cs-601 physical level

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CS 601

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Physical Level


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Overview of data (analog & digital), signal (analog

& digital), transmission (analog & digital) &transmission media (guided & non-guided)

TDM FDM WDM

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Circuit switching: time division & space divisionswitch, TDM bus

Telephone network


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Periodic Analog Signal

OneCycle

0

+Amplitude

Time

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Digital Binary Data and Signal

Signal

Data

Time

01000101100


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Analog Data Analog SignalTelephone

Digital Data Analog SignalModem

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Analog Signal Digital SignalCodec

Digital Data Digital SignalDigital

Transmitter


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Phase of Periodic Analog Signals

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900

2700

Signal A Signal B Signal C


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Bandwidth

The range of frequencies that a medium can pass without losing the powercontained in the signal is bandwidth. If the bandwidth of a medium does not

match the spectrum of a signal, some of the frequencies are lost. Forexample, the square wave signal has a spectrum that expands to infinityand no transmission media has such a bandwidth. So, passing a squarewave through any medium will always deform the signal. If we use a

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ransm ss on ne o an w o z e ween an , welose some frequency of our voice.

Voice Spectrum


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Unipolar

In a Unipolar coding scheme, only a positive or a negative voltage (but not both)represent the data. Unipolar coding is typically used in TTY (teletype) interfaces and PC

interfaces that are TTY compatible. Such coding requires a separate clock signal.

Drawback

DC componentLack of synchronization for a series of 0s and 1s.

Susceptible to noise corruption


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Polar

In a polar coding scheme, both positive and negative voltages are used. Forexample, a 1 may be denoted by 3V and a zero by +3V. Popular polar codingare:

RZ (Return to Zero)NRZ (Non-Return to Zero)ManchesterDifferential Manchester

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Before a network service can be shared, networked computers musthave a pathway to contact other computers. Computers use electronicvoltage or electromagnetic waves or light pulses to send signals. Thephysical path through which the above travel is called transmissionmedia. It can be classified as

Cable (Guided Media)Wireless (Unguided Media)

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Cable media are wires or fibers that conduct electricity or light

Cable media provides a conductor for the electrical signal whereaswireless media do not

Cable media is typically used in LANs

Wireless media is typically used in WANs


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Types of Cables(Guided)

Twisted Pair Cable Coaxial Cable Fiber Optic Cable

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Twisted Pair Cable:

Copper wire is used

Formed by two insulated copper wires thatare twisted each other

Multi le airs are combined within a

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common jacket

Following types are available:

Unshielded - Set of twisted pairs within asimple plastic encasement (UTP)

Shielded - Set of twisted pairs wrappedin a foil shielding (STP)


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UTP

Advantages

Relatively inexpensive Easy to install and manage

Unsuitable for >100MBPS speed High rate of attenuation

Disadvantages

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STP

Advantages Disadvantages

Higher bandwidth available than UTP Less sensitive to EMI than UTP

More expensive Harder to install

Unsuitable for more than 155MBPS


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Coaxial Cable:

Made of two conductors that share a common axis

The center is a insulated stiff solid copper wire

This is in turn surrounded by a metallic foil as a shield from EMI

Outer cover is usually made of non-conducting material such as

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,


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Coaxial Cable

Thinnet Thicknet

Diameter 0.25 inch Transmit signals up to 185 meter

without attenuation

Diameter 0.5 inch Transmit signals up to 500 meter

without attenuation

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More cost

Advantages of using Coaxial cable: Simple to install Less sensitive to EMI than TP cable

Disadvantages of using Coaxial cable: Moderately high attenuation Cabling is hard to manage and reconfigure Moderately susceptible to EMI

Generally not used for more than 10MBPS


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Fiber Optic Cable:

Made of a light conducting glass or plastic core

This is surrounded by more glass called cladding

The cladding is covered by a tough outer sheath

The core rovides the li ht ath and the claddin is

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provided to reflect light back into the core

Available in Single Mode and Multi Mode configuration


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Different rays of light bounce along the fiber at different angles.Consequently, different parts of the signal arrive at differenttimes at the destination, resulting in some amount of signaldispersion and distortion during transmission.

Multimode fiber suffers from attenuation.

For these reasons, early multimode fiber was limited to 500meters.

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Graded-index multimode reduces the problems and can functionup to 1000 meters.

