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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.

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
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