Chapter 2: Networking

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19

Illustrations contained in this document are for representation only.

Communications

Data communication involves the flow of packets

of data from one device

to another. These

devices include personal computers, Ethernet and USB hubs, cable modems, digital routers and

switches, and highly integrated devices that combine functions, like the Wireless Cable Gateway.

The gateway integrates the functionality often found in two separate devices into one. It’s both

a cable modem and an intelligent wireless gateway networking device that can provide a host

of networking features, such as NAT and firewall. Figure 2 illustrates this concept, with the cable

modem (CM) functionality on the left, and networking functionality on the right. In this figure,

the numbered arrows represent communication based on source and destination, as follows:

Type of Communication

1.

Communication between the Internet and your PCs

Example: The packets created by your request for a page stored at a web site, and the

contents of that page sent to your PC.

2.

Communication between your cable company and the cable modem side

Example: When your cable modem starts up, it must initialize with the cable company,

which requires the cable company to communicate directly with the cable modem itself.

3.

Communication between your PCs and the networking side

CM

CM IP

IP Stack 1

CPE IP

IP Stack 2

WAN

LAN

Networking

Cable Modem Section

Networking Section

Cable service

Connected to your PC via:

• Ethernet

• 802.11b/g

• USB

1

2

3

1

Fig. 11

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Example: The Wireless Cable Gateway offers a number of built-in web pages which you can

use to configure its networking side; when you communicate with the networking side, your

communication is following this path.

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 DOCSIS Standard

cable modem technol-

ogy. 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 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 pro-

vides 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

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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 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.

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CM/Networking

IP Stack 1

111.111.1.11

IP Stack 2

192.168.100.1

WAN

LAN

Cable Modem (CM) Mode

Fig. 12

Fig. 13

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CM (Cable Modem) Mode provides basic home networking. In this mode, two IP stacks are active:

•

IP Stack 1 - for use by the cable company to communicate with the cable modem section

only. This stack receives its IP address from the cable company during CM initialization. It uses

the MAC address printed on the label attached to the Wireless Cable gateway.

•

IP Stack 2 - for use by you, the end user, to communicate with the cable modem and

Networking sections, to access the internal web page diagnostics and configuration. This stack

uses a fixed IP address: 192.168.100.1. It uses a MAC address of MAC label + 1 (the MAC label

is found on the bottom of the unit). E.g., if the MAC address is 00:90:64:12:B1:91, this MAC

address would be 00:90:64:12:B1:92.

With CM Mode, your cable company must provide one IP address for the CM section, plus one for

each PC you connect from their pool of available addresses. Your cable company may have you

or your installer manually enter these assigned addresses into your PC, or use a DHCP Server to

communicate them to your PCs, or use a method that involves you entering host names into your

PCs.

Note that in CM Mode, packets passing to the Internet to/from your PCs do not travel through

any of the IP stacks; instead they are directly bridged between the WAN and LAN sides.