Cisco 3521 Administrator's Manual page 122

About Dialing from the ISDN Network to the IP Network
Table 4-1
Routing Method
DID
TCS4
Administrator's Guide for Cisco IPVC 3521 BRI Gateway, Cisco IPVC 3526 PRI Gateway, and Cisco IPVC 3540 PRI Gateway
4-4
Routing Methods
Chapter 4 Using the Cisco IPVC 3500 Series Gateway
Explanation
The gateway supports two forms of DID (Direct
Inward Dialing): Multiple Subscriber Network
(MSN) and sub-addressing.
MSN—The telephone company assigns a
•
group of phone numbers to a particular ISDN
line by the telephone company. PRI or BRI
ISDN lines are usually assigned multiple
numbers in the US and in Europe.
When MSN is used, an ISDN terminal or
phone can dial directly to an IP network
endpoint. The call is still routed through the
gateway but the gateway is transparent to the
person dialing from an ISDN terminal.
An H.323 endpoint on the IP network
registers with the gatekeeper using one of the
MSN numbers. When an ISDN terminal dials
the MSN number, the call routes through the
gateway ISDN port connected to the line with
the MSN service to the endpoint that
registered using the requested number.
Sub-addressing—Sub-addressing is a
•
one-stage DID dialing mechanism in which a
phone sends two numbers. One number is for
routing on the circuit switched network. The
other number is forwarded to the gateway
inside a Q.931 sub-addressing information
element for IP address resolution by the
gatekeeper.
For PRI gatekeepers, sub-addressing can also
be used for implementing ISDN fallback
when not enough bandwidth is available for
routing an IP-oriented call over IP.
Implementing ISDN fallback requires the
support of the gatekeeper.
TCS4 is a special routing method for incoming
H.320 video calls. TCS4 allows direct inward
dialing to an endpoint on the IP network through
the gateway when DID is not available. H.323
endpoints on the IP network register with the
gatekeeper using extension numbers. When an
ISDN terminal dials one of the gateway phone
numbers followed by a TCS4 extension, the call is
routed directly to the corresponding IP endpoint
registered with that extension.
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