UNIT-1

What is a network?

A network can be anything from a simple collection of computers (two connected computers qualify as a network) at one location that have been tied together using a particular connectivity medium (such as network cabling or wireless technology) to a giant global network, such as the Internet, that uses a number of different connectivity media, including microwave and satellite technology. The network can then be used to transmit data, voice, and even video between users on the network.

Local Area Network (LAN): A LAN supports fast, low−error data transfer on a physical network infrastructure that covers a small, limited geographic area, such as within a single building or on a single floor of a building.

Defining LANs, MANs, and WANs

Metropolitan Area Network (MAN): A MAN is a network that spans an area larger than a LAN but is less dispersed geographically than a WAN. A MAN network may connect several LANs on a single companys campus, or interconnect the LANs of several companies and businesses in one part of town,

Wide Area Networks (WAN): A WAN,, is a network that interconnects LANs and MANs across a broad geographic area

A local Area Network

A Wide Area Network

Users can share resources and communicate

File sharing. Hardware sharing (printers, CD-ROM drives,

and hard drives ) Program sharing User communication. Multiplayer gaming

Why Network Your Computers?

Change: changes made to one layer, the impact on the other layers is minimized.

Design: protocol designers can specialize in one area (layer) without worrying about how any new implementations affect other layers.

Learning: The layered approach reduces the complexity and makes learning ,understanding the actions of each layer and the model on the whole much easier.

Reasons why a layered−model is used

Troubleshooting: The protocols, actions, and data contained in each layer of the model relates only to the purpose of that layer. This enables troubleshooting efforts to be pinpointed on that layer.

Standards: Probably the most important reason for using a layered model is that it establishes a prescribed guideline for interoperability between the various vendors developing products that perform different data communications tasks.

OSI Reference model

The Open Systems Interconnection Reference Model, the OSI model was developed by the ISO (International Standards Organization) and released in 1984.

The OSI model, as it is called for short, defines the rules,mechanisms, formats, and protocols used to guide how data flows from one device to another.

1.All People Seem To Need Data Processing2. Please Do Not Throw Salami (or Sausage if you prefer) Pizza Away

The Physical layer of the OSI model defines the electrical and mechanical specifications used in networking,including transmission distances, the various types of media available, and electrical issues.

Physical Layers

The Data Link Layer

Physical addressing Network topology Error notification Access to the physical medium (a.k.a.

arbitration) Flow control

The Network Layer

• Message addressing

• Path determination between source and destination nodes on different networks

• Routing messages between networks

• Controlling congestion on the subnet

• Translating logical addresses into physical addresses

When the message (which moves down through the seven OSI layers on Johns computer before its sent out on the local network in binary form) arrives at Router 1, it moves up from the Physical layer to the Data Link layer to the Network layer. At Layer 3, its determined that the message is not on a network attached to Router 1 and the message is sent down through the Data Link layer to the Physical layer and on to Router 3.

Segment and assemble upper−layer applications

Transport segments from one host to another host

Establish and manage end−to−end operations

Error recovery

The Transport Layer

A session is a series of related connection−oriented transmissions between network nodes.

Session Layer, establishes, manages, and terminates sessions between applications.

The session layer provides a name space that is used to tie together the potentially different transport streams that are part of a single application.

Session layer is its role in deciding whether a communications session uses a simplex, half−duplex, or full−duplex transmission mode.

The Session Layer

Data encryption

Data compression

Data formatting

Data conversion

Presentation Layers

Application layer defines the communication services used by the users applications to transmit data over the network.

FTP (File Transfer Protocol) E−mail clients Web browsers Telnet SNMP (Simple Network Management Protocol) BBS (bulletin board system) servers EDI (Electronic Data Interchange) and other

transaction services

The Application Layer

A Quick Review of the OSI Model

OSI vs TCP/IP

OSI introduced concept of services, interface,

protocols. These were force-fitted to TCP later

⇒ It is not easy to replace protocols in TCP. In OSI, reference model was done before protocols.

In TCP, protocols were done before the model OSI: Standardize first, build later

TCP: Build first, standardize later OSI took too long to standardize.

TCP/IP was already in wide use by the time. OSI became too complex. TCP/IP is not general. Ad hoc.

