Asynchronous Transfer Model is another important connection oriented Network.
Why we call it asynchronous is most of the transmission in telephone systems is synchronous (closed tied to a clock) but ATM is not such type.
ATM was designed in 1990’s, it was the cell ray protocol designed by the ATM forum and was adopted by ITU-T.
- A technology is required that provides large data rates with the high data rate transmission media available (Optical Fiber Communication) and this media requires less susceptible to noise.
- The system must interface with existing systems to provide wide-area inter connectivity.
- The cost for such a system should not be more.
- The new system must be connection-oriented type.
- The new system must be able to work with the existing tele-communication hierarchies like local loops, long distance carriers etc.
The problems associated with existing networks:-
The design goals come into picture for ATM, since there exists some problems that are associated with the existing systems.
Before ATM we have data communications at DLL are based on frame switching and frame networks .
i.e, different protocols use frames of varying size (frame has data and header). If header size is more than that of actual data there is a burden so some protocols have enlarged the size of data unit relative to the header.
if there is no data in such cases there is a wastage , so there is to provide variable frame sizes to the users.
Mixed N/w Traffic:-
If there exists variable frame sizes
- The switches Multiplexers and routers must require an elaborate Software to manage variable size frames.
- Internet working among different frame N/w ‘s become slow and expensive too.
suppose we have two networks generating frames of variable sizes that is N/W 1 is connected to line 1 and the frame is X. N/W 2is connected to line 2 and of having 3 frames of equal sizes A,B,C are connected to a TDM.
If X has arrived a bit earlier than A,B,C (having more priority than X) on the output line . The frames has to wait for a time to move on to the output line, this causes delay for line 2 N/W.
i.e, Audio and video frames are small so mixing them with conventional data traffic often creates unacceptable delays and makes shared frame links unusable for audio and video information.
but we need to send all kinds of traffic over the same links.
so a solution to frame internet working is by adopting a concept called cell networking.
In a cell N/W we use a small data unit of fixed size called cell so all types of data are loaded into identical cells and are multiplexed with other cells and are routed through the cell N/W.
because each cell is small and of same size the problems associated with multiplexing different sized frames are avoided.
ATM uses asynchronous TDM- hence the name Asynchronous Transfer Model.
i.e, it multiplexes data coming from different channels. it also uses fixed size slots called cells.
ATM Mux’rs fill a slot with a cell from any input channel that has a cell and slot is empty if there is no cell.
ATM was going to solve all the world’s networking and tele-communications problems by merging voice, data, cable TV, telex,telegraph…… and everything else into a single integrated system that could do everything for everyone.
i.e, ATM was much successful than OSI and is now widely used in telephone system for moving IP packets.
ATM is a cell-switched N/W the user access devices are connected through a user-to- N/W interface (UNI) to the switches inside the N/W. The switches are connected through N/W-to-N/W interface (NNI) as shown in the following figure
two end points is accomplished through transmission paths (TP’s), Virtual Paths (VP’s) and Virtual Circuits (VC’s)
ATM Virtual Circuits:-
Since ATM N/w’s are connection-oriented, sending data requires a connection , first sending a packet to setup the connection.
- as setup packet travels though the sub net all the routers on the path make an entry in their internal tables noting for existence and reserving the resources.
- connections are often called virtual circuits and most ATM N/W’s support permanent virtual circuits. i.e, for permanent connections b/w two hosts.
- after establishing a connection either side can transmit data.
- all information is in small, fixed size packets called cells.
- cell routing is done in Hard ware at high speed.
- fixed size cells makes the building of Hard ware routers easier with short, fixed length cells.
- variable length IP packets have to be routed by Software which is a slower process.
- ATM uses the Hardware that can setup to copy one incoming cell to multiple output lines (ex:-TV).
- All cells follow the same route to the destination.
- cell delivery is not guaranteed but their order is.
- if cells lost along the way it is up to higher protocol levels to recover from lost cells but this also not guarantee.
- ATM N/W’s are organized like traditional WAN’s with lines and switches.
- the most common speeds for ATm are
- 155 Mbps-used for high definition TV.
- 155.52 Mbps-used for AT & T’s SONET transmission system
- 622 Mbps-4 X 155 Mbps channels can be sent over it.