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  • VoIP Protocols

  • Telephones

  • QoS (Quality of Service)

  • Codecs

  • Asterisk

  • 3CX PBX

  • SER (Sip Express Router)

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      SIP Protocol  

        Architecture   Components   Messages   Error Messages   Headers   Addressing   SDP   Example  


    SDP Protocol (Session Description Protocol) RFC 2327 is used to describe multicast sessions in real time, being useful for invites, announcements, and any other type session setup.

    The original proposal of SDP was designed to announce necessary information for the participants and multicast MBONE (Multicast Backbone) applications. At this moment, its use is extended for the announcement and the negotiation of a multimedia session.

    Since SDP is a description protocol, SDP messages can be transported by means of different protocols, for example SIP, SAP, RTSP, electronic mail with MIME applications or HTTP-like protocols. SDP uses text codification. A SDP message is made up of lines, called fields, where names are identified by a single letter. The SDP was not designed to be easily extensible.

    The only way to add new capabilities to SDP is to define a new attribute. Nevertheless, the unknown attributes can be ignored. In the following table all the fields are explained.

    Type Description

    V Protocol Version (mandatory)
    o Identifier (mandatory)
    S Session Name (mandatory)
    I Session Information (mandatory)
    U Description URI
    e E-mail
    p Telephone Number
    C Connection Information
    b Bandwidth
    Z Correction Time
    K Key
    a Attributes
    T Session Start (Start y stop) (mandatory)
    R Repetition Time
    m Transport Protocol Information (media) (mandatory)

    Session Description Protocol Version (v): 0
    Owner/Creator, Session Id (o): Cisco-SIPUA 26425 12433 IN IP4
    Owner Username: Cisco-SIPUA
    Session ID: 26425
    Session Version: 12433
    Owner Network Type: IN
    Owner Address Type: IP4
    Owner Address:
    Session Name (s): SIP Call
    Connection Information (c): IN IP4
    Connection Network Type: IN
    Connection Address Type: IP4
    Connection Address:
    Time Description, active time (t): 0 0
    Session Start Time: 0
    Session Stop Time: 0
    Media Description, name and address (m): audio 17338 RTP/AVP 0 8 18 101
    Media Type: audio
    Media Port: 17338
    Media Proto: RTP/AVP
    Media Format: ITU-T G.711 PCMU
    Media Format: ITU-T G.711 PCMA
    Format: ITU-T G.729
    Media Format: 101
    Media Attribute (a): rtpmap:0 PCMU/8000
    Media Attribute (a): rtpmap:8 PCMA/8000
    Media Attribute (a): rtpmap:18 G729/8000
    Media Attribute (a): rtpmap:101 telephone-event/8000
    Media Attribute (a): fmtp:101 0-15


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