Codecs in Wireless Networks and Transcoding
A wide variety of codecs are deployed in 2G and 3G wireless networks, and it
is beyond the scope of this book to describe them.
Conversion between the various codecs deployed in wireline, wireless, and
IP networks introduces both distortion and delay that is sometimes percepti-
ble. Ideally, broadband wireless IP networks should not constrain the choice of
codecs deployed in the endpoints, and there should not be any difference in
codecs depending on the type off access, wired or wireless. Most 3G wireless
networks have their own codec types, and one more reason to choose 4G wire-
less networks is not be constrained by the types of codec the SIP UA must use.
Codec Bandwidth
Codec bandwidth can be a consideration on frugal access links. Table 18.1
shows some typical voice codec bandwidths.
The bandwidths shown in Table 18.1 do not show the additional bandwidth
consumed by the packet overhead for the encapsulation of the codec payload
into RTP and IP packets. The RTP and IP headers can increase the effective
bandwidth over the network 2–3 times the codec bandwidth shown here. The
overhead for the RTP and IP headers is 40 bytes, and the exact bandwidth over
the network is a somewhat more complex function of the frame lengths used
for various codecs, typically 10, 20, or 30 ms. Most compressed codecs (this
excludes G.711) have effective network bandwidths in the 16–32 kb/s range.
As mentioned, the network load from voice is negligible compared to video
and various P2P file-sharing applications.
Table 18.1 Typical Codec Bandwidths
CODEC TYPE CODEC BW IN KB/S
G.711 64
G.729A 8
G.723.1 6.3
iLBC 15.2
SPEEX 4 - 44Quality of Service for Real-Time Internet Communications 307