Specifications

Overview of Dial Interfaces, Controllers, and Lines
Circuit-Switched Digital Calls
DC-9
Cisco IOS Dial Technologies Configuration Guide
Virtual Asynchronous Interfaces
A virtual asynchronous interface is created on demand to support calls that enter the router through a
nonphysical interface. For example, asynchronous character stream calls terminate or land on
nonphysical interfaces. These types of calls include inbound Telnet, LAT, PPP over character-oriented
protocols (such as V.120 or X.25), and LAPB-TA and PAD calls. A virtual asynchronous interface is also
used to terminate L2F/L2TP tunnels, which are often traveling companions with Multilink protocol
sessions. Virtual asynchronous interfaces are not user configurable; rather, they are dynamically created
and torn down on demand. A virtual asynchronous line is used to access a virtual asynchronous interface.
Figure 7 shows a variety of calls that are terminating on a virtual asynchronous interface. After the calls
end, the interface is torn down.
Figure 7 Asynchronous Character Stream Calls Terminating on a Virtual Asynchronous Interface
Circuit-Switched Digital Calls
Circuit-switched digital calls are usually ISDN 56-kbps or 64-kbps data calls that use PPP. These calls
are initiated by an ISDN router, access server, or terminal adapter that is connected to a client
workstation. Individual synchronous serial digital signal level 0 (DS0) bearer (B) channels are used to
transport circuit-switched digital calls across WANs. These calls do not transmit across “old world”
lines.
Figure 8 shows a Cisco 1600 series remote office router dialing in to a Cisco 3640 router positioned at
a headquarters gateway.
Virtual asynchronous
interface
Telnet call
X.25 PAD
call
PPP stream
coming in over
a V.120 line
L2F/L2TP tunnel
needing to be
terminated
LAT call
S6488