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Topic7-PhysicalLayer.pptxITECH1102 Networking and Security
Topic 7 – The Physical Layer
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Last week
Last week: The Transport Layer
Segmentation/Reassembly
Individual identification of applications (Port numbers – Well known port numbers for server, others for clients)
Transport Layer Services: - UDP (unreliable service) - TCP (reliable transport service)
TCP connections (establishment and termination) (3 way handshake - SYN, SYN/ACK, ACK)
Flow control
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This week
The Physical layer is the connection between computers and/or other devices on the network.
This week we will investigate:
Analog and Digital data
Data flow
Multiplexing
Manchester encoding (in Ethernet)
Transmission Media
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Two types of data
There are two distinct types of data of concern.
Digital data
Analog data
Digital computers store information in digital format. Text files, programs, songs, videos etc etc. are all stored in binary on associated digital media such as hard drives, USB devices etc.
The main characteristic of digital information is that it is entirely made up of binary ones and zeros. Any bit of data is either a one or zero, nothing in between.
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Analog data
Analog information, for instance voice data can take on an infinite number of levels. The diagram below shows Analog voice data that was captured using a program called Audacity.
To transmit analog data over digital networks (for instance for VoIP, streaming audio and other applications) the analog signal can be digitized into digital format and transmitted digitally.
In such cases the higher the sampling rate the better the quality.
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Digital and Analog transmission media
Like data; transmission media is either digital or analog.
The cables we use to connect computers was developed for the transmission of digital data.
Radio devices like WiFi Access points and Telephone system Microwave dishes are analog by nature.
By suitably modulating analog transmissions, digital data can be sent over analog transmission media.
the presence of an analog signal could signify a 1 bit.
the absence of the signal a zero bit.
As such we are sending digital data over analog transmission lines.
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Point-to-point and Multipoint circuits
Point to point circuits are a direct connection between two devices. Such media is not shared with any other device.
Multipoint circuits share the media with multiple devices. Ethernet systems that use hubs & WiFi fall into this category because they share the media with other devices.
In shared systems there needs to be protocols in place to ensure multiple devices can get access to the media.
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Data Flow: Simplex, Half duplex and Full duplex systems.
Systems can also be categorised according to the way data flows between devices.
Simplex system can only send data in one direction.
Half-duplex system allow data flow in both directions. But only in one direction at any one time.
Full duplex allows data flow in both directions concurrently.
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Multiplexing
Multiplexing is used to divide up a communications channel so the channel can transmit multiple conversations at the one time.
Some multiplexing methods are:
Time division Multiplexing (TDM)
Frequency division multiplexing (FDM)
Wavelength division Multiplexing (WDM)
In Time division Multiplexing different stations get to transmit at different times. Station 1 gets the first timeslot, station 2 the second timeslot etc. etc. Significant bandwidth can be wasted with TDM.
A variation called Statistical TDM can allocate more access to devices that require it. This reduces waste in the channel bandwidth.
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Multiplexing (continued)
Frequency Division Multiplexing
Different channels transmit on a different frequency. AM radio is a good example of FDM.
FDM was also used in the old analog telephone systems so that multiple voice conversations could be sent over a single telephone wire.
Wavelength Division Multiplexing
This type of multiplexing is used in Fibre optic cables. Instead of one light beam being transmitted down the fibre, multiple colours of light (different wavelengths) are sent at the same time, each carrying different data.
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Digital transmission of Data
Digital transmissions occur between a sender and a receiver. This could be for instance two Network Cards on an Ethernet network.
To ensure the receiver understands the data sent from the sender the Ethernet protocol mandates voltage levels, cable lengths, how the receiver identifies the start and end of transmissions and many other properties.
The encoding of the data bits is one such property.
Encoding can be described as the way in which binary zero bits and binary one bits are represented on the network media.
Ethernet uses an encoding method called Manchester encoding. This method is self clocking and so has advantages over some other methods.
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How Manchester Encoding works.
Binary values are represented by a transition in the middle of a bit period. A positive transition represents a binary 1, a negative transition represents a binary zero.
Because there is always a transition in the middle of each bit period, the sender and receiver have no problems staying is synchronization.
Positive Transition is a Binary 1
Negative transition is a Binary 0.
The signal is positive half the time and negative the other half, so no net DC component.
