How is sdp employed by voip

These numbers correspond to Coder-Decoders codecs that package the voice portion of the call to be transmitted over the Internet. You can maximize your Internet bandwidth by listing a compressed codec first, and then an uncompressed codec.

The destination SIP node will respond with the first codec listed that it supports. Provide more than one codec option when negotiating the RTP.

Table lists some of the active RTP payload types currently available. In this area of the SDP code, multiple lines are allowed for a section. But the a section of the SDP requires several entries because it defines the requirements for the payload types and adds any additional requirements requested by the originating SIP node for call completion.

The first line of the preceding code clarifies that the RTP payload 18 represents the G. Different codecs have different clock rates as identified in Table , so the exact rate appears in the rtpmap information, ensuring that both ends of the call are sampling at the same rate.

The other rtpmap offerings clarify the payload type of 0 as PCMU, and pay-load type as a telephone-event. The second line of code lists an fmtp source attribute. The media attributes can be listed in any order. The fmtp listed on this line identifies that, for the G. Annex B is a special variant of the generic G.

If silence suppression is enabled, white noise is played during these times so that neither party believes the call has been disconnected. Modifications to codecs are released, but those modifications are listed as new extensions or annexes of the original codec. The clarification of the payload type in the fmtp line can include several parameters. The second fmtp line of code pertains to the telephone-event and identifies as its additional formatting information.

DTMF is nothing more than a fancy way to refer to the tones generated by your phone when you press the numbers on your keypad. DTMF tones include sounds not made by the keypad of your phone.

More than DTMF telephone-events apply to the telephone-event line, representing non-verbal sounds that can be transmitted in the audio portion of the VoIP call. The DTMF events are grouped based on their application, their geography, or the signaling to which they apply.

This information then allows to decide if the participation in a session is required or not. Figure 1 - Example SDP communication. Session initiation protocol Rfc document list User datagram protocol Real-time transfer protocol Rtcp protocol H protocol.

Net developers. E-mail: info ozeki. Quick start. Example projects Check out our example projects. Sitemap voip-sip-sdk. Installation steps A step-by-step guide on installing the SDK.

Package contents Read about the SDK package contents. Accordingly, SDP bodies are classified into three description levels:. The session description consists of a number of fields and optional attributes that provide details around the session, such as the name of the session, the originator of the session, and bandwidth constraints for the session. The session description can optionally contain attributes as well. Communication sessions can either be unbounded or bounded in time.

The timing field has the following format:. This field is self-explanatory: start-time and stop-time simply encode the time when the session starts and ends, respectively.

The encoding of the start-time and stop-time determines whether the communication session is bounded, unbounded, or permanent. A bounded session has an explicit start-time and stop-time. An unbounded session does not have a stop-time , whereas a permanent session does not have a start-time or stop-time.

The encoding of the timing field is useful for multicast communication sessions. The media description section of SDP bodies provides sufficient detail about the media and transport characteristics of the communication session. Participants use this information to join a multicast session or negotiate common capabilities for unicast sessions. The media description section includes the following information:. Fields and attributes in SDP bodies can be either mandatory or optional.

In either case, they must follow the rigid ordering structure shown in Table The session-level section of SDP bodies provides default values for various fields that are to be used and interpreted. For example, if a user agent wants to use the same media connection IP address for all media streams within the session, it can encode an SDP body with a session-level description of media connection information.

However, if further granularity is required on a per-media-stream basis, the user agent can encode an SDP body with one or several media-level descriptions. The actual SIP message is omitted from this example for brevity. Session-level descriptions serve as default values to be interpreted and used if no corresponding media-level description s is available.

Example is a snippet of an SDP body where the direction attribute has a session-level description and a media-level description. You should be aware that certain SDP fields and attributes can be present concurrently at different levels of the SDP body. When this occurs, the media-level field or attribute overrides the session-level field or attribute.

So, in the case of the direction attribute appearing twice in Example , the media-level description of the direction attribute is given higher precedence. SDP was originally conceived as a way to describe multicast sessions over Mbone short for multicast backbone. SDP scales really well for multicast, as there is a unified view of the session for all participants. For example, for multicast communication sessions, each participant requires a single media address and port to join a communication session.

While SDP has the capability of describing unicast communication sessions, it is a slightly more challenging proposition than describing multicast sessions. For a unicast session between two participants, each participant has a localized view of the session; each participant has its own media IP address and port pair, its own set of supported media types, and its own set of supported codecs per media type.

To obtain a complete view of a unicast session, the participants must exchange information elements and agree on a common set of parameters. The protocol has to be stateful to maintain context around the exchange between an offerer and an answerer, as there may be several SDP exchanges during the course of a call.

It is important for the higher-level protocol to accurately map requests and responses. For example, for SIP delayed offer calls, the user agent being invited to a communication session is the one that constructs the SDP offer. Figure shows the SIP three-way handshake for a delayed offer call versus the same handshake for an early offer call. Regardless of which user agent constructs the offer, the SDP body must consist of a session description, a time description, and a media description section.

This strict encoding format ensures that the peer user agent is provided sufficient information to participate in the communication session. The session description section contains all the mandatory fields for example, v, o, s as well as optional attribute values.

Each media line in the SDP body must encode sufficient information about the media stream to convey the following:.

It is a common misconception that the port number signifies the port number from which media is sourced. Without the connection data field, the remote user agent is only aware of a port number and has no information about the remote IP address. The connection data field can be scoped to include a session-level or media-level definition. Table lists a selection of these mappings for common audio codecs.

To better understand this concept, see the sample SDP body provided in Example For example, when using the OPUS codec, many attributes can be utilized. One example defined here is the maximum average bitrate for the session via the maxaveragebitrate attribute. As the example shows, many attributes can be applied to a given media format on a single line by using the semicolon ; as a separator. Media lines have their interpretations tightly coupled with the SDP direction attribute.

A direction attribute can have a session-level scope or a media-level scope. For unicast streams, the offerer can specify the directionality of a media stream by using the SDP direction attribute. Accordingly, a stream can be marked as sendonly, recvonly, inactive, or sendrecv. Table summarizes the meaning of each Direction Attribute. When the direction attribute has a sendrecv or recvonly value, it signifies the IP address and port number on which the sender would expect to receive media RTP from its peer.

Instead, it signifies the address and port on which the offerer expects to receive media. If a user agent sets the direction attribute to inactive, it means that the user agent wants to simply establish a communication session without transmitting or receiving media. Regardless of the value of the direction attribute, there is a continuous passage of RTCP traffic between communicating entities.

While constructing an SDP body, if the user agent does not specify an explicit value for the direction attribute, it always defaults to sendrecv.

As mentioned earlier, the user agent constructing the SDP offer can include one or more media lines such that the media lines can correspond to the same media type, different media types, or a combination of the two. Conventionally, the offerer must use a valid, nonzero port number for each media line within the offer. This is because the use of port zero for a media line s within the offer has no useful semantics. On receiving the offer, the answerer must construct an SDP body following the guidelines of RFC It must include a session description, a time description, and a media description.

On-line chat Ask for live help. E-mail Write us an email. Telephone Contact us on phone. Office address Office location. Download: sip-message-modification. Generic; using Ozeki. CreatePhoneLine account ; phoneLine. NotRegistered e. Error Console.