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Computer network unit 2 : The physical layer
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DIGITAL SIGNALS
In addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.
Bit Rate Bit Length Digital Signal as a Composite Analog
Signal Application Layer
Topics discussed in this section:
Figure : Two digital signals: one with two signal levels and the other with four signal levels
3.4
A digital signal has eight levels. How many bits are needed per level? We calculate the number of bits from the formula
Example
Each signal level is represented by 3 bits.
3.5
A digital signal has nine levels. How many bits are needed per level? We calculate the number of bits by using the formula. Each signal level is represented by 3.17 bits. However, this answer is not realistic. The number of bits sent per level needs to be an integer as well as a power of 2. For this example, 4 bits can represent one level.
Example
3.6
Assume we need to download text documents at the rate of 100 pages per sec. What is the required bit rate of the channel?SolutionA page is an average of 24 lines with 80 characters in each line. If we assume that one character requires 8 bits (ascii), the bit rate is
Example
Assume we need to download text documents at the rate of 100 pages per sec. What is the required bit rate of the channel?
SolutionA page is an average of 24 lines with 80 characters in each line. If we assume that one character requires 8 bits (ascii), the bit rate is
Example
A digitized voice channel, as we will see in Chapter 4, is made by digitizing a 4-kHz bandwidth analog voice signal. We need to sample the signal at twice the highest frequency (two samples per hertz). We assume that each sample requires 8 bits. What is the required bit rate?
SolutionThe bit rate can be calculated as
Example
What is the bit rate for high-definition TV (HDTV)?
Solution
HDTV uses digital signals to broadcast high quality video signals. The HDTV screen is normally a ratio of 16 : 9. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Twenty-four bits represents one color pixel.
Example
The TV stations reduce this rate to 20 to 40 Mbps through compression.
Figure : The time and frequency domains of periodic and nonperiodic digital signals
Figure Baseband transmission
A digital signal is a composite analog signal with an infinite bandwidth.
Note
Figure Bandwidths of two low-pass channels
Figure : Baseband transmission using a dedicated medium
Baseband transmission of a digital signal that preserves the shape of the digital
signal is possible only if we have a low-pass channel with an infinite or very wide
bandwidth.
Note
An example of a dedicated channel where the entire bandwidth of the medium is used as one single channel is a LAN. Almost every wired LAN today uses a dedicated channel for two stations communicating with each other. In a bus topology LAN with multipoint connections, only two stations can communicate with each other at each moment in time (timesharing); the other stations need to refrain from sending data. In a star topology LAN, the entire channel between each station and the hub is used for communication between these two entities.
Example
Figure Rough approximation of a digital signal using the first harmonic for worst case
Figure Simulating a digital signal with first three harmonics
Note
In baseband transmission, the required bandwidth is proportional to the bit rate;if we need to send bits faster, we need
more bandwidth.
Table Bandwidth requirements
What is the required bandwidth of a low-pass channel if we need to send 1 Mbps by using baseband transmission?
SolutionThe answer depends on the accuracy desired.a. The minimum bandwidth, is B = bit rate /2, or 500 kHz. b. A better solution is to use the first and the third harmonics with B = 3 × 500 kHz = 1.5 MHz.
c. Still a better solution is to use the first, third, and fifth
harmonics with B = 5 × 500 kHz = 2.5 MHz.
Example
We have a low-pass channel with bandwidth 100 kHz. What is the maximum bit rate of this channel?
SolutionThe maximum bit rate can be achieved if we use the first harmonic. The bit rate is 2 times the available bandwidth, or 200 kbps.
Example
Figure : Bandwidth of a band pass channel
If the available channel is a bandpass channel, we cannot send the digital signal
directly to the channel; we need to convert the digital signal to an
analog signal before transmission.
Note
Figure Modulation of a digital signal for transmission on a bandpass channel
An example of broadband transmission using modulation is the sending of computer data through a telephone subscriber line, the line connecting a resident to the central telephone office. These lines are designed to carry voice with a limited bandwidth. The channel is considered a band pass channel. We convert the digital signal from the computer to an analog signal, and send the analog signal. We can install two converters to change the digital signal to analog and vice versa at the receiving end. The converter, in this case, is called a modem.
Example
3.27
A second example is the digital cellular telephone. For better reception, digital cellular phones convert the analog voice signal to a digital signal (see Chapter 16). Although the bandwidth allocated to a company providing digital cellular phone service is very wide, we still cannot send the digital signal without conversion. The reason is that we only have a bandpass channel available between caller and callee. We need to convert the digitized voice to a composite analog signal before sending.
