Cable

Cable is currently the most common broadband service. It is readily available, and has constant connectivity and high-speed asymmetric access. Its biggest disadvantage is that the bandwidth is shared among all users in a particular area; during high utilization, users can see a decrease in performance. Originally called community antenna television (CATV), cable refers to the use of coaxial cable for transmitting signals. Many of today's cable networks have moved from an all coaxial network to extensive use of fiber, which allows for longer runs and a reduced number of amplifiers. Cable can be very cost-effective and can offer a variety of services such as virtual private network (VPN), interactive television, and voice and data capabilities.

Terminology

Before we examine how a cable system works, it would be best to cover some of the terminology. The terms are discussed in order of use, starting at the cable company and ending in your home:

  • Antenna site This location is where antennas and satellite receivers receive broadcast signals.

  • Headend Similar to a telephone company's central office (CO), the headend is where the signals are processed and formatted for transmission onto the distribution network.

  • Distribution network The trunk or backbone made of fiber and coaxial cabling brings the signal to the subscriber drop.

  • Subscriber drop This connection from your television to the distribution network consists of the cable, set-top box, grounding, and attachment hardware.

  • Transportation network Transportation networks often appear between the antenna site and headend or the headend and the distribution network. They are used when necessary to maintain the link.

Now that you have a basic understanding of the different sections in the transmission path, let's look at some additional terms and definitions of cable technology:

  • Broadband A type of data transmission in which a single medium (a wire) can carry several signals at once. Usually, the transmission involves frequency division multiplexing (FDM).

  • Hybrid Fiber Coax (HFC) Provides two-way, high-speed data access to the home using a combination of fiber optics and coaxial cable. Each channel, upstream and downstream, gets a 6MHz channel to transmit and receive its signals. Downstream gets a 50 to 860MHz range, and upstream gets a 5 to 42 MHz range.

  • Radio Frequency (RF) Any frequency within the electromagnetic spectrum associated with radio wave propagation.

  • Coaxial cable A type of wire that consists of a center wire surrounded by insulation and a grounded shield of braided wire. Coaxial cable suffers from attenuation, which is the weakening of the signal due to resistance.

  • Downstream The transmission from the headend to a subscriber, also called the forward path.

  • Upstream The transmission from a subscriber to the headend, also called the return or reverse path.

  • Data Over Cable Service Interface Specification (DOCSIS) Developed by CableLabs and approved by the International Telecommunication Union (ITU) in March 1998. DOCSIS defines interface standards for cable modems and supporting equipment. Version 1.0 was the first standard, 1.1 added VoIP capabilities; Version 2.0 is currently in the works and should allow for 30Mbps in the upstream path.

  • Cable Modem Termination System (CMTS) A system of devices located in the headend that allows cable providers to offer high-speed Internet access. The CMTS provides many of the same functions provided by the DSLAM in a DSL system.

  • Cable Modem (CM) A modulator-demodulator at subscriber locations for use in conveying data packets on a cable television system.

  • Spectrum reuse Takes advantage of a "sealed" cable or network. A cable company can place signals on a wire that it could otherwise not use. The fact that the signal is trapped within means that it doesn't conflict with other signals.

  • National Television System Committee (NTSC) Responsible for setting television and video standards in the United States, which use a 6MHz modulated signal.

  • Phase Alternating Line (PAL) The dominant television standard in Europe, which uses a 6MHz, 7MHz, or 8MHz modulated signal, depending on the version.

Now that you have a good understanding of the terms, you should already have an idea of the interaction between the different devices and components. Internet capabilities are added to the headend and sent out over the distribution network. The signal that eventually enters the CM is an RF signal. The CM tunes into the signal and demodulates it into digital data ready for the PC. That covers the trip downstream; now check out the trip back. The CM takes the digital data signal from the PC, converts it into an RF signal at the appropriate frequency, and sends it on up. The signal is eventually received at the headend by the CMTS, where it is converted back to a digital data signal and routed to the Internet. It is worth noting that two cable modems in neighboring houses can never communicate directly; the CMTS must act as a go-between during the "conversation."

Provisioning

The last thing to examine is the process of provisioning the CM, which follows a number of steps:

  1. On powering up, the CM scans for the RF channel used for the downstream.

  2. Next, it examines maintenance messages in the downstream path on how to use the upstream path.

  3. Layer 1 and 2 communications are established with the CMTS.

  4. The CM requests an IP address through Dynamic Host Configuration Protocol (DHCP). The DHCP server must be RFC-2131 compliant.

  5. Now that it has an address, the CM downloads a DOCSIS configuration file from a Trivial File Transfer Protocol (TFTP) server. The TFTP server must be RFC-1350 compliant.

  6. The CM registers with the CMTS and negotiates any quality of service (QoS) parameters.

The CM is now ready for use, and workstations can request their own address and begin using the Internet.

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Cable modems receive IP addresses from DHCP and DOCSIS configuration files via TFTP.




CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
ISBN: N/A
EAN: N/A
Year: 2003
Pages: 183

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