1. Rivier College CS575: Advanced LANs ATM Adaptation Layer

2. ATM Adaptation Layer Functions
The ATM Adaptation Layer (AAL) is designed to support different types of applications and different types of traffic, such as voice, video, imagery, and data
Its basic function is the enhanced adaptation of services provided by the ATM layer to the requirements of the higher layer
It maps higher layer PDUs into the information field of the ATM cell
AAL is divided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) Sublayer
The Convergence Sublayer is service dependent and provides the AAL services at the AAL-SAP
The functions of the SAR sublayer are segmentation of higher layer PDUs into a suitable size for the information field of the ATM cell (48 octets) at the transmitting end and reassembly of the information fields into higher layer PDUs at the receiving end
CS575
ATM Adaptation Layer

3. ATM Adaptation Sublayers
Convergence Sublayer (CS):
Handling lost/misdelivered cells
Timing recovery
Interleaving
Segmentation and Reassembly Sublayer (SAR):
Split frames/bit stream into cells for transmission
Reassemble frames/bit stream for receiving
Support multiple protocols
Higher Layers
ATM Adaptation Layer (AAL)
Convergence Sublayer (CS)
Segmentation and Reassembly Sublayer (SAR)
ATM Layer (ATM)
Virtual Channel (VC)
Virtual Path (VP)
Physical Layer (PL)
Transmission Convergence Sublayer (TC)
Physical Medium Sublayer (PM)
The AAL functions are divided into two sublayers: segmentation and reassembly sublayer (SAR) and convergence sublayer (CS). On transmission, the SAR sublayer performs segmentation of higher layer PDUs into a suitable size for the information field of the ATM cell. On receiving, it reassembles the information field of the ATM cells into higher layer PDUs. The functions performed in the CS sublayer are service dependent. It performs mapping of the higher layer PDUs into the information field of an ATM cell. Four AAL service classes have been specified depending on the timing relationship, bit rate, and connection mode.
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ATM Adaptation Layer

4. AAL Protocols and Service Classes
To minimize the number of AAL protocols, ITU-T proposed a service classification specific to the AAL
The classification was made with respect to the following parameters:
Timing relationship between sender and receiver
Related
Not related
Bit rate
Constant bit rate
Variable bit rate
Connection mode
Connection-oriented
Connectionless
CS575
ATM Adaptation Layer

5. AAL Protocols and Service Classes (Concluded)
Four AAL protocols have been defined to support four AAL service classes
AAL service classes
Class A
Class B
Class C
Class D
AAL protocols
AAL 1
AAL 2
AAL 3/4
AAL 5
CS575
ATM Adaptation Layer

6. AAL Services CS575 ATM Adaptation Layer Class A Class B Class C
Class D
Related
Not Related
Service
Timing
between
Source and
Destination
Bit Rate
Constant
Variable
Connection
Mode
Connection Oriented
Connectionless
Circuit
Emulation
Connection-
Examples
Variable Bit
oriented
Connectionless of
Rate Video
Constant Bit
Data
Date Transfer
Services
and Audio
Rate Video
Transfer
and Audio
AAL 3/4
To minimize the number of AAL protocols, ITU-T (CCITT) proposed a service classification which is specific to the AAL This classification was made with respect to timing relation, bit rate, and connection mode. Four classes of services have been defined: class A, class B, class C, and class D.
Four AAL protocols have also been developed: AAL 1, AAL 2, AAL 3/4, and AAL 5. AAL 1 was designed to provide class A service. AAL 2 was designed for class B traffic. AAL3 and AAL 4 were combined into a single AAL 3/4 protocol to provide class C and class D services. A more efficient protocol AAL 5 was developed for class C traffic. However, no strict relationship between the the AAL service classes and the AAL protocol types is requested. In some applications, the AAL layer may be empty and the AAL functions are just the reception/delivery of the ATM-SDUs.
AAL
AAL3/4 AAL 5
AAL 1
AAL 2
TYPE
AAL 5
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ATM Adaptation Layer

