Tip

Duties of an IP telephony gateway when integrating VoIP and the PSTN

In order to integrate a Voice over IP (VoIP) solution with the Public Switched Traditional Network (PSTN), an IP telephony gateway is used to convert the traditional voice traffic (analog) to digital data so that it can be transmitted over a data infrastructure. This tip explains the various features and responsibilities of an IP telephony gateway.

Destination Mapping
Traditional voice calls are routed based on country code, area code, and exchange and line numbers through Local Exchange Carriers (LECs), Inter-exchange Carriers (IXCs), and exchange switches. However, the VoIP gateways that receive these dual-tone multifrequency (DTMF) tones via PSTN signaling are looking to map the 10-digit number to the IP address of a gateway that can terminate the call. Within each gateway area code/exchanges are mapped to other gateways' IP addresses. Based on the size of the network, these mappings can range from simple to complex.

IP Connectivity
Once a gateway determines the remote gateway(s) necessary to complete a call, a "connection" must be made over the packet network. Today, there are two prominent standards-based protocols available to establish and maintain VoIP connections: the ITU-T H.323 specification and the Session Initiation Protocol (SIP). These protocols provide the suite of functions in order to make VoIP calls.

Voice Digitization and Compression
The most prominent task required of the gateway is to convert

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the voice signal to a digital one in order to be transferred through a digital network. Usually, the digitization takes place in data rates of 64 kbps. IP Telephony gateways can additionally compress data rates for voice calls to a rate of 24 to 5.3 kbps per call. From a bandwidth utilization perspective, the gateways can provide up to 10 times the number of calls as compared to digital calls transmitted over a PSTN infrastructure.

Decompression and reconfiguration
If IP Telephony Gateway device functions as the IP connectivity endpoint to complete a call, the voice traffic must be decoded. Although the decoding is not necessary for the sender to transmit the call, it is necessary to decode the voice traffic so that the telephony equipment (fax or phones) can receive the correct signaling.

Authorization, Access, and Accounting
Similar to the functionality of a PSTN switch, the VoIP gateway is responsible for assuring the proper security, system access and billing records for used services. From a security perspective, the gateways are responsible for not allowing specified phones to have control input. From an access control perspective, the gateway must only allow those services contractually agreed upon to the end user. From a billing perspective, the gateway must be able to provide the various billing models used in the PSTN networks today. Two examples of billing models are monthly fixed fee agreements or "pay per usage" plans.


Richard Parsons (CCIE#5719) is a Manager of Professional Services for Callisma Inc., a wholly owned subsidiary of SBC. He has built a solid foundation in networking concepts, advanced troubleshooting, and monitoring in areas such as optical, ATM, VoIP, routed, routing, and storage infrastructures. Rich resides in Atlanta GA, and is a graduate of Clemson University. His background includes senior and principal consulting positions at International Network Services, Lucent, and Callisma.


This was first published in October 2004

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