Telindus 1031, User And Reference Manual

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Telindus 1031 Router Chapter 6
User manual Configuring the WAN encapsulation protocols
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What are the ATM layers?
The ATM reference model is composed of the following ATM layers:
What is multi-protocol over ATM?
As its name implies, multi-protocol encapsulation over ATM provides mechanisms for carrying traffic
other than just IP. There are two ways to do this:
LLC encapsulation is provided to support routed and bridged protocols. In this encapsulation format,
PDUs from multiple protocols can be carried over the same virtual connection. The type of protocol is
indicated in the packet's SNAP header. By contrast, the virtual connection multiplexing method allows
for transport of just one protocol per virtual connection.
Layer Description
physical layer Analogous to the physical layer of the OSI reference model, the ATM physical
layer manages the medium-dependent transmission.
ATM layer Combined with the ATM adaptation layer, the ATM layer is roughly analogous to
the data link layer of the OSI reference model. The ATM layer is responsible for
the simultaneous sharing of virtual circuits over a physical link (cell multiplexing)
and passing cells through the ATM network (cell relay). To do this, it uses the VPI
and VCI information in the header of each ATM cell.
ATM Adaptation
Layer (AAL) Combined with the ATM layer, the AAL is roughly analogous to the data link layer
of the OSI model. The AAL is responsible for isolating higher-layer protocols from
the details of the ATM processes. The adaptation layer prepares user data for con-
version into cells and segments the data into 48-byte cell payloads.
higher layers Finally, the higher layers residing above the AAL accept user data, arrange it into
packets, and hand it to the AAL.
Layer Description
Logical Link Control
(LLC) encapsulation In this method, multiple protocol types can be carried across a single con-
nection with the type of encapsulated packet identified by a standard LLC/
SNAP header.
Virtual connection mul-
tiplexing In this method, only a single protocol is carried across an ATM connection,
with the type of protocol implicitly identified at connection setup.