multiple subscribers share the same fiber, media access control and authentication between ONTs and OLTs need to be in place.

ONTs and OLTs communicate with each over with the OMCI protocol (ONU Management and Control Interface). Via OMCI a mutual authentication between ONT and OLT takes place, and encryption keys are exchanged. Each ONT needs to be assigned on the OLT, simply plugging an ONT into the network won’t bring it online immediately. This gives the operator a form of control over which devices connect to their network. Assigning a ONT to the OLT creates an own OMCI channel to manage this ONT.

Once assigned, the OLT and ONT agree on encryption keys and the OLT sends service configuration to the ONT. It can also query performance parameters and alarms from the ONT. So while the ONT is a dedicated device, it can be fully managed from the OLT and all relevant service configuration will be applied to the OLT only. The negotiated encryption keys are used to encrypt all subscriber traffic, so other stations on the same fiber cannot easily sniff and intercept communication in cleartext.

Media Access Control

Also via OMCI information for media access control will be exchanged between the ONTs and the OLT. In downstream connection (from OLT to ONT), this happens on the OLT itself, so the OLT sends traffic destined to a specific ONT with their encryption key, and all other ONTs will ignore those messages.

For the upstream direction (ONT to OLT) however, a different mechanism is needed, since multiple ONTs co-exist on the same fiber. Here a Time Division Multiplexing where each ONT has a timeslot assigned by the OLT is used. The time and length of these timeslots are usually dynamically assigned by the OLT and communicated to the ONTs via OMCI. One parameter in the timeslot assignment is the distance between the OLT and the ONT, since this directly influences the transmission time of a message on the network: ONTs further away need more time to send the same amount of data due to the signal run-time. Each ONT is only allowed to send data in their assigned timeslot, and if it does not conform this specific fiber segment can be malfunctioning for all subscribers.

Summary

Finally, we are looking at what this means for a single subscriber. Let’s compare a GPON network with a split ratio of 1:32 with a XGSPON network with a split ratio of 1:64, and a fully populated PON port.

	GPON	XGSPON
Max. Linerate Downstream	2.488 Gbit/s	9.953 Gbit/s
Max. Linerate Upstream	1.244 Gbit/s	9.953 Gbit/s
Split Ratio	1:32	1:64
Min. Downstream Bandwidth/Subscriber	77 Mbit/s	155 Mbit/s
Min. Upstream Bandwidth/Subscriber	38 Mbit/s	155 Mbit/s

So with a XGSPON network and a moderate 1:64 split ratio, each subscriber gets a minimum of 155 Mbit/s. This would mean that 64 subscribers are connected to one port, and each one is fully utilising their connection. In reality this leaves plenty of additional bandwidth, which can be distributed to active subscribers by dynamic bandwidth allocation.

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