What happens to data being sent into a circuit-switched network when the incoming data rate exceeds the bandwidth of the network?
In the modern state of networks and from a wide-area networking perspective, circuit switched links are basically merely last mile circuits. Such links are often categorized as analog or digital, such as T1's, PRI's, or telephone lines (sometimes called 1FB's by business customers).
The reality is once the traffic is onto the carriers network, it's all packetized and packet switched. So, beyond your last mile circuit, there is no nailed up and wasted capacity in your carriers system.
You physically can't push more into your carriers system than the last mile link will support.
Such problem couldn't happen with circuit switching because, in circuit switching, a route and bandwidth is reserved from source to destination. Sometimes circuit switching can be relatively inefficient because capacity is guaranteed on connections which are set up but are not in continuous use, but rather momentarily. However, the connection is immediately available while established.
examples of circuit switching are PSTN , ISDN , Optical mesh network such types of network where the media designed to carry fixed number of calls for example in case of PSTN .
Please make use of this link Circuit switching