ATM is one of the three key layers of Broadband Integrated
Services Digital Network (B-ISDN). The other two layers are the Physical Layer
and the ATM Adaptation Layers (AALs).
Layer is responsible for the electrical or optical transmission and receipt
along the physical media between two devices. The physical layer supports
different physical media and different media interface rates. The physical layer
maps the cells into time-division multiplexed frames, which are then sent over
the physical medium.
ATM layer is placed above the Physical Layer. The
responsibility of ATM layer is to handle multiplexing and switching and
maintains the Quality of Service (QoS). Whenever an ATM connection is made, a
traffic contract is established between the user and the network. The
characteristics of the user data are established by this traffic contract. Thus
by determining the type of data, the network allocates the amount of resources
(bandwidth) required to the user to ensure that the traffic can be supported.
The user communicates such details as the average and burst traffic rates and
the acceptable loss and delay levels. These parameters determine the QoS. The
ATM layer is responsible for maintaining the QoS for each connection.
- The ATM layer performs the following functions
- Cell header generation/extraction.
- Multiplexing and de-multiplexing of cells.
- Managing of cell flow and sequencing.
- Handling dropped cells.
- Switch-based routing using virtual paths and virtual circuits.
ATM Adaptation Layer (AALs)
The ATM Adaptation layer adapts user traffic into cell-format.
AALs break data of various types, having different characteristics into 48
byte-cells. At the receiving end the cells are reassembled into the original
form by the AAL. Different AALs exist for different types of traffic.
are divided into 2 sub-layers namely Convergence Sub-layer (CS) and the
Segmentation and Reassembly (SAR) sub-layers. The function of the CS layer is to
describe the method in which non-ATM traffic is converted into ATM traffic. The
function of SAR layer is to insert data into ATM cells and adds header
information. There are different types of AALs, each supporting a different type
of traffic or service.
- Type1: Constant Bit Rate (CBR) service: AAL1 supports a
connection-oriented service in which the bit rate is constant. Examples of
this service include 64 Kbit/sec voice, fixed-rate uncompressed video and
leased lines for private data networks.
- Type2: Variable Bit Rate (VBR) service: AAL2 supports a
connection-oriented service in which the bit rate is variable but requires a
bounded delay for delivery. Examples of this service include compressed
packetized voice or video. The requirement on bounded delay for delivery is
necessary for the receiver to reconstruct the original uncompressed voice or
video. AAL2 has not been fully developed yet.
- Type3: Connection-oriented data service: Examples of this service include
connection-oriented file transfer and in general, data network applications
where a connection is set up before data is transferred. This service has
variable bit rate and does not require bounded delay for delivery. The ITU
originally recommended two types of AAL protocols to support this service
class, but these two types have been merged into a single type, called AAL3/4.
Type4: Connectionless data service: Examples of this service include
datagram traffic and in general, data network applications where no connection
is set up before data is transferred. Either AAL3/4 or AAL5 can be used to
support this class of service.