Computer Networks Module 2: Physical Layer Dr. Vikram Shete St. Francis Institute of Technology.
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Transcript of Computer Networks Module 2: Physical Layer Dr. Vikram Shete St. Francis Institute of Technology.
![Page 1: Computer Networks Module 2: Physical Layer Dr. Vikram Shete St. Francis Institute of Technology.](https://reader030.fdocuments.in/reader030/viewer/2022020208/56649e425503460f94b35778/html5/thumbnails/1.jpg)
Computer NetworksModule 2: Physical Layer
Dr. Vikram Shete
St. Francis Institute of Technology
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Physical Layer and Tx. Medium
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Medium Classification
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Guided Media
• Guided media:– Provide a conduit from one device to another– Eg. twisted-pair cable, coaxial cable, and
fiber-optic cable
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Guided Media (Twisted Pair Cable)
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Effect of Noise (Parallel Lines)
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Effect of Noise (Twisted Pair)
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Commercial Unshielded Twisted Pair (UTP) Cable
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UTP Cable
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Shielded Twisted Pair (STP)
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Coaxial Cable
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Optical Fiber Cable
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Optical Fiber Cable
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Optical Fiber Cable
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Optical Fiber Cable
• Multimode– Multiple beams from light source can travel
through the core in different paths– Step index fiber:
• Core density is constant from center to edges• At the edge there is a abrupt change due to lower
density• Step index is derived from this sudden change and
causes distortion
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Optical Fiber Cable
• Multimode– Graded index fiber
• Core density is highest at the center and gradually decreases toward the edges
– Single mode (SM)• Uses step index fiber• A highly focused source limits beams to small
range of angles close to the horizontal• Diameter of SM fiber is very small with a
substantially lower density
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Optical Fiber Cable
– Single mode (SM)• Results in a critical angle close to 90o resulting in
horizontal beam propagation• Propagation of beams is almost identical and
hence delays are negligible• All beams arrive at the same time
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Optical Fiber Cable
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Unguided Media
• Spectrum of wireless transmission waves
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Unguided Media
• Propagation methods
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Unguided Media
• Ground Propagation:– Radio waves travel through the lowest portion
of the atmosphere – Waves emanate from antenna in all direction
and follow earths curvature
• Sky Propagation:– Waves travel to the atmosphere and are
reflected back.– Greater distance at lower power
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Unguided Media
• Line of Sight Propagation:– Very high frequency signals transmitted in
straight lines directly from antenna to antenna
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Bands
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Unguided Media
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Unguided Media
• Radio Waves– 3kHz - 1GHz– Omnidirectional
• Rx and Tx do not have to be aligned
– Susceptible to interference– Radio waves in sky mode good
candidate for longer distance (broadcasting - AM)
– Radio waves and low frequencies can penetrate walls (its an advantage and disadvantage)
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Unguided Media
• Microwaves– 1GHz - 300 GHz– Unidirectional in nature and can be narrowly
focussed– Rx and Tx have to be aligned– Line of sight propagation
• Curvature of earth and other blockages can interfere with communication
– Cannot penetrate walls– Band is relatively wide and hence wider sub-
bands and hence higher data rates
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Unguided Media
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Network Hardware Components
Repeaters, Hubs, Bridges, Routers and Gateways
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Network Components
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Repeaters (Physical Layer)
• Repeaters appear at the Physical layer– Analog devices connected to 2 cable
segments
– Signal is amplified and forwarded
– Do not read/understand frames, packets or headers
– Eg: Classic ethernet allows 4 repeaters to extend length from 500 m to 2500 m
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Hubs (Physical Layer)
• Hubs has many inputs. All are connected electrically– Frames arriving on one are sent on
all– May result in collision– A hub forms a single collision domain– Hubs do not amplify– All input lines must operate at the
same speed– Do not read/examine packets or
frames
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Bridges (Data Link Layer)
• Bridges connect two or more LANs– Extracts information from the frame – Identifies destination– Uses look-up tables and forwards to
appropriate ports– Each line of a bridge is its own
collision domain
• Discard if destination and source LANs are same
• Forward if dest. and src LANs are different• If dest. LAN unknown, use flooding
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Bridges
• Bridges are used to split large LANs
• These operate at the data link layer
• Reasons to have bridges1. Goals of departments are different and yet
interaction may be required
2. Cheaper to have individual LANs rather than one huge geographically spread network
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Bridges
3. It’s better to split labs to keep workload local.• Huge bandwidth required if a single LAN is used
Hub
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Bridges
– Total physical distance can be covered• Insert bridges to split LANs instead of using ethernet
cables to its limit (2.5 km)
– “What happens in Vegas stays in Vegas” (Reliability)
• Bridges can isolate LANs• A single berserk node will not bring down whole of
the network
– Admins can keep add security by inserting bridges at critical points.
• Bridges can be programmed to use discretion
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Switches (Data Link Layer)
• Switches are similar to bridges– Switches connect
individual computers
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Switches (Data Link Layer)
• Each switch port is connected to single computer
• Switches have space for more line cards than bridges
• Each line card provides a buffer for frames• Each port is its own collision domain and so
switches never lose frames• However switches may run out of buffer
space due to which frames might be lost
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Switches (Data Link Layer)
• This is due to higher data rates at arriving port
• To alleviate this modern switches use “cut-through approach”– Frames forwarded as soon as header
received
• All modern switches and bridges have similar features making the difference a more of marketing gimmick
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Routers (Network Layer)
• Routers isolate broadcast and collision domains– Strip off frame headers– Peek into the packet
headers– Concerned with logical
addressing – Do not care about the LAN
from which a frame arrives