An external modem comes in its own case, about the size of a small book or even smaller. Usually the front of the case shows the model name of the modem along with a row of status lights. The status lights are handy for diagnosing modem problems. The back of the external modem case has two RJ-11 telephone connectors like the internal modem. The external modem also has connectors for a power supply adapter and a serial (modem) cable.