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The following table shows a subnet mask that “masks” the first twenty-four bits of the

IP address, in both its decimal and binary notation.

This shows that in this subnet, the first three octets (

192.168.1

, in the example IP

address) define the main network, and the final octet (

1

, in the example IP address)

defines the computer’s address on the subnet.

The decimal and binary notations give us the two common ways to write a subnet

mask:



Decimal: the subnet mask is written in the same fashion as the IP address:

255.255.255.0

, for example.



Binary: the subnet mask is indicated after the IP address (preceded by a forward

slash), specifying the number of binary digits that it masks. The subnet mask

255.255.255.0

masks the first twenty-four bits of the IP address, so it would be

written as follows: 192.168.1.1

/24

.

3.1.3

DHCP

The Dynamic Host Configuration Protocol, or DHCP, defines the process by which IP

addresses can be assigned to computers and other networking devices

automatically, from another device on the network. This device is known as a DHCP

server, and provides addresses to all the DHCP client devices.

In order to receive an IP address via DHCP, a computer must first request one from

the DHCP server (this is a broadcast request, meaning that it is sent out to the whole

network, rather than just one IP address). The DHCP server hears the requests, and

responds by assigning an IP address to the computer that requested it.

If a computer is not configured to request an IP address via DHCP, you must

configure an IP address manually if you want to access other computers and devices

on the network. See

IP Address Setup

on page

22

for more information.

By default, the CGN3 is a DHCP client on the WAN (the CATV connection). It

broadcasts an IP address over the cable network, and receives one from the service

provider. By default, the CGN3 is a DHCP server on the LAN; it provides IP

addresses to computers on the LAN which request them.

Table 10:

Subnet Mask: Decimal and Binary

255

255

255

0

11111111

11111111

11111111

00000000

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3.1.4

DHCP Lease

“DHCP lease” refers to the length of time for which a DHCP server allows a DHCP

client to use an IP address. Usually, a DHCP client will request a DHCP lease

renewal before the lease time is up, and can continue to use the IP address for an

additional period. However, if the client does not request a renewal, the DHCP server

stops allowing the client to use the IP address.

This is done to prevent IP addresses from being used up by computers that no longer

require them, since the pool of available IP addresses is finite.

3.1.5

MAC Addresses

Every network device possesses a Media Access Control (MAC) address. This is a

unique alphanumeric code, given to the device at the factory, which in most cases

cannot be changed (although some devices are capable of “MAC spoofing”, where

they impersonate another device’s MAC address).

MAC addresses are the most reliable way of identifying network devices, since IP

addresses tend to change over time (whether manually altered, or updated via

DHCP).

Each MAC address displays as six groups of two hexadecimal digits separated by

colons (or, occasionally, dashes) for example

00:AA:FF:1A:B5:74

.

NOTE:

Each group of two hexadecimal digits is known as an “octet”, since it

represents eight bits.

Bear in mind that a MAC address does not precisely represent a computer on your

network (or elsewhere), it represents a network device, which may be part of a

computer (or other device). For example, if a single computer has an Ethernet card

(to connect to your CGN3 via one of the

LAN

ports) and also has a wireless card (to

connect to your CGN3 over the wireless interface) the MAC addresses of the two

cards will be different. In the case of the CGN3, each internal module (cable modem

module, Ethernet module, wireless module, etc.) possesses its own MAC address.

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3.1.6

Routing Mode

When your CGN3 is in routing mode, it acts as a gateway for computers on the LAN

to access the Internet. The service provider assigns an IP address to the CGN3 on

the WAN, and all traffic for LAN computers is sent to that IP address. The CGN3

assigns private IP addresses to LAN computers (when DHCP is active), and

transmits the relevant traffic to each private IP address.

NOTE:

When DHCP is not active on the CGN3 in routing mode, each computer on

the LAN must be assigned an IP address in the CGN3’s subnet manually.

When the CGN3 is not in routing mode, the service provider assigns an IP address to

each computer connected to the CGN3 directly. The CGN3 does not perform any

routing operations, and traffic flows between the computers and the service provider.

Routing mode is not user-configurable; it is specified by the service provider in the

CGN3’s configuration file.

3.1.7

Configuration Files

The CGN3’s configuration (or config) file is a document that the CGN3 obtains

automatically over the Internet from the service provider’s server, which specifies the

settings that the CGN3 should use. It contains a variety of settings that are not

present in the user-configurable Graphical User Interface (GUI) and can be specified

only by the service provider.

3.1.8

Downstream and Upstream Transmissions

The terms “downstream” and “upstream” refer to data traffic flows, and indicate the

direction in which the traffic is traveling. “Downstream” refers to traffic from the

service provider to the CGN3, and “upstream” refers to traffic from the CGN3 to the

service provider.

3.1.9

Cable Frequencies

Just like radio transmissions, data transmissions over the cable network must exist

on different frequencies in order to avoid interference between signals.

The data traffic band is separate from the TV band, and each data channel is

separate from other data channels.

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3.1.10

Modulation

Transmissions over the cable network are based on a strong, high frequency periodic

waveform known as the “carrier wave.” This carrier wave is so called because it

“carries” the data signal. The data signal itself is defined by variations in the carrier

wave. The process of varying the carrier wave (in order to carry data signal

information) is known as “modulation.” The data signal is thus known as the

“modulating signal.”

Cable transmissions use a variety of methods to perform modulation (and the

“decoding” of the received signal, or “demodulation”). The modulation methods

defined in DOCSIS 3 are as follows:



QPSK

: Quadrature Phase-Shift Keying



QAM

: Quadrature Amplitude Modulation



QAM TCM

: Trellis modulated Quadrature Amplitude Modulation

In many cases, a number precedes the modulation type (for example

16 QAM

). This

number refers to the complexity of modulation. The higher the number, the more data

can be encoded in each symbol.

NOTE:

In modulated signals, each distinct modulated character (for example, each

audible tone produced by a modem for transmission over telephone lines) is

known as a symbol.

Since more information can be represented by a single character, a higher number

indicates a higher data transfer rate.

3.1.11

TDMA, FDMA and SCDMA

Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA)

and Synchronous Code Division Multiple Access (SCDMA) are channel access

methods that allow multiple users to share the same frequency channel.



TDMA allows multiple users to share the same frequency channel by splitting

transmissions by time. Each user is allocated a number of time slots, and

transmits during those time slots.



FDMA allows multiple users to share the same frequency channel by assigning a

frequency band within the existing channel to each user.

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

SCDMA allows multiple users to share the same frequency channel by assigning

a unique orthogonal code to each user.

3.2

The System Information Screen

Use this screen to see general information about your CGN3’s hardware, its software,

and its connection to the Internet.

NOTE:

Most of the information that displays in this screen is for troubleshooting

purposes only. However, you may need to use the MAC Address information

when setting up your network.

Click

Status

>

System Information

. The following screen displays.

Figure 12:

The Status: System Information Screen

The following table describes the labels in this screen.

Table 11:

The Status: System Information Screen

HW Version

This displays the version number of the CGN3’s

physical hardware.

SW Version

This displays the version number of the software that

controls the CGN3.

Serial Number

This displays a number that uniquely identifies the

device.

RF MAC

This displays the Media Access Control (MAC) address

of the CGN3’s RF module. This is the module that

connects to the Internet through the

Cable

connection.