a stackable network unit comprises a ‘ down ’ port and an ‘ up ’ port , an arbitration path for data packets from the down port to the up port , a repeat path for data packets from the up port to the down port , a link detector for detecting tile absence of another operative unit connected to the down port to cause data packets on the return pat to bypass the down port and proceed on the arbitration path and for detecting the absence of another operative unit connected to the up port to cause data packets on the arbitration path to bypass the up port and proceed on the repeat path .