Asynchronous Transfer Mode (ATM) in Computer Network
Why ATM networks?
- Driven by the integration of services and performance requirements of both telephony and data networking : “broadband integrated service vision” (B-ISON).
- Telephone networks support a single quality of service and is expensive to boot.
- Internet supports no quality of service but is flexible and cheap.
- ATM networks were meant to support a range of service qualities at a reasonable cost- intended to subsume both the telephone network and the Internet.
Asynchronous Transfer Mode (ATM):
It is an International Telecommunication Union- Telecommunications Standards Section (ITU-T) efficient for call relay and it transmits all information including multiple service types such as data, video or voice which is conveyed in small fixed size packets called cells. Cells are transmitted asynchronously and the network is connection oriented.
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ATM is a technology which has some event in the development of broadband ISDN in 1970s and 1980s, which can be considered an evolution of packet switching. Each cell is 53 bytes long – 5 bytes header and 48 bytes payload. Making an ATM call requires first sending a message to set up a connection.
Subsequently all cells follow the same path to the destination. It can handle both constant rate traffic and variable rate traffic. Thus it can carry multiple types of traffic with end-to-end quality of service. ATM is independent of transmission medium, they maybe sent on a wire or fiber by themselves or they may also be packaged inside the payload of other carrier systems. ATM networks use “Packet” or “cell” Switching with virtual circuits. It’s design helps in the implementation of high performance multimedia networking.
ATM Cell Format –
As information is transmitted in ATM in the form of fixed size units called cells. As known already each cell is 53 bytes long which consists of 5 bytes header and 48 bytes payload.
Asynchronous Transfer Mode can be of two format types which are as follows:
- UNI Header: which is used within private networks of ATM for communication between ATM endpoints and ATM switches. It includes the Generic Flow Control (GFC) field.
- NNI Header: is used for communication between ATM switches, and it does not include the Generic Flow Control(GFC) instead it includes a Virtual Path Identifier (VPI) which occupies the first 12 bits.
Working of ATM:
ATM standard uses two types of connections. i.e., Virtual path connections (VPCs) which consists of Virtual channel connections (VCCs) bundled together which is a basic unit carrying single stream of cells from user to user. A virtual path can be created end-to-end across an ATM network, as it does not routs the cells to a particular virtual circuit. In case of major failure all cells belonging to a particular virtual path are routed the same way through ATM network, thus helping in faster recovery.
Switches connected to subscribers uses both VPIs and VCIs to switch the cells which are Virtual Path and Virtual Connection switches that can have different virtual channel connections between them, serving the purpose of creating a virtual trunk between the switches which can be handled as a single entity. It’s basic operation is straightforward by looking up the connection value in the local translation table determining the outgoing port of the connection and the new VPI/VCI value of connection on that link.
ATM vs DATA Networks (Internet) –
- ATM is a “virtual circuit” based: the path is reserved before transmission. While, Internet Protocol (IP) is connectionless and end-to-end resource reservations not possible. RSVP is a new signaling protocol in the internet.
- ATM Cells: Fixed or small size and Tradeoff is between voice or data. While, IP packets are of variable size.
- Addressing: ATM uses 20-byte global NSAP addresses for signaling and 32-bit locally assigned labels in cells. While, IP uses 32-bit global addresses in all packets.
- ATM Adaption Layer (AAL) –
It is meant for isolating higher layer protocols from details of ATM processes and prepares for conversion of user data into cells and segments it into 48-byte cell payloads. AAL protocol excepts transmission from upper layer services and help them in mapping applications, e.g., voice, data to ATM cells.
- Physical Layer –
It manages the medium-dependent transmission and is divided into two parts physical medium-dependent sublayer and transmission convergence sublayer. Main functions are as follows:
- It converts cells into a bit stream.
- It controls the transmission and receipt of bits in the physical medium.
- It can track the ATM cell boundaries.
- Looks for the packaging of cells into appropriate type of frames.
- ATM Layer –
It handles transmission, switching, congestion control, cell header processing, sequential delivery, etc., and is responsible for simultaneously sharing the virtual circuits over the physical link known as cell multiplexing and passing cells through ATM network known as cell relay making use of the VPI and VCI information in the cell header.
- ATM WANs –
It can be used as a WAN to send cells over long distances, router serving as a end-point between ATM network and other networks, which has two stacks of protocol.
- Multimedia virtual private networks and managed services –
It helps in managing ATM, LAN, voice and video services and is capable of full-service virtual private-networking, which includes integrated access of multimedia.
- Frame relay backbone –
Frame relay services are used as a networking infrastructure for a range of data services and enabling frame relay ATM service to Internetworking services.
- Residential broadband networks –
ATM is by choice provides the networking infrastructure for the establishment of residential broadband services in search for highly scalable solutions.
- Carrier infrastructure for telephone and private line networks –
To make more effective use of SONET/SDH fiber infrastructures by building the ATM infrastructure for carrying the telephonic and private-line traffic.