Thomson DWG875 Router Manual PDF (Setup & Configuration Guide)

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Chapter 3: Networking

Illustrations contained in this document are for representation only.

Chapter 3: Networking

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

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.

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.

Communication between your PCs and the networking side

Fig.53 Communication between your PCs and the network side

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

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Chapter 3: Networking

Illustrations contained in this document are for representation only.

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

technology. 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 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 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

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Chapter 3: Networking

Illustrations contained in this document are for representation only.

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.

Cable Modem (CM) Mode

Fig. 54 Cable Modem Mode

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Chapter 3: Networking

Illustrations contained in this document are for representation only.

Fig. 55 Two IP stacks are activated in cable modem mode

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 con

guration. 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.

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Chapter 3: Networking

Illustrations contained in this document are for representation only.

Residential Gateway (RG) Mode

Fig. 56 Residential Gateway Mode

Fig. 57 Three IP stacks are activated in Residential mode

RG (Residential Gateway) Mode provides basic home networking plus NAT (Network Address

Translation). In this mode, three 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 3 - for use by you to remotely (i.e. from somewhere on the WAN side, such as at

your remote workplace) communicate with the Cable Modem and Networking sections, to

remotely access the internal web page diagnostics and configuration. This stack is also

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