Computer NetworksModule 2: Physical Layer

Dr. Vikram Shete

St. Francis Institute of Technology

Physical Layer and Tx. Medium

Medium Classification

Guided Media

• Guided media:– Provide a conduit from one device to another– Eg. twisted-pair cable, coaxial cable, and

fiber-optic cable

Guided Media (Twisted Pair Cable)

Effect of Noise (Parallel Lines)

Effect of Noise (Twisted Pair)

Commercial Unshielded Twisted Pair (UTP) Cable

UTP Cable

Shielded Twisted Pair (STP)

Coaxial Cable

Optical Fiber Cable

Optical Fiber Cable

Optical Fiber Cable

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

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

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

Optical Fiber Cable

Unguided Media

• Spectrum of wireless transmission waves

Unguided Media

• Propagation methods

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

Unguided Media

• Line of Sight Propagation:– Very high frequency signals transmitted in

straight lines directly from antenna to antenna

Bands

Unguided Media

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)

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

Unguided Media

Network Hardware Components

Repeaters, Hubs, Bridges, Routers and Gateways

Network Components

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

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

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

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

Bridges

3. It’s better to split labs to keep workload local.• Huge bandwidth required if a single LAN is used

Hub

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

Switches (Data Link Layer)

• Switches are similar to bridges– Switches connect

individual computers

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

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

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