Aynchronous Transfer Mode

Aynchronous Transfer Mode – Atm Essay, Research Paper Introduction Aynchronous Transfer Mode (ATM) has been accepted universally as the transfer mode of choice for Broadband Integrated Services Digital Networks(BISDN). ATM can handle any kind of information i.e. voice, data, image, text and video in an integrated manner. ATM provides a good bandwidth flexibility and can be used efficiently from desktop computers to local area and wide area networks. ATM is a connection- oriented packet switching technique in which all packets are of fixed length i.e. 53 bytes(5 bytes for header and 48 bytes for information). No processing like error control is done on the information field of ATM cells inside the network and it is carried transparently in the network. ATM meets the

following objectives for BISDN networks. 1) Supports all existing services as well as emerging services in the future 2) utilizes network resources very efficiently 3)minimizes the switching complexity 4)minimizes the processing time at the intermediate nodes and supports very high transmission speeds. 5)minimizes the number of buffers required at the intermediate nodes to bound the delay and the complexity of buffer management 6)guarantees performance requirements of existing and emerging applications. Click here to ATM Tutorial page Return To ATM Module Home Page Basic Concepts in ATM Information Transfer Routing ATM Resources ATM Cell Identifiers Throughput Quality Of Service Usage Parameter Control Flow Control Information Transfer: ATM is a fast packet oriented transfer mode

based on asynchronous time division multiplexing and it uses fixed length(53 bytes) cells. Each ATM cell consists of 48 bytes for information field and 5 bytes for header. The header is used to identify cells belonging to the same virtual channel and thus used in appropriate routing. Cell sequence integrity is preserved per virtual channel. ATM Adaptation layers(AAL) are used to support various services and provide service specific functions. This AAL specific information is contained in the information field of the ATM cell. Basic ATM cell structure can be shown as follows. Look on the internet for the cell structure! Routing: ATM is a connection oriented mode. The header values(i.e. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched

from one section to other. Signaling information is carried on a separate virtual channel than the user information. In routing, there are two types of connections i.e. Virtual channel connection(VCC) and Virtual path connection(VPC). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC. ATM Resources: ATM is connection-oriented and the establishment of the connections includes the allocation of a VCI i.e. virtual channel identifier and/or VPI i.e. virtual path identifier and also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call.

ATM Cell Identifiers:ATM cell identifiers, i.e. Virtual Path Identifier, Virtual Channel Identifier and Payload Type Identifier(PTI) are used to recognise an ATM cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium. Throughput: Peak Cell Rate(PCR) can be defined as a Throughput parameter which in turn is defined as the inverse of the minimum interarrival time T between two consecutive basic events and T is the peak emission interval of the ATM connection. PCR applies to both constant bit rate(CBR) and variable bit rate(VBR) services for ATM connections. It is an upper bound of the cell rate of an ATM connection and there is another parameter sustainable cell rate(SCR) allows the ATM network