Network Architectural Functions
In network topology, a cable modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The cable modem bridges Ethernet frames between a customer LAN and the coax network. Technically, it is a modem because it must modulate data to transmit it over the cable network, and it must demodulate data from the cable network to receive it.
With respect to the OSI model of network design, a cable modem is both Physical Layer (Layer 1) device and a Data Link Layer (Layer 2) forwarder. As an IP addressable network node, cable modems support functionality at other layers.
Layer 1 is implemented in the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. The term cable modem refers to this cable-specific PHY. The Network Layer (Layer 3) is implemented as an IP host in that it has its own IP address used by the network operator to maintain the device. In the Transport Layer (Layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the Application Layer (Layer 7), the cable modem supports certain protocols that are used for management and maintenance, notably Dynamic Host Configuration Protocol (DHCP), SNMP, and TFTP.
Some cable modems may incorporate a router and a DHCP server to provide the LAN with IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit, sometimes called a residential gateway. So, the cable modem function will have its own IP address and MAC address as will the router.
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