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Cable Modem (CM) Section
The cable modem (or CM) section of your gateway uses DOCSIS or EURO-DOCSIS Standard cable modem
technology. DOCSIS or EURO-DOCSIS specifies that TCP/IP over Ethernet style data communication be
used between the WAN interface of your cable modem and your cable company.
A DOCSIS or EURO-DOCSIS modem, when connected to a Cable System equipped to support such
modems, performs a fully automated initialization process that requires no user intervention. Part of this
initialization configures the cable modem with a CM IP (Cable Modem Internet Protocol) address, as shown
in Figure 3-2, so the cable company can communicate directly with the CM itself.
Networking Section
The Networking section of your gateway also uses TCP/IP (Transmission Control Protocol/ Internet Protocol)
for the PCs you connected on the LAN side. TCP/IP is a networking protocol that provides communication
across interconnected networks, between computers with diverse hardware architectures and various
operating systems.
TCP/IP requires that each communicating device be configured with one or more TCP/IP stacks, as
illustrated by Fig.3-2. On a PC, you often use software that came with the PC or its network interface (if you
purchased a network interface card separately) to perform this configuration. To communicate with the
Internet, the stack must also be assigned an IP (Internet Protocol) address. 192.168.100.1 is an example of an
IP address. A TCP/IP stack can be configured to get this IP address by various means, including a DHCP
server, by you directly entering it, or sometimes by a PC generating one of its own.
Ethernet requires that each TCP/IP stack on the Wireless Cable Gateway also have associated with it an
Ethernet MAC (Media Access Control) address. MAC addresses are permanently fixed into network devices
at the time of their manufacture. 00:90:64:12:B1:91 is an example of a MAC address.
Data packets enter and exit a device through one of its network interfaces. The gateway offers Ethernet and
802.11b/g/n wireless network interfaces on the LAN side and the DOCSIS network interface on the WAN
side.
When a packet enters a network interface, it is offered to all the TCP/IP stacks associated with the device
side from which it entered. But only one stack can accept it — a stack whose configured Ethernet address
matches the Ethernet destination address inside the packet. Furthermore, at a packet’s final destination, its
destination IP address must also match the IP address of the stack.
Each packet that enters a device contains source MAC and IP addresses telling where it came from, and
destination MAC and IP addresses telling where it is going to. In addition, the packet contains all or part of a
message destined for some application that is running on the destination device. IRC used in an Internet
instant messaging program, HTTP used by a web browser, and FTP used by a file transfer program are all
examples of applications. Inside the packet, these applications are designated by their port number. Port 80,
the standard HTTP port, is an example of a port number.
The Networking section of the router performs many elegant functions by recognizing different packet types
based upon their contents, such as source and destination MAC address, IP address, and ports.
Three Networking Modes
Your gateway can be configured to provide connectivity between your cable company and your home LAN
in any one of three Networking Modes: CM, RG, and CH. This mode setting is under the control of your
cable company, who can select the mode to match the level of home networking support for which you have
subscribed. All units ship from the factory set for the RG mode, but a configuration file which the cable
company sends the cable modem section during its initialization can change it.
