Cell in ATM

Cell in ATM is a 53-byte packet of data, the standard packet size used by Asynchronous Transfer Mode (ATM) communication technologies.

What is Cell in ATM?

A 53-byte packet of data, the standard packet size used by Asynchronous Transfer Mode (ATM) communication technologies. Cells are to ATM technologies what frames are to Ethernet networking. In other words, they form the smallest element of data for transmission over the network.

How Cell in ATM Works

ATM cells are standardized at a fixed-length size of 53 bytes to enable faster switching than is possible on networks using variable-packet sizes (such as Ethernet). It is much easier to design a device to quickly switch a fixed-length packet than to design a device to switch a variable-length packet. (Switching a fixed-length packet is easier because the device knows in advance the exact length of the packet and can anticipate the exact moment at which the last portion of the packet will be received. With variable-length packets, the device must examine each packet for length information.) Using fixed-length cells also makes it possible to control and allocate ATM bandwidth more effectively, making support for different quality of service (QoS) levels for ATM possible.

The functions of information stored in the 5-byte header of an ATM cell include the following:

  • Providing information about the physical layer transmission method being used
  • Providing flow control to enable a steady flow of cell traffic and to reduce cell jitter
  • Specifying virtual path or channel identification numbers so that multiplexed cells belonging to the same ATM connection can be distinguished from cells belonging to other ATM connections, and so that cells can be switched to their intended destination
  • Specifying the nature of the payload contained in the cell—that is, whether it contains actual user data or ATM cell-management information
  • Specifying the priority of the cell to determine whether the cell can be dropped in congested traffic conditions
  • Providing error checking by means of an 8-bit field containing cyclical redundancy check (CRC) information for the header itself

There are two kinds of header formats used in ATM cells:

  • User-Network Interface (UNI) format:
    Used for communication between end nodes and an ATM network

     

  • Network-Node Interface (NNI) format:
    Used within the ATM network itself after the cell has been multiplexed for transmission over its virtual path

     

NOTE

Why a 48-byte data payload for ATM cells? This is the result of a trade-off between larger 64-byte payloads that contain more data but take longer to package and unpackage—and are therefore not suitable for real-time transmissions such as voice or multimedia—and shorter 32-byte payloads that provide better real-time transmission but are inefficient for larger amounts of data. By compromising at a 48-byte payload size, ATM has good transmission capabilities for both voice and data communication, providing efficient packet transfer with low latency.