Alcatel-Lucent omniacces-voip gateway Manuel D’Utilisation

 

Page 1-14

 

cations nodes in the network. ISDN does not use T1 (or DS-1) robbed bit signaling, where 
bits are taken from voice data to carry signaling. Alcatel H.323 gateways support in-band and 
out-of-band signaling.

 

Once signaling types are determined they are analyzed and converted by the appropriate DSP 
voice coder (vocoder) into digital signals, which are ultimately converted and expanded back 
(re-modulated) into real voice. More specifically, after signal recognition and analysis, DSPs 
convert (encode) the amplitude of incoming analog signals into digital form using codecs, or 
CODer/DECoders.

The basic encoding schemes, or companding methods in use today, are for PCM which 
“encodes” analog signals into digital signals. Although the PCM companding methods used for 
T1, which follows Mu-Law, and E1 which follows A-Law, differ mainly in their algorithms, 
their purpose is much the same. (Companding is a contraction for compression and expan-
sion.) However, A-Law and Mu-Law are incompatible. They use different methods, for exam-
ple, to sample analog signals.

Next, the digitally encoded signals are compressed using industry standard vocoders. These 
are devices that use speech compression/decompression algorithms to analyze and convert 
analog waveforms into digital signals and reduce related bandwidth requirements.

 

 

The appropriate vocoder used for VoIP calls is then negotiated by the H.323 VoIP gateway 
prior to call placement. As an added bonus, but with some variations in protocols, the same 
DSP technology that is used for voice compression also works with fax modems. (For that 
reason, it can be assumed that references to voice signal packets inherently include “fax” 
packets.)

The codecs and vocoders used in enterprise VoIP adhere to the ITU recommendations that 
fall under the H.323 IP network call control umbrella of interoperability standards for multi-
media communications over packet-switched local area networks (part of the Series H Recom-
mendations for Audiovisual and Multimedia Systems). The ITU’s H.323 suite of specifications 
includes the H.245 in-band call control specifications. For the signaling vocoders (G.711, PCM; 
G.723.1, Internet Speech; G.729/G.729a; Standard Telephone Speech), the algorithms in the 
Series G Recommendations for Transmission Systems and Media, Digital Systems and 
Networks are used.