3.1.3 Long-Distance Exchanges

The long-distance telecommunication network in Iran has a three-layer hierarchical structure. In other words, switching centers are operating in three levels:

• Secondary Centers (SCs): These are the regional transit exchanges which are established in 8 major cities. Specifically, the centers are located in Tehran, Tabriz, Mashad, Isfahan, Shiraz, Ahvaz, Hamedan, and Babol. To enhance reliability of the network, redundant configurations have been deployed. Namely, in each city two SCs have been established. The two centers in Tehran and Hamedan are both digital, while other cities have one digital and one analog center each. The analog centers, however, are expected to be converted to digital centers soon.

• Primary Centers (PCs): These are the long-distance transit exchanges. According to TCI's latest annual report, in 1997, 59 long-distance transit centers were active, 48 of which were digital. It is expected that 25 new PCs will be established which will increase the number of PCs to 84 by the end of 1999.

• Terminating Exchanges (TXs): In 1997, 1,800 terminating exchanges were deployed to provide telecommunication services for the farthest reaches of Iran. The number of TXs is expected to increase to 3,600 by the end of 1999.

In 1997, the national long-distance telecommunication network provided reliable services in 1,930 nodes throughout the country using about 182,000 trunks.