Computer Communication & Networks

Lecture # 07Physical Layer: Transmission Media

Course Instructor:Engr. Sana Ziafat

Physical Layer Topics to CoverSignals

Digital Transmission

Analog Transmission

Multiplexing

Transmission Media

Transmission Media

Anything that carry information from source to destination.

Physical path between transmitter and receiver in data communication .

Media IssuesFrequency range

Some media support higher frequencies than others

Impairments

• Different media deform signals differently

• Some are more susceptible to noise and distortion

Cost

• We’re in the real world…

Number of receivers

• Broadcast vs. point-to-point

Transmission Medium and Physical Layer

Types of transmission media

Transmission media is divided in to two: Wired or Wireless

Wired Media (Guided Media) , Is most common and is further divided in to three different types of cabling: Coaxial, Twisted pairs and fiber optic cables.

Wireless Media (Unguided Media), which is , in a sense, no media at all, is gaining popularity. Wireless transmission use radio waves or infrared light to transmit data

Guided Media

Waves are guided along solid medium. Guided media is also known as bounded

media, since the data signals are bounded system.

Cabling technology is not limited to copper wire only. Cables can be any physical or conductive media like wires, coaxial cables or fiber optics.

Twisted-pair Cable

Categories of unshielded twisted-pair cables

Twisted Pair Cable

(a) Category 3 UTP (16MHz)(b) Category 5 UTP (100MHz)

UTP connector

Types of Twisted Pair Cable

Two types: Shielded twisted pair Unshielded twisted pair (most commonly

used)

Twisted Pair Cables (Example) ADSL

Ethernet networks

- 10BASE-T

- 100BASE-TX

- 1000BASE-T

- 1000BASE-TX (Cat5e (enhanced))

UTP Performance

Twisted Pair Cable (Pros & Cons)pro: easy to understand mass production - low cost most widely used medium

Cons: prone to electromagnetic interference

in power plants, airport buildings, military facilities, cars…

Note:In-building networks at our university are almost all

twisted pair

Coaxial Cable

It carries high frequency signals than in twisted pair cable.

Less susceptible to interference or crosstalk.

Coaxial cable

BNC connectors

Performance Coaxial Cable

Coaxial cable has much higher bandwidth, the signals weakens rapidly and requires the frequent use of repeaters.

Applications

Cable- TV Long distance telephone transmission.

Optical Fiber

Made of glass or plastic and transmits signal in form of light.

Signal is sent using internal reflection. Relies on total internal reflection

Light waves bounce of edge of fiber Channels waves to destination

A glass or plastic core is surrounded by a cladding of less dense glass or plastic.

Bending of light ray

Optical fiber

Propagation Modes

Multimode

Multiple signals, multiple rays will pass and reflected with different angles

Light waves bounce off at different angles. Two types:

1. Step Index

2. Graded Index

Modes

Fiber Types

Defined by the ratio of diameter of their core to the diameter of their cladding.

Fiber types

Fiber construction

Fiber-optic Cable Connectors

Performance Optical Fiber

Optical Fiber (Pros & Cons)

Pros: Low attenuation Large bandwidth

Cons: Relatively “new” technology “Expensive”

Comparing optical fiber to UTPPros:

Immune to electro-magnetic interference no crosstalk

Reduced need for error detection and correction Enables longer link distances Attenuation unaffected by transmission rate Easier network upgrade Can combine different services: telephony, TV, internet… Lighter than copper cables Corrosion resistant

Cons: Optical components have higher cost Expensive deploying protocols

Unguided Media: Wireless

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.

Wireless

Modern wireless digital communication began in the Hawaiian Islands

What is “the best” frequency to use for communication?

Propagation Methods

Bands

Wireless Transmission Waves

Radio Waves

Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves.

They are omnidirectional (sends signal in all directions).

Radio waves are used for multicast communications, such as radio and television, and paging systems.

Omni directional Antenna

Microwaves

Having range from 1 to 300 GHz. They are unidirectional. Microwave propagation is line-of-sight. Very high frequency microwaves can not

penetrate walls. Microwaves are used for unicast communication

such as cellular telephones, satellite networks,and wireless LANs.

Unidirectional Antennas

Infrared waves

Range from frequencies 300 GHz to 400 THz.

Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.

Readings

Chapter 7 (B.A Forouzan) Section 7.1, 7.2

Q & A