 We have appreciated what is the role of multiplexing in transmission systems in terms of the telecommunication infrastructure. Now, we are going to discuss the variants of multiplexing, especially with a focus on digital multiplexing techniques. In this module, we'll discuss, as historical perspective, the analog multiplexing. And then we are going to discuss the multiplexing for digital signals, both on the long haul, that is, over the long distance, and on the access side. The analog signal to begin with that were to be carried on telephony networks was voice. So, to carry such voice signals, which were analog in nature, the multiplexing had to recognize that the typical human frequency range that humans could utter varies between 300 hertz to 3500 hertz. So, it is band limited to around 4 kilohertz centered around zero frequency. If the multiplexing of such voice channels was to be done for multiple users, since they share the same frequency range, it's not possible to send them as is, that is, as a baseband signal. Therefore, some analog modulation technique was required. The well-known examples are either the amplitude was modulated, that is, AM, or frequency modulation, or the phase modulation. This was the most common and primitive way of multiplexing the analog signals. As regards the digital signals, since the digital signals are already represented in terms of ones and zeros, so there was no need as such to translate them into broadband. So, the digital signals are actually multiplexed right in the baseband. The typical example is the time division multiplexing in which either bit level interleaving or byte level interleaving was carried out for the connected users. Likewise, frequency division multiplexing is another technique where each user's data was carried on separate frequency band. Code division multiplexing is another technique that used orthogonal codes. However, code division multiplexing or CDM did not find much applicability and viability on the long haul. Therefore, it was confined or mostly restricted to wireless networks, especially on the access side. For instance, Wi-Fi uses direct sequence spread spectrum and 3G uses CDMA. Likewise, in telecommunication systems, the predecessors of 3G like IS-95 and WCDMA also use CDMA. Let's quickly go through how a typical time division multiplexing system works. On the multiplexer end, we have users particularly in the given example, we have 32 users. Each user is generating its analog signals. These signals are translated into the digital form using analog to digital conversion. The well-known technique is known as PCM or the pulse code modulation. This technique is going to be discussed in the later lectures. However, for the sake of this particular scenario, we assume that somehow 64 kilobits per second bit stream is coming from every user. Now, since each user is being offered a certain space on that multiplex signal, which is an interleaving for these 32 users. So for a total time period of 125 microseconds, which is the time slot given to each individual user. Now, this time slot is being shared between 32 users. So effectively, each user in an interleaved manner would be transmitting 8 bits of its data in a duration of 3.9 microseconds. In other words, the total time period of every user is now being shared amongst 32 users. As a consequence, at one point in time, a user can only send 8 of its bits. So resultantly, the overall multiplexed stream would have the resultant data rate that is the product of the total number of users to the total data rate of each user. So you can see it is 32 multiplied by 64 kilobits per second or 2.048 megabits per second. Although we are going to discuss it later, but you can just remember it that it is called an E1 stream. It is the European standard for digital transmission. Multiplexing is also carried out at the excess side. It is required because multiple competing users are to be given a chance either in a probabilistic manner or in a deterministic manner. Certain techniques such as frequency division multiple access, time division and cold division multiple access, these schemes provide a fair or deterministic kind of access mechanism to competing users. But in case of probabilistic schemes such as aloha or pure and slotted aloha, the users may be lucky to have their chance to transmit. If two users transmit at the same time, then collisions may happen.