The following example is adapted from an example given by Hughes Escort. This example may help to explain TDM/TDMA (Time Division Multiplexing/Time Division Multiple Access )technology in a very simple way.
It seems many of us are working backwards in this age of technology. It is amusing, but true to a large extent. Instead of explaining the new technologies in a simple way, we end up bombarding the audience with a broadcast of jargon.
As a result, instead of developing an appreciation and clear conceptual understanding for the technology, the listener becomes entangled in a game of scrabble, where he tries to arrange the jargon in a meaningful sequence, but unfortunately, often fails to do so. In the following, an attempt has been made to be different.
TDM/TDMA technology, which is in operation in various flavours in more than 75% of the VSATs installed globally and more than 90% of VSATs installed in India, has been explained through a simple analogy. At the end of the analogy, the technological terms used in conjunction with TDM/TDMA are explained in relation to the selected analogy so that the exact meaning and function of these terms can be clearly understood.
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Consider a class of students with a teacher. Let us closely observe, analyse and understand the way the teacher conducts the class and the way the students communicate among themselves. After all, we are talking about effective and efficient communication.
The rule of the institution instructs the students to communicate among each other through the teacher. Therefore, the students speak with the teacher (and mention who the communication is intended for) and the teacher repeats the information to the appropriate recipient.
Thus, the students cannot communicate among themselves directly; they have to necessarily communicate through the teacher. However, the teacher does not analyse or validate the content of the information.
The teacher only verifies the integrity of the information (like checking spelling and grammar in a document). But the teacher has the option to analyse the content in case the students are unable to communicate among themselves properly. Additionally, to ensure the smooth conduct of the class, the teacher enquires from all the students regarding their status (desire for communication) at regular periodic intervals.
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Now the teacher has several options to make the students communicate with each other. The first mode is to allow everyone to speak whenever anyone wants to. In this case, if two students speak simultaneously, then the teacher cannot understand what each one is trying to communicate.
Hence the teacher requests the students to express their opinion again assuming that they will not speak at the same time again. This assumption would be true if the class size is very small or if everyone is trying to send very short messages.
Another way for students to communicate is on the basis of student requests, i.e. if the student wants to communicate, he/she requests the teacher. In the request he/she informs the amount of information he/she has to send and the time required to send it.
The teacher in turn informs the student his/her allocated time slot. Thus, the student communicates in that allocated time slot. If the student is not able to finish the conversation in the allocated time interval then he/she again sends a request to the teacher for an additional time slot.
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Yet another way adopted by the teacher is to divide the period in several time slots or time intervals. The teacher then informs/instructs each student when he or she can speak, i.e. in which time interval the student can speak. Thus each student takes his/her turn to speak to the teacher who in turn communicates it to other students.
Now there are a few students in the class who are more intelligent and hence want to speak for a longer time. The teacher recognises them and permits them to speak for a longer duration, the duration being an integral multiple of the basic duration allocated to every student. The teacher, unfortunately, does not know who is really intelligent and requires more time.
Hence the teacher reserves a time slot in the period (say the last ten minutes), wherein the few students who have more information to convey, can speak. The teacher allocates more time duration only when requested to do so by the students. The teacher being an intelligent person can handle requests from multiple students nearly concurrently. Therefore, the teacher handles multiple student groups simultaneously.
It is possible that in a class a student requests for a time slot, but the teacher finds that the reserved time slots are already allocated to other students. So the teacher looks at the other class he/she is managing and realizes that here, there are fewer students and time slots are available in this class. The teacher then shifts the student to this class.
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In order to avoid overlapping communication (i.e. a student starts speaking before the other finishes), the teacher allocates a minimum time interval after a student stops speaking and the next student starts speaking. Though this eats up time, the apparent time efficiency is offset by gain is terms of efficient and clear communication. In other words, this is a necessary evil.
The student, in order to derive maximum benefit from the allotted time slot, uses several means to send maximum information in the allocated time slot.
For example, the student uses acronyms, abbreviated statements and symbolic statements to send the maximum possible information. The student also prioritizes the information that he/she desires to send. The student keeps the teacher well informed while carrying out these actions.
In a class different students can have different needs. Some may want to exchange multiple short messages, while others might exchange long messages. So the teacher can logically divide the class into several groups and each group will have its own way to communicate among themselves.
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Now let us closely examine what happens when a new student joins the class. The teacher has to first include the student as a part of the class. The teacher does so by documenting the various details about the student in the register. In the next step, the teacher allocates a definite time slot to the student when he/she can speak or convey his/her information.
