SIP Standard

The VoIP world is dominated by SIP lately. SIP is a standard that is defined by a number of RFCs but searching through here for answers to questions is ridiculously difficult. I would like to use this post to answer any SIP questions that I come across in my daily travels. If you have a question that has not been answered feel free to drop it in the comments. I'll do my very best to find an answer for you!

Q: What is the maximum size of a SIP packet?
A: The maximum allowed size is equal to the maximum size of a UDP packet, or 65,507 bytes (65,535B - 20 B IP Header -8 B UDP Header). Some software may not accept packets of this size. Often software will limit the packet size to 4096B.

Q: What is a SIP User Agent (UA)?
A: A User Agent refers to a component in the SIP communication between two devices. There are two types of User Agents: Client and Server. A UA Client (UAC) sends a SIP request and a UA Server (UAS) sends a response. Most of the time a SIP server or endpoint will act as both a UAC and a UAS, depending on the circumstance. A call may terminate to a phone in which case the SIP server is the UAC and the phone is the UAS. The phone may then place the call on hold or conference another user, at which point the phone sends the request and becomes the UAC.

Q: What is a B2BUA?
A: A B2BUA (Back to Back User Agent) is a device that receives a request from a UAC and forwards that request out to another device, acting initially as the UAS and as the second leg as a UAC. A B2BUA will remain in the middle of the conversation so that the endpoints do not communicate directly for signaling purposes. It is still possible to have a B2BUA with direct media path for RTP packets.

Q: What is the maximum speed for faxing with SIP?
A: Contrary to what most people believe, there is no difference in the maximum speed of a fax machine on analog or PRI versus SIP. The SIP standard doesn't care - this is a function of the codec negotiation. But that doesn't answer the real question here. When I'm setting up a PBX I always suggest the customer limit their fax speed to 14,400bps. The PBX will see the 14,400 speed (or lower) and will use the T.38 codec for faxing. Most phone systems will only allow T.38 if the PBX will be holding up the RTP (i.e. no direct media path) as the system will switch to T.38 when it detects the fax tones being sent from the originating fax machine. Modern fax machines (also known as Super G3 fax machines) support speeds of up to 38,400bps, although the realized throughput is often lower. When using a Super G3 fax machine the codec will negotiate to G.711. Network reliability is incredibly important when using Super G3 speeds. These machines are incredibly intolerant to jitter and packet loss.


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