Türk Telekom

ATM took its name from the first letters of Asynchronous Transfer Mode. It forms the infrastructure of access applications requiring more band width in the wide area. Losing its popularity day by day consequent to the development of the IP-Ethernet, ATM subsists as a transportation system. It performs the cell transfer, which can be considered as a type of packet switching transmission technique, by also benefiting from the advantages of the circuit switching technology.

It carries the traffics having different structural features such as sound, image and data through the same platform. In this transmission technique, in which the hardware based cell switching perfectly suitable for real time data transmission, the carrier cells have a fixed size of 53 bytes. The frames larger than 53 bytes are converted into the suitable format by the ATM adaptation level and the transmission is performed.

In the ATM service, the scalable parameters from Mbit level to Gbit level and the QoS (Quality of Service) parameters are assigned / guaranteed before the data transmission.

Figure (clockwise): Voice, Video, Data, ATM cells, 48-byte Payload, “ATM 53-byte cells are switched on hardware base.

Hardware based switching, the existence of fixed and small packets have allowed the ATM system to be fast and reliable. Virtual paths and channels have brought flexibility to the system. Turk Telekom provides this service with bandwidths varying from 2 Mbps and 622 Mbps to its customers.

ATM’S WAY OF OPERATING

The ATM system is a connection oriented transmission technique. Therefore, one of the parties sends a packet to the opposite party in order the connection to be made. This packet saves the information of the sources it requires in the paths and switches it passes from. These paths are called virtual paths (Private Virtual Paths). Each connection carries a unique identity and this information is hidden on these switches in case there would be a permanent connection.

When the connection is made, as the cells forming the transmission also carry the header information, the ATM switches know from which path they would transmit the incoming cells. The cells go through these virtual paths follow each other but it is not checked whether there is a cell loss or not.

Figure (counter-clockwise from the blue part): “ATM physical trunk”, “such as E3, STM-1…”, “Virtual Path (VP) can carry more than one VC’s”, “VC the logical link between the ATM end devices”, “Connection Indicator = VPI/VCI”

The cells are switched through the use of (VPI/VCI) channel indicators and the connection based, that is before the data transmission is initiated, resource allocation is performed.

ATM’S CELL STRUCTURE

The standard size every cell being sent in the ATM technology is 53 bytes. This standard sized 53 byte cell consists of a 5 byte header and a 48 byte payload.

Figure Above: User End
Figure Below: Carrier End