It may be hard to believe that the entire operation of the TDM/TDMA system has been explained in the above analogy, along with various bandwidth allocation schemes. This is explained below.
The entire system comprising of the teacher, the students, their modes of communication with each other correspond to the Integrated Service Business Network or ISBN. It’s simple to observe that the teacher has all the attributes of the HUB and the students are the remote VSATs.
The fact that all the students have to communicate through the teacher emphasises double hop nature of VSAT connectivity, i.e. all the VSATs have to communicate through the Hub. The Hub is the central intelligent entity that controls and monitors all the remote VSATs.
Each class represents an In-route. In-route is a frequency channel, which is used by all the VSATs on a shared basis, though the ISBN system provides schemes to dedicate bandwidth to a VSAT The way the students communicate with the teacher denotes the In-route access method.
The logical grouping of students in a class denotes the fact that in an In-route different VSATs can use different In-route access methods depending on their specific data transmission requirements. The fact a teacher is controlling multiple classes denotes the fact that the Hub supports multiple In-routes.
The fact that if a student requests for additional time duration and if the required time duration is not available, then the teacher transfers the student to another class illustrates the concept of In-route Switching.
This is beneficial since the traffic pattern for a particular device connected to a VSAT may not be consistent throughout the day and hence it is desirable to change the bandwidth access mechanism when its traffic pattern changes.
The single most important parameter that determines the efficiency of any TDM/TDMA system is bandwidth allocation. In the analogy, the teacher divides the available time interval to allow multiple students to communicate. This illustrates the basic characteristic of Time Division Multiple Assess or TDM A operation.
In the first mode of communication where each student communicates randomly illustrates the User Aloha bandwidth assignment technique. Here a part of the In-route is set aside to be contended for by an assigned set of remote VSATs. The User Aloha technique is useful for light traffic that has small, uniform message sizes.
In the second mode of communication where the student requests the teacher to allocate a time slot to communicate illustrates the Transaction Reservation Bandwidth Assignment technique. Here, when a remote VSAT is configured for transaction determines that it has one or more packets ready for transmission, it sends a transaction request to the Hub.
The request includes the number of packets and their sizes. The Hub allocates a time slot and returns a transaction response message to the VSAT. The remote VSAT then transmits as many packets as will fit into the allocated time slot. If there are more packets, then the VSAT sends another transaction request.
In the third mode of communication where each student is pre-allocated a time slot for communication illustrates Stream Bandwidth Assignment.
This technique provides fixed, periodic transmission opportunities during each super-frame to a remote VSAT. The Stream technique can be most efficient capacity allocation technique for high throughput applications, and it can be used to provide very good and consistent response time.
The use of means like acronyms, abbreviated statements, and symbolic statements to send maximum information illustrates the features of data compression and data prioritization provided by the VSAT to improve transmission efficiency.
It minimizes average response time, but on account of packet retransmission, bandwidth utilisation is poor. Packet retransmission—refers to resending some bits of information that has been unable to reach its destination.
With the greater acceptance of satellite transmission by the business community, there has been considerable development in satellite communications.
While satellite transmission started growing rapidly after the introduction of Very Small Aperture Terminals, the original technology used for these was Time Division Multiplexing/Time Division Multiple Access or TDM/TDMA. The concept of the hub to be used to facilitate transmission was based on this technology.
In this technology, while the one-time cost of establishing two VSAT terminals is not very high, the service provider also charges for the usage of satellite transmission facilities. Naturally, this cost varies with the volume of data and voice transmitted. However, in order to avoid any conflict between customers, the service provider usually allocates a permanent pipeline for a particular bandwidth, to each customer.
Thereafter, the service provider charges each customer on the basis of this permanent bandwidth provided to the customer. Very often companies take pipelines based on their peak requirements.
This means that since the average usage is very much below their peak usage, they end up paying for services contracted for but not used. Ideally, the customer would like to pay for the services they use and to have the freedom to pay for only what they use.
Secondly, the delay in set up between two persons in voice communications can be quite irksome at times. It may be noted that this delay is not particularly dependent on the physical distance between the two parties. This delay is more or less the same whether the two parties in conversation are 1,000 km apart or 1 km apart.
This is because in either case, the signal must be bounced back twice from the satellite before it reaches the other party and most of the time is taken in these two transmissions; since the satellite is some 36,000 km away, the physical distance between the two parties is hardly of any consequence.