barcode dll for vb net G.723.1 ACELP G.723.1 ACELP can operate at either 6.3 Kbps or 5.3 Kbps in Software

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G.723.1 ACELP G.723.1 ACELP can operate at either 6.3 Kbps or 5.3 Kbps
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with the 6.3 Kbps providing higher voice quality. Bit rates are contained in the coder and decoder, and the transition between the two can be made during a conversation. The coder takes a bank-limited input speech signal that is sampled a 8,000 Hz and undergoes uniform PCM quantization, resulting in a 16-bit PCM signal. The encoder then operates on blocks or frames of 240 samples at a time. Each frame corresponds to 30 milliseconds of speech, which means that the coder causes a delay of 30 milliseconds. With a lookahead delay of 7.5 milliseconds, the total algorithmic delay is 37.5 milliseconds. G.723.1 gives an MOS of 3.8, which is highly advantageous in regards to the bandwidth used. The delay of 37.5 milliseconds one way does present an impediment to good quality, but the round-trip delay over varying aspects of a network determines the final delay and not necessarily the codec used.
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G.729 G.729 is a speech coder that operates at 8 Kbps. This coder uses
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input frames of 10 milliseconds, corresponding to 80 samples at a sampling rate of 8,000 Hz. This coder includes a 5-millisecond look-ahead, resulting in an algorithmic delay of 15 milliseconds (considerably better than G.723.1). G.729 uses an 80-bit frame. The transmitted bit rate is 8 Kbps. Given that it turns in an MOS of 4.0, G.729 is perhaps the best trade-off in bandwidth for voice quality. The previous paragraphs provide an overview of the multiple means of maximizing the efficiency of transport via the PSTN. We find today that TDM is almost synonymous with circuit switching. Telecommunications engineers use the term TDM to describe a circuitswitched solution. A 64 Kbps G.711 codec is the standard in use on the PSTN. The codecs described in the previous pages apply to VoIP as well. VoIP engineers seeking to squeeze more conversations over valuable bandwidth have found these codecs very valuable in compressing VoIP conversations over an IP circuit.7
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Signaling
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Signaling describes the process of how calls are set up and torn down. Generally speaking, there are three main functions of signaling: supervision, alerting, and addressing. Supervision refers to monitoring the status of a line or circuit to determine if there is traffic on the line. Alerting deals with the ringing of a phone indicating the arrival of an incoming call. Addressing is the routing of a call over a network. As telephone networks matured,
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Ibid.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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The Public Switched Telephone Network (PSTN)
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individual nations developed their proprietary signaling systems. Ultimately, there become a signaling protocol for every national phone service in the world. Frankly, it is a miracle that international calls are ever completed given the complexity of interfacing national signaling protocols. Signaling System 7 (SS7) For much of the history of circuit-switched networks, signaling followed the same path as the conversation. This is called Channel-Associated Signaling (CAS) and is still in wide use today. R1 Multifrequency (MF) used in North American markets and R2 MultiFrequency Compelled (RFC) used elsewhere in the world are the best examples of this. Another name for this is in-channel signaling. The newer technology for signaling is called Common Channel Signaling (CCS), also known as out-of-band signaling. CCS uses a separate transmission path for call signaling and not the bearer path for the call. This separation enables the signaling to be handled in a different manner to the call. This enables signaling to be managed by a network independent of the transport network. Figure 2-4 details the difference between CAS and CCS.
Figure 2-4 CAS and CCS
Speech and Signaling
Switch
Channel Associated Signaling
Switch
Signaling
S.T.P. Speech
S.T.P.
Switch
Common Channel Signaling
Switch
8 Stallings, William. ISDN and Broadband ISDN with Frame Relay and ATM. New York: Prentice Hall, 1995. p.292.
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The Public Switched Telephone Network (PSTN)
The Public Switched Telephone Network (PSTN)
Signaling System 7 (SS7) is the standard for CCS with many national variants throughout the world (such as Mexico s NOM-112). It routes control messages through the network to perform call management (setup, maintenance, and termination) and network management functions. Although the network being controlled is circuit switched, the control signaling is implemented using packet-switching technology. In effect, a packet-switched network is overlaid on a circuit-switched network in order to operate and control the circuit-switched network. SS7 defines the functions that are performed in the packet-switched network but does not dictate any particular hardware implementation.8 The SS7 network and protocol are used for the following:
Basic call setup, management, and tear down Wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication Local number portability (LNP) Toll-free (800/888) and toll (900) wireline services Enhanced call features such as call forwarding, calling party name/number display, and three-way calling Efficient and secure worldwide telecommunications
Signaling Links SS7 messages are exchanged between network elements over 56 or 64 Kbps bidirectional channels called signaling links. Signaling occurs out of band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-band signaling provides faster call setup times (compared to in-band signaling using MF signaling tones), more efficient use of voice circuits, support for Intelligent Network (IN) services that require signaling to network elements without voice trunks (such as database systems), and improved control over fraudulent network usage. Signaling Points Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message. Three kinds of signaling points are used in the SS7 network: service switching points (SSP), signal transfer points (STP), and service control points (SCP), as shown in Figure 2-5. SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling messages to other SSPs to set up, manage, and release voice
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The Public Switched Telephone Network (PSTN)
Figure 2-5 SS7 signaling points (Source: Performance Technologies)
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