程序代写CS代考 Week 2 – Physical Layer COMP90007 Internet Technologies – cscodehelp代写

Week 2 – Physical Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021
1

What is the Physical Layer ?
 Recall the layer hierarchy from network reference models
 In OSI model, the physical layer is the lowest layer
 In TCP/IP model, the physical layer’s properties are in the
“host-to-network” division.
 The physical layer is concerned with the electrical, timing and mechanical interfaces of the network  Electrical: voltage levels, signal strength …
 Timing: data rate …
 Mechanical: material, cable length …
2

Outline
 Timing aspect
 BandwidthandLatency
 Mechanicalaspect:transmissionmedia
 Twistedpair
 Co-axial
 Fibreoptics
 Wireless: EM waves, satellites  Electrical aspect
 Datacommunicationusingsignals  Digitalmodulation
 Capacity of a channel
 Maximum data rate  Multiplexing
3

Link Model
 We can abstract the physical channel as a link
 Simplified Link Model: Consider the network as a
connected link between computers
Browser
Server
HTTP
HTTP
Length = L metres
TCP
TCP
IP
IP
PHY
PHY
10111..
10111..
4

Link Model
 Bandwidth is usually treated as the rate of transmission in bits/second.
 Delay is the time required for the first bit to travel from computer A to computer B.
5

Example
 We need about 1 kbit/sec to transmit voice.
 Bandwidth of single mode fibre can reach
1 Tbit/sec.
 How many voice calls can be transmitted through a Fibre Optic Cable?
1012 / 103= 1 billion calls Tbit/s kbit/s
6

Message Latency
 Latency is the time delay associated with sending a message over a link
 This is made of up two parts
 Transmission delay
 T-delay = Message in bits / Rate of transmission  = M/R seconds
 Propagation delay
 P-delay= length of the channel/ speed of signals
 = Length / Speed of signal (2/3 of speed of light for wire)
 Latency = L = M/R + P-delay
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Example-1
 A home computer is connected to an ISP server through 56 K bps modem. Assuming a frame size of 5600 bits, compute P-Delay and T-Delay for the link. Assume speed of signal = 2/3 C and length of the link is 5 K metres.
 T-delay =
 P-delay =
 Latency = 100.025 m sec
5600 (bits)/ 56 000 (bps) = 100 m sec 5 (km)/200000 (km/s) = 0.025 m sec
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Example-2
 Now for the previous question, assume a countrywide optical broadband link of length 1000 kms of bandwidth 100 M bits/sec. Assuming a frame size of 5600 bits, compute P- Delay and T-Delay for the link. Assume speed of signal = C = 300000 km/sec.
 T-delay =
 P-delay = 1000 (km) /300000 (km/s) = 3.33 m sec
 Latency =
5600 (bits)/ 100 000 000 (bits/s) = 0.056 m sec
3.386 m sec
9

The Growth of Bandwidth
 CPU speeds increase by a factor of ~20 per decade
 1981: PC 4.77MHz vs. 2020: PC 4GHz
 Current CPU speed now approaching physical limits – constrained by physical properties pertaining to granularity of engraving on silicon
 Bandwidth increases by a factor of ~125 per decade
 1981: Modem 56kbps
 Current bandwidth available up to 65 Tbps – vastly exceeding the rate at which we can convert electrical impulses to optical pulses
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Outline
 Timing aspect
 Mechanical aspect: transmission media  Electrical aspect
 Capacity of a channel  Maximumdatarate
 Multiplexing
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Transmission Media
 How many different types of physical media can you think of?
 Wired: twisted pair, co-axial, fibre optics
 Wireless: electromagnetic waves and satellites
 Various physical media can be used to transmit data, but the performance is affected by physical properties.
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Signal Attenuation
 The loss or reduction in the amplitude (strength) of a signal as it passes through a medium.
 Signal attenuation impacts how far and how much data a medium can carry.
Image source: https://www.signalintegrityjournal.com/articles/1734-how-to-reduce-attenuation-in-a-differential-channel
13

Wires – Twisted Pair
 Two insulated copper wires, twisted in helical (DNA) form.
 Twisting reduces interference: canceling out electromagnetic
interference from external sources
 Distance up to 5km, repeaters can extend this distance
cable with four twisted pairs
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Properties and Types of Twisted Pair
 Bandwidth depends on distance, wire quality/density
 Cat 3 – 2 wires, 4 pairs in sheath, 16MHz
 Cat 5 – 2 wires, 4 pair in sheath, more twists = less interference, higher quality over longer distance, 100 MHz
 Cat8–2000MHz
Don’t worry about this unit for now, just higher value is better!
15

Coaxial Cable (Co-ax)
 Copper core with insulation, mesh, and sheath
 Better shielding than twisted pair = higher speeds
over greater distances
 Bandwidth approaches 1GHz
 Still widely used for cable TV/Internet
16

Fibre Optics
 Fibre has enormous bandwidth (THz) and tiny signal loss
 Data transmission over a fibre of glass
 Common for high rates and long distances
 e.g. backbone links between ISP facilities, Fibre- to-the-Home (FTTH)
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Transmission of Light Through Fibre
 3 components: light source, transmission medium, detector
 Semantics: light = 1, no light = 0 (basic binary system)
 Signalling using LED’s or semiconductor lasers
 A detector generates electrical pulse when light hits it
 Refraction between air/silica boundary is compensated for by design – total internal reflection
Light source (LED, laser)
Light trapped by total internal reflection
Photodetector
18

Fibre Optic Cables
Single-mode
 Narrow core (10um), light can’t even bounce around
 Used with lasers for long distances, e.g., 100km
Multi-mode
 50um core, light can bounce
 Used with LEDs for cheaper, shorter distance links
19

Fibre Optic Connections
 Connectors and Fibre Sockets (10-20% loss)  Mechanical Splice (10% loss)
 Fusion (<1% loss) Example: mechanical splice 20 Fibre Optic Networks  Fibre optic cable is a scalable network media - LAN, WAN, long distances  Fibre optic cable networks can be organised either as a ring or as a bus network (series of point-to-point connections) Fibre Optic Ring 21 Comparison: Wires and Fibre Comparison of the properties of wires (i.e. twisted pairs and co-ax cable) and fibre: Property Wires Fibre Distance Short (100s of m) Long (tens of km) Bandwidth Moderate Very High Security Easy to tap Hard to tap Cost Inexpensive More Expensive Convenience Easy to use Harder to use 22