VSAT - Access Technologies

Consider a class of students with a teacher. Lets closely observe analyze 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 student to communicate among each other through the Teacher. So the students speak with the teacher (and mention whom it 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 analyze or validate the content of the information.

The teacher only verifies the integrity of the information (like running spelling and grammar check on a document).

But the teacher has the option to analyze the content in case the students are unable to communicate among themselves properly. Additionally to ensure that the smooth conduct of the class; the teacher enquires from all the students regarding their status at regular periodic intervals.

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 every one is trying to send very short messages.

Another way for student to student communication 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 it has to send and the time required to send it. The teacher in turn informs the student its 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 it again sends a request to the teacher for another time slot.

Yet another way adopted by the teacher is to divide the period in several time slots or time intervals.

The teacher then and informs/instructs each student when he or she can speak i.e. in which time interval the student can speak.

Thus each student takes its turn to speak to the teacher who in turn tells it to other students. Now there are a few students in the class who are more intelligent and hence want to speak for longer time. Thus the teacher recognizes them and permits them to speak for longer duration and the duration being an integral multiple of the basic duration allocated to every student. Now the teacher 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); where in the few student who have more information to convey can speak. Now the teacher allocates more time duration only when requested by the student.

Now the teacher being an intelligent person can handle requests from multiple students nearly concurrently. So the teacher handles multiple student group simultaneously. Now 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 there are fewer students and time slots are available.

So the teacher shifts the student to another class.

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, but 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, symbolic statements to send maximum information. The student also prioritizes the information that he/she desires to send.

The student keeping the teacher well informed carries 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.

Now lets closely examine what happens when a new student joins the class. The teacher has to first include the student as the 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 where he/she can speak or convey his/her information.

Perhaps it is 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.

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 emphasizes 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.

Now each class represents an Inroute. Inroute is a frequency channel, which is used by all the VSATs in 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 Inroute access method. The logical grouping of students in a class denotes the fact that in an Inroute different VSATs can use different Inroute 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 Inroutes.

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 Inroute 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 parameter that determines the efficiency of any TDM/TDMA System is bandwidth allocation. In the analogy teacher divides the available time interval to allow multiple students to communicate. This illustrates the basic characteristic of Time Division Multiple Assess or TDMA operation.

In the first mode of communication where each student communicates randomly illustrates the User Aloha bandwidth assignment technique. Here a part of the Inroute 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 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 superframe 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 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 utilization is poor.

Multiple - Frequency Time Division Multiple Access(M - FTDMA)

Sharing Bandwidth (FDMA)

Using the analogy above, FDMA is simply having all the students talking together but only listening to one teacher.

In the same way, the teacher has to tune in to one student and filter out the rest. This is perhaps simple to understand since radio and tv has traditionally used this principle, albeit one way from transmitter to receiver.

Sharing Bandwidth & Time (FTDMA)

Now if the students take turns to talk to the teacher then the teacher can talk to every student. The teacher can choose which student can talk and when they can talk. Sounds like a dream situation for some teachers.

M-FTDMA is the same system, each student has a frequency or groups of students have a frequency and the teacher has a frequency. The result is an extremely efficient access scheme for many VSAT users to communicate with the network via satellite.

Code Division Multiple Access(CDMA)

ACCESS SCHEMES

For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

CDMA uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be despread and will not pass through the filter.

Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.

CODES

CDMA codes are not required to provide call security, but create a uniqueness to enable call identification. Codes should not correlate to other codes or time shifted version of itself. Spreading codes are noise like pseudo-random codes, channel codes are designed for maximum separation from each other and cell identification codes are balanced not to correlate to other codes of itself.

THE SPREADING PROCESS

WCDMA uses Direct Sequence spreading, where spreading process is done by directly combining the baseband information to high chip rate binary code. The Spreading Factor is the ratio of the chips (UMTS = 3.84Mchips/s) to baseband information rate. Spreading factors vary from 4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs (Spreading factor 128 = 21dB gain).

POWER CONTROL

CDMA is interference limited multiple access system. Because all users transmit on the same frequency, internal interference generated by the system is the most significant factor in determining system capacity and call quality.

The transmit power for each user must be reduced to limit interference, however, the power should be enough to maintain the required Eb/No (signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No of every user is at the minimum level needed for the acceptable channel performance.

Demand Assigned Multiple Access(DAMA) & Pre-Assigned Multiple Access(PAMA)

TECHNOLOGY SIMPLIFIED – SCPC (DAMA/PAMA)

Possessiveness is a basic human instinct. One really doesn’t like to share resources. You want your own PC. You want a dedicated LAN connection. You want your own home page. You want your own telephone. Well the list is long. But this is in direct contradiction to the fact that Networking was born and has evolved on the philosophy of resource sharing, first across premises and eventually across geographies.

So we are going to talk about two things –

a) Dedicated resources in a shared environment and

b) Shared resources in a dedicated environment.

This means that if you don’t want to travel in a bus you can buy a car (it’s your dedicated resource), but you cannot buy the road (it’s a shared resource).

Let’s come to the point directly. We are talking about Satellite Communication. Today there is a plethora of products and technologies available in the market place. It is indeed a formidable task to select the appropriate technology. The even more difficult task is to select the appropriate product. This documents aims to demystify the concepts involved with an established technology – the Single Channel Per Carrier or SCPC technology.

The service provider providing SCPC connectivity services has a deep resemblance to a Courier company. After all it is also a service provider. We ask the service provider much the same questions as we do while selecting a Courier company. Now since we all are familiar with Courier business, lets take a close look how the business operates, how the customer’s interests are taken care of and what are the trade-off involved.

