HP 200 Series Services And Applications page 212

Improving Network Availability
Switched Circuit Types
Events
Figure 9. Connection Sequence for Manual Adapters
Figure 9 shows the events in the connection sequence for two routers using
manual adapters. Router B is configured to connect when the circuit is
enabled (during router bootup). Terminal adapter B has been configured for
auto-answer (that is, not to dial a number when the router enables the
circuit but rather to wait for an incoming call).
Router A is configured to connect when data is available. Data available
means that the the router has received a packet which must be sent to router
B. Terminal adapter A is configured to dial terminal adapter B's phone
number when router A raises DTR (data terminal ready).
In this configuration, router A always initiates calls and router B always
receives calls. In other words, applications on systems attached to router A
always initiate the connections to systems attached to router B. This is
frequently the situation for client systems at a sales/support location
connecting to servers at a central or regional office for database access,
credit verification, electronic mail, etc.
When circuit-switching network connections are used as backup circuits,
and manual terminal adapters are used, the configuration just discussed is
recommended.
When the establishment of connections must be bilateral, that is, when
either router may initiate the connection establishment, a V.25 bis adapter
must be used. Connecting with V.25 bis adapters is discussed further below.
3-18
TA
a
Network
data
rec'd
DTR
call TA "B"
DSR
*** Connection Established ***
ISDN
TA
ready to
receive call
DSR
b
DTR