VoIP Tele Phone Service - Interaction with H.323 protocol

Since the beginning of use of Voice over IP services, it was basically decided to use h.323 as the basis of VoIP; therefore, VoIP or Voice over IP is seen as a clarification of h.323 protocol making sure that in order to avoid some differences between standards and in case of conflicts, it was decided since the past that priority would be always given to h.323 on VoIP. VoIP or Voice over IP services have focused on ensuring interoperability between VoIP devices from different types of manufacturers for example fixing issues with voice routing and coding, silence suppression, and setting up new elements to enable connectivity to traditional telephone system. These elements are concerned with transmission of dual-tone multi-frequency or in simple abbreviated form DTMF.

H.323 protocol is used for making IP calls with NetMeeting, etc. The H.323 protocol allows us a many types of interacting elements, which include:

Terminals are the users who take the first step in using a VoIP connection. Users or terminals are only able to connect with each other and if additional user access is required for communication, this needs incorporation of some additional elements namely gatekeepers which operate as follows:

1. Address Translation Services like DNS or Domain Name System so that the user is able to use names instead of IP addresses.
2. Admission control and authentication to deny and allow user access plus managing bandwidth.
3. Gateways points of reference for conversion TCP/IP - PSTN.
4. MCUs or Multipoint Control Units to allow holding of a conference.

H.323 is not only used for Voice over IP, but is also used for exchange of video and data communication. H.323 includes a set of standards and is basically supported by series of protocols which cover different configurations of communication:

For Address:

Registration, admission and status or RAS is a communication protocol, which allows one h.323 station to locate another h.323 station via the gatekeeper.

Domain name system or DNS is a service through which the Internet domain names are located and then translated into IP addresses. The DNS serves the same purpose as the RAS protocol but it is done via a DNS server and not a gatekeeper.

For Markings:

Initially requires Q.931 for call signaling.

H.225 protocol is required for call control, i.e., call signaling and call setup, registration and admissions, and synchronization/ packetization of the flow of voice.

H.245 protocol controls messages to specify closing and opening of channels for voice flows.

Voice Compression:

Requires: G.711 and G.723

Optional: G.728, G.729 and G.722

Voice Transmission:

User Datagram Protocol or UDP: Transmission of voice takes place on UDP voice packets, but the UDP cannot provide data integrity, as use of bandwidth while using UDP is higher than TCP.

Real time protocol or RTP manages aspects of scoring and timing of UDP packets with the necessary information for efficient delivery of packets to the reception.

Transmission Control:

Real time control protocol or RTCP is used for detecting situations of network congestion and it takes appropriate corrective actions wherever necessary.

VoIP application is built by different number of elements already available in the market and these are:

• IP Phones
• Adapters for personal computers
• Telephone hubs.
• Gateways like PSTN/IP
• Gatekeeper
• Multiple control units or audio units
• Directory services