System information

Troubleshooting ATM Switching Environments 21-435
Virtual Channel Connections and Virtual Paths
Figure 21-5 Cell Passing Through a Boston Switch
In all cases, transporting cells through the use of label switching requires a connection. Information
about the connections is provided in the routing tables (sometimes called lookup tables) of switching
and multiplexing units. ATM uses virtual channel connections and virtual paths to accomplish
routing functions.
Virtual Channel Connections and Virtual Paths
A virtual channel connection (VCC) is a series of virtual channel links (VCLs) between two ATM
points. A VCL is a means of bidirectional transport of ATM cells between a point where a virtual
channel identifier (VCI) value is assigned and the point where the same value is either reassigned or
terminated. The VCI identifies the VCL to which a cell belongs and determines where the cell should
go next. Figure 21-6 shows the relationship between VCLs and VCCs in an ATM network.
Figure 21-6 The Relationship Between VCLs and VCCs in an ATM Network
VCCs are sometimes transported within virtual paths (VPs). A VP is identified by its virtual path
identifier (VPI). VPs provide a convenient way of bundling traffic directed to the same destination
or traffic requiring the same Quality of Service (QoS) in the network (see Figure 21-7).
VPI/VCI
Input ports
Output ports
Switch
value Data
Incoming cell
= Path of cell
through Boston
switch
To next
ATM switch
in network
M
VPI/VCI
value Data
Outgoing cell
X
2
3
5
6
7
1
Boston
Node 1
VCL
VCC
Node 2 Node n
VCL
ATM
endpoint
Host
ATM
endpoint
Host
Trunk
Virtual channel
connection(VCC)
or
Virtual channel
link (VCL)