OLT supposed to control user traffic and dynamically assign bandwidth to ONT (ONU). At the same time, for all (up to 32 or 64) ONU use the same wavelength, so it needs some synchronization that will help to avoid collisions from different ONUs. There are three main methods to solve this problem. Easiest one to do is TDMA (Time-Division Multiple Access), when every user have specified time slot and prearranged data rate. This method is not efficient in cases when some users don’t have information to send, which will make a signal have many empty time slots. Another method is DBA (Dynamic Bandwidth Allocation), which is more effective, because empty and not used time slots are reassigned to another user, who is active at this time. Also the amount of time-slots for user vary from many different factors, such as user priorities, the speed and quality for specific customer (guaranteed), amount of time slots requested by user, etc. Last method is TDM (Time-Division Multiplexing) is combining voice and data streams.
Figure 1. Operation of a time-division multiplexing process.
It is having N outputs for N different users; each output is running at R bps rate. At the input it is having one stream at N x R bps rate. Multiplexed signal is sent to all ONTs, and packages are accepted or declined, depending on packet address header. For safe streaming and privacy, encryption may be used, since all the ONTs get all the information. Sending information upstream is more complicated, since all the users are sharing one wavelength. For avoiding collisions the TDMA protocol is used by system. OLT is sending permissions to all ONTs, so they have specified time slot.
Figure 2. Operation of a time-division multiple access protocol.
These permissions are synchronized so the data floats from different ONTs don’t collide. All the ONTs are allocated on different distance from central office, so OLT is using range calculating technique, which allows OLT to prevent all the collisions, and adjust timing correctly.
Want to see practical product examples of OLT and ONU (ONT)?
Then check out our GEPON product range: https://www.ad-net.com.tw/index.php?id=176