1. International Telecommunications Union-Telecommunications Standardisation Sector (ITU-T), Principles of Intelligent network architecture, Recommendation Q1201, ITU-T, Geneva, Switzerland, October 1992.
2. International Telecommunications Union-Telecommunications Standardisation Sector (ITU-T), Network grade of service parameters and target values for circuit-switched services in the evolving ISDN, Recommendation E.721, ITU-T, Geneva, Switzerland, 1992.
3. J..M. Lennox, Services for IP telephony, PhD dissertation, Columbia University, New York, 2004.
4. SIP: Understanding Session Initiation Protocol, vol. 1;Johnston,2004