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Transmission Media
There are many types of transmission media used in networked and communication systems, including:
Unshielded twisted pair
Shielded twisted pair
Coaxial cable
Optical Fibre
Terrestrial Microwave
Satellite Microwave
Infrared
Twisted Pair
Consists of two insulated copper wires arranged in a regular spiral pattern
Separately insulated
A pair of wires acts as a single communication link
Often a number of pairs are bundled together into a cable
Commonly installed in a building when built
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By far the most common guided transmission medium for both analog and digital signals is twisted pair. It is the most commonly used medium in the telephone network (linking residential telephones to the local telephone exchange, or office phones to a PBX), and for communications within buildings (for LANs running at 10-100Mbps). Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with.
A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.
Twisted pair is the most common medium. Common uses:
Telephone network
Between house and local exchange
Within buildings
To private branch exchange (PBX)
For local area networks (LAN)
10Mbps, 100Mbps or 1000Mbps
Twisted Pair - Applications
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Unshielded vs Shielded TP
Unshielded Twisted Pair (UTP)
Unshielded Twisted Pair cabling has many different uses.
Cat 1 - Telephone systems (cheap and easy to install)
Cat 2, 3, 4 - Token ring networking
Cat 5, Cat 6 & Cat 7 - Used for networking (higher categories higher speed)
UTP does suffers from external EM interference
Shielded Twisted Pair (STP)
metal braid or sheathing that reduces interference
higher data rate
more expensive
harder to handle (thick, heavy)
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Twisted pair comes in two varieties: unshielded and shielded.
Unshielded twisted pair (UTP) is ordinary telephone wire. Office buildings, by universal practice, are prewired with excess unshielded twisted pair, more than is needed for simple telephone support. This is the least expensive of all the transmission media commonly used for local area networks and is easy to work with and easy to install. However UTP is subject to external electromagnetic interference, including interference from nearby twisted pair and from noise generated in the environment.
A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference. This shielded twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more difficult to work with than unshielded twisted pair.
Twisted Pair cables come in a variety of categories. Originally most office buildings were prewired with a type of 100-ohm twisted pair cable commonly referred to as voice grade. Because voice-grade twisted pair was already installed, it was an attractive alternative for use as a LAN medium, and this use was recognised by the Electronic Industries Association standard EIA-568, Commercial Building Telecommunications Cabling Standard, published in 1991. Unfortunately, the data rates and distances achievable with voice-grade twisted pair are limited. As users migrated to higher-performance workstations and applications, there was increasing interest in providing LANs that could operate up to 100 Mbps over inexpensive cable.
Cheap
Easy to work with
Low data rate
Short range
Higher attenuation
Twisted Pair - Pros and Cons
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Coaxial Cable
Consists of two conductors, inner and outer
Outer conductor is braided shield
Inner conductor is solid metal
Separated by insulating material
Usual diameter 1-2.5 cm
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Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor (see the figure). The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. A single coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.
Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including:
• Television distribution - aerial to TV & CATV systems
• Long-distance telephone transmission - traditionally used for inter-exchange links, now being replaced by optical fiber/microwave/satellite
• Short-run computer system links
• Local area networks
Coaxial Cable - Transmission Characteristics
Superior frequency characteristics to Twisted Pair but performance is limited by attenuation & noise
Analog signals
Requires amplifiers every few km or closer if carrying higher
frequencies.
Digital signals
repeater every 1km or closer for higher data rates
Co-axial cables were common in early Ethernet systems. 10Base2 (very common) and 10Base 5.
10Base2 had a maximum cable length of 185 metres @ 10Mbps.
10Base5 had a maximum cable length of 500 metres @ 10Mbps.
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Coaxial cable is used to transmit both analog and digital signals. It has frequency characteristics that are superior to those of twisted pair and can hence be used effectively at higher frequencies and data rates. Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise, and intermodulation noise. The latter is present only when several channels (FDM) or frequency bands are in use on the cable.
For long-distance transmission of analog signals, amplifiers are needed every few kilometers, with closer spacing required if higher frequencies are used. The usable spectrum for analog signaling extends to about 500 MHz. For digital signaling, repeaters are needed every kilometer or so, with closer spacing needed for higher data rates.
Optical Fiber
Consists of three parts: core, cladding, jacket
Glass or plastic core
Laser or light emitting diode
Specially designed jacket
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An optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers. An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket (see above figure). The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8 to 50 µm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core and a diameter of 125 µm. The interface between the core and cladding acts as a reflector to confine light that would otherwise escape the core. The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing, and other environmental dangers.
Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in military applications is growing. The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local area networking. Five basic categories of application have become important for optical fiber: Long-haul trunks, Metropolitan trunks, Rural exchange trunks, Subscriber loops & Local area networks.
Optical Fiber - Benefits
Greater capacity
data rates of hundreds of Gbps over tens of kilometers
Smaller size & weight
lighter than UTP and cable
considerable advantage in laying
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
10s of km at least
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The following characteristics distinguish optical fiber from twisted pair or coaxial cable:
Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to 100 Mbps to 10 Gbps over a few tens of meters for twisted pair.
Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or bundled twisted-pair cable. For cramped conduits in buildings and underground along public rights-of-way, the advantage of small size is considerable. The corresponding reduction in weight reduces structural support requirements.
Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair, and is constant over a wide range.
Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do not radiate energy, so there is little interference with other equipment and there is a high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap.
Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. Repeater spacing in the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers have been demonstrated.
Between main switches in LAN’s
Long-haul trunks
Metropolitan trunks
Rural exchange trunks
Subscriber loops
NBN initiation in Australia – fibre to home
Optical Fibre Applications
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Optical Fiber - Transmission Characteristics
Uses total internal reflection to transmit light
effectively acts as wave guide for 1014 to 1015 Hz
Can use several different light sources
Light Emitting Diode (LED)
cheaper, wider operating temp range, lasts longer
Injection Laser Diode (ILD)
more efficient, has greater data rate
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Optical fiber transmits a signal-encoded beam of light by means of total internal reflection. Total internal reflection can occur in any transparent medium that has a higher index of refraction than the surrounding medium. In effect, the optical fiber acts as a waveguide for frequencies in the range of about 1014 to 1015 Hertz; this covers portions of the infrared and visible spectra.
Two different types of light source are used in fiber optic systems: the light-emitting diode (LED) and the injection laser diode (ILD). Both are semiconductor devices that emit a beam of light when a voltage is applied. The LED is less costly, operates over a greater temperature range, and has a longer operational life. The ILD, which operates on the laser principle, is more efficient and can sustain greater data rates.
There is a relationship among the wavelength employed, the type of transmission, and the achievable data rate. Both single mode and multimode can support several different wavelengths of light and can employ laser or LED light sources.
Optical Fiber Transmission Modes
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The above figure shows the principle of optical fiber transmission. Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material. This form of propagation is called step-index multimode, referring to the variety of angles that will reflect. With multimode transmission, multiple propagation paths exist, each with a different path length and hence time to traverse the fiber. This causes signal elements (light pulses) to spread out in time, which limits the rate at which data can be accurately received. This type of fiber is best suited for transmission over very short distances.
When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass: the axial ray. This single-mode propagation provides superior performance for the following reason. Because there is a single transmission path with single-mode transmission, the distortion found in multimode cannot occur. Single-mode is typically used for long-distance applications, including telephone and cable television.
Finally, by varying the index of refraction of the core, a third type of transmission, known as graded-index multimode, is possible. The higher refractive index (discussed subsequently) at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Rather than zig-zagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and higher speed allows light at the periphery to arrive at a receiver at about the same time as the straight rays in the core axis. Graded-index fibers are often used in local area networks.
Optical Fiber - Transmission Characteristics
LAN – Local Area Network
WDM - Wavelength Division Multiplexing
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In optical fiber, based on the attenuation characteristics of the medium and on properties of light sources and receivers, four transmission windows are appropriate, as shown in the above table. Note the tremendous bandwidths available. For the four windows, the respective bandwidths are 33 THz, 12 THz, 4 THz, and 7 THz. This is several orders of magnitude greater than the bandwidth available in the radio-frequency spectrum.
The four transmission windows are in the infrared portion of the frequency spectrum, below the visible-light portion, which is 400 to 700 nm. The loss is lower at higher wavelengths, allowing greater data rates over longer distances. Many local applications today use 850-nm LED light sources. Although this combination is relatively inexpensive, it is generally limited to data rates under 100 Mbps and distances of a few kilometers. To achieve higher data rates and longer distances, a 1300-nm LED or laser source is needed. The highest data rates and longest distances require 1500-nm laser sources. nb. WDM = wavelength division multiplexing.
