ping command and modem lights

The ping command and modem lights are used to debug SLIP communication problems.

A ping is an echo request packet, sent out of the machine, and an echo response packet is returned. This sequence of events is useful if the administrator can see the modem lights.

For example, a local system constructs the echo request packet and sends it to the remote system. The Send Data (SD) light on the local modem illuminates. This means that the local TCP/IP, slattach, and tty were able to group information and send it out of the modem to the remote system.

The remote modem receives the packet and the receive data light flashes but its SD light does not. This means that the remote system was not able to send (or return) the local system's ping request. As a result, the user on the local system may see the ping command hang, requiring a Ctrl-C to exit the condition.

The most common cause of this problem is the use of XON/XOFF flow control in one or both modems, however, the user should not overlook the possibility of routing or address conflicts on the systems.