![]() ![]() ![]() It is not media sent to the SIP device from either the far-end or the communications system. However, if a sound is played it’s a local sound. Your SIP device can do whatever it wants with those messages including playing sounds or displaying messages. 486 Busy Here informs you that the far-end is busy. 180 Ringing tells you that far-end is ringing. In SIP, we don’t use sounds to indicate call progress. So, instead of your eyes, you use your ears to monitor how your call is progressing. If you think about it, how else can they inform you of call progress? Those old analog telephones don’t have displays. These sounds are sent from the telephone company to tell you something about your call. Whether it’s ring-back or re-order tone, this is media. This “fast busy” is often used to indicate that all circuits are busy and the call cannot be placed. You might also hear a message telling you that “the number you dialed is no longer in service.” Lastly, it’s possible to hear what we telephone people know of as re-order tone. You might also hear a busy signal if the other party is on an existing call. That tells you that the called party’s telephone is alerting. Depending on the number you dialed and the state of the called party you will hear a variety of different sounds. You hear the dial tone, you enter the telephone number, and the call is launched. This is simply a friendly, comfort noise that acts like the on/off light on a piece of electronic equipment. The phone company sends dial tone to let you know that the line is working and is ready to accept digits. When you pick up that home phone, what’s the first thing you hear? Dial tone, of course. The point is you need to focus on telephony prior to SIP. For those of you who don’t own a land line, you can think about the phone at grandma’s house. In order to make sense of this, you need to shift from thinking about SIP and VoIP to that analog telephone back home. You can have late offer and early media and its possible to have early offer and late media. Simply put, early media indicates that media is sent prior to the call being answered and late media indicates that media waits until the call has been answered.Įarly and late media actually have nothing to do with early and late offer. With early offer, you can put SDP in the INVITE request, but that doesn’t mean that media will be sent prior to the call being answered.Įarly and late media have to do with when media starts to flow. Early and late offer simply define when media capabilities are exchanged. However, this has nothing to do with when media actually starts. The called party decides with early offer and the calling party decides with late offer. This changes who gets to decide which codec will be used for the session. With early offer you put SDP in the INVITE message and with late offer you put SDP in the ACK. Late Offer, but it all boils down to a simple notion. You are welcome to read the article, SIP Media Management: Early Offer vs. In a previous blog, I addressed the concepts of early offer and late offer. ![]()
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