By Cheap Phone Cards
Figure 1. HFC uses broadcasts to transport data.
Only one device at a time can broadcast a packet.
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The combination of shared bandwidth and broadcast transportation makes cable HFC networks vulnerable to hackers. Therefore, access to the network is controlled by a Cable Modem Termination System (CMTS).
Figure 2. The CMTS controls access to the HFC network.
The CMTS's responsibilities include time slot enforcement, bandwidth allocation, and Quality of Service (QoS) management. To prevent transmission collisions, the CMTS allocates a time slot to each network device. During this time slot, only the assigned network device can transmit data.