Interconnection Networks

Local access networks often connect to other networks (such as the PSTN, PTT or Internet) via switching systems or gateway connection. Various types of gateway connections can connect the local telephone switching system (often called the “End Office” or “Central Office”) to other public (e.g. Internet) or private (e.g. corporate) networks. A gateway transforms data that is received from one network into a format that can be used by a different network. It usually has more intelligence (processing function) than a bridge as it can adjust the protocols and timing between two dissimilar computer systems or data networks. A gateway can also be a router when its key function is to switch data between network points.

Interconnections to other public telephone networks are classified by type of connection. Basically, the lower the connection type number, the more simple (and more limited) is the connection. The connection types include the basic customer type POTS (type 1) and inter-switch types 2. Type 1 POTS connection provides for basic signaling and low speed (audio) connection. The higher types of connection include various capabilities such as types of information services available (operator assist, emergency number support). In the United States, the typical interconnection types include those designated as type 2A, 2B and other variants of type 2, each serving a specific purpose. Type 2 interconnections link the LEC into a tandem (standard local switch interconnect) office. When using the type 2 connections, the CO appears as a standard end office switching facility.

Networks commonly increase their data transmission capacity or quality levels as you move towards the top of the network (away from the endpoints). For cable networks, this is called feeders and for telephone networks, these are called high-speed backbone interconnects. High-speed interconnection lines between switches and tie lines are called trunks.

Figure 1.5 shows some of the different types of interconnection networks. These vary from distribution networks (no switching functions such as cable television), centrally controlled networks (such as the public telephone system), and packet switching networks (such as the Internet).