A packet switched communications network comprises a controller (11) and a plurality of controlled units (12, 13, 14) arranged in a ring. Each of the controlled units comprises a switch (31) at its input, each switch including control means (44) that are responsive to control instructions transmitted by the controller (11). Depending on the instructions the switch (33) will either be configured to pass control instructions and data into or out of the controlled unit (12, 13, 14), or to forward the control instructions and data to a further controlled unit (12, 13, 14) that is connected to the controller (11) or back to the controller (11). In order for units in the ring, that are not registered with the controller (11), to acquire an address, the controller (11) transmits periodically broadcast packets, which contain a unique registration address to be used by an unregistered unit. Once registered, the unit can receive control instructions from the controller (11) as well as data.