This read-only INTEGER counts frames that were not properly transmitted because the transmit channel was starved of data, i.e. the local system could not feed data to the HDLC transmitter fast enough. A growing count points to a local resource bottleneck, such as CPU contention or a slow upstream data source, rather than a problem on the wire itself. For example, an admin seeing transmitter underruns on a router forwarding traffic from a heavily loaded LAN interface would investigate whether the local system is CPU-bound.