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Kabel Modems and VoIP

Kabel Modem – With the advent of Voice over Internet Protocol (VoIP) telephony, kabel modems have been extended to provide telephone service. Some companies which offer kabel TV service also offer VoIP phone, allowing customers who purchase kabel TV to eliminate their plain old telephone service (POTS). Because many telephone companies do not offer naked DSL (DSL service without POTS line service), VoIP use is higher amongst kabel modem users.[citation needed] Any high-speed Internet service subscriber can use VoIP telephony by subscribing to a third-party service (e.g., Skype), the problem is that doing so, one needs to turn on their computer to use the telephone, while kabel modems have a port to connect the phone directly, without using a computer. However, there are also stand-alone VoiP systems available that connect directly to a broadband router (e.g., Vonage, MagicJack+ and NetTALK).

Kabel Modems and VoIP

Kabel Modems and VoIP

Many cable operators offer their own VoIP service, based on PacketCable. PacketCable allows multiple system operators (MSOs) to offer both high-speed Internet and VoIP through the same kabel transmission system. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed quality of service across their entire transmission path, so call quality can be assured.

When using kabel operator VoIP, a combined customer premises equipment device known as an embedded multimedia terminal adapter (E-MTA) will often be used. An E-MTA is a kabel modem and a VoIP adapter (MTA, multimedia terminal adapter) bundled into a single device.

Network Architectural Functions

In network topology, a kabel modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The kabel 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 kabel modem is both Physical Layer (Layer 1) device and a Data Link Layer (Layer 2) forwarder. As an IP addressable network node, kabel 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 kabel 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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