Below is the list of VOIP Interview questions , that will cover most of the interview questions
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VoIP & SIP
- Difference between Via and Route?
- What is Dialog and transaction? How many transactions are there in single call?
Dialog/Call Leg is same.
In the example 1,
(a) to (c) are one transaction.
(d) is second transaction and
(e)& (f) constitute third transaction.
Together, these three transaction constitute one dialog
- What is Loop and what parameters you will see to detect A loop or how to detect?
- How to detect the response of immediate request?
This is called transaction and it’s identified by branch Id in via header.
Via: SIP/2.0/UDP 10.255.1.30:5060;branch=z9hG4bK3763983732-38
You can see branch- id in the via header that is the transaction number...
Since sip is a transaction protocol so it required
transaction number to send a request and get response for this
transaction.
- What is different between Session Vs Dialog?
After dialog established, if the call is successful and media starts
Flowing (RTP), then it is said to session established.
For Register request, dialog establishes but there is no media, so no
Session.
- What is call forking?
- What is early dialogue and confirmed Dialogue?
Early Dialog:- A dialog established by a non final response.
Confirmed Dialog:- After getting 2xx response.
- Difference between Record-route & route?
- Explain important headers in RTP?
- Why RTP uses UDP?
- What is Refer? And where it is used?
- What are the Mandatory general headers you will see in SIP Requests and Responses?
- What is SIP Protocol Structure /Stack?
- Draw Call Flow of Call waiting and 3way conference?[Samsung]
- What is present in Auth Headers of INVITE in case of IMS ?[Samsung]
- What are P-headers present in SIP message when using IMS ?[Aricent,Samsung]
P-Associated-URI
– URIs associated to an address of records
P-Called-Party-ID
– Identity the session was addressed to
P-Visited-Network-ID
– Check roaming agreements and authorize registrations
P-Access-Network-Info
– Information about the access network used by the terminal
P-Charging-Function
– Charging entities for a session
P-Charging-Vector
– Charging identifiers and networks performing charging
P-User-Database
– Optimization for queries to the HSS
- what are headers/differences in INVITE and CANCEL message ?[Samsung]
- What is difference between Via ,Route,Record-route,Path field?[Samsung,Radisys]
- What is difference between Session and Dialog?[Aricent]
- Who adds record route in Case of IMS?[Samsung]
- How DTMF is send in VOIP?
3 common ways of sending DTMF on SIP calls
>>SIP INFO packets
-------
INFO sip:7007471000@example.com SIP/2.0
Via: SIP/2.0/UDP alice.uk.example.com:5060
From: <sip:7007471234@alice.uk.example.com>;tag=d3f423d
To: <sip:7007471000@example.com>;tag=8942
Call-ID: 312352@myphone
CSeq: 5 INFO
Content-Length: 24
Content-Type: application/dtmf-relay
Signal=5
Duration=160
-----
>>As specially marked events in the RTP stream - see: RFC 2833
>>inband as normal audio tones in the RTP stream with no special coding or markers
- What is Loose Routing and Strict Routing? [Radisys]
- What is P-asserted Identity Header?[Aricent]
- Difference between Proxy and B2BUA ?[Sonus,Tata Elixis,Aricent]
- Draw Call flow of Attended Transfer ?[Aricent]
- What is OMA Specs?[Aricent]
- Which Layer of TCP/IP SIP is involved?[Radisys]
- In OSI model, how SIP packet is formed and traversed?[Radisys]
- Which field is added in SIP packet at each process in IMS Server?[Radisys,Samsung]
- Draw the call flow from host A to host B in IMS Server?[Radisys]
- What are components of IMS-Core and function of Each and where located (Home or Visited Network)?[Aricent,Samsung]
- The P-CSCF resides in the visited network.
It acts as the UE's gateway into the UE's home network. The P-CSCF is responsible for:
Identifying the home IMS network.
Routing traffic to and from the home IMS network.
Establishing the IPSec security association
· The I-CSCF resides in the home network. It acts as the front-end of the Home IMS network.
The I-CSCF is responsible for:
Interfacing with the P-CSCF in the visited network.
Selecting the S-CSCF by querying the HSS.
· The S-CSCF resides in the home network.
The S-CSCF is responsible for:
Handling the registration request from the I-CSCF.
Pulling the authentication vectors from the HSS
Passing the authentication vectors to the P-CSCF via the I-CSCF.
Authenticate the user in the second registration attempt.
- What are different SIP Request?[Samsung,Aricent]
- Draw call flow of Cancelled INVITE?
- When we should TCP or UDP for Sending SIP Message?[Samsung]
TCP should be used when message size is too large to be fit in one frame ( 1500 Bytes)
This is normally needed in case of IMS as SIP messages has too many headers.
Also when there is no PRACK available.
- What is via, Branch, Tag?
- What is Early dialog?
- Give me an example when servers send 5XX response?
- What are Branch ID and Tag ID in SIP?
- What is path Header?
The Path header field is a SIP extension header field with syntax
very similar to the Record-Route header field. It is used in
conjunction with SIP REGISTER requests and with 200 class messages in
response to REGISTER (REGISTER responses).
- how to add multiple contact in register and remove one contact from register.
- stateless vs stateful proxy ?what is used in gateway?
- How to implement ACL for invite
- Explain Answer/Offer model in details
- 3-Way conference Call flow
- How to match response of a request?
- What is 301 Transfer
- IPC mechanisms in Asterisk
..Today only I got to knew that there is no need of CSMA/CD in today’s 
