COMPUTER NETWORK LECTURE NOTES-- ATM Adaptation Layers (AAL)

The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer.

Examples of services that need adaptations are Gigabit Ethernet, IP, Frame Relay, SONET/SDH, UMTS/Wireless, etc.

The main services provided by AAL (ATM Adaptation Layer) are:

• Segmentation and reassembly
• Handling of transmission errors
• Handling of lost and misinserted cell conditions
• Timing and flow control

The following ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T. It is meant that these AALs will meet a variety of needs. The classification is based on whether a timing relationship must be maintained between source and destination, whether the application requires a constant bit rate, and whether the transfer is connection oriented or connectionless.

• AAL Type 1 supports constant bit rate (CBR), synchronous, connection oriented traffic. Examples include T1 (DS1), E1, and x64 kbit/s emulation.
• AAL Type 2 supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented, synchronous traffic. Examples include Voice over ATM. AAL2 is also widely used in wireless applications due to the capability of multiplexing voice packets from different users on a single ATM connection.
• AAL Type 3/4 supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25.
• AAL Type 5 is similar to AAL 3/4 with a simplified information header scheme. This AAL assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services that use AAL 5 are classic IP over ATM, Ethernet Over ATM, SMDS, and LAN Emulation (LANE). AAL 5 is a widely used ATM adaptation layer protocol. This protocol was intended to provide a streamlined transport facility for higher-layer protocols that are connection oriented.

     AAL 5 was introduced to:

• reduce protocol processing overhead.
• reduce transmission overhead.
• ensure adaptability to existing transport protocols.

T  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.

AAL2

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     Channelidentification.
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

HEC
Header error control.

AAL2

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.

CRC-10
The 10-bit CRC.

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 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:

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.

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
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