OSI vs TCP Reference Models

TCP/IP Concepts

Network Attachment Point (NAP)

Transmission Media

Overview

Electromagnetic Spectrum

Transmission Media: Twisted Pair, Coax, fiber,wireless

Unshielded Twisted Pair (UTP) categories

Reflection and Refraction

Antennas: Isotropic, directional, omni-directional

Terrestrial and Satellite Microwave

Electromagnetic Spectrum

Guided: Twisted Pair Coaxial cable Optical fiber

Unguided: Microwave Satellite Wireless

Transmission Media

Twists decrease the cross-talk Neighboring pairs have different twist length Most of telephone and network wiring in

homes and offices is TP.

Twisted Pair (TP)

Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheap, Flexible Easiest to install No shielding Suffers from external EM interference Used in Telephone and Ethernet

Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy) Used in token rings

Unshielded and Shielded TP

Cat 3 Up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 Up to 20 MHz. Not used much in practice. Cat 5 Up to 100MHz Used in 10 Mbps and 100 Mbps Ethernet Twist length 0.6 cm to 0.85 cm Cat 5E (Enhanced), Cat 6, Cat 7

UTP Categories

Higher bandwidth than UTP. Up to 500 MHz. Used in cable TV

Coaxial Cable

Index of Refraction = Speed of light in Vacuum/Speed in glass

= 300 m/μs / 200 m/μs =1.5 Refracted light bends towards the higher index

medium

Reflection and Refraction

Attenuation and Dispersion

A cylindrical mirror is formed by the cladding The light wave propagate by continuous reflection in the

fiber Not affected by external interference =>low bit error rate Fiber is used in all long-haul or high-speed communication Infrared light is used in communication

Optical Fiber

Multimode Fiber: Core Diameter 50 or 62.5 mm Wide core => Several rays (mode) enter the

fiber Each mode travels a different distance Single Mode Fiber: 10-μm core. Lower

dispersion.

Types of Fibers I

Dispersion-Shifted Fiber: Zero dispersion at 1310nm

EDFAs/DWDM systems operate at 1550 nm

Special core profile zero dispersion at 1550 nm Dispersion Flattened Fiber: 3 ps/nm/km 1300-

1700nm

Use 1300 nm now and 1550 in future

Low dispersion causes four-wave mixing

DSF/DFF not used in multi-wavelength systems

Types of Fibers II

2GHz to 60GHz Terrestrial Microwave, Satellite Microwave Highly directional Point to point

30MHz to 1GHz Omni-directional Broadcast radio

3 x 1011 to 2 x 1014 Infrared Short distance

Wireless Transmission Frequencies

Transmitter converts electrical energy to electromagnetic

waves Receiver converts electromagnetic waves to electrical

energy Same antenna is used for transmission and reception Omni-Directional: Power radiated in all directions Directional: Most power in the desired direction Isotropic antenna: Radiates in all directions equally Antenna Gain = Power at particular point/Power with

Isotropic Expressed in dBi

Antenna

Parabolic Antenna

Used in Terrestrial microwaves Line of sight communication 10-60 GHz Higher frequencies for higher data rates

Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates

Terrestrial Microwave

Satellite Microwave

Relay station => Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Geo-stationary orbit: Height of 35,784km Point to Point or Direct broadcast satellite

Omni-directional FM radio, UHF and VHF television Line of sight Suffers from multi-path interference

(Reflections)

Broadcast Radio

Used in TV remote control IRD port of computers Modulate infrared light Line of sight (or reflection) Blocked by walls

Infrared

Ground wave: Follows contour of earth. Up to 2MHz. AM radio

Sky wave: Signal reflected (Actually refracted) from ionosphere layer of upper atmosphere.

Amateur radio, BBC world service, Voice of America

Line of sight: Above 30MHz. Density of atmosphere decreases with height. Results in radio waves bending towards earth

Wireless Propagation

Free space loss: Signal disperses with distance Greater for lower frequencies (longer wavelengths)

Atmospheric Absorption: Water vapour and oxygen

Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz Rain and fog scatter radio waves

Multipath: Signal can be reflected causing multiple copies to be received. May be no direct signal at all. May reinforce or cancel direct signal

Line of Sight Transmission

Unshielded twisted-pair (UTP) vs STP Single mode and multimode optical fiber Optical communication wavelengths Isotropic vs omni directional vs directional

antennas Parabolic antenna for microwave Ground wave, sky wave, line of sight

Summary