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ISCW – Broadband Cable breakdown

Though not as sexy as MPLS and IPSec VPN, Cable Modem and DSL seem like an important topic for the exam. The natural focus for this chapter is PPPoE and PPPoA but the more I read about it, the more I understand how much emphasis Cisco put on this sub-topic and since exam day is looming I cannot allow myself to ignore it.

I will go over the major components of the cable system and layout the provisioning of cable modem.

Components of Cable System

There are five basic components:

  • Antenna site – this is where we receive TV signals (from either antenna or dish).
  • Headend site – this is where TV signal is converted for distribution to the end users.
    Here we also convert data for transport to\from users.
  • Transportation network – this is where the antenna site and headend get connected.
    It can use either a coaxial or a fiber cable.
  • Distribution network – this is how signals are carried between the user and transportation network.
    Feeder cables connect the distribution network to the subscriber drops.
  • Subscriber drops – this is where the distribution network connects with the customer equipment, the cable box.

DOCSIS, Data Over Cable Service Interface Specifications is the world standard that defines Layer 1 and 2 (physical and data) requirements.

Steps to provision cable modem (DOCSIS definition):
Scan downstream frequency -> modem startup
Obtain upstream parameters-> modem listen to message from the network
Establish Layer 1\2 communications -> connection to CMTS
Acquire IP configuration via DHCP -> get IP, DNS and TFTP addresses
Acquire DOCSIS –> connect to TFTP and download DOCSIS file
Register with CMTS -> QoS registration
IP network initialization

More in-depth reading on DOCSIS at Wiki and Cisco.

Few more details from this exam topic that you shouldn’t skip…

Around the world there are three major standards for TV systems:
NTSC – Analog standard used in North America. 6MHz channel width
PAL – Color TV standard used in most of the world. 6, 7,8MHz channel width.
SECAM – Standard used in France. 8MHz channel width

DSL types:

ADSL, Asymmetric Digital Subscriber Line has a large download bandwidth and smaller upload bandwidth. It has a 18,000 feet limit and can coexist with POTS (Plain Old Telephone Service)
Signal loss over distance is sometimes called Attenuation.
ADSL POTS splitter is installed at the headend side.
The splitter pass the ADSL traffic to the modem and block voice frequencies from the modem (and that is how we can use both internet and phone at the same time)
The splitter also pass the voice frequencies to POTS device and block ADSL traffic from POTS device.

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