Example
1 Polyethylene2 "Mylar" tape3 Stranded metal (steel) wires4 Aluminum water barrier5 Polycarbonate6 Copper or aluminum tube7 Petroleum jelly8 Optical fibers
SUB MARINE COMMUNICATION CABLE
Transmission Media• Transfer of Data from source to destination is
called TRANSMISSION.• Medium used for transmission of data is called
MEDIA.• Transmission of data through any medium is
called Transmission Media
• Transmission media is actually located below the physical layer and Directly control by physical layer.
• Transmission media between two people : AIR• Transmission media is usually free space or any
cable.• Information : Signal that is result of a conversion
of data from another form (like o and 1 to signal)
• Definitions
• Attenuation : Loss of Signals.• Noise : Interference in
transmission
GUIDED MEDIA• Wired • Waves are guided along solid medium• Data signals are guided along the path like cabling system. There is
no other way to go• Types : -
Twisted pair cable. Coaxial cable. Fiber optic cable.
• Twisted pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current.
• Optical fiber is the cable that accepts and transports signal in the form of Light
Twisted-Pair Cable
• A twisted pair consists of two conductors (copper), Each with own plastic insulation, twisted together.
• Twisted pair cable may be used to transmit Both Digital and Analog transmission
• One will used to carry signal to the receiver and the other used as a ground reference. Receiver uses the difference between the two.
• Twisted pair cable is good for transferring balanced differential signals.
• If the two wires are parallel, The effect of these unwanted signals is not same in both wires because location is different relative to noise and cross talk.
• Result in a difference at receiver . So by twisting the pair ,A balance is maintained. And receiver will not receive unwanted signals.
• The twisting of the pairs of copper cable is one method used to protect the cable's signals from interference and crosstalk. This type of protection is known as cancellation.
Twisted pair cable
Shielded Unshielded
TWISTED PAIR CABLE
• Most common twisted pair cable used in communication referred as Unshielded twisted-pair.
• Shielded Twisted pair cable : Developed by IBM
Unshielded Twisted pair cable
• A common implementation of UTP is 100BaseT.• 100 Base T stands for…
Data Transmission Rate of 100Mbps, i.e. 100 Uses baseband transmission, i.e. Base The media is twisted pair, i.e. the T.
• Categories : EIA (Electronic industries association) has developed
standards to classify UTP cable into 7 categories.
Categories of unshielded twisted-pair cables
Category Bandwidth Data Rate Digital/Analog Use
1 very low < 100 kbps Analog Telephone
2 < 2 MHz 2 Mbps Analog/digital T-1 lines
3 16 MHz 10 Mbps Digital LANs
4 20 MHz 20 Mbps Digital LANs
5 100 MHz 100 Mbps Digital LANs
6 (draft) 200 MHz 200 Mbps Digital LANs
7 (draft) 600 MHz 600 Mbps Digital LANs
Category Grade Business Application Frequency Range
Category 1 voice grade voice-grade telephone networks only; not for data transmissions
750 kHz
Category 2 voice grade voice-grade telephone networks, as well as IBM dumb-terminal connections to mainframe computers
1 MHz
Category 3 data grade voice-grade telephone networks, 10Mbps Ethernet, 4Mbps Token Ring, 100BaseT4 Fast Ethernet, and 100VG Any LAN
16 MHz
Category 4 data grade 16Mbps Token Ring networks 20 MHz
Category 5 data grade 100BastTX Fast Ethernet, SONET, and OC-3 ATM networks
100 MHz
Category 5e data grade Gigabit (1000Mbps) Ethernet 100 MHz
Category 6 data grade Gigabit (1000Mbps) Ethernet 250 MHz
Category 6A data grade Gigabit (1000Mbps) and 10 Gigabit Ethernet 500 MHz
Connectors
• Most common conductor is RJ 45 . (Registered Jack).
Performance• Performance of cable is based on comparison of Loss of
Data(Attenuation) versus Frequency and Distance
STP (Shielded twisted Pair cable)
• STP is similar to UTP but with each pair covered by an additional copper braid jacket or foil wrapping.
• This shielding helps protect the signals on the cables from external interference.
• Improves the quality.• STP is more expensive.• Used in IBM Token ring.
Application
• Used in telephone line to provide voice and data channel.
• Local loop : Line that connects subscriber to the central telephone office commonly consists of UTP.
• LAN with 10 base t , 100base T , uses twisted pair cable.
