Existing community cable television systems are evolving into bidirectional Hybrid Fiber Coaxial networks.

An HFC architecture:

The architecture of the HFC has the following important features that affect MAC protocol design:

• Tree and Branch Topology:
  Point-to-multipoint downstream and multipoint-to-point upstream. It is a point-to-multipoint, tree-and-branch access network in the downstream direction, but a multipoint-to-point bus-like access network in the upstream direction. This makes the packets subject to collisions, so shared upstream channel needs an efficient scheme to avoid and resolve collisions.
  
• The inability to detect collisions by stations:
  Stations can only listen to the downstream traffic, which differs from an Ethernet where adaptors can detect when collisions occur. Thus, stations rely on the headend to notify them of the results of upstream transmissions.
  
• Large propagation delay:
  The maximum round-trip-delay (RTD) is significantly longer than that of Ethernet. Therefore, a channel should be utilized to transmit other data frames during the RTD of a transmitted data frame. Neutralizing the effect of propagation delay is of synchronization concern so that the transmissions from stations arrive at the right time slots assigned by the headend. Because of this, the MAC protocol should have a ranging scheme to measure the propagation delay for each station.
  
• Asymmetric upstream and downstream:
  The downstream data rate is substantially larger than that of the upstream. Thus, the efficiency of upstream channels is critical.
  
• Non-uniform user distribution:
  Most subscribers are distributed over the last few miles of the network. Their propagation times to the headend are quite close to each other. Repeated collisions may occur for a straightforward ranging algorithm that does not consider this factor.