Take transportation for example: Board any train today and you will most likely notice a panel with a button and a lamp—this is the passenger’s connection to the outside. In the SIP environment, pressing the button will automatically route the call to an available source. This is because the SIP protocol also supports automatic call forwarding services to enable that automatic routing, often via blind transfers to ensure interoperability with automated devices.
The SIP call refer/forwarding capability is significant for emergency situations where it is imperative the call is received. The integration of a SIP server establishes that mediation point with central routing intelligence. This enables the system to recognize that a call is being targeted for a group, while also identifying destinations that are unable to accept such a request do to being busy, offline or otherwise unavailable.
Schools represent another example where SIP-driven emergency intercom makes sense. Here, a press-to-talk button in the classroom opens a direct two-way channel to the main office and SIP automatically transfers unanswered calls to the local police station. This represents a controlled telephone system, typically operating completely independent of a certified evacuation system, where a teacher or student presses a button and waits for an external source to answer the call.
It is possible to use the SIP protocol without an IP PBX, employing a series of paging stations and receivers to enable one- and two-way communications. As a real-world example, a paging station acts as a specialized phone, with press-to-talk buttons that establish connection to an IP phone or another receiving device. This would require the necessary intelligence in the paging stations so that they can operate with the SIP protocol “peer-to-peer” mode.
This kind of system can later be seamlessly integrated into an IP PBX phone system to expand functionality; however, it can remain separate if desired. SIP is not tied to the fixed environment—roaming security guards, for example, can receive SIP calls on mobile phones and VHF radios. Those same guards can easily make calls from their phones by dialing into the PBX and entering a security code to page a specific location or zone. This is most typical in a “SIP Gateway” scenario, where a guard can unlock the announcement capability on any connected device. Here, security personnel can then select a group or zone and the SIP Gateway will translate the information into audio over IP.
SIP beyond security
SIP delivers another benefit through automatic negotiation of codec choice. SIP devices typically enable negotiations of the audio codec used on a per-connection basis, chiefly because bandwidth may be more important than audio quality in telephone communications. In other cases, a communication partner may not support the standard codec used for most connections.
The SIP protocol can automatically negotiate the use of a matching, VoIP supported codec when SIP-capable devices are operating within a larger audio over IP domain. Although naturally inclined to use high-quality audio codecs, the SIP-enabled audio over IP device will accept 8 kHz voice quality if that is what is provided by the VoIP system.
The introduction of a SIP Gateway also unlocks a new universe that can combine both security and more common applications. This is most useful when directing background music, paging and public address to loudspeaker systems as opposed to simply routing phone calls.
Let’s assume an integrator is installing 100 devices in an office-wide communication system, with background music streaming in high-quality, MP3 encoding, to each device from 10 channels. This can be easily accomplished without any need for SIP by using 10 multicast MP3 encoded streams and 100 receivers with local control for channel selection. Let’s further assume that the end-user wants to make announcements to these 100 devices from a SIP-based phone system. This traditionally means that each device must be subscribed to the PBX system. The PBX needs to talk to each device to communicate that it wants to send an announcement, while also negotiating the codec. This can take some time before all the devices in the system are online and willing to listen. And it can be costly to add 100 telephone licenses to the PBX.