IP Telephony can be broken up into two key part – Trunks and Handsets.
Trunks; The telephone trunk is the connection to the outside world. Therefore an IP Trunk is a connection using the TCP/IP suite of protocols, which is the communications protocol of nearly all business networks and the internet. IP trunks use what is know as a Session Initiated Protocol (or SIP) to connect from point to point, thus providing a connection. Telephone conversations (or Voice calls) are converted to data packets and are translated from point A to point B using the SIP protocol. Quality of Service (QoS) is a means by which IP equipment between point A and point B can monitor and ensure a given quality of connection, thus clarity of voice. Some Internet Service Providers (ISP’s) offer SIP connections whilst others don’t. You will need to check the service is available and that your internet connection is suitable before using IP telephony. SIP Trunks can be brought into more traditional telephone systems along side other traditional trunks such as PSTN & ISDN etc. Programming within the PBX equipment can ensure the appropriate trunk is chosen for each call placed. This is a process called Least Cost Routing. Making SIP calls does not require an IP Handset.
Handsets; IP handsets are connected to the site’s main equipment via Unshielded Twisted Pair (UTP) cabling being Cat-5 as a minimum. The main equipment typically provides power to the handset via Power over Ethernet (PoE) connections and issues an IP address to the handset on boot up. Like computers on a Local Area Network (LAN) every handset is is issued a unique IP address. This IP address allows all handsets within a network to be uniquely identified. Most LANs are connected to the Internet, known as the Wide Area Network (or WAN). Thus, IP phones are able to connect directly with other IP phones via the WAN. This is the main feature of an IP handset over using traditional digital proprietry handsets. A real world example might be a national organisation that has offices in Adelaide, Melbourne, Sydney, Brisbane and Perth. Each handset can connect to another via the WAN is what is essentially an ‘internal call’. In doing so, telecommunication costs for the connection are reduced. Note however that data connections the call is not ‘free’, because it requires an internet connection and and ISP neither of which come free.
Note; in the case of IP Telephony, the main equipment means an IP PBX, and may include modems, routers, & switches.