ATM Technology
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Appendix: ATM Technology
BASICS
Cells
ATM (Asynchronous Transmit Mode) is a switching data transmission technology based on fixed-length cells
(5-byte header and 48-byte payload). Each cell contains information in its header, essential to routing the cell
and ensuring that the cell reaches its destination.
1
6
53
Header
Payload
Figure 4: ATM cell format
Virtual Circuits
ATM is a connection-oriented technology. Before transmitting data, a source must establish virtual circuit (a
bi-directional connection).
ATM circuits are of two types: virtual paths (VP) and virtual channels (VC). A virtual path is a bundle of
virtual channels (Figure 5.).
VP
VC1
VC2
VCn
Figure 5: Virtual channels and virtual path
The header of each cell includes the virtual path identifier (VPI) and the virtual channel identifier (VCI). The
VPI/VCI combination identifies a unique virtual circuit in a particular switch.
A virtual circuit can be permanent or created at connection time. The network administrator must manually set
up permanent Virtual Circuits (PVC). Switched Virtual Circuits (SVC) are automatically established and
closed through signaling protocols.
Signaling Protocols
In order to set up or to close SVC, signaling protocols send and receive packets on a well-known virtual
channel (VPI=0, VCI=5). The format of the packets depends on the type of interface. Two types of interfaces
defined by the ATM Forum coexist in ATM networks:
UNI (user-network interface) applying to the connections between final switches and systems (computers,
routers, etc), or public switches and private switches.
NNI (network-network interface) applying to the connections between switches of the same level.
The ATM Model
ATM standards define protocols that operate at the data-link layer level of OSI model. These protocols are
distributed in two layers: the ATM adaptation Layer (AAL) and the ATM layer. These two layers are
