Week 2 – Physical Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021
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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 …
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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
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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..
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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.
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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
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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
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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
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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!
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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
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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
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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
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Fibre Optic Connections
Connectors and Fibre Sockets (10-20% loss) Mechanical Splice (10% loss)
Fusion (<1% loss)
Example: mechanical splice
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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
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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
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