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• ATM is a high-speed switching
network architecture
• ATM can be used to carry data,
voice, and video
– separately or simultaneously
over same network path
• ATM has a robust quality of
service (QoS)
– can provide seamless
interconnectivity between LAN’s and WAN’s
– supports a wide range of data
rates:
• 25 to 155 Mbps over copper
• 100 to 622 Mbps and higher over
fiber
• common implementation is
155-Mbps ATM
• ATM is specified via a
three-layer reference model:
• Physical layer (OSIs physical
layer)
• ATM layer (generally OSIs data
link layer)
• ATM adaptation layer (AAL)
(generally OSIs higher-level layers (transport,
session, and application)
• Physical layer (2 sublayers)
– Physical medium PM (lower
sublayer)
• definition for the medium
• the bit-timing capabilities.
– Transmission convergence (TC)
(upper sublayer)
• makes sure that valid cells are being created and
transmitted
• involves breaking off
individual cells from the data stream of the
higher layer (the ATM layer)
• checking the cell’s header
• Encoding the bit values
• ATM layer
– service-independent layer
– creates cell headers and
trailers
– defines virtual channels and
paths and gives them unique identifiers
– cells are multiplexed or
demultiplexed.
– ATM layer creates the cells and
uses the physical layer to transmit them.
• ATM adaptation layer (AAL) (2
sublayers)
– Segmentation and reassembly SAR
(lower sublayer)
• packages variable size packets
into fixed-size cells at the
transmitting end
• repackages the cells at the
receiving end
• responsible for finding and
dealing with cells that are out of order
or lost
– convergence sublayer CS (upper
sublayer)
• provides the interface for the
various services (e.g. data, voice, and
video).
• users connect to CS through
service access points
• (SAPs).
• ATM cells are always 53 bytes
long
• partitioned into
– 5 byte header è
contains
addressing information
– 48 byte payload è
contains
user data
• ATM virtual connections consist
of either
– permanent or switched virtual
circuits
– that logically connect source
and destination sites
– Virtual circuits are identified
by specific virtual channel identifiers (VCIs).
• A collection of virtual
channels that all have the same endpoints is called a virtual
path connection (VPC)
• VPCs are specified by virtual
path identifiers (VPIs)
• Virtual connections established
– VCI and VPI assignments are
made dynamically by ATM end nodes and
switches at the time data are to
be transmitted
– VCI is not of interest to e.g.
public switches they would only use the VPI
ATM LAN
• Local area network emulation
(LANE) interface
– can provide a service interface
for the network layer that functions exactly
as the same as Ethernet/802.3 and token ring
– LANs with this interface –
Emulated LANs (ELAN)
• involve special client/server
processes that enables MAC-to-ATM address resolution
• support connectionless nature of local area
networks
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