Скачать или смотреть Asynchronous Transfer Mode - 7 : ATM Development

Full playlist of #ATM covers various aspects like ATM basics, ATM operation, ATM #QoS & integration with #LAN
   • 46 : ATM -  Fundamentals of Communications  

Before we look how ATM works, let's discuss how the protocol developed. ATM was developed by the WAN telecommunications industry as a new and improved way to support different types of traffic, such as voice, video and data, across the WAN. It was only during the development of ATM that it was seen as a protocol that could be used for all networks-not just the WAN, but the LAN and MAN too.


So, because it can support different types of traffic, ATM is a very flexible protocol that can detect the different classes of traffic and assign them the appropriate quality of service. Voice gets one level, video gets another and data a third-yet all co-exist on a single wire.


This graphic gives you a good view of the problem with existing networks that don't use ATM. In this example, two frames of information arrive at a switch. Remember, a frame is a variable length packet of data. One frame of information is very small but has high priority. The other frame is very large; however, its priority is low.


In this example, the low priority traffic arrives at the switch first. Since switches work on a first come, first served basis, the other traffic, even though it has higher priority, must wait.


Prior to ATM, the only way to deal with this problem was to increase the bandwidth, which made the delay shorter. But this did not resolve the underlying problem.


Let's take a look at an example to compare how switches work to something you might see in your everyday life. Have you ever been driving in a car and had to wait at a railroad crossing for a train? The train gets there first, so you have to wait. This is how switches work-the packet that gets there first gets the priority, no matter what. Now imagine you are in an ambulance and the same thing happens-even though it's an emergency, you still have to wait for the train to pass. The same is true of switches. The priority of the packet doesn't matter-the one that gets to the switch first gets processed first.