Atm Virtual Connections; Atm And Quality Of Service - Allied Telesis AT-AR240E User Manual

AT-AR240E ADSL Router Web Interface Manual

ATM VIRTUAL CONNECTIONS

ATM networks are fundamentally connection oriented, which means that a
virtual channel (VC) must be set up across the ATM network prior to any
data transfer. (A virtual channel is roughly equivalent to a virtual circuit.)
Two types of ATM connections exist: virtual paths, which are identified by
virtual path identifiers, and virtual channels, which are identified by the
combination of a VPI and a virtual channel identifier (VCI).
Virtual Path Identifier/ Virtual Circuit Identifier.
Routing is performed using a two layer hierarchical scheme. The higher layer
involves virtual paths and switching according to the VPI only. A particular
virtual path may carry a number of different virtual channels corresponding
to individual connections. When switching is performed according to the VPI
all cells on that particular virtual path are switched regardless of VCI. An
ATM switch may route according to VCI, VPI or VCI and VPI.
An example of how this routing scheme may be useful is setting up a virtual
path across a public ATM network between two sites. All cells from one site
to the other will follow the same virtual path. Within each site routing will be
performed according to the VCI.

ATM and Quality of Service

ATM Networks are designed to transmit data with varying characteristics.
Different applications need various Qualities of Service (QoS). Some
applications like telephony may be very sensitive to delay, but rather
insensitive to loss, whereas others like compressed video are quite sensitive to
loss.
Using the Web interface it is possible to define the following
connection types, each of which is characterized by a different expected
Quality of Service (QoS):
•
CBR (Constant Bit Rate)
•
VBRrt (real-time Variable Bit Rate)
•
VBRnrt (non-real-time Variable Bit Rate)
•
ABR (Available Bit Rate)
•
UBR (Unspecified Bit Rate)
The relationship between Virtual Circuits and Virtual Paths
99
different