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Rivier College
CS575: Advanced LANs
ATM Adaptation Layer
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ATM Adaptation Layer
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ATM Adaptation Layer Functions
0 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
0 Its basic function is the enhanced adaptation of services provided by the
ATM layer to the requirements of the higher layer
0 It maps higher layer PDUs into the information field of the ATM cell
0 AAL is divided into the Convergence Sublayer (CS) and the Segmentation
and Reassembly (SAR) Sublayer
0 The Convergence Sublayer is service dependent and provides the AAL
services at the AAL-SAP
0 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
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ATM Adaptation Sublayers
0 Convergence Sublayer (CS):
- Handling lost/misdelivered cells
- Timing recovery
- Interleaving
0 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)
ATM Layer (ATM)
Physical Layer (PL)
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Convergence Sublayer (CS)
Segmentation and Reassembly Sublayer (SAR)
Virtual Channel (VC)
Virtual Path (VP)
Transmission Convergence Sublayer (TC)
Physical Medium Sublayer (PM)
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AAL Protocols and Service Classes
0 To minimize the number of AAL protocols, ITU-T proposed a service
classification specific to the AAL
0 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
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AAL Protocols and Service Classes (Concluded)
0 Four AAL protocols have been defined to support four AAL service classes
0 AAL service classes
- Class A
- Class B
- Class C
- Class D
0 AAL protocols
- AAL 1
- AAL 2
- AAL 3/4
- AAL 5
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AAL Services
Service
Class A
Timing
between
Source and
Destination
Bit Rate
AAL
TYPE
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Class C
Related
Variable
Connectionle
ss
Connection Oriented
Circuit
Emulation
Constant Bit
Rate
Video
and Audio
Variable Bit
Rate Video
and Audio
Connectionoriented
Data
Transfer
AAL 3/4
AAL 1
Class D
Not Related
Constant
Connection
Mode
Examples
of
Services
Class B
AAL 2
ATM Adaptation Layer
AAL 5
Connectionless
Date Transfer
AAL3/4
AAL 5
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AAL Type 1 Protocol
0 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)
0 Receives constant bit rate stream with a well defined clock from source
and delivers the same to the destination
0 Provides for timing recovery (using SRTS), synchronization, and
indication of lost information not recovered by AAL1
0 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
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AAL Type 1 SAR-PDU
0 Consists of 1 octet header (PCI) and 47 octets of payload
0 Sequence Number (SN): A 1-bit Convergence Sublayer Indication
and 3-bit sequence count to detect deletion or misinsertion of cells
0 Sequence Number Protection (SNP): 3-bit CRC with even parity
for detecting and correcting SN error
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4 bits
4 bits
SN
SNP
47 Octets
SAR-PDU Payload
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AAL Type 1 Sequence Number and Sequence
Number Protection
0 The 4 bit RTS is transferred by the CSI bit in successive SAR-PDU
headers with an odd SN (SN = 1, 3, 5, 7)
0 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)
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Sequence Number (SN)
3 bit
1 bit
Cyclic Redundancy Check
Even Parity
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AAL Type 1 CS PDU
0 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
47 Octets AAL User Information
CS-PDU Non-P Format
1-Octet SP
46 Octets AAL User Information
CS-PDU P Format
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AAL Type 2 Protocol
0 Designed to support Class B (VBR) traffic
0 Supports variable bit rate data where a strong timing relationship
0
0
0
0
0
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
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AAL Type 2 Voice Application Examples
0 PBX-to-PBX trunking for compressed voice
0 ATM trunking on public-switched telephone network
0 ATM backbone for cellular systems and personal communications
services (PCS)
0 ATM backbone connectivity to packet telephone
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PBX-to-PBX Connectivity
PBX
PCM Voice
PBX
PCM Voice
Compression
Silence Suppression
Packetization
Compression
Silence Suppression
Packetization
AAL
ATM
PHY
