Network infrastructure

The latest configuration of our IP telephony network is shown in Fig. 1. Total of 12 institutions are connected using 10 voice gateways. More voice gateways could have been added from the last update, please contact the VoIP group in CESNET for current information about availability in specific locations. Several thousands of users of regular phones in these institutions can call each other over the IP telephony network free of charge. We developed our own IP telephony accounting application. Currently we use H.323 signalling in production network and we are working on incorporating SIP clients. Calling instructions can be found here.

Fig. 1: IP telephony network in CESNET2

Connection to the public telephony network

Our IP telephony network is connected to the public telephony network (PSTN) via Aliatel telecommunication operator. Users in the IP telephony network can now call all phone numbers in the public telephone network in Czech Republic and selected international numbers.

International peerings

We have a peering with IP telephony network to CERN and SANET in routine operation. Other peerings to SLAC, AARNET, Fermilab, Texas A&M University, Hungarnet, TASK, and UFRJ are in pilot testing. Calls to these peering locations are free of charge. We use Kerio gatekeeper from Kerio Technologies for registration of international peerings. Users can also call all numbers in selected countries via Aliatel communication operator for the agreed upon fees.

History of the IP telephony group in CESNET

The IP telephony group in CESNET, operating the CESNET2 network (which was preceded by the TEN-155 CZ network) was established in the mid-1999. The aim of the group has been set to perform testing, measurement, development and practical deployment of technology for the transmission of voice over a data network. The group is supported by CESNET.

The goal of the first phase of our project was to interconnect telephone exchanges (PBXs) of several universities across the country using IP telephony and to connect this IP telephony network to the public telephone network. The motivation was to reduce cost of telephone calls, create a testbed for IP telephony experimental work and prepare for development of new services in the next phase.

Selecting technology

One of the requirements was to incorporate existing hardware - PBXs in universities and routers in the TEN-155 CZ network. The PBXs are of different brands and types (e.g., Ericsson MD110, Siemens Hicom 300, Alcatel and Matra) and they are equipped with different types of data interfaces. The network is based mainly on Cisco routers, providing both ATM and IP services. We performed a series of experiments with various telephony equipment, particularly testing available PBX interfaces for their suitability for the connection to a data network. Upon completing these tests, we decided to use the VoIP (voice over IP) technology (as oposed to voice over ATM) and digital ISDN PRI/BRI and analog E&M and FXS interfaces. Based on our further experience, we finally decided to use only digital ISDN PRI/BRI interfaces.

Connection of universities in Prague, Ostrava, Brno and Ceske Budejovice

We interconnected PBXs of four universitites in different cities across the country in a pilot project. The configuration is shown in Fig. 2. There were two voice gateways based on Cisco AS5300 routers located in Prague and Ostrava. The PBXs of the other two universities were also connected to these two voice gateways over ATM PVC tunnels. Call routing was performed by the gatekeeper based on Cisco 2620 router. As a part of the pilot project, the VoIP connection to CERN, Switzerland was also implemented.

Fig. 2: Configuration of the pilot project