Transmits a single beam of light through the cable. The light

reflects in only one pattern. Allows faster transmission andlonger distances - up to 100 kilometers.

Uses lasers (rather than LEDs) and are therefore more expensivethan multimode.


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An optical transmission system has three components

Transmission medium

Light source

Detector

The light source is either an LED (Light Emitting Diode) or a laser diode.

The detector is a photodiode, which generates an electrical pulse when lightfalls on it.

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Advantages of using Fiber Optic cable:

Supports very high bandwidth (many GBPS) Low attenuation

Immune to interference it will not corrode and is unaffected by most chemicals.

usually much smaller and lighter in weight than a wire or coaxialcable with similar information carrying capacity.

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Disadvantages of using Fiber Optic cable:

Relatively expensive cable and hardware Connections require high precision manufacturing andcomplex installation

Relatively complex to configure and install


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Wireless Connectivity:

Wireless media transmit and receive electromagneticsignals without an electrical or optical conductor

Various forms of electromagnetic waves are used to carrysignals

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Wireless Media

Radio Wave Micro Wave Infrared Light


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Radio wave:

Lies between 10KHz to 1GHz of the Electromagnetic spectrum

Advantages:

Stations can be stationer or mobile.

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Easier to set up and establish.

Disadvantages:

Require frequency licensing and approved equipment Highly susceptible to external interference and jamming Only low bandwidths are possible (


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Microwave: Can exist in two forms : Terrestrial Satellite

Terrestrial: Uses directional parabolic antennas that require unobstructed lineof sight

Advantage:Much more easily and cost effectively established than cablein inhospitable terrain or congested area

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Disadvantage:Complex installation and line of sight required

Satellite: Uses beamed line of sight between directional parabolic antennas

located on earth and geosynchronous orbiting satellite Advantage:

Can cover extremely large distances Disadvantage:

Extremely expensive setup, but end user cost may not be that high


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Infrared System:

Used LEDs to exchange data. Signals may be picked up by line ofsight receivers or after reflection from walls and ceilings

Susceptible to high intensity light and atmospheric conditions

Easy to install but signals can not penetrate opaque objects

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Disturbances

During Data Transmission Signals can be affected by

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Attenuation Crosstalk EMI


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Attenuation:

When Signal travels over Cables, theresistance of cable wire interferes with

signal transmission. Therefore, the signalbecomes weak and distorted. This distortionof Signal is called Attenuation.

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How to Overcome:

Regenerate or Amplify the Signal

For Digital Signal use REPEATERFor Analog Signal use Amplifier


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EMI (Electromagnetic Interference):

During transmission, Signals can getcorrupted due to Electromagnetic wavesemitted by adjacent cable. This phenomenon

is called Electromagnetic Interference.

How to Overcome:Use Fiber-Optic Cables

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Crosstalk:

Crosstalk occurs when signals of one cable

get mixed with signals of adjacent cable due toEMI.

How to Overcome:Use STP (Shielded Twisted Pair) cables


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Media Connectivity Hardware:

The interfaces and devices that are used to connect computingdevices and transmission media are called Media ConnectivityHardware.

Media Connectivity Can be classified as

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Internetwork Connectivity Hardware

Network connectivity hardwareconnect individual devices to a single network.e.g., connecting a computer or printer to the LAN

Internetwork connectivity hardwareconnect multiple independent networks together.e.g., two campus LANs or multiple cities


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Network Connectivity Hardware

Connectors Interface Card Modem

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Internetwork Connectivity Hardware

HUB SWITCH MULTIPLEXER ROUTER


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Transmission Media Connectors

Every medium has one or more physical connectors to whichvarious devices can be attached

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BNC - Coaxial CableRJ-45 - UTP Cable

SC/ST or MT-RJfor Fiber Optic Cable


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Network Interface Boards

Installed in a computer (typically) to connect it to the transmission mediathrough the transmission media connector

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Network InterfaceCard (NIC)

Connection between NICand transmission media

(UTP cable)Connection between PCSusing NIC & transmission

media (Co-axial cable)


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Modems

Convert your computers digital signal to analog for use withtelephone lines and vice-versa

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Modem


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HUB:

A Piece of network equipment that connects PCs together It passes traffic to the network regardless of the intended destination

Generate lot of unnecessary traffic into the Network May be used for very small Network

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IBM

Server PC Printer Mainframe

HUB


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Switch:

