A method for selecting an operating mode for a frame-based communications network
consisting of a plurality of stations attached to a transmission medium. The plurality
of stations include both a first type station and a second type station. The first
type station is capable of transmitting and receiving first protocol frames in
accordance with a first protocol. The second type station is capable of transmitting
and receiving both first protocol frames and second protocol frames in accordance
with a second protocol. The first protocol and the second protocol each use different
signals on the transmission medium. The first type station is not capable of reliably
detecting second protocol frames. The first protocol has a first protocol frame
format containing at least two reserved bits in a first protocol frame header which
are ignored in received frames by first type stations and always sent with a same
fixed value by first type stations. The first protocol frame format is redefined
to provide an updated first protocol frame header wherein two reserved bits in
the first protocol frame header are allocated as a mode selection indicator field
in the updated first protocol frame header. The mode selection indicator field
has meaning for second type stations.
