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WLAN and IP
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Skype |
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Skype
Skype is an proprietary P2P VOIP software application. It can
be used either by free computer software or by purchasing a
special Skype phone to attach to your computer via USB, or
directly to your router (ie: no need for a computer in the
latter case) (Note: SIP phones can not be used for Skype).
Niklas Zennstrom (founder of Skype) explains
here the P2P nature of Skype: "Without
being too technical, each Skype client is always connected to a
SuperNode (any Skype client can become a SuperNode, the
SuperNode is acting as a hub). SuperNodes are always on routable
open IP addresses. When a call is set up the established TCP
connection with the SuperNode is used to signal that a call is
coming. Dependent on the firewall status of the client the data
stream is set up either as UDP (if firewall allows) or in worse
case as outgoing TCP which is almost always allowed. If both
clients are only allowed to do outgoing TCP calls are routed
through another peer."
This is contrast to SIP where a fixed SIP server stores user
info and sets up calls, and the voice is then communicated via RTP (UDP)
directly between users. An analysis of the Skype P2P technology
was examined by Columbia University in 2004 in this
document(280KB) The
security and other general aspects was examined by EADS in 2006
in this
presentation (2MB).
The main advantages of Skype over SIP are it's ease of use, in
terms of set-up and it's ability to automatically overcome NAT
and firewall issues. The downside of Skype is that it is a
"walled garden" since it is not an open source. This means less
interoperability and flexibility with other networks compared to
SIP.
A short comparison of Skype and SIP:
Skype
SIP
-Open Source
No Yes
-Telecom standard
No Yes
-Traffic even when not used (ping, relays)
Yes No
-All traffic (voice included) is heavily encrypted
Yes No
-Easy set-up
Yes No
-Easy firewall/NAT transversal
Yes No
-Selectable Codecs / HD Audio
No Yes
-Audio stream type
PCM RTP
-Connection Type
UDP or TCP UDP
-Connection set up P2P
Supernode SIP Server
-On-line peers are visible
Yes
No
Skype to Skype calls are always free, but to call out of the
Skype network to PSTN or SIP phones costs a fee.
To call Sykpe-to-SIP for free is possible with some work-around
software, such as:
(Free)
http://www.mhspot.com/siptheeskype.html
($40)
http://www.nch.com.au/skypetosip/
or integrated into a SIP PBX based on Unbuntu called Vasuntu:
http://zhink.com/site/main/WBR>index.php/20081101vasuntu/
Skype can be downloaded for free here:
http://www.skype.com
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