A Piece of network equipment that connects PCs together It passes traffic based on address information on each packet It learns which devices are connected to its ports & forward packets to

appropriate port only It allows simultaneous communications , improving Bandwidth Reduces the amount of unnecessary traffic Normally used in large Network

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IBM

Server PC Printer Mainframe

SWITCH


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Router:

A network equipment that connects two or more separate networks It passes traffic based on distance or cost It maintains a table of all available routes & then forward packets

through the best path Used for large network

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IP Address : 10.236.145.x IP Address : 10.236.147.x

Router Router


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Circuit Switching

Circuit Switching is a connection technique that directly connects the senderand the receiver in an unbroken path. With this type of switching technique,once a connection is established a dedicated path exists between both ends

unless the connection is terminated.

Circuit Switching network operates much the same way as in the telephonesystem. A complete path, end-to-end, must exist before communication can

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ta e p ace. e computer n t at ng t e ata trans er must as or a

connection to the destination. Once the connection has been initiated andcomplete to the destination device, the destination device must acknowledgethat it is ready and willing to carry on a transfer.


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Advantages

No congestion, because the communication channel (once

established) is dedicated. Virtually no channel access delay, because the channel is dedicated.

Disadvanta es

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Inefficient use of the communication channel, because the channel isnot used when the connected devices are not using it. Can be more expensive than other methods, because a dedicatedchannel is required for each connection. Possible long wait to establish a connection, during which no data canbe transmitted.


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Message Switching

With message switching there is no need to establish a dedicated path between

two stations. Rather, when a station sends a message, it appends a destinationaddress to the message. The message is then transmitted through the network,in its entirely, from node to node. Each node receives the entire message, storeit briefly, and then transmit the message to the next node. This type of network

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s some mes ca e a s ore-an - orwar ne wor .

A message-switching node is typically a general-purpose computer. The deviceneeds sufficient storage capacity to buffer the incoming messages, which maybe quite long. A delay is introduced using this type of scheme due to the timerequired to find the next node in the transmission path and to queue and send

the message.


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Advantages:

Line efficiency can be greater than circuit-switched system because more devicesare sharing network bandwidth. Traffic congestion can be reduced as message are temporarily stored-n-route.

Message priorities can be established. Low-priority message can be delayed sothat higher-priority messages can be forwarded first. One message can be sent to many destinations. Broadcast addresses can beappended to the message, specifying that at some convenient or logical place in the

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network, the message is retransmitted to more than one destination.

Disadvantages:

Message switching is not compatible with most real-time applications. Applicationswith a relatively higher rate of interaction may suffer from the store-and-forward

imposed delay. For example, message switching cannot be used for voiceconnections. Store-and-forward devices are often expensive, because they must have largedisks to hold potentially long messages.


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Characteristics

No requirement of establishing the Physical pathPackets handled in two waysAs Datagram in packet switchingAs Virtual circuits in Virtual Circuit Switching

Packet Switching

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Advantages

Resources can be shared

Limitations

Each packet has to carry entire address Ex. Internet (TCP/IP)


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A

D

B

C

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E


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Advantages and Disadvantages of Packet Switching

Advantages:

Packet switching is cost effective, because switching devices do not needmassive amounts of secondary storage. Packet switching offers improved delay characteristics, because there are nolong messages in the queue and because switches can be tuned for optimaldelivery given the maximum packet size.

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. Packet switching can maximize link efficiency by making optimal use of linkbandwidth.

Disadvantages:

Protocols for packet switching are typically more complex and can add someinitial costs in implementation. Packets are more easily lost (for reasons explored in later sectors) whentransmitted over non-dedicated paths, necessitating retransmission of somedata.


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Virtual Circuits

Virtual circuits involve the establishment of so-called local connections between thesender and the receiver. A logical connection is formed when a sender andreceiver exchange messages at the outset of a conversation. The messages allowthe sender and receiver to agree on conversational parameters such as maximummessage size, path to be taken and other variables necessary to establish andmaintain the conversation. Virtual circuits usually imply acknowledgements, flow

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, .is, they posses the capability to inform upper protocol layers if a transmissionproblem occurs. Virtual circuits may be temporarily (lasting through oneconversation) or permanent (lasting as long as the sending and receivingcomputers are up).

