Virtual Channels
12-3
Virtual Channels
ATM is a connection-oriented transport service, much like a telephone. As
opposed to Ethernet, where data is sent out on a common bus, ATM
requires that a
virtual channel
connecting the source station to the
destination station be established before data transmission can begin. A
virtual channel does not necessarily correspond to a single physical line
and, in fact, may pass through many different network devices and cables
before reaching its destination.
Virtual Channel
Connection
ATM seeks to establish an optimum path for a cell to travel through the
ATM network from source to destination. This path is called a
virtual
channel connection
(VCC). Its specification is coded into a
VCC identifier
which is then placed in each cell header. Using this information,
high-speed buffer management and control schemes route ATM cells
between ports along the path. To access the ATM network, a transmitting
station requests a VCC between itself and a destination station, using the
signalling protocol to the ATM switch. At connection time, a
VCC
is
established between the stations and a VCC identifier is assigned to the
connection.
Virtual Path
Connection
In addition to the VCC, another type of virtual connection called a
virtual
path connection
(VPC) can exist on the same physical link. The VPC is a
higher level “container” connection which includes one or more VCCs.
A VCC is uniquely identified by the combination of
Virtual Path Identifier
(VPI) and
Virtual Channel Identifier
(VCI) values and a VPC is uniquely
identified by the VPI value.
VPCs and VCCs can be set up either dynamically via signalling, in which
case it is called a
switched virtual circuit
(SVC), or permanently by
management where it is called a
permanent virtual circuit
(PVC).
Permanent Virtual
Channels
A PVC connection is set up manually by management (for example,
SNMP). A PVC connection, once established, is not automatically released
and is not dynamically checked and reinitiated.
The characteristics of the PVC connection include:
Q
Connections initiated by network administrator
Q
Connections established and released manually
Q
Long-term duration connections
abtthgde.book Page 3 Tuesday, June 23, 1998 10:29 AM