7. AAL Type 1 Protocol
Supports Class A traffic, i.e., constant bit rate data with specific requirements for delay, delay jitter, and timing, e.g., PCM voice, CBR video, and emulation of T-carrier circuits (DS1, DS3)
Receives constant bit rate stream with a well defined clock from source and delivers the same to the destination
Provides for timing recovery (using SRTS), synchronization, and indication of lost information not recovered by AAL1
Summary of AAL 1 functions
Segmentation and reassembly of user information
Handling of cell delay variation
Handling of cell payload assembly delay
Handling lost and misinserted cells
Recovery of sending clock frequency at receiver
Checking and handling AAL PCI (header) error
AAL 1 supports Class A traffic. Timing information is transferred between source and destination. If necessary, information about the data structure can also be conveyed. Indication of lost or errored information is sent to the higher layer if these failures cannot be recovered within the AAL.
The functions performed by the AAL are as follows:
• Segmentation and reassembly of user information
• Handling of cell delay variation
• Handling of cell payload assembly delay
• Handling of lost and misinserted cells
• Source clock frequency recovery at the receiver
• Recovery of the source data structure at the receiver
• Monitoring of AAL-PCI for bit errors as well as handling of those errors
• Minitoring of the user information field for bit errors and possible corrective action
In the case of circuit emulation, monitoring of the end-to-end QoS is necessary, which will be located in the CS. For this purpose, a CRC may be calculated for the information carried in one or more cells. The result is transferred to the receiver within the information field of a cell or in a special OAM cell.
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ATM Adaptation Layer

8. AAL Type 1 SAR-PDU
Consists of 1 octet header (PCI) and 47 octets of payload
Sequence Number (SN): A 1-bit Convergence Sublayer Indication and 3-bit sequence count to detect deletion or misinsertion of cells
Sequence Number Protection (SNP): 3-bit CRC with even parity for detecting and correcting SN error
4 bits
4 bits
47 Octets SN
SNP
SAR-PDU Payload
The SAR-PDU consists of 48 octets.
the PCI is subdivided into a 4-bit sequence number (SN) and a 4-bit sequence number protection (SNP) field. The SN consists of a convergence sublayer indication (CSI) bit and a 3-bit sequence count. The CSI bit can be used to transfer timing information and/or information about the data structure.
The SN field makes it possible to detect the loss or misinsertion of cells.
The SNP field provides error detection and correction capabilities. The following two step approach will be used to allow the correction of all 1 bit errors and the detection of all 2 bit errors:
1. The SN is protected by the polynomial G(x) = x3 + x + 1.
2. The resulting 7 bit code word is protected by an even parity check.
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ATM Adaptation Layer

9. AAL Type 1 Sequence Number and Sequence Number Protection
The 4 bit RTS is transferred by the CSI bit in successive SAR-PDU headers with an odd SN (SN = 1, 3, 5, 7)
For P format operations, the CSI value in SAR-PDU headers with an even SN (SN = 0, 2, 4, 6) is set to 1
1 bit
3 bits
Convergence Sublayer
Indication (CSI)
Sequence Number (SN)
Cyclic Redundancy Check
Even Parity
1 bit
3 bit
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ATM Adaptation Layer

10. AAL Type 1 CS PDU Two CS PDU formats
Non-P format: No CS header (CS PCI), 47 octet user information for transfer of unstructured data such as circuit emulation of full DS1 or DS3
P format: 1 octet header (Structure Pointer SP), and 46 octet user information for transfer of octet-aligned data such as N x 64 kbps (e.g., fractional DS1) services
The functions of the CS depend strongly on the service to be supported. Some of these functions are listed below:
• Handling of cell delay variation. A buffer is used to support this function.
• Handling of lost and misinserted cells.
• Source clock frequency recovery. The 4-bit residual time stamp (RTS) is transferred by the CSI bit in successive SAR-PDU headers with an odd number of sequence count.
• Transfer of structure information between source and destination. For the P format an additional pointer field (1 octet) is provided in the SAR-PDU payload. This pointer indicates the start of a structured block. The CSI in SAR-PDU header with an even SN is set to 1.
• Forward error correction may be used to ensure high quality for some video and audio applications.
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ATM Adaptation Layer

