A Survey of Bandwidth Optimization Techniques and Patterns in VoIP Services and Applications <s> a) De(Multiplexing) Applications <s> We describe a new method to multiplex a number of low bit rate audio streams into a single RTP stream between IP telephony gateways. Audio frames from different users are assembled into a multiplexed RTP payload thus reducing the overhead of RTP/UDP/IP headers. To maintain distinction between the multiplexed audio streams sharing a single RTP stream, a 2-byte mini-header is added to each frame of an audio stream. A unique channel identifier is associated between the peer gateway entities by means of signaling procedures. Major applications of this method will be in the areas of IP telephony gateways that interconnect PSTN/PBX systems and cellular access networks. Results have indicated that the overhead is reduced by 50% to 80% depending on the size of the audio frame. Other advantages of the proposed scheme include fewer UDP/IP connections between gateways, a reduced possibility of congestion at intermediate IP routers, and a reduction of processing delays in multiprotocol label switching (MPLS), differentiated service (diff-serv) and IP security (IPsec) environments. <s> BIB001 </s> A Survey of Bandwidth Optimization Techniques and Patterns in VoIP Services and Applications <s> a) De(Multiplexing) Applications <s> In packet voice communication systems, silence suppression algorithms are employed to achieve bandwidth efficiency by suppressing the inactive part of the speech. In this paper, we consider a Nortel fixed-point silence suppression algorithm to investigate means of reducing the computational complexity with a little or no loss in the performance. In view of the compact structure and superior performance of the existing Nortel algorithm, certain modifications are introduced without altering the structure of the original design. Based on these modifications, two algorithms are proposed. Results from objective and subjective quality tests show that the performance of each of the two modified algorithms is comparable to that of the Nortel algorithm. As for the average computational complexity, simulation tests based on TMS320C5402 DSK show that reduction of about 40% and 22% can be achieved for the two modified algorithms. <s> BIB002
Multiplexing approach was applied on IP-Telephony Gateways (IP-TG) that linked PSTN/IPBX to IP networks  . It was also seen to be appropriate for IP-TG networks linking Cellular Access Networks (CAN) with Mobile Switching Centres (MSC). This practice multiplexes multiple VoIP packets from different sources in single RTP header. A Test application showed that resultant overhead reduced from 50% to 80%. Another application of multiplexing was expressed in BIB001 involving voice packets generated by Session Initiation Protocols (SIP) applications. The scheme depended on the hypothesis that there are multiple SIP VoIP LANS linked via one SIP WAN VoIP Gateway (SWVG). This implied an increase in traffic on the SWVG gateway thus improving the multiplexing process. The concept is such that the sender SWVG gateway multiplexes the packets destined to the same destination, while the receiver SWVG gateway demultiplexes the packets and dispatches them to their various destinations. The outcome is such that header overhead is reduced, thus saving bandwidth. And quite supplementary is that the number of packets sent is reduced influencing reduction in the total overhead on network hips BIB001 . 3) Silence Suppression. This approach describes the technique for saving bandwidth wastage in a VoIP system. A silence suppression technique utilizes algorithm functions comprising of an encoder and a decoder. On the encoder side, voice activity detection (VAD) mechanism is incorporated with the task of monitoring received signals for voice activity. If this is secured, a low bit-rate encoder is initiated before padding the transmission details. And when activity is not detected for a preconfigured time period, the encoder output is prevented from being transported across the network; resulting in additional bandwidth savings  ,  This is achieved using a special packet called silence insertion descriptor (SID) packet that holds some characteristic parameters of the background noise, created and released to the far end. And discontinuous transmission (DTX) algorithm defines SID packet transmission frequency. On the decoder side, a signal representative of the silence in between conversation is generated by a comfort noise generation (CNG) engine. This is meant to fill in the breaks that the original silence should have occupied. At such, the transmission rate during the inactive periods of speech is reduced, and thus, the average transmission rate is condensed as well BIB002 . Quite assuredly, VoIP frames will not be generated continuously for each user, since there are silent periods within talk periods. These periods are noted to follow exponential distribution such that each sound alternates between OFF-state and ON-state appropriately  . Figure 7 shows the voice activity model, while table 2 shows the parameter valuations. 