The primary difference between Datagram packet switching and virtual-circuitpacket switching is that virtual circuits entail establishing a logical connection.Datagram service is more flexible but inherently less reliable. Datagram servicecan also be faster, because hare is little administrative overhead.


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Virtual Circuit vs. datagram

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PSTN Structure

The PSTN includes the following distinct classes of components, each with itsown ownership, responsibilities, and functions:

Subscriber wiring and equipment Local phone company local loops and central offices (COs) Local phone company trunk lines and other switching offices Long-distance carrier facilities.

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How a Call is Made

The CO provides some services we take for granted, like dial tone and busy signal.The phoning process is as follows:

1. The calling phones receiver is lifted, closing the hook switch.

2. Current flows in the local loop, indicating the phone is off the hook.3. The CO outputs a dial tone to indicate that the phone is ready to dial.4. The phone number is dialed using signaling tones or pulses.5. The CO reads the digits and establishes a circuit that controls the number dialed.

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6. The dialed CO checks the state of the line being called. If the phone is off the

hook, the dialed CO sends a busy signal to the local CO. If the phone is on thehook, the dialed CO sends two unsynchronized ringing signals: 90-volt AC ringingpulses to the bell of the phone being called and audio tone ring pulses to thereceiver of the phone that is calling.

7. When the called phone goes off the hook, the CO stops the ringing signals and

completes the circuit between the phones. Voice communication usesmechanical circuit switching. For the duration of any call; a pair of wires is literallyconnected all the way from phone A to phone B. Such a connection is circuitswitched. You will most likely get a different set of connections each time you callthe same number.


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Broadband Systems

Broadband systems use analog signaling. Many different signals can occupythe cable. For example, one cable may support transmission of video, radio, andcomputer data.

On a typical broadband network, each network device transmits over 75-ohmcoaxial cable. Broadband networks span greater distances than baseband (up totens of kilometers).

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Attenuation and noise do not degrade the modulated analog signals of

broadband systems as much as they do the digital signals of baseband systems.

In some broadband networks, the transmit and receive channels use the samecarrier frequency, but the transmit channel is on one cable and the receivechannel is on another cable. This is called a dual-cable configuration.

In other broadband networks, the transmit and receive channels use the samecable, but the transmit channel is on one frequency and the receive channel is onanother frequency. This is called a split configuration.


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Multiplexing Definition and Uses

Multiplexing is uses to combine and transmit several low bandwidth channelsacross a single high-bandwidth channel. Demultiplexing separates out the inputchannels after transmission. The multiplexer and demultiplexer use the same rules(except that the demultiplexer applies them in reverse of the multiplexer) for

combining and separating signals on the transmission line. A piece of equipmentthat multiplexes and demultiplexes is sometimes called a mux.

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Frequency-Division Multiplexing (FDM)

FDM uses separate analog carrier signals to establish multiple channels across themedia. FDM can utilize any modulation system (ASK, FSK, PSK, or combinations)for any channel.

Each channel centers on a different carrier frequency. For example, a CATV coaxcable with a bandwidth of about 500 MHz can carry more than 80 televisionchannels (of 6-MHz bandwidth each). Further multiplexed within each 6-MHz band

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are the channels own audio subcarrier, video subcarrier, and color subcarrier - plus

a guardband on each side separating it from other FDM channels on the cable.These channels function almost like separate wires or circuits.

Some telephone trunks use FDM to split the cables bandwidth into many 4-kHzvoice channels and other higher-bandwidth channels.

FDM is also used in broadband LANs to separate different direction traffic on thecable and to provide special services like dedicated connections betweenmachines.


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Time-Division Multiplexing (TDM)

TDM systems interleaved pieces of slower channels into a single faster channel,and then reconstitute the slower channels at the other end. The data can bebits, blocks of bits, bytes, or large blocks.

Time division is the only multiplexing technique that can be used on a basebandline. It could be used on an individual channel in an FDM system.

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.are sometimes called synchronous TDM because the pieces of each inputchannel are always the same length and come at the same time in each frameon the multiplexed channel. The muxes synchronize, and in each frame on themultiplexed channel. The muxes synchronize, and all transfer is fixed. Manydifferent synchronous TDM systems are in use.

Conventional TDM wastes bandwidth if many of the time slots are unused.Statistical TDM multiplexers (usually called stat muxes) solve this problem bydynamically allocating time slots to active devices. A control field identifies slotowners.


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cs-601 physical level

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