The Voice Over Internet Protocol; Why Sip - Snom 4S Administrator's Manual

The Voice over
Internet Protocol
Today there is a wide choice of different switched network
products. Telephones have now been being built for more than a century,
and their technology is well-understood and proven. Why choose a
different technology?
Modern communication infrastructures transport much more than
just one application: email, http, files, instant messages, videos, music,
so it is only natural to include voice in the list of applications and use one
infrastructure for all of them.
Voice is a real time application. Sending voice over the Internet
Protocol is called "VoIP". The delay between sending a packet to the
network and receiving it needs to be minimal and constant and this makes
specific demands on this application.
Most network equipment can already fulfil this real time
requirement. Virtually all switches currently on sale support a VLAN with
different priorities in the network, and the vast majority of higher layer
network equipment supports some means of transporting packets with
different qualities (DiffSrv). The LAN usually supports a bandwidth of
100 MBit/s, which is more than enough to allow voice to flow through
the network, and adherence to a certain set of rules ensures that this
bandwidth is enough to supply superior telephone quality. The Internet
backbone's ability to transport large loads is increasing on a daily basis,
and global communications are now ruled by the Internet. Making calls
over the public Internet for example between Europe and North America
works with a surprisingly good quality.

Why SIP?

There has been a "protocol war" regarding the "best" way to set
up a phone call. In the mid 90s, H.323 was the first attempt to unify the
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