In TDM transmission using T1 or E1 circuits, digital data is sent as a continuous stream of ones and zeros at a constant data rate. There are no gaps in transmission on a TDM circuit.
Now think about a simple transmission system (for example, a microwave link) that receives a TDM signal on one port, buffers the data, and transmits the same signal at a different port. A similar arrangement applies for the reverse direction:
The system must, on average, transmit data at exactly the same rate that it receives it. If it transmits data too slowly, the internal buffer will eventually overflow, with the result that some data is discarded. In this case, the transmitted signal is still continuous, but some of the received data will not make it into the transmitted signal. On the other hand, if our system transmits data too quickly, the buffer will eventually empty, with the result that the transmitter has no data to send. In this case, the transmitter will send some invented data until real data becomes available in the buffer.
It’s really important in a TDM system to understand how the transmitter data rates are set. Most TDM systems support several timing modes. Here are some of the options:
The transmitter uses a free-running clock from within the system
The transmitter uses a clock from outside the system. That might be a free-running clock or a clock synchronized to some other source.
Things start to get more interesting here. In Loopback timing, the transmitter uses a clock based on the data rate recovered from data at the receiver section of the same port. In other words, the port sends the timing back the way it arrived.
In Through timing, the transmitter uses a clock based on the data rate at the receiver section of a different port. This allows the system to propagate the timing along with the data. Through timing is the closest we can get to the model of a conventional copper connection.
The TDM solution for PTP 650 and PTP 700 provides Through timing between the T1 or E1 ports of one NIDU and the corresponding port of the NIDU at the remote end of the link. Through timing is independent between the left-to-right and right-to-left directions and independent between the eight ports on the NIDU. The NIDU does not support (and does not need to support) internal, external or loopback timing modes.
There are very many ways to organize the timing in a TDM network. One thing they have in common is that there is always a free-running clock to which all other clocks are ultimately synchronized. As a simple example consider this link between two systems:
Here the system on the left uses a free-running clock, and the system on the right uses Loopback. The clock received at the left-hand system is synchronized to the free-running clock via the right-hand system.
We can add an intermediate transmission system with Through timing (for example PTP 650 NIDUs) like this:
Beware the dreaded timing loop
A timing loop is a common cause of problems in TDM networks. In this simple example, both ends are configured for Loopback and there is no free-running reference. The consequence is that the clock frequency will be unstable, and eventually one or both of the systems will reach the maximum excursion that it can tolerate. At this point the two systems are no longer synchronized.
In practical networks many more systems may be involved and timing loops are sometimes quite difficult to detect.