Editing Asynchronous Transfer Mode (section)

===Cell structure===
An ATM cell consists of a 5-byte header and a 48-byte payload. ATM defines two different cell formats: [[user–network interface]] (UNI) and [[network-to-network interface|network–network interface]] (NNI). Most ATM links use UNI cell format.

{| width="100%"
|-----
|
'''Diagram of a UNI ATM cell'''

{| style="width: 20em; text-align: left;" border="1" cellpadding="2" cellspacing="0"
|-----
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 7
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 4
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 3
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 0
|-----
| colspan="4" rowspan="1" style="background-color: rgb(102, 255, 255); vertical-align: top; text-align: center; white-space: nowrap; width: 10em;" | GFC
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 204, 255);" | VPI<br />
|-----
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 204, 255);" | VPI<br />
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 255, 153);" | VCI<br />
|-----
| colspan="8" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 20em; background-color: rgb(255, 255, 153);" | VCI<br />
|-----
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 255, 153);" | VCI
| colspan="3" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 7.5em; background-color: rgb(255, 255, 204);" | PT
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em; background-color: rgb(255, 204, 204);" | CLP
|-----
| colspan="8" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 20em; background-color: rgb(153, 255, 153);" | HEC
|-----
| colspan="8" rowspan="1" style="background-color: rgb(204, 204, 204); vertical-align: top; text-align: center; white-space: nowrap; width: 20em;" | <br />
<br />
Payload and padding if necessary (48 bytes)<br />
<br />
|}
|
'''Diagram of an NNI ATM cell'''

{| style="width: 20em; text-align: left;" border="1" cellpadding="2" cellspacing="0"
|-----
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 7
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 4
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 3
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em;" | 0
|-----
| colspan="8" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 20em; background-color: rgb(255, 204, 255);" | VPI<br />
|-----
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 204, 255);" | VPI<br />
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 255, 153);" | VCI<br />
|-----
| colspan="8" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 20em; background-color: rgb(255, 255, 153);" | VCI<br />
|-----
| colspan="4" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 10em; background-color: rgb(255, 255, 153);" | VCI
| colspan="3" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 7.5em; background-color: rgb(255, 255, 204);" | PT
| style="vertical-align: top; text-align: center; white-space: nowrap; width: 2.5em; background-color: rgb(255, 204, 204);" | CLP
|-----
| colspan="8" rowspan="1" style="vertical-align: top; text-align: center; white-space: nowrap; width: 20em; background-color: rgb(153, 255, 153);" | HEC
|-----
| colspan="8" rowspan="1" style="background-color: rgb(204, 204, 204); vertical-align: top; text-align: center; white-space: nowrap; width: 20em;" | <br />
<br />
Payload and padding if necessary (48 bytes)<br />
<br />
|}
|}

;GFC
:The generic flow control (GFC) field is a 4-bit field that was originally added to support the connection of ATM networks to shared access networks such as a distributed queue dual bus (DQDB) ring. The GFC field was designed to give the User-Network Interface (UNI) 4 bits in which to negotiate multiplexing and flow control among the cells of various ATM connections. However, the use and exact values of the GFC field have not been standardized, and the field is always set to 0000.<ref>{{cite web|title=ATM Cell Structure|date=11 September 2008 |url=https://technet.microsoft.com/en-us/library/cc976978.aspx|access-date=13 June 2017}}</ref>
;VPI
:[[Virtual path identifier]] (8 bits UNI, or 12 bits NNI)
;VCI
:[[Virtual channel identifier]] (16 bits)
;PT
:Payload type (3 bits)
::Bit 3 (msbit): Network management cell. If 0, user data cell and the following apply:
::Bit 2: Explicit forward congestion indication (EFCI); 1 = [[network congestion]] experienced
::Bit 1 (lsbit): ATM user-to-user (AAU) bit. Used by [[AAL5]] to indicate packet boundaries.
;CLP
:Cell loss priority (1-bit)
;HEC
:[[CRC-based framing|Header error control]] (8-bit CRC, polynomial = X<sup>8</sup> + X<sup>2</sup> + 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 [[ATM adaptation layer]]s (AAL). If the [[most significant bit]] (MSB) of the PT field is 0, this is a user data cell, and the other two bits are used to indicate network congestion and as a general-purpose header bit available for ATM adaptation layers. If the MSB is 1, this is a management cell, and the other two bits indicate the type: network management segment, network management end-to-end, resource management, and reserved for future use.

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 (data)|flow control]] and sub-multiplexing system between users. This was intended to allow several terminals to share a single network connection in the same way that two 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 2<sup>12</sup> VPs of up to almost 2<sup>16</sup> VCs each.{{efn|In practice some of the VP and VC numbers are reserved.}}