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ATM Network
ATM Adaptation Layer
AAL
ATM
PHY
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AAL Type 2 Packets
0 AAL2 provides bandwidth-efficient transmission of low-rate, short,
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0
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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
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AAL Type 2 Protocol Sublayers
Service Specific Convergence Sublayer
(SSCS)
Common Part Sublayer (CPS)
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AAL 2 Common Part Sublayer
0 Defines an end-to-end AAL connection as a concatenation of AAL2
0
0
0
0
0
0
0
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
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AAL 2 Structure
AAL-SAP
Service Specific
Convergence
Sublayer (SSCS)
SSCS-PDU Header
(if present)
SSCS-PDU Payload
SSCS-PDU Trailer
(if present)
SSCS-PDU
Packet Payload
(PP)
Packet Header
(PH)
Packet
Common Part
Sublayer (CPS)
Start Field
(STF)
CPS-PDU Payload
CPS-PDU
ATM-SAP
ATM Layer
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Cell Header
Cell Payload
ATM Adaptation Layer
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Format of AAL2 Packet
CID
8 bits
LI
6 bits
UUI
5 bits
HEC
5 bits
Information
1 to 45/64 octets
Packet Payload (PP)
Packet Header (PH)
Packet
0 Channel Identification (CID): Uniquely identifies the individual user channel (LLC)
within the AAL2, and allows up to 248 (8 - 255) individual users within each AAL2
structure.
0 Length Indicator (LI): Identifies the length of the LLC packet associated with each
individual user, and assures conveyance of variable payload.
0 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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Format of AAL2 CPS-PDU
Cell Header
5 octets
OSF
6 bits
SN
P
1 bit 1 bit
Start Field
CPS Information
PAD
0 to 47 octets
CPS-PDU Payload
CPS -PDU
0 Packets are combined into CPS-PDU payload
0 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
0 Data integrity is protected by the Sequence Number (SN)
0 The Start Field is protected from error by a Parity bit (P)
0 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
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AAL 2 Service Specific Convergence Sublayer
0 SSCS is the link between the AAL2 CPS and the higher layer
applications of the individual AAL2 users
0 Standards for SSCS are being developed in ITU-T and ATM Forum
0 A null SSCS satisfies most mobile voice applications
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AAL Type 3/4 Protocol
0 Supports variable bit rate data where there is no timing
0
0
0
0
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
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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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AAL Type 3/4 CPCS-PDU
1
CPI
1
0-3
2
Btag BASize
CPCS-PDU Payload
PAD
1
AL
1
2
Etag Length
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
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AAL Type 3/4 SAR-PDU
2 bits
ST
4 bits
10 bits
SN
MID
352 bits
6 bits
SAR-PDU Payload
LI
10 bits
CRC-10
2-octet header, 44-octet payload, and 2-octet trailer
ST
SN
MID
LI
CRC-10
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Segment Type
Sequence Number
Multiplexing Identifier
Length Indication
CRC Code
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AAL Type 5 Protocol
0 AAL5 is a simple and efficient AAL (SEAL) to perform a subset of the
functions of AAL3/4
0 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
1
1
2
4
CPCS
UU
CPI
Length
CRC-32
0 - 47
CPCS-PDU Payload
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PAD
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AAL Type 5 SAR-PDU
CPCS-SDU
CPCS-PDU
CPCS-PDU Payload
PAD
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
Payload
SARPDU
SARPDU
SARPDU
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SAR-PDU
Payload
SARPDU
ATM Adaptation Layer
CPCS-PDU
Trailer
SAR-PDU
Payload
SARPDU
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AAL Type 5 Protocol (Concluded)
0 The CPCS-PDU is divided into 48 octets SAR-PDUs
0 Since CPCS-PDU is 48-octet aligned, there is no need for a length
field in the SAR-PDU
0 The AAL5 SAR-PDU is 48 octets of data with no overhead of SARPDU header or trailer
0 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-ofMessage (COM)
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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.
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