TDM & IP Telephony Comparison Ports and Timeslots in TRANSip
In a TDM (circuit-switched) call setting, there is an assigned connection between the two endpoints of the call. The entire bandwidth assigned to the call is maintained for the exclusive use of the call for the duration of that call.
In the IP world, packet switching is used and there is no dedicated bandwidth between the two endpoints. Data is sent on a voice-packet by voice-packet basis. Between the individual voice packets, the IP network bandwidth is continually reassigned for the purpose of carrying all IP network traffic including voice.
In the TDM world, all or most of the “intelligence” resides at the network level and switching equipment does more than just switching. All of the end terminals need the support of this central unit.
In IP Telephony, a (SIP-based VoIP) telephone instrument, unlike its TDM counterpart, can perform simple call functions including call setup with another VoIP telephone without the help of any external controller. The two VoIP phones simply exchange messaging to coordinate their own call through the IP network. However, their capabilities in this respect are limited. Commonly, the VoIP telephone relies on a remote (SIP) Call Controller to orchestrate call setup and feature management. In addition to gaining access to richer and more standardized feature capabilities, this shifts the burden of understanding the initial phone setup requirements from the VoIP instrument user to a system level manager.
Each active VoIP call leg (channel) occupies a port. On the HDX•C and SLICE 2100, TDM and analog interfaces (i.e. line boards, trunk boards, service boards) occupy ports and timeslots. The number of occupied ports is directly related to the number of circuits that would be used. For example, a line board with 8 circuits occupies 8 ports from the available pool of ports in TRANSip.
In addition to the ports, each interface that is connected to TRANSip may occupy a timeslot. Timeslots are required for making connections with the TDM services and line circuits, trunk circuits or even customized announcements. There may be situations where certain timeslots can be shared across certain circuits. For example, 16 line circuits may share 10 timeslots meaning that only 10 line circuits can have service at any given time.
Please note that the term 'port' used in relation to REDCOM system resources and TRANSip is unrelated to the term 'port' used in IP networking (for example, Telnet uses TCP port 23, SIP uses UDP port 5060).