June 2005
Intermediate to advanced
502 pages
21h 48m
English
Content preview from Switching to VoIP
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Chapter 6: Replacing the Voice Circuit with VoIP
Ethernet isn’t the only data link suitable for carrying VoIP packets—ATM, frame-
relay, point-to-point circuits, and other technologies can be used, and each intro-
duces its own overhead factors.
Decoding and Playback
When a VoIP packet is received, it is decoded according to the codec employed to
encode it. It is then played back on the analog hardware of the receiving endpoint—a
speaker—while undergoing DAC, or digital-to-analog conversion. Decoding
generally takes about as much processing power as encoding, depending on the
codec employed.
Figure 6-4. An Ethernet-encapsulated 20 ms VoIP packet
Table 6-2. VoIP codec bandwidth consumption
Codec Encoded sound bandwidth Ethernet overhead bandwidth Total bandwidth
G.711 64 kbps 31.2 kbps 95.2 kbps
G.726 32 kbps 31.2 kbps 63.2 kbps
G.728 16 kbps 31.2 kbps 78.4 kbps
a
a
G.728 uses four voice frames at 16 kbps per packet. This accounts for the deviation in overhead bandwidth.
G.729A 8 kbps 31.2 kbps 39.2 kbps
GSM 13 kbps 31.2 kbps 44.2 kbps
IP
Header
IP
Payload
UDP
Header
UDP
Payload
RTP
Header
RTP
Payload
G.711
Payload
160 bits
64 bits
96 bits
1,280 bits
Ethernet
Header
Ethernet
Header
128 bits
Ethernet CRC and Bumper
176 bits
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