elecommunications Network: An Overview Detect off-hook condition, analyze the dialed number, and determine if a route is available. Connect the subscriber to a trunk exchange for longer distance calls. Connect the subscriber to another in the same local area. Determine if the called subscriber is free and connect ringing signal to her. Provide metering and collect charging data for its own subscribers. Convert 2W local access to 4W circuit of the network. Convert analog speech into a digital signal (PCM). The size of local exchanges varies from hundreds of subscribers up to tens of thousand subscribers or even more. A small local exchange is some- times known as a remote switching unit (RSU) and it performs the switching and concentration functions just as all local exchanges do. A local exchange reduces the required transmission capacity (number of speech channels) typi- cally by a factor of 10 or more; that is, the number of subscribers of the local exchange is 10 times higher than the number of trunk channels from the exchange for external calls. The number of required trunk circuits is analyzed in Section 2.12. Figure 2.15 shows some different subscriber connections to a local exchange and the way they are physically installed. 2.7.2 Distribution Frames All subscriber lines are wired to the main distribution frame (MDF), as shown in Figure 2.15, which is located close to the local exchange. It is a large con- struction with huge number of connectors. Subscriber pairs are connected to one side and pairs from the local exchange to the other. Between these con- nector fields there is enough space for free cross-connections. Cables and connectors are usually arranged in a logical way considering the subscriber cable network structure and switching arrangements. This fixed cabling stays the same over long periods of time, but connections between sides change daily, for example, because a subscriber moves to another house in the same switching area. A cross-connection in the MDF is usually done with twisted open pa