When we talk about VoIP, the most prominent and widely offered protocol is conference Initiation Protocol (SIP). But SIP doesn’t operate alone. Over there are countless other protocols connected in IP telephony that duty alongside SIP or even in the location of it. It is essential for a telecom technician to be familiar with these protocols, to know what they do and how they can be leveraged on the telecommunications network.

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In this article, fine look at the companion protocols that work-related in conjunction through SIP. In a future article, we will focus on protocols that have the right to be supplied as options to SIP.

Quick evaluation of SIP

To see where these protocols to the right in, let’s quickly review SIP and also how the works.

SIP is a protocol used by both voice and video endpoints to provide call setup, signaling, and also call teardown for interaction sessions. A VoIP endpoint have the right to be a physics IP phone, a softphone, a videoconferencing terminal, a voice gateway, or even a gaming console – essentially any device capable of permitting voice and/or video communication. Every one of these endpoints it is registered to a SIP server, also known together an IP PBX, i beg your pardon is offered to coordinate progressed features such as speak to transfer, call hold, music ~ above hold, and other timeless and amplified telephony features.

To enable even more dedicated services, such as speak to routing, speak to queuing, interaction Voice Response, multiparty voice and video conferencing, integration with internet services, and interconnectivity with the classic PSTN, additional servers and also devices have the right to be added to a VoIP network.

Keep in mind the SIP walk not bring the voice or video itself. Instead, the operates in conjunction v several other protocols that bring the session media. Let’s look at these in much more detail.

Companion protocols that job-related in conjunction through SIP

The adhering to diagram shows the assorted protocols that are supplied in conjunction with SIP in a typical VoIP telephony conversation. This protocols room mapped to their loved one layers within the OSI model framework.

Actually, the OSI model has actually no official an interpretation for SIP or the companion protocols we’re reviewing right here (SIP, SDP, RTP, and RTCP), for this reason they don\"t actually fit right into the OSI model. But due to the fact that the OSI design is so widely used, we can say castle sit on optimal of the move Layer. This way that SIP is live independence of the protocols offered at lower layers, so it can work with any type of Transport great protocol. UDP is normally employed, return if SIP messaging is secured using transfer Layer security (TLS), then TCP is utilized for the SIP signaling.


Session summary Protocol (SDP) – if SIP is supplied for VoIP endpoints to exchange signaling y2kcenter.orgrmation, SDP is to work to define multimedia sessions. Specifics it allows endpoints to negotiate the media type, format, and all of its associated properties. SDP walk not lug the media itself, nor is it sent out via any kind of Transport great protocol. Rather, it is consisted of as a payload that the SIP messages themselves.

Real-time carry Protocol (RTP) – RTP tote the actual media streams, whether voice, video or both. While SIP creates connections across the network, RTP transports the actual voice packets over the provisioned connections. Save in mind the unlike SDP, which is a payload the SIP, RTP sessions run individually of and also in parallel to SIP sessions, and also are regulated via those SIP and SDP sessions. RTP generally runs end UDP and constantly functions in conjunction through RTCP.

RTP control Protocol (RTCP) – RTCP is regularly referred to together the sister protocol the RTP. RTCP collection out-of-band statistics and control y2kcenter.orgrmation for RTP sessions. Out-of-band just means that the exchange of this y2kcenter.orgrmation wake up in a separate, parallel session rather than within the RTP media stream. The objective of RTCP is to administer feedback on the high quality of company (QoS), includingstatistics such together packet counts, packet loss, jitter, and round-trip delay time. This y2kcenter.orgrmation is shared in between endpoints, which can react come these alters by limiting packet operation or by transforming to another available codec. Some IP phone call models can display screen network statistics either in ~ an embedded web internet browser on the call itself, or top top the phone’s LCD screen. Y2kcenter.orgrmation such together codec, jitter, received packets, and lost packets can be tracked. This data is all accumulated using the RTCP protocol.Like many other VoIP protocols, RTCP additionally operates with UDP together its transfer Layer protocol.

Transmission regulate Protocol/User Datagram Protocol (TCP/UDP) – uneven you explicitly configure them otherwise, all of these companion protocols use UDP together the underlying carry Layer protocol. UDP has actually much reduced overhead 보다 TCP since it doesn’t have any kind of flow control, error checking, or packet ordering mechanisms and thus is an ext suitable for transporting media. A steady circulation of data is much an ext important 보다 a correct ordering that packets or perfect packet arrival, because voice and video are very forgiving approximately a particular level of such irregularities. Fifty percent a shed syllable or an untrue pixel top top a single frame is the no repercussion in a voice or video conversation. This is why, if TCP is ever before used, the is because that signaling protocols and also only really rarely because that media-carrying protocols such as RTP. Instead, the media will certainly be carried on UDP, also when TCP is used to carry the signaling packets.

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The adhering to diagram puts these protocols right into some perspective within the frame of a VoIP voice call between two endpoints:



SIP is a revolutionary protocol that delivers phenomenal flexibility and also capabilities come voice and video clip communications. Nevertheless, that does not function alone. Knowing and also understanding the companion protocols is vital to many aspects of network management such as making procurement decisions, troubleshooting, or network optimization.