ATM

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

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UNI/NNI Cells

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:

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 indication.

CLP
Cell loss priority.

HEC
Header error control.

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AAL0

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

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

SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16). The sequence number is comprised of the CSI and the SN.

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

SC
Sequence count. The sequence number for the entire CS PDU, which is generated by the Convergence Sublayer.

SNP
Sequence number protection. Comprised of the CRC and the EPC.

CRC
Cyclic redundancy check calculated over the SAR header.

EPC
Even parity check calculated over the CRC.

SAR PDU payload
47-byte user information field.

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ITU-T I.366.2

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. AAL type 2 is subdivided into the Common Part Sublayer (CPS ) and the Service Specific Convergence Sublayer (SSCS ).

The CPS packet consists of a 3 octet header followed by a payload. The structure of the AAL2 CPS packet is shown in the following illustration.

LI
Length indicator. This is the length of the packet payload associated with each individual user. Value is one less than the packet payload and has a default value of 45 bytes (may be set to 64 bytes).

UUI
User-to-user indication. Provides a link between the CPS and an appropriate SSCS that satisfies the higher layer application. Values may be:

HEC
Header error control.

Information payload
Contains the CPS/SSCS PDU as described below.

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

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.

SAR PDU payload
Information field of the SAR PDU.

PAD
Padding.

The SSCS conveys narrowband calls consisting of voice, voiceband data or circuit mode data. SSCS packets are transported as CPS packets over AAL2 connections. The CPS packet contains a SSCS payload. There are 3 SSCS packet types.

Type 1 Unprotected; this is used by default.

Type 2 Partially protected.

Type 3 Fully protected: the entire payload is protected by a 10-bit CRC which is computed as for OAM cells. The remaining 2 bits of the 2-octet trailer consist of the message type field.

AAL2 SSCS Type 3 Packets:

The type 3 packets are used for the following:

• Dialled digits
• Channel associated signalling bits
• Facsimile demodulated control data
• Alarms
• User state control operations.

The following illustration gives the general sturcture of AAL2 SSCS Type 3 PDUs. The format varies and each message has its own format according to the actual message type.

Redundancy	Time stamp	Message dependant information	Message type	CRC-10
2	14	16	6	10 bits
AAL2 SSCS Type 3 PDU

Redundancy
Packets are sent 3 times to ensure error correction. The value in this field signifies the transmission number.

Time stamp
Counters packet delay variation and allows a receiver to accurately reproduce the relative timing of successive events separated by a short interval.

Message dependant information
Packet content that varies, depending on the message type.

The following message type codes exist:

CRC-10
The 10-bit CRC.

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AAL3/4 consists of message and streaming modes. It provides for point-to-point and point-to-multipoint (ATM layer) connections. The Convergence Sublayer (CS) of the ATM Adaptation Layer (AAL) is divided into two parts: service specific (SSCS ) and common part (CPCS ). 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:

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.

CPCS SDU
Variable information field up to 65535 bytes.

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.

AAL3/4 SAR PDU

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.

Functions of AAL3/4 SAR include identification of SAR SDUs; error indication and handling; SAR SDU sequence continuity; multiplexing and demultiplexing.

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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 AAL5 CPCS PDU is composed of the following fields:

CPCS payload
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.

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 user information without the Pad.

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

The structure of the AAL5 CS PDU is as follows:

The fields are as described for the AAL5 CPCS PDU.

IP frames encapsulated over ATM

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

CRC-10
Cyclic redundancy check: G(x) = x 10 +x 9 +x 5 +x 4 +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 same VPI as the user cells, however, they use two different reserved VCIs, as follows:

VCI=3 Segment OAM F4 cells.
VCI=4 End-end OAM F4 cells.

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

PTI=100 (4) Segment OAM F5 cells processed by the next segment.
PTI=101 (5) End-to-end OAM F5 cells which are only processed by end stations terminating an ATM link.

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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 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.

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.

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

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SSSAR

http://www.itu.int/ITU-T/ ITU-T RECOMMENDATION I.366.1.

The Segmentation and Reassembly Service Specific Convergence sublayer of the ATM Adaptation Layer (AAL) type 2 (SSSAR) allows bandwidth-efficient transmission of low-rate, short, and variable length packets in delay sensitive applications. The Segmentation and Reassembly Service Specific Convergence sublayer may be deployed on one or more AAL type 2 user information streams. The SSSAR protocol defines the sublayer structure and the procedures for the segmentation and reassembly process, as well as the optional transmission error detection and assured data transfer.

With this Segmentation and Reassembly Service Specific Convergence sublayer applied for a Service Specific Convergence sublayer for the AAL type 2, it is possible to transport a packet size of more than the maximum length specified in the CPS and also to multiplex with low-rate and short length packets in delay sensitive application.

The Segmentation and Reassembly Service Specific Convergence sublayer is subdivided into the Service Specific Segmentation and Reassembly sublayer (SSSAR), the Service specific Transmission Error Detection sublayer (SSTED), and the Service Specific Assured Data Transfer sublayer (SSADT). The protocol header structure is as follows:

Format of the SSSAR-PDU :

Format of the SSTED-PDU:

CI-Congestion Indication (1 bit)
CRC-Cyclic Redundancy Check (4 octets)
Length-Length of SSTED-SDU (2 octets)
LP-Loss Priorit y (1 bit)
Reserved-Reserved Field (set to zero) (6 bits)
SSTED-UU-SSTED User-to-User indication (1 octet)

SSTED User-to-User indication (SSTED-UU) field
The CPCS-UU field is used to transparently transfer CPCS user-to-user information.

Congestion Indication (CI)
This field is provided for compatibility with the service of the CPCS of the AAL type 5. It is transported transparently from the user of the transmitter to the user of receiver.

Loss Priority (LP)
This field is provided for compatibility with the service of the CPCS of the AAL type 5. It is transported transparently from the user of the transmitter to the user of receiver.

Length field
The Length field is used to encode the length of the SSTED-PDU payload field. The Length field value is also used by the receiver to detect the loss or gain of information. The length is binary encoded as number of octets. The Length field value of "0" is used to indicate that the received SSTED-PDU is to be aborted.

CRC field
The CRC-32 is used to detect bit errors in the SSTED-PDU.

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