The increase in the number of users and the amount of information exchanged in communication networks has led to the development of solutions to increase network capacity. Multiplexing techniques have thus been developed.

Time division multiplexing (TDM-PON)

The concept of deployment of the PON network architecture is based on the time sharing of a single wavelength among multiple users (subscribers). This is TDM (Time Division Multiplexing). In fact, TDM multiplexing consists of allocating the entire frequency band to all users sequentially (in turn for each of them) during an interval of fixed time called “Time Slots”. Therefore, each user transmits his data in the "Time Slot" assigned to him. The principle of TDM-PON time division multiplexing is described in figure below.

In a downstream TDM transmission (Downstream, DS), the time division multiplexing allocates “Time slots” for each client in a data frame transmitted from the OLT to the ONUs. In this case, all the ONUs each receive the entire signal coming from the CO and each ONU selects the part of the signal corresponding to its time-slot (predefined). On the other hand, for uplink transmission (Upstream, US), each customer synchronizes with the exchange to know when to transmit its packet so as not to interfere with the various packets transmitted.

Wavelength division multiplexing (WDM-PON)

Unlike the TDM-PON system, the sharing of the material resource between several subscribers is based on wavelength division multiplexing in which each subscriber is assigned a specific wavelength. This type of optical multiplexing is used in access networks in order to increase the bandwidth, up to a speed of the order of 10 Gbps. The figure below describes the principle of WDM-PON multiplexing.

In the downstream direction, the OLT transmits all wavelengths on the same fiber where the communication between the OLT and each ONU is based on a specific wavelength.
The WDM-PON network in this case consists of several virtual point-to-point links between the OLT and the ONUs, which guarantees excellent security at the physical layer level and avoids collision between subscribers. A set (coupler - filter) or a wavelength splitter or an AWG (Arrayed Waveguide Grating) type demultiplexer makes it possible to select the wavelength dedicated to each customer. In the uplink direction, the ONUs send different wavelengths back to the OLT. Additionally, with WDMPON, we can have protocol-level transparency between different ONUs (colorless ONUs).

Hybrid WDM and TDM multiplexing (WDM/TDM-PON)

The WDM/TDM-PON hybrid technology consists of combining the two multiplexing techniques, time and wavelength, in order to be able to increase the bandwidth allocated to each user and share in a flexible and dynamic manner the bit rate carried by a length of data. wave between several subscribers. Its architecture is shown in figure below. This topology consists of at least two distribution levels: the first 1 to N by multiplexing/demultiplexing of WDM channels based on an AWG and the second 1 to K for the power division of each wavelength based optical couplers (splitter).

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