SCPC provides clear channel communication. To understand this lets the situation where you have to send large equipment. Now then there are two ways to send it. You either send it as a single piece or you disassemble it into small parts and then send it. In the later option you have to first disassemble the equipment, send adequate information with each component to that it can be re-assembled in the proper order. Now if one of the components does not arrive in time then the whole consignment waits till it arrives. The former case represents clear channel communication and the later represents packetised data/voice transmission.

The Courier Company picks up the goods from your premises and delivers to you premises in another location and thus it provides end-to-end delivery. Similarly SCPC provides end-to-end data, voice and video connectivity. Its just not sufficient to deliver the information, it should be delivered at the earliest. Now there are two ways to it. If you observe the geographic distribution of your organization, you can surely identify locations between which you transfer goods very frequently. So the Courier Company can reserve a definite space in their flights to carry your goods everyday. If your goods volume is very large then you can ask the Courier Company to reserve a flight for you.

Of there is a cost attached to it. At the same time once the flight is dedicated to you, any type of goods can be sent through. However if you don’t load the flight fully, then you are at loss as the cost of operating the flight still remains the same. But there is a way to optimize or distribute the cost. Use the flight to cove all your locations sequentially, taking a pair at a time.

In parlance of SCPC connectivity, the flight illustrates the concept of a frequency carrier. Thus the frequency carrier can either be permanently assigned between two VSATs giving way to Permanently Assigned Multiple Access (PAMA). Thus a permanently assigned frequency channel provides dedicated bandwidth, through which you can send data, voice or video. This illustrates the concept of Dedicated Resource in Shared Environment. Here the frequency channel is dedicated to you but the basic Satellite resource is shared by many.

Now the assigned frequency carrier in PAMA can either be used for voice or for data. But what if you want to use one carrier for data and voice. Of course this is possible. How ever it calls for the use of a call of device called Voice Data Multiplexer (VDM) which combines or aggregates several data and voice channels into one trunk line which in turn is interfaced to the VSAT equipment. The VDM is chosen primarily keeping in mind the number of voice channels required. A typical VDM configuration comprises of one LAN interface and multiple voice interface.

Alternately the frequency carrier can be assigned between any two VSATs on a demand basis giving way to Demand Assigned Multiple Access (DAMA). Thus several VSATs share a floating link. Now as the number of VSAT grows, one floating channel becomes insufficient. Thus a pool of floating is assigned for the group of VSATs. It should be noted that in DAMA after the link is established it becomes equivalent in every respect to PAMA. Thus DAMA involves a call establishment cycle which is not there in PAMA.

The link establishment in DAMA is quite similar to making a long distance telephone call. If your phone has STD facility then you just dial the destination number and get connected. In the absence of the ISD felicity you request the telephone operator to dial the destination number and connect you through. But once you are connected the operator does not come in picture. Thus in DAMA call setup procedure the end equipment first data transmission request to the VSAT. The VSAT then sends another request to the Network Control Center (NCS). The NCS then sends another request to the destination VSAT to confirm if it is busy. If it is not busy then it allocates a pair of frequency to the two VSATs. Thus the clear channel circuit is established end-to-end. Thus the NCS is only involves in call establishment.

A logical question that arises here is what should be the capacity of the satellite channel established between two locations.

Going back to the courier analogy, if you notice there are weight slabs fixed by the courier company e.g. 0 to 1Kg, 1 to 5 Kg, 5 to 20 kg, 20 to 50kg and so on. This implies that whether you want to send a 1.5 kg object or a 4.9 Kg object, the charges are the same. Of course you would like the range to be narrower. Similarly in the case of SCPC the capacity of the Satellite channel cannot be arbitrary. The commonly available steps are 4.8 Kbps, 9.6 Kbps. 16 Kbps, 19.2 Kbps, 32 Kbps, 64 Kbps and 128 Kbps. Thus depending on traffic requirement appropriate channel size is allocated.

A network can have a mix of both PAMA and DAMA Links. Generally PAMA is preferred for data and DAMA for voice. Also there can be multiple DAMA and PAMA from the same location. Finally the DAMA link can be easily transformed into a PAMA link. This does not call for any hardware chance. This can be seamlessly carried out from the Network Control System (NCS).

Now an obvious question is what determines how many frequency channels or carriers can be established from one location. Well there are two things in this. One is the Channel Unit and the other is the RF Unit. The Channel Unit is the satellite modem (the counterpart of the familiar Landline modem). Every frequency carrier PAMA or DAMA requires a Channel Unit. These Channel Units are housed in a modular chassis having multiple slots. So it is obvious that if you are using a four slot chassis, you can have up to four carriers. Now the RF Unit, which is collocated with the antenna, actually transmits four frequency carriers. Now there is certain amount of power associated with each carrier depending on its bandwidth i.e. the RF unit has to radiate more power to transmit a 64 Kbps carrier that a 19.2 Kbps carrier. Thus every RF unit has a definite power rating and that has to be considered while determining the number of carriers supported. Normally RF units are available with power ratings of 2W, 5W, 10W and 20W. For example a 5W RF unit can support either two 64 Kbps Carriers or seven 19.2 Kbps Carriers or fourteen 9.6 Kbps Carriers. Thus if you need three voice channels (@ 16 Kbps per channel) and two 19.2 Kbps and one 64 Kbps data channel then you will require a 10W RF unit.

Sources / References :

Direcway Satellite Services - TDM/TDMA :
http://www.dwge.com/services/networking/tdma.htm

Direcway Satellite Services - SCPC (DAMA/PAMA) :
http://www.dwge.com/services/networking/scpc.htm