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.

This ATM section includes information on the following:

 

UNI/NNI cell
AALO  
AAL1 PDU  
AAL2  
AAL3/4  
AAL5  
F4/F5 OAM  
RM Cells  
Reserved VPI/VCI Values  
SSSAR  

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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:

4

8 bits

GFC

VPI

VPI

VCI

VCI

VCI

PTI (3 bits)

CLP

HEC

UNI cell header

4

8 bits

VPI

VPI

VCI

VCI

VCI

PTI (3 bits)

CLP

HEC

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 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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AAL1 PDU

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

SN

SNP

 

CSI

SC

CRC

EPC

SAR PDU Payload

1 bit

3 bits

3 bits

1 bit

47 bytes
AAL1 PDU

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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AAL2

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 ).

AAL2 CPS Packet

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.

CID

LI

UUI

HEC

Information payload

8 bits

6 bits

5 bits

5 bits

1-45/64 bytes

AAL2 CPS packet
CID
Channel identification. Values may be as follows:
0 Not used
1 Reserved for layer management peer-to-peer procedures
2-7 Reserved
8-255 Identifies AAL2 user (248 total channels)


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:

1-15 Encoding format for audio, circuit mode data and demodulated fascimile image data using SSCS type 1 packets.
16-22 Reserved.
23 Reserved for SSCS type 2 packets.
24 SSCS type 3 packets except alarm packets.
25 Non-standard extension.
26 Framed mode data, final packet.
27 Framed mode data, more to come.
28-30 Reserved.
31 Alarm packets.

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.

Start field

CPS-PDU payload

 

OSF

SN

P

AAL2 PDU payload

PAD

 

6 bits

1 bit

1 bit

 

0-47 bytes

AAL2 CPS 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.

SAR PDU payload
Information field of the SAR PDU.

PAD
Padding.

AAL2 SSCS Packet

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.

Message type
The message type code.

The following message type codes exist:

Information stream

Message type code

Packet format

Dialled digits

000010

Dialled digits

Channel associated signalling

000011

CAS bits

Facsimile demodulation control

100000

T.30 Preamble

100001

EPT

100010

Training

100011

Fax Idle

100100

T.30 Data

Alarms

000000

Alarm

User state control

000001

User state control

CRC-10
The 10-bit CRC.

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AAL3/4

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.

AAL3/4 CPCS PDU

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:

Header

Info

Trailer

CPI

Btag

Basize

CPCS SDU

Pad

0

Etag

Length

1

1

2

0-65535

0-3

1

1

2 bytes

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.

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:

Segment type Value Meaning
BOM 10 Beginning of message
COM 00 Continuation of message
EOM 01 End of message
SSM 11 Single segment message


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:

Segment type LI
BOM, COM 44
EOM 4, ..., 44
EOM (Abort) 63
SSM 9, ..., 44

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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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).

AAL5 CPCS PDU

The AAL5 CPCS PDU is composed of the following fields:

Info

Trailer

CPCS payload

Pad

UU

CPI

Length

CRC

0-65535

0-47

1

1

2

4 bytes

AAL5 CPCS PDU

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.

AAL5 SAR PDU

The structure of the AAL5 CS 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

The fields are as described for the AAL5 CPCS PDU.

IP frames encapsulated over ATM

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F4/F5 OAM

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) = 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 type

Value

Function type

Value

Fault Management

0001

Alarm Indication Signal (AIS)

0000

Far End Receive Failure (FERF)

0001

OAM Cell Loopback

1000

Continuity Check

0100

Performance Management

0010

Forward Monitoring

0000

Backward Reporting

0001

Monitoring and Reporting

0010

Activation/ Deactivation

1000

Performance Monitoring

0000

Continuity Check

0001

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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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:

ATM Header: VCI=6 and PTI=110 (5 bytes)

RM protocol identifier (1 byte)

Message type (1 byte)

ER (2 bytes)

CCR (2 bytes)

MCR (2 bytes)

QL (4 bytes)

SN (4 bytes)

Reserved (30 bytes)

Reserved (6 bits) + CRC-10 (10 bits)

RM-VPC cell format

RM protocol identifier
Always 1 for ABR services.

Message type
This field is comprised of several bit fields:

Bit Name Description
8 DIR Direction of the RM cells: 0=forward, 1=backward.
7 BN BECN: 0=source is generated; 1=network is generated.
6 CI Congestion Indication: 0=no congestion, 1=congestion.
5 NI No increase: 1=do not increase the ACR.
4 RA Not used.

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.

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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. The following table contains a list of all reserved VPI/VCI values and their designated meanings:

VPI

VCI

Description

0

0

Idle cells. Must also have GFC set to zero. Idle cells are added by the transmitter to generate information for non-used cells. They are removed by the receiver together with bad cells.

0

1

Meta signalling (default). Meta-signalling is used to define the subchannel for signalling (default value: 0,5).

Non-zero

1

Meta signalling .

0

2

General broadcast signalling (default). Can be used to broadcast signalling information which is independent of a specific service. Not used in practice.

Non-zero

2

General broadcast signalling.

0

5

Point-to-point signalling (default). Generally used to set-up and release switched virtual circuits (SVCs).

Non-zero

5

Point-to-point signalling.

3

Segment OAM F4 flow cell. OAM cells are used for continuity checks as well as to notify and acknowledge failures.

4

End-to-end OAM F4 flow cell.

6

RM-VPC cells for rate management.

0

15

SPANS . The Simple Protocol for ATM Network Signalling is a simple signalling protocol, developed by FORE systems and used by FORE and other manufacturers working in cooperation with FORE, for use in ATM networks. Refer to Chapter 3 for more information.

0

16

ILMI . The Interim Local Management Interface is used to manage and compare databases across an ATM link. This is used for signalling address registration, RMON applications, SNMP, etc. Refer to ILMI in this book for more information.

0

18

PNNI signalling .

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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 :

UUI
MSB LSB
 
SSSAR-PDU Payload
CPS-Packet Header (CPS-PH)
CPS-Packet Payload (CPS-PP)
<------------------------------------>
<---------------------------------------------->

Format of the SSTED-PDU:

SSTED-PDU payload (SSTED-SDU)
SSTED-PDU
Trailer
SSTED-PDU
SSTED-UU
Reserved
C
I
L
P
Length
CRC
SSTED-PDU Trailer

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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Additional Information