Copyrights © All rights Reserved. Asterisk and VoIP issues Chetan Vaity March 2007

Copyright © GS Lab All rights Reserved. What is Asterisk  Asterisk is an open source software IP PBX  Digium is the primary developer and sponsor of Asterisk  In addition, Digium develops PBX hardware and provides contract support and development services for Asterisk  Protocols  SIP (Session Initiation Protocol)  SIP is primarily used in setting up and tearing down voice calls  Media protocols  RTP  RTCP

Copyright © GS Lab All rights Reserved. Getting started…  Get a Linux box  Install Asterisk  Get source from  Create SIP users by adding entries to sip.conf  Modify the dialplan by editing extensions.ael  Install softphones on client computers and register  Try X-Lite 3.0 from  Make a call!

Copyright © GS Lab All rights Reserved. Getting started…  Example of a user entry in sip.conf  [user1] secret=123user1 type=friend host=dynamic context=sipextensions mailbox=301 callerid="user1" dmtfmode=rfc2833 canreinvite=yes  Example dialplan in extensions.ael  context sipextensions { 301 => { Dial(SIP/user1); Hangup(); }; 302 => { Dial(SIP/user1); Hangup(); }; };

Copyright © GS Lab All rights Reserved. Asterisk features  Connect to the PSTN world  Hardware - PCI cards from Digium to connect to analog PSTN or digital (PRI) lines  Asterisk can then act as a VoIP to PSTN gateway.  Feature-rich and Extensible  Extension patterns  Conference, Voic  Recording  TTS integration with Festival  AGI – Run your own programs!  Say cricket score

Copyright © GS Lab All rights Reserved. Voice Quality  Bad links  Packet loss  Media goes over UDP  Jitter  Delay  Disordered packet arrivals  Codecs should be resistant to packet loss  iLBC used by Skype, GoogleTalk etc. (developed by GIPS)

Copyright © GS Lab All rights Reserved. Delay and Jitter  A delay of less than 150 ms is acceptable and usually goes unnoticed by humans  With delay greater than 400 ms, conversation starts becoming irritating  Coder delay is the time taken to compress a block of PCM samples  This delay varies with the codec used and processor speed  For G.729, delay is around 30ms  Packetization delay is the time taken to fill a packet payload with encoded speech  Queuing delay and Propagation delay at various network components  Jitter buffer delay  Variation in delay of packets is called Jitter  The effects of jitter can be mitigated by storing voice packets in a buffer upon arrival, before playing out  Increases delay by the length of the buffer

Copyright © GS Lab All rights Reserved. Echo  Echo in telephony systems is caused by two main phenomena  Electrical echo due to imperfect impedance matching  Acoustic echo due to microphone pickup of audio output  Echo becomes noticeable only when there is a delay between speaking and hearing your voice echoed. (more than about 50 ms)  In PSTN calls, there is always echo, but it remains unnoticed because the delay is quite small  VoIP intrinsically has packetization, depacketization and processing delays built into its protocols  VoIP phones don't cause echo. They just make it audible by introducing an extra delay  Echo cancellation: Subtract from the received signal  Based on the response of the system to a short spike of sound  Echo cancellation is a hugely CPU-intensive process

Copyright © GS Lab All rights Reserved. Advantages of VoIP  Reduction in costs  Uses the internet for long distance calls  Uses underutilized existing network capacity  Functionality  Especially for computer users – (click on name to call)  Merging of Data and Voice infrastructures  No need for separate cabling  Mobility  Wherever you are connected to the Internet, you can receive VoIP calls

Copyright © GS Lab All rights Reserved. Disadvantages of VoIP  Quality  Due to low/variable bandwidth  Echo  Internet connection  VoIP usage is entirely dependent on the quality, reliability and speed of the internet connection  If the net is down, you have no telephony  Power  No phone calls in a power outage  Security (lack of it)  Anyone listening on the LAN can potentially capture packets and reconstruct the conversation