VoIP Protocols - RTP and UDP
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As well as SIP and H.323, there are other protocols used with VoIP. This article discusses these VoIP Protocols in more detail.
As mentioned previously in the ‘IP Suite and VoIP’ article, Internet data transmissions are composed of several layers. The network layer consists of the IP which establishes a connection between two computers. The transport layer provides the rules required for sending the data and the application layer determines how the data will be processed once it arrives at its destination.
Most data travelling over the Internet uses the Transmission Control Protocol (TCP) for the transport layer because it guarantees data delivery and integrity.
VoIP does not need the kind of delivery guarantee which TCP provides, so most VoIP transmissions use the faster User Datagram Protocol (UDP) as the transport layer.
Once VoIP data arrives at its destination, the application layer interprets it and presents it to the user. The most commonly used application layers for VoIP are SIP and RTP.
Real-Time Transport Protocol (RTP) is the Internet protocol which transmits real-time data such as audio and video. RTP does not exclusively guarantee real-time delivery of data, but it does provide mechanisms for the sending and receiving applications to support streaming data.
As VoIP doesn’t use TCP (Transmission Control Protocol), RTP runs on top of the User Datagram protocol (UDP) instead. VoIP uses UDP as the transport layer. The UDP protocol provides only a direct method of sending and receiving data over an IP network and offers very few error recovery services. UDP has no mechanisms in place to notify the application of any loss in transmission whilst delivering packets of data; it also sends data unordered with no guarantees of the data being presented in the receiving application. All re-ordering of data into the correct format it was sent, is handled by the RTP.
In VoIP, voice packets are inserted into data packets using RTP, which in turn are inside UDP packets. Once VoIP data arrives, the application layer interprets it and the data is presented to the user.
There are also other protocols used in VoIP such as RSVP (Resource ReServation Protocol), which is a signalling protocol and an ideal solution for ensuring that sufficient bandwidth is reserved for VoIP. RSVP can manage QoS (Quality of Service). QoS refers to the likelihood that voice data will be delivered rapidly and meets a certain standard i.e. without background noise QoS is used where a high degree of reliability is required such as in VoIP and multimedia streaming applications. As you can see there are a number of VoIP Protocols that need to work together for successful VoIP delivery.
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Eddie Edwards
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