PON has a “multipoint-to-point” structure in the upstream direction (from ONU to OLT), and multiple ONUs send data to one OLT. The optical splitter/combiner ensures that the signal sent by each ONU will not be detected by other ONUs, but it cannot be avoided: the signals sent by different ONUs at the same time may “collide”.In this way, a channel segmentation mechanism should be adopted in the upstream direction of the PON to avoid data “collision”, and then the backbone fiber and resources can be shared fairly.
Regarding this “channel segmentation mechanism”, the industry uses two ideas:
- One is to use Wavelength Division Mutiple Access (WDMA) to distinguish the uplink channels of different ONUs.
- The other is to use Time Division Multiple Access (TDMA) to distinguish the uplink channels of different ONUs.
Therefore, a large classification of PON technology has been produced: WDMA PON and TDMA PON.
The key point of WDMA PON is to allow each ONU to work on different wavelengths. In theory, this is a simple solution, but in practice it is not feasible due to the high cost.
On the one hand, the WDMA solution requires the OLT to receive multiple wavelength channels, either with a tunable optical receiver or a receiver array.
On the other hand, the bigger problem is that WDMA PON requires network operators to maintain multiple ONUs with different wavelengths instead of one type of ONU. Each ONU has to use narrow-spectrum lasers, which is much more expensive. When a faulty ONU is replaced with an ONU with the wrong wavelength, it will also interfere with other ONUs. ONUs with tunable lasers can solve the problems of multiple types of ONUs, but it will significantly increase the cost of the terminal, and operators will not buy it.
For these reasons, WDMA PON is currently not an attractive solution.
There are several other implementations of WDMA PON:
First, the use of wavelength routed PON (Wavelength Routed PON, WRPON), that is, the main use of waveguide array grating wavelength division multiplexer instead of wavelength-independent optical splitter.
Second, the uplink and downlink channels use the same wavelength, and the ONU uses an external modem to modulate the signal it receives from the OLT (as an uplink carrier signal). This method is also very costly: it places an optical amplifier near the ONU to compensate for signal attenuation, and requires more expensive optical devices to reduce reflection;at the same time, for independent transmission of multiple ONUs, the OLT must have multiple receivers.
Third, the ONU can also use low-cost LEDs, and then its wide spectrum is filtered by the waveguide array grating in the upstream optical path. This method requires multiple receivers in the OLT. If the OLT uses a tunable optical receiver, the OLT can only receive a signal from one ONU at a certain time, which is essentially a time division multiple access (TDMA) PON.
In order to avoid collisions of signals sent by multiple ONUs at the optical combiner, TDMA PON requires each ONU must only send data in its own sending window (also called a time slot).
The main advantage of TDMA PON is that all ONUs work on the same wavelength and use the same devices, and the OLT also only needs one receiver. The ONU transceiver must work at the line rate, even if the bandwidth obtained by the ONU is lower than the line rate bandwidth. However, TDMA PON can effectively change the bandwidth allocated to the ONU by changing the length of the time slot, and use statistical multiplexing to make full use of the PON channel capacity.
In the user access network, most of the traffic is usually the dowstream traffic from the network to the user, not the upstream traffic from the user to the network, and it is not peer to peer. Therefore, it is reasonable to separate the uplink and downlink channels. There are two ways of separating channels:
A simple separation method is air-division multiplexing, where the uplink and downlink signals are transmitted in two optical fibers respectively, that is, dual-fiber bidirectional PON. But this will take up fiber resources, and at the same time will introduce more optical splitters/combiners, so the second type is created.
The other is to use a single fiber for bidirectional transmission, namely single fiber bidirectional PON, in order to save fiber and reduce maintenance costs. At this time, two wavelengths need to be used. λ1 is used for uplink transmission, and λ2 is used for downlink transmission (as shown in the figure below). The capacity of each wavelength channel can be flexibly allocated among ONUs through time slot sharing.
Since only one upstlink wavelength and one OLT optical transceiver are required, time slot sharing (TDMA PON) is a low-cost method for optical channel sharing in the access network, and is commonly used in existing networks.