11. AAL Type 2 Protocol Designed to support Class B (VBR) traffic
Supports variable bit rate data where a strong timing relationship between source and destination is required, e.g., VBR audio and video
Data passed to AAL2 from higher layers at the source at fixed intervals and must be passed to the destination at the same rate
The amount of data passed to AAL2 may vary with each transfer
Supports voice compression and silence suppression
Supports idle voice channel deletion
Supports multiple user channels with varying bandwidth on a single ATM connection
AAL type 2 is proposed for VBR services with a timing relation between source and destination (class B traffic, e.g., VBR audio and video).
AAL type 2 is not well defined yet.
The functions performed by the AAL 2 are as follows:
• Segmentation and reassembly of user information
• Handling of cell delay variation
• Handling of lost and misinserted cells
• Recovery of the source clock at the receiver
• Monitoring of AAL-PCI for bit errors as well as handling of these errors
• Minitoring of the user information field for bit errors and possible corrective action
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ATM Adaptation Layer

12. AAL Type 2 Voice Application Examples
PBX-to-PBX trunking for compressed voice
ATM trunking on public-switched telephone network
ATM backbone for cellular systems and personal communications services (PCS)
ATM backbone connectivity to packet telephone
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ATM Adaptation Layer

13. PBX-to-PBX Connectivity
Compression
Silence Suppression
Packetization
PBX
PCM Voice
Compression
Silence Suppression
Packetization
PBX
PCM Voice
AAL
ATM
PHY
ATM Network
AAL
ATM
PHY
CS575
ATM Adaptation Layer

14. AAL Type 2 Packets
AAL2 provides bandwidth-efficient transmission of low-rate, short, and variable-size packets for delay sensitive applications
AAL2 uses one ATM connection between two points to carry packets from multiple native connections
The ATM payloads from successive cells of the ATM connection are used as a byte stream on which packets from different native channels, called logical link channels (LLCs), are packed without regard to the cell boundaries
A channel identification (CID) field is used in the packet header to identify the LLC to which a packet belongs
A length indicator (LI) field is used to identify the boundaries of variable-length LLC packets
CS575
ATM Adaptation Layer

15. AAL Type 2 Protocol Sublayers
Service Specific Convergence Sublayer (SSCS)
Common Part Sublayer (CPS)
CS575
ATM Adaptation Layer

16. AAL 2 Common Part Sublayer
Defines an end-to-end AAL connection as a concatenation of AAL2 channels
Each AAL2 channel is a bi-directional virtual channel, with the same channel identifier value for both directions
AAL2 channels are established over an ATM layer PVC, SPVC, or SVC
Provides basic structure for identifying the users of the AAL
Assembling/disassembling the variable payload associated with each individual user
Error detection and correction
Multiplexing multiple AAL channels (merging multiple streams of CPS packets) onto a single ATM connection
Provides QoS through the choice of AAL-SAP for data transfer
CS575
ATM Adaptation Layer

17. AAL 2 Structure Service Specific Convergence Sublayer (SSCS)
AAL-SAP
Service Specific
Convergence
Sublayer (SSCS)
SSCS-PDU Header
(if present)
SSCS-PDU Trailer
SSCS-PDU Payload
SSCS-PDU
Packet Header
(PH)
Packet Payload
(PP)
Packet
Common Part
Sublayer (CPS)
Start Field
(STF)
CPS-PDU Payload
CPS-PDU
ATM-SAP
ATM Layer
Cell Header
Cell Payload
CS575
ATM Adaptation Layer

18. Format of AAL2 Packet
CID
8 bits
Information
1 to 45/64 octets LI
6 bits
UUI
5 bits
HEC
Packet Header (PH)
Packet Payload (PP)
Packet
Channel Identification (CID): Uniquely identifies the individual user channel (LLC) within the AAL2, and allows up to 248 ( ) individual users within each AAL2 structure.
Length Indicator (LI): Identifies the length of the LLC packet associated with each individual user, and assures conveyance of variable payload.
User-to User Indication (UUI): Provides a link between CPS and an appropriate SSCS that satisfies the higher layer application. Different SSCS protocols may be defined to support specific AAL2 user services, or groups of services. The SSCS may also be null.
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ATM Adaptation Layer

