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Cable
modems
By
Suryakant Naik
In
the market for a cable modem? Here is all you need to know
about cable modems before making a buying decision
Cable
modems offer "24 x 7 x 365" connectivity. There is no need
for you to dial into the Internet service provider. Simply
click on your browser and you are on the Internet. No more
waiting, no busy signals
 Looking
for fast Net access? A cable modem is an alternative to otherwise
slow and unreliable Net access over plain old telephone lines
off course if your cable guy provides you the facility.
Broadband
is in and with the widespread cable connectivity, cable modems
are the ideal choice for making a jump onto the fast Net access
bandwagon.
Besides
providing stable connectivity and high access speeds, cable
modems are easy to install, maintain and use.
Traditional
dialup modems provide
Net access through public telephone
networks at speeds up to 56kbps. A cable modem on the other
hand provides high speed Internet access through a cable television
network at more than 1 Mbps. This is at least 20 times faster
than a traditional
modem. If you frequently download
large files such as video clips/audio clips or software, cable
modems can make Net access much faster.
The
other advantage offered by cable modem access is "Neither
Dial Nor Disconnect, Always On". That means it offers "24
x 7 x 365" connectivity. There is no need for you to dial
into the Internet service provider. Simply click on your browser
and you are on the Internet. No more waiting, no busy signals.
Cable
modems deliver multi-megabit speeds by using your local cable
company's cable TV network. A cable television system is meant
for typically 60 or more channels. Cable modems use one or two
TV channels to transmit and receive data over the standard coaxial
cable already laid to receive cable TV services.
A
cable modem has two connectors; one connects the cable network
through coaxial cable and the other connects the computer through
a standard Ethernet Interface.
In
case a user does not have a PC at his home, he still can access
Internet via TV with a set-top box instead of a cable modem.
A
splitter at the user end breaks the coaxial cable lines to serve
the cable modem and TV outlet. Cable modems currently connect
to an Ethernet card in the PC with CAT5 cabling and RJ-45 connectors.
Forthcoming cable modem products will also offer Universal Serial
Bus (USB) connections.
It
is important to note that if a cable company is using a particular
brand of networking equipment, the same brand of cable modem
has to be installed at the user end. That is why most cable
companies usually lease modems to subscribers on a monthly rental
basis.
Cable
companies are just beginning to develop cable modem standards
based on DOCSIS Standard (Data Over Cable Service Interface
Specification). DOCSIS Standard was developed by a technology
organization called Cable Labs; to ensure that cable modem equipment
built by various manufacturers are compatible with each other.
Therefore, it is better that you lease
the
modem from the cable service operators for now and perhaps buy
a new Cable Labs certified DOCSIS cable modem down the road.
Features
The
following are some parameters to be taken into consideration
while purchasing cable modems:
-
MAC Protocol: It should be compliant to DOCSIS, MCNS
DOCSIS 1.0 (Euro DOCSIS for European Markets). DOCSIS 1.0
provides the bandwidth and latency guarantees required to
offer constant toll quality voice, dedicated business class
data services and multimedia applications across a shared
cable modem access network. l
Protocol: It should support most of the Internet protocol
such as TCP/IP, UDP, ARP, ICMP, DHCP, SNMP, TPTP, BOOTP,
SYSLOG and so on.
-
Interface:
-
RF Interface that normally connects to the cable network,
should be F-type Female 75-ohm connector.
-
CPE Interface, which connects the computer, should have
RJ-45 connector supporting 10/100 Ethernet.
-
Apart from the above, some cable modems have USB B type
interface, to connect USB port to the home PC.
-
Frequency Range/Bandwidth: To deliver DOCSIS data
services over a cable television network, one 6 MHz RF (bandwidth)
channel in 50-850 MHz spectrum range is typically allocated
for downstream traffic to homes. Similarly, for upstream,
another channel of any of the following RF channels—200K,
400K, 800K 1.6KA, 3.2 MHz—in the 5-42 MHz frequency
band is utilized. Therefore, cable modems should be operated
with the above frequency/bandwidth.
-
Bridging: It should be compliant to IEEE 802.1, and
support transparent bridging, unicast, broadcast and multi-cast
IP packets.
-
Modulation: 64QAM/256QAM auto-detection for downstream.
Similarly, for Upstream it auto-detects QPSK/16 QAM.
-
Data Rate/Data Speed: It basically depends on the modulation
selected.
Downstream
speed--30 Mbps/64 QAM
Downstream
speed--42 Mbps/256 QAM
For
upstream speed it is 320 Kbps to 5 Mbps with QPSK type modulation
and 640 Kbps to 10 Mbps with 16 QAM modulation.
-
Management: It should support SNMP V1/V2C, RFC 1902,
and SMI V2 RFC 1903. Texture conversion apart from the following
MIBs such as RFC 1907, RFC 2011, RFC 2670 etc. are supported.
-
Security: Since cable modem Internet access is "Always
On" connectivity, security policy should be in place
to avoid intruders. It should be compliant to DOCSIS Baseline
privacy (BPC), 56-bit DES and 168-bit triple DES encryption
and configuration for both downstream and upstream traffic.
l
Signal to Noise Ratio (SNR): SNR is a very important factor
for effective, clear data transmission.
Downstream:
64 QAM > 24db@BER<10^-8
Downstream:
256 QAM > 30db@BER<10^-8
Upstream:
QP5K > 15db@BER<10^8
Upstream:
16 QAM > 22db@BER<10^8
-
Flash Memory: It should have at least 4 MB flash memory
with support for software upgrades.
-
Apart from the above parameters, cable modes should have
the front panel with the diagnostic LEDs. It should be stylish,
compact in design to suit the Soho/home segment.
Suryakant
Naik can be reached at surya@lanbitmail.com |