Coaxial Cable• Coaxial Cable carries signals of higher frequency
ranges than those in twisted pair cables.
• Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair
• Inner conductor : Solid or slandered wire usually copper.• Insulator layer : Layer of plastic foam.• Outer metallic wrapping serves both as a shield against noise and as the second conductor which completes the circuit.• cable is protected by plastic cover
Coaxial Cable standards• Coaxial cables are categorized by their RG(Radio government ratings). • Each RG number is based on ..
Wire gauge of inner conductor. Thickness and type of inner insulator. The construction of shield. The size and type of outer casing.
Categories of coaxial cables
Category Impedance Use
RG-59 75 W Cable TV
RG-58 50 W Thin Ethernet
RG-11 50 W Thick Ethernet
Coaxial Cable connectors
• Most common type of connector used today is the Bayone –Neill-concelman(BNC),connector.
• 3 type of connector : The BNC connector .
Used to connect the end of the cable to device like TV Set. The BNC T Connector.
Used in Ethernet n/w to connect with computer. The BNC Terminator.
Used at end of cable to prevent the reflection of the signal.
Performance• Attenuation is much higher in coaxial cables than in twisted pair.• With higher bandwidth , signals becomes weak so repeater is needed
rapidly.
Application
• Widely used in Analog telephone n/w , single coaxial n/w could carry 10,000 voice signals.
• Used in cable TV n/w.• Cable TV uses RG-59 cable.
• Traditional Ethernet LAN
FIBER-OPTIC CABLE
• Transmission Media • Twisted Pair Cable • Coaxial Cable
• Fiber optics cable transmits signal in the form of Light
• Made up of Glass and Plastic.• LIGHT :
Moving through single uniform substance From one substance to another substance of different
density . Than Direction of light will be changed.
Refraction and Reflecion
• I = angle of Incidence means the angle the ray makes with the line perpendicular to the interface between two substance.
• Critical angle : Fixed angle with line.• Refraction : IF I <= Critical angle . • Reflection : IF I > Critical angle.• Reflection means make turn in same surface.• Optical Fibers uses reflection to guide light
through a channel
• A GLASS OR PLASTIC CORE IS SURROUNDED BY CLADDING OF LESS DENCE GLASS OR PLASTIC.
• Difference in density of the 2 materials must be such that a beam of light
moving through the core is reflected off the cladding instead of being refracted into it.
Propagation Modes
• Multi mode : Multiple beams from a light source move through the core in different path. • Move of beams depends on the structure of core.
• Step Index means : Suddenness change of beam leads to distortion of signal at passes through fiber.
• Multimode step index fiber : Density of the core remains constant from center to the edges.
• At the interface , there is an abrupt change due to lower density, This alters the angles of the beam’s motion.
Multimode graded-index fiber
• Decrease the distortion of the signal through the cable.
• Index of refraction is related with density. Density is highest at the center of the core and decrease to its lowest at the edge.
Single Mode
• Single mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles , all close to horizontal.
• Single mode fiber is manufactured with a much smaller diameter than that of multimode fiber with substantially lower density.
• Critical angle = 90 degree so propagation of beam almost horizontal.
Fiber Size• Size of optical fiber = ratio between diameter of
core and diameter of cladding
Type Core Cladding Mode
50/125 50 125 Multimode, graded-index
62.5/125 62.5 125 Multimode, graded-index
100/125 100 125 Multimode, graded-index
7/125 7 125 Single-mode
Fiber optic cable connectors• SC Connector.
Used for Cable TV • ST Connector.
Used for connecting cable to networking device. Reliable than SC
• MT-RJ Connector. Size is same as RJ45
Advantage and Disadvantage• Advantages
Higher bandwidth Less signal attenuation Immunity to electro magnetic interference Resistive to corrosive materials Light weight Greater immunity to tapping.
• Disadvantages Installation and maintenance Unidirectional light propagation cost
Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.
Radio WavesMicrowavesInfrared
Topics discussed in this section:
UNGUIDED MEDIA: WIRELESS
Electromagnetic spectrum for wireless communication
Propagation methods
Bands
Wireless transmission waves
Radio waves are used for multicast communications, such as radio and television, and paging systems. They can penetrate through walls. Highly regulated. Use Omni directional antennas
Note
Omni directional antenna
Microwaves are used for unicast communication such as cellular telephones, satellite networks,and wireless LANs. Higher frequency ranges cannot penetrate walls.Use directional antennas - point to point line of sight communications.
Note
Unidirectional antennas
Thank You….
