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Chapter 3: Networking
Each packet on the Internet addressed to a PC in your home travels from the Internet down- stream on
the cable company's system to the WAN side of your Wireless Cable Gateway. There it enters the
Cable Modem section, which inspects the packet, and, based on the results, proceeds to either forward
or block the packet from proceeding on to the Networking section. Similarly, the Networking section
then decides whether to forward or block the packet from proceeding on to your PC. Communication
from your home device to an Internet device works similarly, but in reverse, with the packet traveling
upstream on the cable system.
Cable Modem (CM) Section
The cable modem (or CM) section of your gateway uses EURO-DOCSIS Standard cable modem
technology. 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 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, 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 Figure 4. 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,
USB, and 802.11b/g wireless network interfaces on the LAN side and the EURO-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
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Illustrations contained in this document are for representation only.
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