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7-Transmission Media
DR. JOHN ABRAHAM
UNIVERSITY OF TEXAS PANAM
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THEORETICAL BASIS
See previous lesson
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Transmission Media
Guided – exact path – physical media Unguided – radio transmission
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Taxonomy by forms of Energy
Electromagnetic, light, electrical Use my notes
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Background Radiation and electrical Noise
Random electromagnetic radiation (noise). When noise hits metal it produces a small
signal which interferes with data signal
Placing enough metal between noise and communication medium lowers interference.
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Twisted pair
Unshielded UTP
Shielded STP
Both wires in a twisted pair is affected equally in case of Noise giving a net difference of 0.
In parallel wires the one closer to the noise source will be affected more, giving a net difference.
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Twisted Pair Ten Base-T UTP Connector Type RJ-45 Cat 3 up to 16MHz Cat 4 up to 20MHz Cat 5 up to 100 MHz
More twists per cm and teflon insulation Cat 5e noise immunity 125 mbps Cat 6 – 200 mbps Cat 7 foil shielded around the entire set of wires. 600 Mbpps
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Twisted pair wiring scheme
1. Orange white2. Orange3. Green white4. blue5. Blue white6. Green7. Brown white8. Brown
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1 White/Orange White/Green 1
2 Orange Green 2
3 White/Green White/Orange 3
4 Blue White/Brown 4
5 White/Blue Brown 5
6 Green Orange 6
7 White/Brown Blue 7
8 Brown White/Blue 8
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Problems
cross talk and skin effect Picks up noise from other wires (distortion) Attenuation (reduction of signal strength)
influenced by distances and bit data being transmitted bandwidth of the cable
Twisting - noise will be picked up by both wires which reduce errors also reduces cross talk
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Hub is a multiport repeater (operates in Layer1 OSI)
Every transmission from one port is amplified and retransmitted on all other ports
The maximum length of any segment is 100 meters
UTP contd
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Using FDM coaxial can carry over 10,000 voice channels
When FDM is used CATV is called Broadband Good for both analog and digital signals Greater attenuation of signals, therefore amplifiers
and repeaters are used frequently. 50 ohm cable is for digital transmission 75 ohm for
TV
Coaxial cable
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Single line - multiple connections FDM - frequency division - most widespread
A number of signals can be carried simultaneously if each signal is modulated onto a different carrier frequency
Multiplexing
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Each modulated signal requires a certain bandwidth (called a channel)
Channels are separated by bands of unused portions
Broadcast TV - each channel requires 6Mhz. Coaxial cable has 500 MHz bandwidth Voice only requres 4 KHz
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Synchronous Time Division Multiplexing Possible when the available data rate of a
medium exceeds the data rate of a digital signal to be transmitted.
Interleave bits or blocks at a time
TDM
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Time Division Multiplexing The general alternative to FDM is
time division multiplexing (TDM)
In TDM sources share a medium by ``taking turns''
There are two types of TDM: Synchronous Time Division Multiplexing
(STDM)arranges for sources to proceed in a round-robin
manneralso known as Slotted Time Division Multiplexing
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Statistical Multiplexing
Works similar to STDM, but if a given source does not have data to send, the multiplexor skips that source
Most NW use a form of statistical multiplexing because computers do not all generate data at exactly the same rate
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Thick 10base5 up to 1650 feet -requires trnsceivers
High Bandwidth - use FDM or TDM Thin Coax 10base2 - up to 607 feet
T-connectors and 50 ohm resitors No more 30 devices on a segment No more than 5 segments in a single LAN Only 3 of these segments may have devices
(90) others are repeaters.
Coaxial cable contd
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Greater capacity 2 Gbps over tens of kilometers Repeaters needed only every 8 KM Smaller size & weight Materials used
Plastic (short haul) Glass Fused Silica (best)
Fiber Optics
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http://www.datacottage.com/nch/fibre.htm lower attenuation electromagnetic isolation Use LED or ILD
light emitting diode, Injection Laser diode Use Photodiode to detect
PIN photodiode APD photodiode
One - short pulse of light, Zero-absence of light
Fiber contd
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Core Cladding
optical properties differ between core and cladding. Cladding has lower refractive index
Jacket to protect against moisture, crushing, etc.
Fiber contd
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Light propagates from one end to another in one of the following ways mono mode (straight line)
the source is laser Most expensive
Multimode stepped index the source is LED Bounces of cladding
Multimode graded index The cladding refractive index increases as it moves away from the
core
Fiber contd
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Fiber Optic Networks
Taps are difficult Two types of interface
Passive interface one end has an LED or laser diode The other end has photodiode
Active interface incoming light is converted to electric signal Signal is generated to full strength Tap into the signal generator
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Wireless Transmission
Radio Microwave Infrared Laser light
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Telephone
We skip most of it
The phone system is divided into 2 partsOutside plant
Local loops and trunks
Inside plantswitches
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Switching1. Circuit SwitchingA physical connection (circuit) is established between the sender
and the receiver. (Similar to operator plugging in – old days)This path is called an end-to-end path.Time used to find the path adds enormous overhead. Entire
bandwidth is reserved even when there are gaps in communiction.
2. Message SwitchingNo physical path is established.The data is sent to the first switching office.Stored, forwarded later, one hop at a time.
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Switching continued.
This is known as store-and-forward network.The message could be typed and saved to punched tape and
forwarded later.There was no block size limit. Therefore, one sender can
monopolize the system.3. Packet SwitchingBlock size has upper limit.No one can monopolize more than a fraction of a sec.Suited for interactive traffic.Each router should have sufficient memory to store data.
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Switching continued
In packet switching each packet can be examined and format changed if need to be.
Packets may arrive at different order. Will have to be rearranged.
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Types of switches
Hierarchical
Crossbar
Space Division
Time Division
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ISDN
Narrow band
Broad band
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Narrow Band ISDN
Primary goal was integration of voice and data.
Telephones can be connected to a computer for statistics.
Voice and data can be sent concurrently
Caller ID
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ISDN System Architecture
ISDN is a bit pipe
All bits can flow in either direction.
Does not matter where the bit comes from – voice, computer, fax, etc.
Additional bit pipes can be added and combined.
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ISDN continued
For each ISDN service a terminating device is need at your location such as NT1.
Up to 8 devices can be added to an NT1, provided you have sufficient bandwidth (pipes).
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ISDN interface
Multiple channels can be interleaved by time division multiplexing.
The Channels that are standardized is given on page 142 of the text book.
Basic rate comes 2B and 1 DPrimary rate: 23B +1Hybrid: 1A +1C
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N-ISDN
Narrow ISDN focused on 64Kbps channels.
Directed at telephone customers
Big failure
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BROADBAND ISDN AND ATM
156 Mbps
This data rate is enough for HDTV
Developed based on ATM technology
ATM is a packet switching technology