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Version 1.1, 08/2013. Copyright
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36
Hitron CGN3 User’s Guide
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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Hitron CGN3 User’s Guide
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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Hitron CGN3 User’s Guide
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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Hitron CGN3 User’s Guide
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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Hitron CGN3 User’s Guide
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.

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