Trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. When dealing with a private branch exchange (PBX), trunk lines are the phone lines coming into the PBX from the telephone provider. This differentiates these incoming lines from extension lines that connect the PBX to (usually) individual phone sets. Trunking saves cost because there are usually fewer trunk lines than extension lines since it is unusual in most offices to have all extension lines in use for external calls at once. Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN or PRI. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.
The trusted old PSTN with its Analog, ISDN BRI, E1 or t1 lines are slowly reducing and near to disappear. With unified communication in place, it is moving from PSTN to much more modern and flexible IP (H.323) and SIP trunks.
Today most of Telephony vendor provide Telephony PABX with H.323 and SIP trunking capability, but the decision to use SIP or H.323 is driven by the unique feature(s) offered by each protocol. There are also a number of external factors that can affect the choice of trunk protocol, such as customer preference or the protocol's maturity and degree of interoperability offered between various vendors' products.
H.323 Trunks: H.323 trunks provide connectivity between multiple Communication Managers and other H.323 devices such as gateways. H.323 trunks support most of the audio and video codecs today. H.323 IP trunks allow ISDN-PRI equivalent tie trunks to be defined over IP networks. H.323 IP trunks support ISDN trunk features such as DCS+ and QSIG, or generic tie trunk connections between different vendors’ H.323 PABX systems.
SIP Trunks: SIP trunking is a Voice over Internet Protocol and streaming media service based on the Session Initiation Protocol (SIP) by which Internet Telephony Service providers (ITSPs) deliver telephone services and unified communications to customers equipped with SIP-based private branch exchange (IP-PBX) and Unified Communications facilities. In addition to VoIP calls, SIP trunks can also carry instant messages, multimedia conferences, user presence information, Enhanced 9-1-1 (E9-1-1) emergency calls, and other SIP-based, real-time communications services.
SIP Trunk Benefits:
- Network savings are just one part of the benefits of moving to SIP trunking from legacy trunking or TDM.
- A role for SBCs in the network for all the multiple SBC functions including protocol conversion, routing, and standardization of dissimilar equipment and SIP trunking.
- Include diversity and redundancy alternatives that are enabled by the conversion to SIP trunking and IP communications.
- Help in consolidating the Unified Communications Applications and extending those to remote and branch locations irrespective of their local PABX features
- Highly scalable solution with better interop between different Solution provider
- Less resource intensive and saves cost.