Zenith Cable Modem - The Zenith MAC Layer

The Zenith MAC Layer

CSMA/CD, or Ethernet “like” was expanded as follows: Data rate reduced to 500 kbit/s for up to 50 (100?) miles plant radius Data rate reduced to 4 Mbit/s for up to 25 miles (40 km) plant radius The transmitting device would first listen for activity on the wire, if there was activity, it would activate the “A” LED, and wait. Activity is defined as any RF energy, intelligible or not, at an absolute amplitude of -25 dBmV into the device receive port within a 1 MHz or 6 MHz passband (500kbit/s or 4 Mbit/s respectively.)

Upon detection of no activity, and appropriate backoff timers (as per Ethernet CSMA/CD standard), the CM would begin transmitting a preamble, then the start of the Ethernet frame. Simultaneously, the transmitting CM would begin listening for its own burst. If the CM were able to decode its own burst, and correctly compare a checksum of the first 17 bytes of the packet, it would declare no-collision, and continue. If there was a checksum error, it would declare a collision, and stop transmitting after the 19th byte. This was easily and directly measured using Novel LANalyzer software on a dedicated Windows 3.1 for Workgroups PC.

The card kept some locally accessible counters on excessive and late collisions, but there was no driver or management of any kind.

By November 20, 1996 Zenith released SNMP MIBs and management for their Channelmizer under software release 9.32, under the title “Node Data Controller SNMP”