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
|
