Specifications

Appendix B Synopsis of Basic VoIP Concepts
A Voice Primer
B-2
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
A Voice Primer
This section describes some basic telephony concepts that might help you
understand VoIP:
• How VoIP Processes a Typical Telephone Call, page B-2
• Numbering Scheme, page B-3
• Analog Versus Digital, page B-3
• codecs, page B-4
• Delay, page B-5
• Echo, page B-7
• Signaling, page B-7
How VoIP Processes a Typical Telephone Call
The general flow of a two-party call follows this process:
1. The caller picks up the handset. This signals an off-hook condition to the
VoIP signaling application in the Catalyst 4224.
2. The session application issues a dial tone and waits for the caller to dial a
telephone number.
3. The caller dials the telephone number. The session application stores the
dialed digits.
4. After enough digits are accumulated to match a configured destination
pattern, the telephone number is mapped to an IP host via the dial plan
mapper. The IP host has a direct connection to either the destination
telephone number or a private branch exchange (PBX) that is responsible for
completing the call to the configured destination pattern.
5. The session application runs the H.323 session protocol to establish
transmission and reception channels for each direction over the IP network.
If the call is being handled by a PBX, the PBX forwards the call to the
destination telephone. If Resource Reservation Protocol (RSVP) has been
configured, RSVP reservations are put into effect to achieve the desired
quality of service (QoS) over the IP network.