What is Time Division Multiplexing? Supporting our customers daily, we sometimes get this question, and mostly because various references mention, that EPON and GPON protocols use TDM – Time Division Multiplexing in a direction from ONU to OLT. We also discuss that more in details in this article, which talks about Layer 1 structure in GPON/EPON.
To understand this, let’s answer the the question – what is multiplexing and what this acronym stands for?
Multiplexing – it technology that allows transmitting multiple signals over the cable simultaneously. The main channel is divided logically to smaller parts – subchannels. Depending on the equipment and transmission media available – there might be different types of multiplexing applied. Sending end of the transmission must have multiplexer (mux) – a device that combines multiple subchannels to one signal. On the receiving end there is demultiplexer (demux) that returns signal to the state of multiple subchannels. Multiplexing technology allows sending multiple data at the same time, therefore increasing network speed and bandwidth. In this article we discuss FDM and WDM type multiplexing, however…
…the type of multiplexing that is dividing channel into time slots is called time division multiplexing. Every node in the network is having personal time slot that is available for carrying data from this node. Slots from different nodes are being sent consequently, one after another. Drawback of this type of multiplexing is that slots are being reserved for the node, even if no data available for transmission. Effectiveness of the network significantly decreases if some nodes in the network transmit data rarely.
Statistical multiplexing is using basically same principle while assigning time slots on the priority and need principle. If the node is not using time slot, it will be reassigned to the node that is transmitting data. The way how slots are being reassigned is depending on the network policies. This way of multiplexing is maximizing bandwidth of the network making it highly efficient.
We suggest to read article about TDM principles in EPON, which is here, for further study.
