 Time division multiplexing is a very primitive and well-known technique that has long been deployed in public switch telephone networks. It is important that we go a little bit more into the details of how TDM was implemented in these networks. For that, we'll have to start with the well-known technique for analog to digital conversion known as the PCM. We then appreciate a digital hierarchical standard which was devised by the ITUT. To compensate and make it interoperable with the American standard, a hybrid hierarchy was also devised. And finally, we'd look at the digital hierarchy that was coined to meet the high bandwidth requirements and the emergence of fiber optic communications as the end-to-end transmission system. Pulse code modulation or PCM is the analog to digital conversion technique that samples user data, user analog voice sample at the rate of 8,000 samples for a voice which is band limited to 4 kHz. Then these samples are quantized in 256 different levels. Each level is encoded using 8 bits. Consequently, for a user analog voice, a digital equivalent of 64 kbps stream is obtained. This user data or digitized voice is then fed into a multiplexing system that takes user data from individual user and so many other users and passes it on to the other side of the communication system. For that, ITUT coined a hierarchy known as the digital hierarchy. On the rightmost side, you see the European hierarchy. In the middle is the American hierarchy. We'll just compare the two, but my primary focus is going to be on the European hierarchy. As you can see that in level 0, in the European hierarchy and the American hierarchy, the user voice is sampled and digitized into 64 kbps stream. In European hierarchy, 32 of such 64 kbps channels are multiplexed together to create a bit stream of 2048 kbps or 2.048 mbps. Of these 32 channels, 30 channels are all for voice. The first time slot that is the zeroth channel is for the purpose of synchronization and alarms. The 16th time slot is used for establishing voice connections, especially for the signaling of it. You would also appreciate that similar to the European hierarchy, the American hierarchy grows from level 0 to level 1 with 24 voice channels, creating an effective data rate of 1.544 mbps. And then you can understand the remaining columns by knowing the multiplexing factor and the resultant bit rate. This ITUT hierarchy is a digital hierarchy that was predominantly European, but it does also consider the American digital hierarchy. So in order to make these two compatible and interoperable, hybrid hierarchy was conceived that allows mapping from the European to American hierarchy and vice versa. The hybrid hierarchy starts from level 0, both for the European and American digital hierarchies. Then the top one is the European hierarchy and the bottom one is the American hierarchy. At level 1, effective data rate for European hierarchy is 2.4048 mbps and for American 1.544 mbps. Depending upon the multiplexing factor, the level 2 and level 3 data rates vary. But there is a provision that from one hierarchy, you can switch to the other hierarchy. For instance, you can move from European level 1 hierarchy and using a multiplexing factor of 3, you can translate it into American digital hierarchy by achieving 6.312 mbps. And then you also see that reverse mapping also exists from the American hierarchy to the European hierarchy, that is at level 3 in the American digital hierarchy. Using a multiplexing factor of 3, you can achieve level 4 at the European digital hierarchy that would otherwise be achieved by using a multiplexing factor of 4 from the level 3 of the European hierarchy. Now, all this digital hierarchy was mostly good and it was acceptable as far as user requirements were concerned. But when user requirements grew, then this hierarchy needed to expand and new requirements emerged in terms of change of medium. When fiber optic communication systems emerged and very, an excellent bit rate performance were reported because of very low electromagnetic interference, then this digital hierarchy naturally formed part of the please synchronous digital hierarchy. Here the word please synchronous means almost synchronous digital hierarchy. For that the clocking was required because the sender and the receiver need to be in sync. This clocking was done through clocks provided at every exchange point. However, since this hierarchy was using clocks at different locations, so there was a chance of maybe systems system going out of synchronization. And essentially this formed a natural predecessor to the SDH or the synchronous digital hierarchy. Synchronous digital hierarchy in fact uses a central primary reference clock, which is based on a radioactive material called cesium. This cesium based clock actually is so precise and it is so well timed. All this timing information is then fed to all the multiplexers in this hierarchical system. In SDH as we have seen the previous hierarchies, in SDH the basic building block or the data block is known as synchronous transmission or transport module number one. It has a data rate of 155.2 megabits per second. So if you just go back you would appreciate that we actually moved from 139.264 megabits per second to 155.52 megabits per second in case of STM-1. So this additional data rate is achieved by bit stuffing and padding. This is beyond the scope for now, but you see that the basic digital hierarchy helped us to reach the synchronous digital hierarchy that is used on fiber optic communication systems. The sister technology of SDH, which is primarily European, is American technology called SONET or synchronous optical network. The synchronous optical network provides a different hierarchy. It has the basic envelope for transmission at the rate of 51.85 megabits per second. In this particular table, you can see a comparison between SDH and SONET hierarchies and a one-on-one comparison shows that STS-1 that is the American technology has a basic data rate of 51.840 megabits per second. But one STS-1 is embedded into three STS's and that makes an equivalent of one STM-1 at the data rate of 155.52 megabits per second. So essentially STM-1 is equivalent to three STS's or STS-3 Likewise, you can look at other columns. For instance, the STM-256 is equivalent to STS-768 at 39.813 gigabits per second. These multiplexing techniques are currently used for Internet whereas these have always been used to transmit voice. With Internet, we are transmitting voice, video, data, multimedia content and messaging using time division multiplexing.