PON - passive optical networks

PON technology

PON (Passive Optical Network)— technology of passive optical networks.

One of the main challenges facing modern telecommunications access networks is the so-called "last mile" problem, providing as much bandwidth as possible to individual and corporate subscribers at minimal cost.

The essence of PON technology is that between the transceiver module of the central node OLT (Opterminal line) and remote subscriber nodes ONT (Optical network terminal) a completely passive optical network with a tree topology is created. Passive optical splitters (splitters) are placed in the intermediate nodes of the tree - compact devices that do not require power and maintenance. One transceiver module OLT allows you to transmit information to a plurality of ONT subscriber devices. The number of ONTs connected to one OLT can be as large as the power budget and the maximum speed of the transceiver equipment allow.

Rice. 1. PON network architecture

To transmit the forward and reverse channels, one optical fiber is used, the bandwidth of which is dynamically distributed between subscribers, or two fibers in case of redundancy. Downstream from the central node to subscribers is at a wavelength of 1490 nm and 1550 nm for video. Upstreams (upstream) from subscribers are at a wavelength of 1310 nm using the Protocol multiple access with time division (TDMA).

To build a PON, a point-to-multipoint topology is used, and the network itself has a tree structure. Each fiber optic segment is connected to one transceiver in the central node (as opposed to the point-to-point topology, which also provides significant savings in equipment cost. One fiber optic segment of the PON network can cover up to 32 subscriber nodes within a radius of up to 20 km for EPON / BPON technologies and up to 128 nodes within a radius of up to 60 km for GPON technology.Each subscriber node is designed for an ordinary residential house or office building and, in turn, can cover hundreds of subscribers.All subscriber nodes are terminal, and disconnection or failure one or more subscriber nodes does not affect the operation of the others.

The central PON node can have ATM, SDH (STM-1), Gigabit Ethernet network interfaces for connecting to backbone networks. The subscriber node can provide service interfaces 10/100Base-TX, FXS (2, 4, 8 and 16 ports for connecting analog SLTs), E1, digital video, ATM (E3, DS3, STM-1c).

Fig.2. Technology Comparison

PON network testing

When testing a PON network, an operator usually has two main concerns:

• Real attenuation in the optical line between the central node and the subscriber device (operating or preparing for connection).
• The location of the problem area, if the actual attenuation in the line turned out to be higher than expected (calculated or reference).

To answer the first question, it is enough to make simple measurements using an optical tester. The second question is more complex and requires the use of an optical reflectometer (OTDR), as well as some experience in reflectogram interpretation.

As a rule, it is desirable that all necessary measurements can be carried out on a working PON network without disconnecting subscribers (except possibly the one under test). Such testing is carried out at a non-working wavelength using additional devices (DWDM wave multiplexers, filters) so that the radiation of the measuring equipment does not interfere with the useful signal. As already mentioned, in the PON network for the direct channel (from the center to subscribers) the wavelength is 1490 or 1550 nm (for video), for the reverse - 1310 nm. For PON network testing, a wavelength of 1625 nm is commonly used.

The radiation of the measuring equipment (tester, reflectometer) is injected into the fiber immediately after the OLT using a wave multiplexer (DWDM). This radiation can cause interference on the optical receiver of the subscriber unit, so a filter must be installed in front of each ONT subscriber unit. In order to be able to carry out testing without disconnecting the network, the wave multiplexer and filters must be permanently connected to the optical path, (see Fig. 3).

Rice. 3. Scheme of connecting the wave multiplexer and filters to PON

An optical tester at 1625 nm is used to measure the attenuation in the optical link between the OLT and ONT. The tester's transmitter is connected to the free end of the wave multiplexer on the OLT. The tester's receiver is connected to the free end of the fiber before the filter, (see Fig. 4).

Rice. 4. Attenuation measurement with disconnection of the subscriber device

It is possible to measure the attenuation without disconnecting the subscriber device. To do this, it is necessary to use not a filter on the ONT, but a wave multiplexer, as on the central node (see Fig. 5).

Rice. 5. Attenuation measurement without disconnecting the subscriber device

The attenuation at a wavelength of 1625 nm is slightly higher than at 1550 and 1490 nm (by 10% on average). Therefore, attenuation testing at 1625 nm provides an upper estimate for attenuation at operating wavelengths. If this estimate is within the allowable budget (23 dB), then the attenuation at operating wavelengths certainly satisfies the budget requirements. If the attenuation at a wavelength of 1625 nm exceeds the allowable value, then in order to accurately determine the attenuation at operating wavelengths, it is necessary to recalculate based on the passport of the optical cable.

Measurement in PON using an optical tester allows you to get the real attenuation value in the section from OLT to ONT, but does not answer the question of where the problem area is if this attenuation turned out to be higher than expected (calculated or reference). To localize the problem area, a more complex device is used - an optical reflectometer (OTDR).

A scatterometer with a 1625 nm test module is connected to the free end of the wave multiplexer on the OLT, (see Fig. 6). The reflectometer radiation propagates along the PON tree and, due to reflection on obstacles and backscattering in the optical fiber, partially returns to the input of the reflectometer. Thus, the reflectogram of the PON tree is taken - a graph of attenuation in the line depending on the distance. Each attenuation peak or step in this graph corresponds to a specific network element or fiber event.

Rice. 6. Retrieval of the reflectogram of the PON tree

The technique for testing the PON network using a reflectometer is as follows. After each change in the network topology (connection of a new subscriber, replacement of a splitter, etc.), a reference (reference) reflectogram is taken, corresponding to the normal state of the network. If problems are detected in the network (for example, if the attenuation measured by the optical tester turned out to be higher than the calculated one), a new reflectogram is taken, which is compared with the reference one. New events on the trace locate the location of the problem area, (see Fig. 7).

Rice. 7. Analysis of new events on the reflectogram.

With the OTDR, you can monitor the PON network and detect fiber degradation before problems occur. To do this, it is necessary to regularly (for example, once a week) take a network trace and compare it with a reference trace. When any deviations, and even more so new events, appear on the reflectogram, it is necessary to analyze their possible causes and, if necessary, take adequate preventive measures.