What is the difference between SIP and PRI and Analog?

I have a telecom question.  What is the difference between SIP and PRI and Analog?  Also, are DID numbers related only to PRI?
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Wayne HerbertIT SpecialistCommented:
PRI and SIP both enable multiple phone calls to be carried over the same line.

PRI stands for Primary Rate Interface.  It is not a transmission protocol but rather a dedicated hardwired line that runs from your telco to the PBX in your office.  PRI can carry a maximum of 23 phone calls at a time.  PRI is considered more secure because you have your own dedicated line, at least into the telco.  Essentially PRI is a hardware to hardware connection.
SIP is an internet protocol that permits voice conversations to be carried on a standard ethernet circuit.  A SIP connection is virtual, defined in software.  If you need more simultaneous calls then you create more virtual SIPS and increase your bandwidth as needed.
SIP is theoretically less secure because you're running your phone conversations through the internet cloud.  SIP also has the potential for poorer call quality because you are not using a dedicated line and you are using internet packet switching technologies to deliver the conversation.  You could, of course, pay for a dedicated line to your telco then run SIP over it.
I judge that PRI is on its way out.  Internet connections continue to improve in speed and quality.  There is a lot of hardware and software out there designed to work with SIP.  SIP is the way we will go in our new office even though Comcast would like to sell me PRI.

As for Analog, it's the forerunner of digital communications.  At one time, everything in the phone system was analog, ie, your voice was carried as waves across the copper.  You can still get analog delivered from AT&T if you wish, although they are moving out of the business.  But these days, your phone call is only analog to the switching station where is is converted to digital to be transmitted across the network.
This gives a pretty good idea of how SIP and PRI work:

Analog is just a reference to normal POTS lines. When you have a large number of them, they tend to be carried in with T1s, which is a bunch of 24 digitized lines. This will connect to a PBX.

DIDs are related to all the technologies (PRI, SIP, and Analog).
PRI is a data connection
SIP is a protocol that travels over Internet carrying voice data.
Analog is old style phone feed/connection.

DID Is the phone number carried over digital trunk to a pbx. PRI Is one of the data connection types over which data flows.
If you PSR a less vague, or less broad question it might be possible to a more pacific answer.
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