19. Format of AAL2 CPS-PDU Packets are combined into CPS-PDU payload
Cell Header
5 octets
OSF
6 bits SN
1 bit P
1 bit
PAD
0 to 47 octets
CPS Information
Start Field
CPS-PDU Payload
CPS -PDU
Packets are combined into CPS-PDU payload
The Offset Field (OSF) identifies the location of the start of the remaining length of the packet that possibly started in the preceding cell and is continuing in the current cell
Data integrity is protected by the Sequence Number (SN)
The Start Field is protected from error by a Parity bit (P)
When it is necessary to transmit a partially filled cell to limit packet emission delay, the remainder of the cell is padded with all zero octets
CS575
ATM Adaptation Layer

20. AAL 2 Service Specific Convergence Sublayer
SSCS is the link between the AAL2 CPS and the higher layer applications of the individual AAL2 users
Standards for SSCS are being developed in ITU-T and ATM Forum
A null SSCS satisfies most mobile voice applications
CS575
ATM Adaptation Layer

21. AAL Type 3/4 Protocol
Supports variable bit rate data where there is no timing relationship between source and destination, e.g., X.25, frame relay, and TCP/IP data
Supports Class C (connection-oriented) and Class D (connectionless) traffic
Convergence sublayer divided into two parts:
Common Part Convergence Sublayer (CPCS)
Service Specific Convergence Sublayer (SSCS)
SSCS layer may provide assured or non-assured services, or may be null
Assured service provides retransmission of missing or corrupted SSCS-PDUs and flow control is mandatory
AAL-SDUs may be lost or corrupted for non-assured service and flow control is optional
CPCS provides message mode or streaming mode services
There are two ways to transport messages in data communications: connection-oriented and connectionless. AAL3 was developed for connection-oriented communications and AAL4 was developed for connectionless communications. Since many functions performed by AAL 3 and 4 are common, the CS sublayer was further divided by ITU-T into two more sublayers, the common part CS (CPCS) and service specific CS (SSCS) so that AAL3 and AAL4 could share the same SAR and CPCS coding. With this layering structure, SSCS would provide AAL3 with connection management, flow control, error recovery, etc., while AAL4 would have no need for this service. In 1992, ITU-T merged AAL3 and 4 into a single AAL3/4 protocol.
SSCS sublayer may provide assured or non-assured services. CPCS supports multiple SSCS services.
Two modes of service have been defined for CPCS: message mode service and stream mode service. The message mode can be used for framed data transfer (e.g., HDLC frames). A single AAL-SDU is transported in one or more CS-PDUs. The stream mode is suitable for the transfer of low speed data with low delay requirements. One or more fixed-size AAL-SDUs are transported in one CS-PDU.
CS575
ATM Adaptation Layer

22. AAL Type 3/4 Protocol Sublayers
Service Specific Convergence Sublayer (SSCS)
Common Part Convergence Sublayer (CPCS)
Segmentation and Reassembly Sublayer (SAR)
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ATM Adaptation Layer

23. AAL Type 3/4 CPCS-PDU 4-octet header and 4-octet trailer
CPI Common Part Indicator
Btag Beginning Tag
BASize Buffer Allocation Size
PAD Padding
AL Alignment
Etag End Tag
Length Length of CPCS-PDU Payload 1 1 2
0 - 3 1 1 2
CPI
Btag
BASize
CPCS-PDU Payload
PAD AL
Etag
Length
The Common Part Indicator (CPI) field is used to indicate the usage of other CPCS-PDU header and trail fields. It varies depending on the modes of operation.
The Beginning Tag (Btag) field allows association of header and trailer of the CPCS-PDU, to ensure that all SAR-PDUs have been received correctly. This is done by putting the same value in both Btag and Etag. The receiving AAL will check whether they are still the same.
The Buffer Allocation Size Indication (BASize) field indicates to the receiving AAL the maximum buffering requirements to receive the CPCS-PDU. For message mode, it is set to the CPCS-PDU length. For stream mode, it is set to equal or greater than CPCS-PDU since the length of the CPCS-SDU (payload of CPCS-PDU) being transmitted is not known to the sending AAL.
The maximum CPCS-PDU payload size supported is 65,535 octets.
The Padding (PAD) field is from 0 to 3 octets to make the CPCS-PDU payload as an integral multiple of 4 octets.
The Alignment (AL) field is used to make 32 bit alignment for the CPCS-PDU trailer.
The Length field indicates the length of the CPCS-PDU payload.
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ATM Adaptation Layer

