Direct Inward Dial
Typically, FXS ports connect to analog phones, but some carriers offer FXS trunks that
support DID. The DID service is offered by telephone companies, and it enables callers
to dial an extension directly on a PBX or a VoIP system (for example, Cisco Unified
Communications Manager and Cisco IOS routers and gateways) without the assistance of
an operator or automated call attendant. This service makes use of DID trunks, which
forward only the last three to five digits of a phone number to the PBX, router, or gateway.
For example, a company has phone extensions 555-1000 to 555-1999. A caller dials
555-1234, and the local CO forwards 234 to the PBX or VoIP system. The PBX or VoIP
system then rings extension 234. This entire process is transparent to the caller.
An FXS DID trunk can receive only inbound calls, thus a combination of FXS, DID, and
FXO ports is required for inbound and outbound calls. Two signaling types exist, loopstart
and groundstart, with groundstart being the preferred method.
Figure 3-22 shows an analog trunk using an FXS DID trunk for inbound calls and a standard
FXO trunk for outbound calls.
158 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Denver
PSTN
FXS-DID Inbound 0/0/0
FXO Outbound 0/1/0
DID 0/0/0
Support
0/1/0
Figure 3-22 Configuring DID Trunks
You could then complete the following steps to enable DID signaling on the FXS port:
Step 1. Configure the FXS port for DID and wink-start.
Router(config)#voice-port 0/0/0
Router(config-voiceport)#signal did wink-start
Step 2. Configure the FXO port for groundstart signaling.
Router(config)#voice-port 0/1/0
Router(config-voiceport)#signal groundstart
Step 3. Create an inbound dial peer using the FXS DID port. Note that direct inward
dial is enabled.
Router(config)#dial-peer voice 1 pots
Router(config-dialpeer)#incoming called-number .
Router(config-dialpeer)#direct-inward-dial
Router(config-dialpeer)#port 0/0/0
Step 4. Create a standard outbound dial peer using the FXO port.
Router(config)#dial-peer voice 910 pots
Router(config-dialpeer)#destination-pattern 9[2-8].........
Router(config-dialpeer)#port 0/1/0
Example 3-5 shows the complete DID trunk configuration.
Example 3-5 DID Trunk Configuration
Chapter 3: Routing Calls over Analog Voice Ports 159
Router(config)#voice-port 0/0/0
Router(config-voiceport)#signal did wink-start
Router(config)#voice-port 0/1/0
Router(config-voiceport)#signal groundstart
Router(config)#dial-peer voice 1 pots
Router(config-dialpeer)#incoming called-number .
Router(config-dialpeer)#direct-inward-dial
Router(config-dialpeer)#port 0/0/0
Router(config)#dial-peer voice 910 pots
Router(config-dialpeer)#destination-pattern 9[2-8].........
Router(config-dialpeer)#port 0/1/0