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Old 2nd September 2009, 12:32   #16
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Originally Posted by Sn1p3r View Post
I am in market for SME server (immediate buy)

Can anyone suggest a decent dependable server. Dell T3400/5500 or comparitive HP/IBM is better?

Thanks!
This query must go to this thread: http://www.team-bhp.com/forum/gadget...-business.html (Server Config + Implementation - Small Business)

IMO, it doesn't belong here.

Other OT posts:
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DELL. Cant go wrong there.
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Originally Posted by NetfreakBombay View Post
These (T3400/5500) are not server products.
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Originally Posted by given2fly View Post
Or also check with Sun.
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Old 2nd September 2009, 15:56   #17
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Originally Posted by DCEite View Post
A lot of us are working in IT and some of us are students, i am sure there is a lot of knowledge pool in the forum, which we can use to solve our day to day technical queries. Lets use this thread as a platform for the same.

Mods: If you deem this thread inappropriate, please let me know.

Some of my queries for today - related to SIP (session initiation protocol)

Q1. Why is ACK treated as a separate transaction , ie why isn't it the part of same transaction as INVITE ?
Q2. Why does INVITE use a 3-way handshake mechanism as against request-response mechanism of other SIP methods ?

though i am not an expert on the SIP side, but with my experience on other circuit switched protocols , let me see if i can clarify on your questions.

Q1. I am not sure of the meaning as to why it isnt part of the same transaction. Ofcourse it is part of the same transaction and this i am saying from the state machine point of view that will be maitained at both the sender and the receiver end for a particular transaction, unless the ACK is not mandatory.

Q2. The invite is basically a handshake before the actual media/voice path can be established between the two ends. the 180 rining is more of alerting and is similar to when you phone rings (during which the RBT is fed back to the caller )when someone is making a call to you and 200 ok comes only when you answer the call/recieve the call. The same is the case with other protocols such as SS7 where you have IAM then the ACM and you recieve an ANM only when someone picks the call. I dont think if this can be achieved in request-response/two message. There are series of handshakes that take place during call initiation which cannot be achieved using a simple request/response.
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Old 2nd September 2009, 17:28   #18
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Post Sip - Rfc 3261

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Originally Posted by DCEite View Post
Q1. Why is ACK treated as a separate transaction , ie why isn't it the part of same transaction as INVITE ?
Hi DCElite, its been a while since I dabbled with SIP but your question is answered by the relevant RFC itself - to quote verbatim from the RFC:

"In the case of a transaction where the request was an INVITE (known as an INVITE transaction), the transaction also includes the ACK only if the final response was not a 2xx response. If the response was a 2xx, the ACK is not considered part of the transaction. The reason for this separation is rooted in the importance of delivering all 200 (OK) responses to an INVITE to the UAC. To deliver them all to the UAC, the UAS alone takes responsibility for retransmitting them (see Section 13.3.1.4), and the UAC alone takes responsibility for acknowledging them with ACK (see Section 13.2.2.4). Since this ACK is retransmitted only by the UAC, it is effectively considered its own transaction".

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Originally Posted by DCEite View Post
Q2. Why does INVITE use a 3-way handshake mechanism as against request-response mechanism of other SIP methods ?
This is also answered by the RFC...

Last edited by im_srini : 2nd September 2009 at 17:35.
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Old 3rd September 2009, 12:28   #19
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Arrow Switching thread

@diabloo : Thank you for pointing in the right direction.

Thank you all for your suggestions. I will continue the discussion on the link marked by diabloo.
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Old 23rd September 2009, 00:15   #20
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Quote:
Originally Posted by DCEite View Post
Some of my queries for today - related to SIP (session initiation protocol)

Q1. Why is ACK treated as a separate transaction , ie why isn't it the part of same transaction as INVITE ?
Q2. Why does INVITE use a 3-way handshake mechanism as against request-response mechanism of other SIP methods ?
Surprised to see such deep level question. Since I have dabbled a little in SIP, and also TDM telephony, I can probably make an attempt.

SIP is way more flexible than traditional telephony, so you need more flexible protocol.

For example, consider John who lives in Bangalore, wants to talk to George in Chennai. The call made by John first goes to the telephone exchange he is connected in Bangalore, travel through the telephony cloud and reaches the
telephone exchange connected to George's phone and then sends a ring to George's phone. By this time, the TDM channel for the entire conversation is already allocated. When George picks the phone, they start talking on that TDM channel.

But VOIP doesn't work that way. SIP (signalling) and RTP(payload) may or may not use the same path. For example, SIP proxies only setup the call, they don't deal with payload or RTP.

SIP Example 1: John calls George, both are using SIP phones, call is made directly.
John -----INVITE -------->George
John<---------RINGING---George
George picks up
John<-----------200 OK----George
Now talk...
John<-------RTP---------->George
John hangs up
John------BYE------------->George
John<-----------OK------George

SIP Example 1: John calls George, both are using SIP phones, call is made via SIP proxy.
John -----INVITE ----Proxy---INVITE---->George
John<----RINGING----Proxy---RINGING---George
George picks up
John<-----200 OK-----Proxy---200 OK--George
Now proxy gives the direct IP address of John to George (SDP contains the direct IP)
Proxy-------ACK------>George
Now talk...
John<-------RTP---------->George
John hangs up
John------BYE---Proxy-----BYE--->George
John<------OK---Proxy----OK---George

Basically ACK is used to give the redirection info for RTP path. The SIP proxy handles only SIP and not RTP. This is my understanding after doing some programming in SIP. Remember I am no expert on SIP. I am not sure whether ACK is initiated by the proxy or the endpoint (John). What happens if there is no direct access between John & George, that too I am not sure. Different SIP PBX and proxies have different behaviors.
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