24. AAL Type 3/4 SAR-PDU
2-octet header, 44-octet payload, and 2-octet trailer
ST Segment Type
SN Sequence Number
MID Multiplexing Identifier
LI Length Indication
CRC-10 CRC Code
2 bits
4 bits
10 bits
352 bits
6 bits
10 bits ST SN
MID
SAR-PDU Payload LI
CRC-10
AAL3/4 SAR split CPCS-PDU into SAR-PDUs.
The Segment Type (ST) field indicates whether the SAR-PDU is the Beginning of Message (BOM, ST = 10), Continuation of Message (COM, ST = 00), End of Message (EOM, ST = 01), or a Single Segment Message (SSM, ST = 11).
The Sequence Number (SN) field is used to detect the loss or misordering of a SAR-PDU.
The Multiplexing Identification (MID) field is used to identify SAR-PDUs belonging to a particular SAR-SDU. MID allows the multiplexing of SAR-PDUs from different CPCS-PDUs and reassembly of them correctly at the receiving AAL.
The SAR-PDU payload field is 44 octets long.
The Length Indicator (LI) indicates how many of the 44 octets of payload actually carrying user data. An EOM or SSM SAR-PDU may not contain 44 octets of data. The value of LI must be a multiple of 4 octets.
The CRC field is used to detect bit errors for the entire SAR-PDU.
CS575
ATM Adaptation Layer

25. AAL Type 5 Protocol
AAL5 is a simple and efficient AAL (SEAL) to perform a subset of the functions of AAL3/4
The CPCS-PDU payload length can be up to 65,535 octets and must use PAD (0 to 47 octets) to align CPCS-PDU length to a multiple of 48 octets
PAD Padding
CPCS-UU CPCS User-to-User Indicator
CPI Common Part Indicator
Length CPCS-PDU Payload Length
CRC-32 Cyclic Redundancy Chuck
The AAL5 CPCS-PDU payload may vary from 0 to 65,535 octets.
The CPCS User-to-User field is used to support future AAL functions.
The AAL5 SAR splits CPCS-PDU into SAR-PDUs and passes them to the ATM layer.
All 48 octets are the SAR-PDU payload. No SAR-PDU header or trailer, no MID; therefore, SAR-PDUs are reassembled on VCI only.
Use Payload Type field in the cell header to indicate whether the SAR-PDU belongs to BOM (PT = 0X0), COM (PT = 0X0), or EOM (PT = 0X1).
The EOM carries the 8 octets trailer of the CPCS-PDU.
CS575
ATM Adaptation Layer

26. AAL Type 5 SAR-PDU CS575 ATM Adaptation Layer CPCS-SDU CPCS-PDU
CPCS-PDU Payload
PAD
CPCS-PDU
Trailer
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
SAR-PDU
SAR-PDU
SAR-PDU
SAR-PDU
CS575
ATM Adaptation Layer

27. AAL Type 5 Protocol (Concluded)
The CPCS-PDU is divided into 48 octets SAR-PDUs
Since CPCS-PDU is 48-octet aligned, there is no need for a length field in the SAR-PDU
The AAL5 SAR-PDU is 48 octets of data with no overhead of SAR- PDU header or trailer
The PTI field of the cell header identifies the beginning or end of the CPCS-PDU
PTI = 0X1: End-of-Message (EOM)
PTI = 0X0: Beginning-of-Message (B0M), or Continuation-of- Message (COM)
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ATM Adaptation Layer

28. References
W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000, Chapter 11
W. Stalling, Data and Computer Communications, 6th edition, Prentice Hall, 2002, Chapters 11-12
A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier College, 2001.
CS575
ATM Adaptation Layer