Specifications

B-3
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Appendix B Synopsis of Basic VoIP Concepts
A Voice Primer
6. The coder-decoder compression schemes (codecs) are enabled for both ends
of the connection using Real-Time Transport Protocol/User Datagram
Protocol/Internet Protocol (RTP/UDP/IP) as the protocol stack.
7. Any call-progress indications (or other signals that can be carried inband) are
cut through the voice path as soon as an end-to-end audio channel is
established. Signaling that can be detected by the voice ports is also trapped
by the session application at each end of the connection. Signaling carried
over the IP network is encapsulated in Real-Time Transport Control Protocol
(RTCP) using the RTCP application-defined (APP) extension mechanism.
8. When either person hangs up the phone, RSVP reservations are torn down (if
RSVP is used) and the session ends. Each end becomes idle, waiting for the
next off-hook condition to trigger another call setup.
Numbering Scheme
The standard Public Switched Telephone Network (PSTN) is a large,
circuit-switched network. It uses a specific numbering scheme, which complies
with the ITU-T’s international public telecommunications numbering plan
(E.164) recommendations. For example, in North America, the North American
Numbering Plan (NANP) is used. NANP consists of an area code, an office code,
and a station code:
• Area codes are assigned geographically.
• Office codes are assigned to specific switches.
• Station codes identify a specific port on that switch.
The format in North America is 1Nxx-Nxx-xxxx, where N = digits 2 through 9,
and x = digits 0 through 9. Internationally, each country is assigned a one- to
three-digit country code, and the country’s dialing plan is dictated by the country
code.
Analog Versus Digital
Analog transmission is not robust or efficient at recovering from line noise.
Because analog signals degrade over distance, they need to be amplified
periodically. This amplification boosts both the voice signal and the ambient line
noise, resulting in degradation of the quality of the transmitted sound.