Terrestrial Microwave
Used for long haul telecommunications and short point-to-point links
It is an alternative to coaxial cable/fiber, such as in mountainous areas where laying cable/fiber is difficult
It requires fewer repeaters but needs line of sight visibility
It uses a parabolic dish to focus a narrow beam onto a receiver antenna
It uses 1-40GHz frequencies (higher frequencies give higher data rates)
The main source of loss is attenuation
distance, rainfall
Terrestrial Microwave also suffers from interference
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The primary use for terrestrial microwave systems is in long haul telecommunications service, as an alternative to coaxial cable or optical fiber. The microwave facility requires far fewer amplifiers or repeaters than coaxial cable over the same distance, (typically every 10-100 km) but requires line-of-sight transmission. Microwave is commonly used for both voice and television transmission. Another increasingly common use of microwave is for short point-to-point links between buildings, for closed-circuit TV or as a data link between local area networks.
The most common type of microwave antenna is the parabolic "dish”, fixed rigidly to focus a narrow beam on a receiving antenna A typical size is about 3 m in diameter. Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and to be able to transmit over intervening obstacles. To achieve long-distance transmission, a series of microwave relay towers is used, and point-to-point microwave links are strung together over the desired distance.
Microwave transmission covers a substantial portion of the electromagnetic spectrum, typically in the range 1 to 40 GHz, with 4-6GHz and now 11GHz bands the most common. The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate. As with any transmission system, a main source of loss is attenuation, related to the square of distance. The effects of rainfall become especially noticeable above 10 GHz. Another source of impairment is interference.
Satellite Microwave
The satellite acts a the relay station
The satellite receives on one frequency, amplifies or repeats the signal and transmits on another frequency
eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz
use of different frequencies for up- & down-link reduces interference
Typically requires geo-stationary orbit
satellite & earth move at speed, so stationary to each other
height of 35,784km
spaced at least 3-4° apart
Typical uses
Television, long distance telephone private business networks; global positioning
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A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground-based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called transponder channels, or simply transponders. The optimum frequency range for satellite transmission is in the range 1 to 10 GHz. Most satellites providing point-to-point service today use a frequency bandwidth in the range 5.925 to 6.425 GHz for transmission from earth to satellite (uplink) and a bandwidth in the range 3.7 to 4.2 GHz for transmission from satellite to earth (downlink). This combination is referred to as the 4/6-GHz band, but has become saturated. So the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GHz).
For a communication satellite to function effectively, it is generally required that it remain stationary with respect to its position over the earth to be within the line of sight of its earth stations at all times. To remain stationary, the satellite must have a period of rotation equal to the earth's period of rotation, which occurs at a height of 35,863 km at the equator. Two satellites using the same frequency band, if close enough together, will interfere with each other. To avoid this, current standards require a 4° spacing in the 4/6-GHz band and a 3° spacing at 12/14 GHz. Thus the number of possible satellites is quite limited.
Among the most important applications for satellites are: Television distribution, Long-distance telephone transmission, Private business networks, and Global positioning.
Satellite Point to Point Link
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The above figure depicts in a general way two common configurations for satellite communication. In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas.
Satellite Broadcast Link
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The above figure depicts in a general way two common configurations for satellite communication. In the second, the satellite provides communications between one ground-based transmitter and a number of ground-based receivers.
Infrared
modulate noncoherent infrared light
end line of sight (or reflection)
are blocked by walls
no licenses required
typical uses
TV remote control
IRD port
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Infrared communications is achieved using transmitters/receivers (transceivers) that modulate noncoherent infrared light. Transceivers must be within the line of sight of each other either directly or via reflection from a light-colored surface such as the ceiling of a room.
One important difference between infrared and microwave transmission is that the former does not penetrate walls. Thus the security and interference problems encountered in microwave systems are not present. Furthermore, there is no frequency allocation issue with infrared, because no licensing is required.
Online resource:
Watch the video at to see characteristics of various media : https://www.youtube.com/watch?v=_6SSiNIzfGc
Readings
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Next Topic
The Cloud & Mobile networking:
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Commonwealth of Australia Copyright Act 1968
Notice for paragraph 135ZXA (a) of the Copyright Act 1968
Warning
This material has been reproduced and communicated to you by or on behalf of Federation University Australia under Part VB of the Copyright Act 1968 (the Act).
The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act.
Do not remove this notice.
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Frequency Range (THz) |
Band Label |
Fiber Type |
Application |
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820 to 900 |
366 to 333 |
|
Multimode |
LAN |
|
1280 to 1350 |
234 to 222 |
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Various |
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196 to 192 |
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