Friday, August 1, 2014

Video Multicast Systems (Introduction) and Drop and Continue logic

"The word Telecom, is no more related to Conversations, it is related to expressions, declarations, user-experience and emotions"

From my Recent Sojourns

My dear friends of the Transmission Fraternity, 

Hello! How are you? Yes, Long time!!!! But better late than never. In the past few months I almost had a kind of writers' block but then it feels really good to come out of that state and being able to lay down to you many things that I experienced in the past few months. Of course all of it cannot be comprised in one post but then there will be many of them I assure you. 

There are times in the life of an engineer that he would want to rediscover himself, he feels stagnated and wants to enter new domains, learn new things, satisfy his technical hunger and for that very reason I had taken a kind of sabbatical from the world of technical blogging, because it felt that I had only the same things to write without much variety. 

Today I bring to you one very important part or rather a section of recent telecom trends and that is a video multicast system.  

1. Multicast Traffic:
To understand this clearly we need to first get a clarity on what exactly is multicast and what is actually not. whenever the communication is done in both the ways with equal priority in both directions and equal adherence this is called as a unicast. Most of the telephone conversations, Internet sessions, P2P are an example of Unicast kind of system. In the communications field almost 95% of the cases are that of Unicast, because then and only then there can be the very important "Communication", but as I said in the first statement the trends in Telecom are changing and today Multicast is a reality. 

Let us in a very simple way understand how a multicast traffic flows. 

General Process of Multicast traffic Flow

As you can see that there is one location which has most of the content that needs to be sent and this is actually sent to a group of users that are actually wanting to view the content. Something like watching television or hearing to radio or things like that. However, one may have a very elementary question and that is "If this is what is Multicast, then what is Broadcast?" Well legitimate question and very nice too. Even I had the same doubt all over because the pattern of traffic flow in Broadcast is also the very same. 

However there are certain minute differences that a Broadcast traffic has with a Multicast traffic and these are. 

1. Broadcast is unconditional to a subnet whereas Multicast can be made conditional.
2. Multicast can be requested whereas Broadcast is not requested it is just received. 
3. There can be grouping of destination and source addresses in the case of Multicast but not Broadcast. 
4. Multicast can be restricted at a user plane, but broadcast no. 

All these four characteristics we will see in four different posts that is going to follow so that there is no overdose of study in our regular life that in itself is so busy. 

Let us now delve into some of the main uses of Multicast. 

2. Any one to Many service. 
3. Multipoint Storage. 

2. Video Transmission of 200 Channels:
Of all the many uses of multicast the best use is that in the case of Video. Video transmission through multicast is one of the best ways of handling the heavy nature of traffic that video has. Imagine you are having around 200 channels and each channel of around 5Mb/s each minimum, if we were do to a unicast transmission then this will have to be like 1gbps per user. the same amount of reservation would also have to be done in the access/aggregate network. However, this is not at all commercially feasible, because if this were done then Television services would cost you a bomb, even more than a business class flight ticket from Mumbai to Los Angeles. 

So what do we do in this case. To understand this let us follow the figure below. 

4 main clauses of Video Multicast

Looking at the clauses above it is apparent that the content is huge but then it is repetitive per user. So there needs to be a kind of a mechanism where the same content is delivered every where and the content is actually sent to a location then replicated over there. 

2.1.Drop and Continue Model
This replication of data is a primary thing in Multicast that gives it an edge in high bandwidth services delivery like Live video. 

To understand more let us see the figure below. 

Drop and Continue Transmission Architecture of Multicast

The figure above is in real sense called as the drop and continue model in the filed of multicast transmission. The Bandwidth of the ring is determined by not the number of users but actually the amount of total content that has to be transmitted. So 200 channels of 5 Mb/s each makes the Bandwidth equal to 1000 Mb/s or 1Gb/s. So the operator has to make a 1Gb/s ring in the system irrespective of how much is the number of users. The topology can also be MESH or whatever based on the number of fall-back paths that the operator desires. 

In this case the operator is actually user volume independent and the bandwidth of the topology/infrastructure remains constant despite use in the number of viewers. 

2.2 Drop and Continue Logic:

Multicast addresses are always of the type 01:00:XX:XX:XX:XX in the MAC sublayer and in the Layer-3 they are all having the Class D IP addresses that are actually more or less reserved for Multicast. So each and every channel has a Multicast IP address that is sent to the user. In the case of Multicast there is no Mac learning so in the Layer-2 the traffic is always in flooding mode without any restriction. 

Keep this in mind that had this traffic been a Broadcast traffic there would be the restriction of a Broadcast Storm Control (BSC) which is not the case in the event of a Multicast traffic. Hence seeing the Multicast category of the Mac address the BSC does not kick in and traffic is flooded to all the ports in the same VLAN. 

As you can see in the drop and continue port the traffic ones it comes in then the traffic is sent to the access and also replicated along the infrastructure. 

So my friends Drop and continue is not a magic, it is actually exploiting the most important faulty state of a switch and that is the flooding state. To exploit flooding property of a switching device in a controlled way leads to an efficient transmission of multicast traffic. 

Till then take care of yourself and always remember whenever you plan with science it is the best, but science only comes in the purest forms when you decide to shun your ego. 



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