MT1: UNIT 4 ATM – Multimedia Systems Technology

In the previous unit we examined the quality of Service and the types of services associated with it. Here, we will be looking at ATM, the Asynchronous transfer Mode in Multimedia Technology. Do make the most of your studies. 2.0 OBJECTIVES At the end of this unit, you should be able to: Identify what an ATM is State the typical features of the Asynchronous transfer Mode

ATM (Asynchronous transfer Mode)

ATM is a new networking technology that is being used increasingly in network backbones and wide area networks. ATM is an attempt to meet the requirements of three different types of network traffic:

Audio (voice telephone over short and long distances) Video (cable television)

Data (computer communications in LAN and WAN environments)

ATM is a high-speed network that uses fiber-optic cables or category 5 copper cables. It is a point-to-point switches network, meaning that central devices called switches are directly connected to end stations and to each other.

ATM is widely used as a backbone technology in carrier networks and large enterprises, but never became popular as a local network (LAN) topology. ATM is highly scalable and supports transmission speeds of 1.5, 25, 100, 155, 622, 2488 and 9953 Mbps. ATM is also running as slow as 9.6 Kbps between ships at sea. An ATM switch can be added into the middle of a switch fabric to enhance total capacity, and the new switch is automatically updated using ATM’s PNNI routing protocol.


Rather than transmitting frames, which can be variably sized, ATM communicates with cells. A cell is exactly 53 octets long. An octet is exactly 8 bits of data. Octets are more precisely defined than bytes, which are usually but not always 8 bits.

Rather than specifying the source and destination addresses of the stations communicating, an ATM cell indicates the path the data will flow through. Small cells all of the same sizes are oused to minimize latency and to make it easy for devices to process a cell, so intermediate devices (called switches) can maintain a very high data rate. The small constant cell size also allows ATM equipment to transmit video, audio and computer data over the same network, and assures that no single type of data hogs the line.


ATM is a circuit-based network, in that a virtual circuit is set up between two devices to communicate over the network. There are two types of circuits in an ATM network:

Permanent Virtual Circuit (PVC): A circuit that is set up once in the switches to allow communication between two devices

Switched Virtual Circuit (SVC): A circuit that is temporarily set up just for the duration of a communication two devices.

ATM is a descendant of packet-switching. Its high-speed advantage comes from transmitting uniform data packets that are subdivided into data frames, and each frame is enclosed within an addressable 53-byte cell and routed by hardware switching. The switching achieves very high-speed data transmission rate, between 155 and 622 Mbps (theoretically, up to 1.2 gigabits per second). ATM offers fast, real-time, demand-responsive switching for efficient use of network resources using broadband and baseband LANs or WANs.

Asynchronous Transfer Mode (ATM) seeks to provide:

A single network interface to communication channels for each media type – audio, video, image and text.

Adaptability of an application’s bandwidth requirements. Flexibility for handling different data types.

A common signalling structure.

The ATM (Asynchronous transfer Mode) provides the following levels of service:

Constant Bit Rate (CBR) guarantees bandwidth for real time voice and video.

Real time variable Bit Rate (RT-VBR) supports interactive multimedia that requires minimal delays.

Non-real time variable bit rate (NRT- VBR) is used for busy transaction traffic.

Available Bit Rate (ABR) adjusts bandwidth according to congestion levels for LAN traffic.

Unspecified Bit Rate (UBR) provides the best effort for noncritical data such as file transfers.


State one limitation and delimitation of the Asynchronous Transfer Mode (ATM)



In conclusion, ATM is replacing frame relay as the digital network over which public telephone systems operate worldwide. It was, however, designed with service for computers in mind and is being used with much success as a high-speed back-bone. ATM will eventually provide direct connection between private networks and public telephone networks for very high-speed access to the Internet.

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