Asynchronous Transfer Mode - Structure of An ATM Cell

Structure of An ATM Cell

An ATM cell consists of a 5-byte header and a 48-byte payload. The payload size of 48 bytes was chosen as described above ("Why cells?").

ATM defines two different cell formats: UNI (User-Network Interface) and NNI (Network-Network Interface). Most ATM links use UNI cell format.

Diagram of the UNI ATM Cell

7 4 3 0
GFC VPI
VPI
VCI
VCI
VCI PT CLP
HEC


Payload and padding if necessary (48 bytes)

Diagram of the NNI ATM Cell

7 4 3 0
VPI
VPI
VCI
VCI
VCI PT CLP
HEC


Payload and padding if necessary (48 bytes)

GFC = Generic Flow Control (4 bits) (default: 4-zero bits)
VPI = Virtual Path Identifier (8 bits UNI) or (12 bits NNI)
VCI = Virtual Channel identifier (16 bits)
PT = Payload Type (3 bits)
CLP = Cell Loss Priority (1-bit)
HEC = Header Error Control (8-bit CRC, polynomial = X8 + X2 + X + 1)

ATM uses the PT field to designate various special kinds of cells for operations, administration and management (OAM) purposes, and to delineate packet boundaries in some AALs.

Several ATM link protocols use the HEC field to drive a CRC-based framing algorithm, which allows locating the ATM cells with no overhead beyond what is otherwise needed for header protection. The 8-bit CRC is used to correct single-bit header errors and detect multi-bit header errors. When multi-bit header errors are detected, the current and subsequent cells are dropped until a cell with no header errors is found.

A UNI cell reserves the GFC field for a local flow control/submultiplexing system between users. This was intended to allow several terminals to share a single network connection, in the same way that two Integrated Services Digital Network (ISDN) phones can share a single basic rate ISDN connection. All four GFC bits must be zero by default.

The NNI cell format replicates the UNI format almost exactly, except that the 4-bit GFC field is re-allocated to the VPI field, extending the VPI to 12 bits. Thus, a single NNI ATM interconnection is capable of addressing almost 212 VPs of up to almost 216 VCs each (in practice some of the VP and VC numbers are reserved).

Read more about this topic:  Asynchronous Transfer Mode

Famous quotes containing the words structure of, structure and/or cell:

    What is the structure of government that will best guard against the precipitate counsels and factious combinations for unjust purposes, without a sacrifice of the fundamental principle of republicanism?
    James Madison (1751–1836)

    Science is intimately integrated with the whole social structure and cultural tradition. They mutually support one other—only in certain types of society can science flourish, and conversely without a continuous and healthy development and application of science such a society cannot function properly.
    Talcott Parsons (1902–1979)

    She that but little patience knew,
    From childhood on, had now so much
    A grey gull lost its fear and flew
    Down to her cell and there alit,
    And there endured her fingers’ touch
    And from her fingers ate its bit.
    William Butler Yeats (1865–1939)