When a
call is initiated between two endpoints it is important to have information of
calling and called party IP address both for signaling and media, codec they
are going to use and several other important information. In a SIP based call (call where SIP protocol is used) we use
SDP to advertise media information like transport
address, transport protocol, codec, ports and other session description information. SDP works at application layer. SDP is defined in RFC 2327, 4566 (newer). Normally we see SDP in Invite, 200 OK
or ACK depending on the call type which we will discuss soon.
Cisco Voice Virtual Lab
Configuring Call manager in VMware, call manager express in GNS3
Sunday 9 October 2016
Thursday 29 September 2016
SIP Response Codes
In this post I am going to discuss
about SIP Response codes. In the previous post we discussed about SIP messages.
SIP Response codes as the name says is a kind of response to any SIP message.
It can be a response to Invite message, Bye message or an Options message. We
have. 6 different classes SIP Response codes which we will discuss now.
1XX - INFORMATIONAL
RESPONSE
2XX - SUCCESSFUL
RESPONSES
3XX - REDIRECTION
RESPONSES
4XX - CLIENT FAILURE
RESPONSES
5XX - SERVER
FAILURE RESPONSES
6XX - GLOBAL
FAILURE RESPONSES
Tuesday 12 July 2016
SIP Messages
When we talk about SIP call flow there are two important things
that need to be understood SIP messages and response code. SIP messages and responses are
used for setup , in session changes, info and for tearing down a session. A response code is a
response which is sent in response to a SIP message, and we can also get a SIP
message in acknowledgement of response code.We will look into some quite
frequent SIP messages in this post and response codes in the next post.
SIP Messages:
INVITE: Invite is
very first message to set up a connection. It is also used to modify a session
as re-invite (which we will discuss in later posts). User Agent sending an
Invite is known as User Agent Client.
ACK: ACK is sent
in response to a 200 OK message. ACK is used only with Invite or a Re-Invite
message.
Tuesday 24 May 2016
SIP CALL FLOW
In SIP Basics we discussed about components of SIP. It's time to understand a sample call flow. Before we start I would like to discuss few components of call flow:
Request: When a UAC (User Agent Client) wants to initiate a call it sends an Invite to the UAS (User Agent Server). In this case Invite is a request .
Response: A reply to UAC's request is called a response. In this case it would be 100 Trying, which we will discuss in a moment.
Tuesday 17 May 2016
SIP BASICS
SIP BASICS
SIP:
SIP Stands for Session Initiation
Protocol. SIP is becoming a global standard for VoIP. SIP is a protocol which
is used for signalling, controlling multimedia sessions such as voice and video
between two endpoints or through a proxy server. SIP is also used for instant
messaging, client presence, user mobility based on its current location.
SIP has been developed by IETF:
Internet Engineering Task Force. This group is responsible for adding new
features and modifying existing features in SIP. All concepts used in SIP are
defined under RFC’s i.e. RFC 3261 which is one of the most important RFC for
VoIP.
Monday 21 December 2015
Analog and Digital Voice System
Analog Systems:
When we thing of
voice systems the very first thing that comes into our mind is a home phone.
Like our home phones our old telephony systems used to operate in analog mode.
It has two cables Ring and Tip and it operates on -48 DC voltage. When the circuit
between the two gets complete it withdraws a dial tone from the central office.
The voice waveform of these kind of calls are analog waveform.
Wednesday 18 March 2015
MEDIA RESOURCES
A media resource is a
software-based or hardware-based entity that performs media processing
functions on the data streams to which it is connected. Media processing
functions include mixing multiple streams to create one output stream
(conferencing), passing the stream from one connection to another (media
termination point), converting the data stream from one compression type to
another (transcoding), streaming music to callers on hold (music on hold), echo
cancellation, signaling, voice termination from a TDM circuit
(coding/decoding), packetization of a stream, streaming audio (annunciation),
and so forth. The software-based resources are provided by the Cisco Unified Communications
Manager (Unified CM) IP Voice Media Streaming Service (IP VMS). Digital signal processor
(DSP) cards provide both software and hardware based resources.
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