In computer networks, ATM stands for Asynchronous Transfer Mode. It is a telecommunication standard and a switching technology. The American National Standards Institute and ITU-T defined it. It can handle both voice and data simultaneously.
Virtual circuits (VCs)
ATM is a connection-oriented technology. It establishes a connection known as a virtual circuit (VC) before transmitting data between endpoints. A VC can be temporary or permanent.
A dial-up or a circuit switch connection establishes temporary VCs as per requirement. They create them before transmitting data and terminate them after the transmission.
Leased lines establish permanent VCs. Permanent VCs always remain available. After establishing a connection, the ATM works similarly on both types of VCs.
How does the ATM work?
ATM breaks up all packets, data, and voice streams into 48-byte chunks and adds a 5-byte routing header to each chunk. A chunk with the routing header is known as a cell.
ATM uses cells for data transmission. All ATM cells are the same in size. They are 53 bytes in length. They contain a header and a payload. The header contains protocol-specific information. The payload contains user data.
Before transmitting cells, the ATM establishes a VC between both endpoints. After establishing a VC, ATM uses it to transmit all cells. All the cells follow the same path connected to the destination.
Advantages of ATM
An ATM network provides the following benefits.
- It provides simple, uniform, and predictable data transmission.
- It can transmit all types and sizes of data efficiently.
- It provides dynamic bandwidth.
- It uses fixed-size cells to transmit data.
- Same-size cells reduce overload and effectively use network bandwidth.
ATM connection types
A connection type defines how an ATM transmits data. There are four types of connections: ABR, CBR, UBR, and VBR. ATM chooses the connection type based on the traffic.
ABR (Available Bit Rate):- It specifies a minimum guaranteed rate. Based on the network condition, it can increase the transmission speed. If traffic is low, it can transfer data at a higher speed.
CBR (Constant Bit Rate):- It specifies a fixed rate. Regardless of network condition, it sends data at a fixed rate. It is used to transmit a steady data stream.
UBR(Unspecified Bit Rate):- It does not specify any rate. It uses all remaining capacity. It is used for applications that can tolerate delays.
VBR(Variable Bit Rate):- It specifies a maximum possible rate. Based on the network condition, it adjusts the transmission speed. It is used for voice and video conferencing.
