IP Telephony- Paper

IP Telephony

Internet Protocol Telephony (IP Telephony) is a technology that uses internet protocol's packet switched connections to exchange voice, fax and other forms of information.

It uses IP based networks to build, provide and access voice, data or other forms of telephonic communications. It roots voice through internet or any other IP based network by converting analog audio signals into digital data. It is designed to replace circuit switched public data network (CSPDN) and PSTN with packet switched IP communication network.

IP Telephony uses QoS router to reduce delay and the highest priority is given to voice on the network. Virtual Private Network (VPN) is required from the service provider if IP Telephony is using on the Internet.

IP Telephony focuses on delivering voice, fax or video packets in a dependable flow to the user on shared lines avoiding tolls of PSDN using internet calls which travel as packets of data.

IP Telephony is having many advantages over circuit switched telephone network such as low set up cost, easier implementation and allows mobility which are briefly explained below:

1.     Cost: IP Telephony provides low cost advantage when compared to circuit switched network because it uses one network to transmit both voice and data.

It provides low setup cost as it uses already existing network which is underutilized and does not require construction of new network.

2.     Functionality and Mobility: IP Telephony is very easy to use after implementation as it provides service wherever in the world if we have an IP phone. The calls can be received without changing the phone number and international calls can be done for cheaper prices.

It also provides call redirection, video conversation, audio conference and others.

IP Telephony also has some disadvantages such as Reliability and Security issues as given below:

1.     Reliability: One cannot use IP Telephony in case of power failure.

In IP Telephony emergency number cannot be called as it cannot be known where the user is.

So, to resolve both the cases common landline has to be used.

2.     Security: Most IP Telephony services do not provide any sort of encryption which makes it insecure allowing other people to eavesdrop on conversations and use services of others without being charged.

Even after implementing encryptions, lowers the quality of the call as it has more complex algorithms and causes latency.

Building blocks of IP Telephony system

1.     Telephone or stations: Allows user to access the capabilities of system.

2.     QoS Gateway: Does the conversion of voice signals into data packets and places those packets on external data network.

3.     Data Network: It can be a VPN or private network.

4.     QoS Router: It is for wide area interface and helps in reducing voice latency.

5.     Voice Switch: Allows the users calls for regular access to PSTN which cannot be carried over private network.

6.     Firewall is required for Security.

7.     Cabling to connect all parts together. 


IP Telephony is implemented using distributed architecture and centralized architecture.

1.     Distributed architecture:

a.     H.323: It is a "packet based multimedia communication system" recommendation from ITU-T. It has terminals (end points), gatekeepers, gateway and multipoint control unit. It was originally designed for multimedia application but nowadays widely used for video conferencing applications. It allows to build large scale networks and supports network intelligence on endpoints or gate keepers.

b.     SIP (Session Initiation Protocol): SIP is an application layer protocol used to establish, modify and terminate multimedia sessions. It was previously designed as multimedia protocol which is used for domain name system for service location and call routing over IP. SIP is easily extensible.

2.     Centralized architecture: It usually includes dumb end points which means they have limited features. Centralized architecture are used where less complex systems are required.

          IP Telephony scenarios

The IP telephony usage scenarios are based on devices terminating an IP call. It includes either a PSTN device (e.g. telephone) or a data-oriented terminal (e.g. personal computer) on each side of a call. It includes four methods as shown below:

1.     PC - to - PC

PC-to-PC communication can be provided for multimedia Personal Computers which have a microphone, speaker and a sound card operating over an IP-based network without connecting to the PSTN. PC applications (and IP-enabled telephones) can communicate using point-to-point or multipoint sessions. The parties need to be equipped to talk at the time of the call.

2.     PC - to - Telephone

The PC-to-Telephone method is used to call any ordinary telephone on a PSTN from his computer. In this case, a gateway converting the IP call into a PSTN call has to be used. The call is converted to a PSTN call at the gateway and is then sent over PSTN to its destination.

3.     Telephone - to - PC

The Telephone-to-PC method allows a user to call from any ordinary telephone on a PSTN to a PC which is connected to the Internet. Here, a gateway converting the PSTN call into an IP call has to be used, and the gateway is required to be located as near to the caller. The call is converted to an IP call at the gateway. The voice data then "hops on" the Internet and finds the PC on the other end by using the unique IP address.

4.     Telephone - to - Telephone

In this scenario, a caller dials into a gateway using a regular telephone. The call is converted to an IP call at the gateway and the voice data "hops on" the Internet. At the end point the voice data hits another gateway and "hops off" the Internet. The voice data is then converted back to PSTN format and sent over the PSTN to its destination.


Some examples of applications of IP Telephony are as follows.

1.     Advanced Intelligent Network Features (AIN): It includes features such as caller ID, voice mail, call waiting, call blocking.

2.     It allows voice calls from mobile laptops and PC's.

3.     It is used in the application of air lines reservations by using a Java applet.

4.     It is used in voice annotated documents for sending them to integrated voice/data mail boxes.

5.     It is implemented in electronic business cards to send a vCard including photo and audio file.

6.     It finds applications in internet call center access and in virtual call centers.


IP Telephony is designed to replace common landlines which still need few years to grow. The disadvantages can be solved by offering QoS by the companies and because of its advantages it is widely used telephone network in the world.

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