A router receives destination address information for a packet and determines, among entries in a first forwarding table, a closest match for the received destination address information. Means are provided for measuring the tension on opposed sides of the web passing downstream from the adjustment roller, and means are provided responsive to the measuring means for automatically approximately equalizing the tension of the web by adjustment of the adjustment roller on one end only of the adjustment roller. The fraction of one of the clock cycles is measured when the head is positioned at a first plurality of radial locations across the disk, and a second plurality of radial locations is estimated where the fraction substantially equals a full one of the clock cycles.