IP Telephony Related Terms & Protocol.
IP Telephony – Internet Protocol
 IP Protocols works at a network level where information is sent in packets
called IP packets.
 The network that we connect our computers devices like laptop etc is an
example of this network.
 Once connected to the IP network, we are able to surf the internet, can
send email, chat etc.
 Each of this service is using a particular IP protocol for example when you
surf a web page. You type http://website-address.com etc
 Now HTTP is a protocol in this case. Where we request a web page from a
web server. It than transport on this protocol and appear in our web
browser.
 In case of voice call the same happen that’s why we call it VoIP
 Voice over Internet Protocol.
 As HTTP is a protocol carries web pages from web server to our web
browser.
 Similarly we need a protocol that can carry our voice call from our IP end
point device like IP phone to the voice server and than voice server
forward it to our dialed destination.
SIP – Session Initiation Protocol.
 SIP is the widely adopted protocol for the VoIP.
 Its IETF – (Internet Engineering Task Force) standard for VoIP communication.
 This means any VoIP device manufactured by vendor. It has to be
compatible with SIP for standard voice communication.
SIP: Little More in Detail…
 SIP is mainly responsible for signaling and establishing session.
 Its uses default port 5060.
 Once the session is established. The voice contents flow using another
protocol known as RTP – Real Time Protocol that uses ports ranging from
10000-20000.
 Along with RTP. There is also another protocol, SDP – Session Description
Protocol.
 The SDP contains information about established session like which voice
codes is being used etc.
PSTN – Public Switch Telephone Network
 PSTN is essentially a circuit-based network.
 This is a traditional telephone network and non IP based.
 To connect to the PSTN, a local telephone company in your city needs to
provide you a cable connectivity and terminate it using RJ 11.
Connecting a phone cable into the
Elastix Server.
 To be able to connect phone cable into the computer server. We require
some kind of interfacing hardware.
Analog Telephony Card.
FXO Module
DIGIUM A4 Series card – 4 Port card
Image Courtesy: http://digium.com
FXS Module
FXO Module
 The port on analog card that you are connecting the phone cable coming
from the local phone company.
 The port on analog that faces the C/O Central Office – local phone
company we need to have FXO module installed on that port.
FXS Module
 The port on analog card where you connect the analog phone.
Note: FXS port require the voltage from inside
computer using Molex connector to generate the
ringing voltage.
The Complete Picture.
Images Courtesy: http://my.digium.com
You don’t require FXS module….
 IP DESK Phones like Yealink, Digium, Polycom, SNOM etc
 Soft Phones – software based phone.
 ATA adapeter – analog to IP adapter.
 Since these IP based devices or softphone uses the IP network and don’t
require ringing voltage etc.