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CS601 - Data Communication - Lecture Handout 24

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Cable Modems

  • Data rate limitation of traditional modems is mostly due to the narrow BW of the local loop telephone line (4Khz)
  • If higher BWs are available, one can design a modem that can handle much higher data rates
  • Fortunately, cable TV provides residential premises with a coaxial cable that has a BW of up to 750 MHz and sometimes even more
  • This BW is normally divided into 6MHz bands using FDM
  • Each band provides a TV Channel
  • Two bands can be left aside to allow a user to download and upload the information from the internet

Cable Modems

  • Instead of the traditional cable box, we show a splitter
  • The splitter directs the TV bands to the TV set and the Internet access bands to the PC

DOWNLOADING:

Downloading requires a 6 MHz BW in the range above 40MHz. The demodulation technique used is 64 QAM (6 bits at a time)

  • This means that a user can download info at a rate of 6MHz * 6 = 36Mbps
  • However PCs are not yet capable of receiving data at this rate
    • Currently rate is b/w 3 and 10 Mbps

UPLOADING:

Requires a 6MHz BW in a range below 40MHz

  • At this low frequency, home appliances can create a noise environment that effects modulation
  • The modulation technique uses is QPSK (4 bits at a time)
  • This means that user can Upload info at a rate of 6 MHz * 2=12 MHz
  • Presently uploading rate is b/w 500Kbps and 1Mbps

Electromagnetic Energy

  • Signals are transmitted from one device to another in the form of electromagnetic energy
  • Electromagnetic signals can travel through Vacuum, Air or other transmission media
  • Electromagnetic energy, a combination of electrical and mechanical fields vibrating in relation to each other includes power, voice, video, radio waves, infrared light, visible light and ultra violet light

Electromagnetic Energy

  • Each of the above constitute a portion of the Electromagnetic Spectrum:
    • Not all the portions of the spectrum are currently usable for Telecommunications
    • Voice-band frequencies are generally tx as current over metal cables, such a twisted pair or coaxial cable
  • Radio frequencies can travel through air or space but require specific transmitting and receiving mechanisms
  • Visible light, the third type of Electromagnetic energy currently used for communications is harnessed using fiber optic cable

Classes of Transmission Media

Classes of Transmission Media

  • Two classes of Transmission Media:
    • Guided Media
    • Unguided Media

Guided Media

Guided Media, are those media that provide a conduit from one device to another

Guided Media

Twisted Pair Cable

  • Twisted pair comes in two forms:
    • Unshielded Twisted Pair (UTP) cable
    • Shielded Twisted Pair (STP) cable

Frequency range for Twisted Pair Cable

Frequency range for Twisted Pair Cable

Unshielded Twisted Pair (UTP) Cable

  • UTP cable is the most common type of Telecommunication Medium in use today
  • Although mostly used in Telephone systems,, its frequency range is suitable for transmitting both data and voice

Unshielded Twisted Pair (UTP) Cable

  • A twisted pair consists of two conductors (usually copper) , each with its own colored plastic insulation.
  • The plastic insulation is color banded for identification
  • Colors are used both to identify the specific conductors in a cable and to indicate which wires belong in pairs and how they relate to other pairs in a large bundle

Parallel Flat Wire

  • In the past, two parallel flat wires were used for communication.
  • However, EM interference from devices such as motor can create noise over those wires

Effect of noise on Parallel Flat Wire

Effect of noise on Parallel Flat Wire

  • If the two wires are parallel, the wire closest to the source of the noise gets more interference and ends up with a higher voltage level than the wire further away
  • This results in an uneven load and a damaged signal

Noise Effect on Twisted-Pair

  • If the two wires are twisted around each other at regular intervals (b/w 2 & 12 twists per foot), each wire is closer to the noise source for half the time and is away for the other half
  • Twisting does not always eliminate the impact of Noise but it does significantly reduce it

Noise Effect on Twisted-Pair

  • With twisting, therefore the cumulative effect of the interference is equal on both wires
  • Each section of wire has a “Load” of 4 when it is on the top of the twist and ‘3’ when it is on the bottom
  • The total effect of the noise at the receiver is therefore 0 (14-14)

Advantage of UTP

  • Advantages of UTP are:
    • Cost
    • Ease of Use
  • Its cheap, flexible and easy to install
  • Higher grades of UTP are used in many LAN technologies including Ethernet and Token Ring

Cable with 5 UTP of wires

Cable with 5 UTP of wires

Categories of UTP Cable

EIA has developed standards to grade UTP cables by quality.
Categories are determined by cable quality, with 1 as the lowest and 5 as the highest
Each EIA category is suitable for certain uses and not for others

Category 1

  • Basic Twisted pair cabling used in Telephone system
  • Fine for voice but inadequate for all but low-speed data communication

Category 2

  • The next higher grade, suitable for voice and for data transmission of up to 4Mbps

Category 3

  • Required to have at least 3 twists per foot
  • Can be used for for data tx ofup to 10Mbps
  • Now the standard cable for most telephone lines

Category 4

  • Must have at least 3 twists per foot
  • Possible tx rate of 16 Mbps

Category 5

  • Used for data transmission of up to 100 Mbps

UTP Connectors

  • UTP is mostly connected to the networked devices via a type of snap-in plug like that used with telephone jacks
  • Connectors are either male (plug) or female (the receptacle)
  • Male connectors snap into female connectors and have a repressible tab (key) that locks them in place

UTP Connectors

  • Each wire in the cable is attached to one conductor (or pin) in the connector
  • The most frequently used of these plugs is an RJ 45 connector with 8 conductors, one for each wire of 4 twisted pairs

Summary

  • Cable Modems
  • Electromagnetic Spectrum
  • Transmission Media and its Types
  • Guided Media
  • Twisted Pair
  • Coaxial Cable
  • Optical Fiber

Reading Sections

  • Section 6.6, 7.1, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan