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STATUS
Version 2.0, 07/2015. Copyright
2015 Hitron Technologies
4
Hitron CGNM/ CGNM-3552 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
10
for more information.
By default, the CGNM/ CGNM-3552 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 CGNM/ CGNM-3552 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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STATUS
Version 2.0, 07/2015. Copyright
2015 Hitron Technologies
5
Hitron CGNM/ CGNM-3552 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 CGNM/ CGNM-3552 via one of the
LAN
ports) and also has a
wireless card (to connect to your CGNM/ CGNM-3552 over the wireless interface) the
MAC addresses of the two cards will be different. In the case of the CGNM/ CGNM-
3552, each internal module (cable modem module, Ethernet module, wireless
module, etc.) possesses its own MAC address.
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STATUS
Version 2.0, 07/2015. Copyright
2015 Hitron Technologies
6
Hitron CGNM/ CGNM-3552 User’s Guide
3.1.6
Routing Mode
When your CGNM/ CGNM-3552 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 CGNM/ CGNM-3552 on the WAN, and all traffic for LAN computers is
sent to that IP address. The CGNM/ CGNM-3552 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 CGNM/ CGNM-3552 in routing mode, each
computer on the LAN must be assigned an IP address in the CGNM/ CGNM-
3552’s subnet manually.
When the CGNM/ CGNM-3552 is not in routing mode, the service provider assigns
an IP address to each computer connected to the CGNM/ CGNM-3552 directly. The
CGNM/ CGNM-3552 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
CGNM/ CGNM-3552’s configuration file.
3.1.7
Configuration Files
The CGNM/ CGNM-3552’s configuration (or config) file is a document that the
CGNM/ CGNM-3552 obtains automatically over the Internet from the service
provider’s server, which specifies the settings that the CGNM/ CGNM-3552 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 CGNM/ CGNM-3552, and “upstream” refers to traffic from the
CGNM/ CGNM-3552 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.
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STATUS
Version 2.0, 07/2015. Copyright
2015 Hitron Technologies
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Hitron CGNM/ CGNM-3552 User’s Guide
The data traffic band is separate from the TV band, and each data channel is
separate from other data channels.
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.
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STATUS
Version 2.0, 07/2015. Copyright
2015 Hitron Technologies
8
Hitron CGNM/ CGNM-3552 User’s Guide
FDMA allows multiple users to share the same frequency channel by assigning a
frequency band within the existing channel to each user.
SCDMA allows multiple users to share the same frequency channel by assigning
a unique orthogonal code to each user.
3.1.12
The Multimedia over Coax Alliance
The Multimedia over Coax Alliance (MoCA) is a non-profit technology alliance, which
defines a set of specifications for the delivery of high-speed data, such as HD video,
over your building’s existing co-axial cabling network. Co-axial, or coax (pronounced
“ko-axe”) cable is already incorporated into most buildings for the transmission of RF
signals, traditionally for relaying television broadcasts from a TV antenna, satellite or
cable box to individual televisions around the building.
MoCA devices allow you use the coax cable network as an extension of your
building’s existing IP network, which includes both wired (Ethernet) and wireless
(WiFi) traffic. Because they bridge the two networks, they are known as Ethernet-to-
Coax Bridges, or ECBs.
Figure 13:
Bridging the Gap Between IP and Coaxial Networks
MoCA traffic on the coax network does not interfere with existing broadcasts from
cable, telco, IPTV or satellite service providers, as it makes use of a previously-
unused segment of the RF spectrum. The medium is ideal for real-time applications,
providing high data throughput (100Mbps~1Gbps) with low latency, jitter or data loss.
Also, coax cabling is generally better-shielded than IP networking media, especially
wireless.
Applications to which MoCA networking is well-suited include:
Video on Demand (VoD)

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