6.4. Key Issues: Replacing the Voice Circuit with VoIP
The two fundamental duties of a softPBX are call signaling and voice transmission
Voice channels in a VoIP network are analogous to loops or circuits in a traditional voice network
Voice transmission occurs across virtual channels on an IP network, most frequently using UDP packets because they have less overhead than TCP packets
Codecs are algorithms that digitize and package sound and/or video for transport across the network.
The most common codec for LAN use is G.711
G.711 uses one of two digitizing scales : m law or Alaw. m law is the standard in North America. Each requires 64 kbps of bandwidth
As a rule, TCP/IP and Ethernet add 32 kbps overhead to the bandwidth required by the codec
Selection of the codec for each call is accomplished by the signaling function of the softPBX or by direct negotiation between two endpoints at the beginning of that call
Transcoding is the real-time translation of one codec or another. This may be necessary when two endpoints cannot use the same codec or when a conferencing application dictates the use of more than one codec to support multiple endpoints.
Different codecs offer different levels of bandwidth economy.
The top causes of perceived quality problems in voice transmission are jitter, lag, and packet loss. Generally, the more bandwidth-conservative a codec is, the more prone to noticeable quality breakdowns caused by these issues
A call path is the chain of virtualized voice channels used to connect a call across the network. Each channel may employ a different codec
A call path that is established directly between two endpoints, without a softPBX, proxy, or transcoding element in between, is an independent call path.
Independent call paths result in lower processing intensity because the softPBX doesn't have to handle the audio. Complex, highly compressed codecs and softPBX call paths result in higher processing intensity
