17.1
This section is presented with the country-specific deviations mapped to the PSTN and VoIP. Catering to required deviations in VoIP can closely match the feel of PSTN-based calls.
17.1.1
Central-Office-Specific Deviations Mapped to VoIP
The VoIP adapter is like a PSTN CO for the telephone user. Hence, most functions of the CO have to be emulated through the VoIP adapter. Figure 17.1(b) is a functional representation of the VoIP adapter with foreign exchange subscriber (FXS) and foreign exchange office (FXO) interfaces. The following major parameters can influence the design and configuration of the VoIP adapter for multiple countries:
• Impedance of VoIP adapter telephone interfaces of FXS and FXO
• REN—number of simultaneous phones that can be used on the same lines
• Subscriber line interface circuit (SLIC) line voltages, battery voltages, line reversals, grounding, and various protections
• Ringing voltage, frequency, and cadence
• Diagnostic methods performed on the telephone interfaces
• Call progress tone generation and specifications
• Dual – tone multifrequency (DTMF) digit generation, detection specifications
• Caller ID, call feature tones, and timings
• Overall loudness rating (OLR), gain/loss planning in send and receive path
• Country-dependent transmission characteristics similar to TR-57 [TR-NWT-000057 (1993)] transmission characteristics
• Call switching timing, which is similar to TR-57 switching characteristics that varies with country
• Based on local PSTN A-law/^-law compression mode, G.711 VoIP calls can use the same preferences for minimal transcoding
17.1.2
Transmission Lines
The deviations in transmission for multiple countries are as follows:
• Telephone cable length, wire gauge, and other cable characteristics between DLC or CO and the telephone equipment
• Various loading coils and matching on telephone lines
• Digital subscriber line (DSL) support on telephone lines [URL (TI - impedance) ]
PSTN service makes use of several 1000-feet lines. Transmission-dependent distortion, drive strength, and losses need to be taken into consideration in PSTN-based systems. VoIP systems use short loop wires, and distortions are insignificant because of the lines. Wire gauge selection will not influence the performance because so few feet lines are used in VoIP.
17.1.3
Telephone Deviations
PSTN- and VoIP-based calls will use the same telephones. The following parameters of a telephone can influence the design for multiple countries:
• Telephone impedance
• Caller ID detection; various indicators such as caller ID display, call wait display, message waiting
• Pulse or tone (DTMF) dialing options, speed and redial timing deviations
• DTMF digits and parameters
• Loudness rating of phones
• On-hook, off-hook, flash hook key, and respective debounce settings
• Telephone connectors and adapters
• Certification of phones to regional requirements
VoIP should consider the above parameter deviations with local country standards. Each country or group of countries may follow its own set of standards in combination with other global standards from the ITU, ETSI, and IEEE. Telephones are tested for meeting electrical and acoustic specifications. A typical test list of instrument-based tests is given at [URL (Microtronix-country)] that lists the tests conducted on a Europe TBR21-complaint phone. Several baseline documents are used that are specific to the country of operation. Instruments like [URL (Microtronix-country)] perform multiple country-specific phone tests.
