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CHAPTER 3: NETWORKING

Communications

Data communication involves the flow of packets of data from one device to another. These devices

include personal computers, Ethernet, 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 devi

ces into one. It’s both a cable

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

networking features, such as NAT and firewall. Fig.3-1 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:

Fig.3-1 Communication between your PCs and the network side

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

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.

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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.

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Cable Modem (CM) Mode

Fig. 3-2 Cable Modem Mode

Fig. 3-3 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 configuration. This stack uses a fixed IP

address: 192.168.100.1. It uses a MAC address 00:10:95:FF:FF:FE.

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

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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.

Residential Gateway (RG) Mode

Fig. 3-4 Residential Gateway Mode

Fig. 3-5 Three IP stacks are activated in cable modem 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

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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 used by your cable company to

deliver packets between the Internet and the gateway’s networking section so they can be routed

to/from your PCs. This stack requires an IP address assigned by the cable company from their pool of

available addresses. Your cable company may have you or your installer manually enter assigned

addresses into your gateway, or use a DHCP Server to communicate them, or use a method that

involves you entering host names. This stack uses a MAC address of MAC label + 2 (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:B1:93.



IP Stack 5 - for use by you to locally (i.e. from somewhere on the LAN side in your home)

communicate with the Cable Modem and Networking sections, to access the internal web page

diagnostics and configuration. This stack is also used by the gateway’s networking section to route

packets between the gateway’s Networking section and your PCs. This

stack uses a fixed IP address:

192.168.0.1. It uses a MAC address of MAC label + 4 (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:B1:95.

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

Networking section, from their pool of available addresses. With RG Mode, each PC you connect gets an

IP address from a DHCP Server that is part of the Networking section of the gateway.