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