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ATM relies on cell-switching technology. ATM cells have a fixed length of 53 bytes which allows for very fast switching. ATM creates pathways between end nodes called virtual circuits which are identified by the VPI/VCI values.

This page describes the ATM UNI cell header structure and the PDU structures for the various ATM/SAR formats including: AAL0, AAL1, AAL2, AAL3/4 and AAL5.

UNI/NNI Cell

The UNI or NNI cell header comprises the first 5 bytes of the ATM cell. The remaining 48 bytes comprise the payload of the cell whose format depends on the AAL type of the cell. The structure of the UNI and NNI cell headers are given here:

UNI Cell header

NNI Cell header

GFC
Generic flow control (000=uncontrolled access).

VPI
Virtual path identifier.

VCI
Virtual channel identifier.
Together, the VPI and VCI comprise the VPCI. These fields represent the routing information within the ATM cell.

PTI
Payload type identifier.

CLP
Cell loss priority.

HEC
Header error control.

AAL0

AAL0 cells are sometimes referred to as raw cells. The payload consists of 48 bytes and has no special meaning.

AAL1 PDU

The structure of the AAL1 PDU is given in the following illustration.

AAL1 PDU

SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).

CSI
Convergence sublayer indicator. Used for residual time stamp for clocking.

SC
Sequence count.

SNP
Sequence number protection.

CRC
Cyclic redundancy check calculated over the SAR header.

Parity
Parity calculated over the CRC.

SAR PDU payload
47-byte user information field.

AAL2

Click here for more information on AAL2 decoding and analysis.

AAL2 provides bandwidth-efficient transmission of low-rate, short and variable packets in delay sensitive applications. It supports VBR and CBR. AAL2 also provides for variable payload within cells and across cells.

AAL2 CPS packet

CID
Channel identifier.

LI
Length indicator.

UUI
User-to-user indicator.

HEC
Header error control.

SAR PDU payload
Information field of the SAR PDU.

The structure of the AAL2 SAR PDU is given in the following illustration.

AAL2 SAR PDU

OSF
Offset field. Identifies the location of the start of the next CPS packet within the CPS-PDU.

SN
Sequence number. Protects data integrity.

P
Parity. Protects the start field from errors.

PDU Payload
Information field of the SAR PDU.

PAD
Padding.

AAL3/4

AAL 3/4 consists of message and streaming modes. It provides for point-to-point and point-to-multipoint (ATM layer) connections. The CS is divided into two parts: service specific and common part. This is illustrated in the following diagram:

AAL3/4 Packet

AAL3/4 packets are used to carry computer data, mainly SMDS traffic.

The functions of the AAL3/4 CPCS include connectionless network layer (Class D), meaning no need for an SSCS; and frame relaying telecommunication service in Class C. The CPCS PDU is composed of the following fields:

AAL3/4 CPCS PDU

CPI
Message type. Set to zero when the BAsize and Length fields are encoded in bytes.

Btag
Beginning tag. This is an identifier for the packet. It is repeated as the Etag.

BAsize
Buffer allocation size. Size (in bytes) that the receiver has to allocate to capture all the data.

User info
The actual information that is being sent by the user.

PAD
Padding field which is used to achieve 32-bit alignment of the length of the packet.

0
All-zero.

Etag
End tag. Must be the same as Btag.

Length
Must be the same as BASize.

IP frames encapsulated over ATM

The structure of the AAL3/4 SAR PDU is illustrated below:

ST	SN	MID	Information	LI	CRC
2	4	10	352	6	10 bits
2-byte header			44 bytes	2-byte trailer
48 bytes

AAL3/4 SAR PDU

ST
Segment type. Values may be as follows:

SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).

MID
Multiplexing identification. This is used for multiplexing several AAL3/4 connections over one ATM link.

Information
This field has a fixed length of 44 bytes and contains parts of CPCS PDU.

LI
Length indication. Contains the length of the SAR SDU in bytes, as follows:

CRC
Cyclic redundancy check.

AAL5

The type 5 adaptation layer is a simplified version of AAL3/4. It also consists of message and streaming modes, with the CS divided into the service specific and common part. AAL5 provides point-to-point and point-to-multipoint (ATM layer) connections.

AAL5 is used to carry computer data such as TCP/IP. It is the most popular AAL and is sometimes referred to as SEAL (simple and easy adaptation layer). The structure of the AAL5 CPCS PDU is composed of the following fields:

AAL5 CPCS PDU

User info
The actual information that is sent by the user. Note that the information comes before any length indication (as opposed to AAL3/4 where the amount of memory required is known in advance).

Pad
Padding bytes to make the entire packet (including control and CRC) fit into a 48-byte boundary.

Control
Reserved bytes which are set to all zeros.

Length
Length of the user information without the Pad.

CRC
CRC-32. Used to allow identification of corrupted transmission.

The structure of the AAL5 SAR PDU is as follows:

Information	PAD	UU	CPI	Length	CRC-32
1-48	0-47	1	1	2	4 bytes
		8-byte trailer

AAL5 SAR PDU

Information
Variable length field containing the CS information.

PAD
Padding used to cell align the trailer which may be between 0 and 47 bytes long.

UU
CPCS user-to-user indication to transfer one byte of user information.

CPI
Common part indicator is a filling byte (of value 0). This field is to be used in the future for layer management message indication.

Length
Length of the Information field.

CRC-32
Cyclic redundancy check computed from the Information field, PAD, UU, CPI and Length fields. It is a 32-generator polynomial.

F4/F5 OAM PDU

The structure of the F4 and F5 OAM cell payload is given in the following illustration.

OAM type	Function type	Function specific	Reserved	CRC-10
4	4	360	6	10	bits
48 bytes

F4/F5 OAM PDU

CRC-10
Cyclic redundancy check: G(x) = x¹⁰+x⁹+x⁵+x⁴+x+1

OAM type / Function type
The possible values for OAM type and function type are listed below:

OAM F4 cells operate at the VP level. They use the save VPI as the user cells, however, they use two different reserved VCIs, as follows:

OAM F5 cells operate at the VC level. They use the save VPI and VCI as the user cells. To distinguish between data and OAM cells, the PTI field is used as follows:

RM Cells

There are two types of Rate Management (RM) cells: RM-VPC, which manages the VP level and RM-VCC, which manages the VC level.

The format of RM-VPC cells is shown in the following illustration:

RM-VPC cell format

RM protocol identifier
Always 1 for ABR services.

Message type
This field is comprised of several bit fields:

ER
Explicit rate.

CCR
Current cell rate.

MCR
Minimum cell rate.

QL
Not used.

SN
Not used.

RM-VCC cells are exactly the same as RM-VPC cells, except that the VCI is not specified. The cell is identified solely by the PTI bits.

Reserved VPI/VCI Values

A number of VPI/VCI values are reserved for various protocols or functions, e.g., 0,5 is used for signalling messages. Following is a list of all reserved VPI/VCI values and their designated meanings:

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