TDM and IP Telephony Comparison

In a TDM (circuit-switched) call setting, there is an assigned connection between the two endpoints of the call. The entire bandwidth as-signed 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 re-mote (SIP) Call Manager 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.

Ports and Timeslots in TRANSip

All of the interfaces (i.e. line boards, trunk boards, service boards) that are connected to TRANSip occupy a port. 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 have service at any given time.

IP ports (aka "socket") are not the same as the previously mentioned voice ports. IP networks also use the concept of a port, but in a much different manner than described above. In the IP world, a port usually refers to the application that would be used. For example, port num-ber 23 is associated with Telnet while port number 80 is associated with HTTP. Since the IP world is mainly packet based, there really is no concept of timeslots.