physical layer and transmission media

8/20/2019 Physical Layer and Transmission Media

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Session 05


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Wired Media

The wired network is the most chosen way to implementnetworks inside buildings and in cooperate institutions.

Since it has the physical link between two locations via acable.

It is often preferred to connect networks in wired mediaunless the terrain of wiring is difficult.

The wired medium still has the best bandwidth (rate thatdata can travel) for sending data which is another plus

factor. There are several Network technology standards in

connecting wires in physical layer, the mostly usedstandard in Ethernet.


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Let us look at most commonly used media for

transmitting network data.

Coaxial Cables

Coaxial cabling has a single copper conductor at its

center.

A plastic layer provides insulation between the centerconductor and a braided metal shield.

The metal shield helps to block any outside interferences.


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10Base2 – Thin Ethernet

This is the IEEE standard for Ethernet cablingwith a data rate of 10 megabits per second

(Mbps) over RG-58 coaxial cable.

Initially denoted a maximum segment (segment

being the length of cable between signalrepeaters) length of 200 meters.

They later discovered that signal degradation

was too severe past 185 meters and changed the

"rules".

This cable type is commonly referred to term:

Thinnet.


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Being an Ethernet standard, Thinnet was used in

networks in typical Ethernet star and bus configurations. It supported longer segments and high bandwidth,

making it a good choice for, say, connecting floors in a

building.

It is a bit unwieldy, and these days it’s incredibly slow

speeds, because of that completely replaced.


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10Base5 – Thick Ethernet

10Base5, also appropriately known as Thicknet is havingvery hard cable.

Probably the only upside this old cable had was its ability

to transmit data 500 meters without needing a signal

repeater.

Other than that, its large size, weight, and hideous visage

(as well as the difficulty of "splitting" it) made it an

unpopular choice that administrators were very pleased

to replace as soon as possible.

Network topologies with this cabling were interesting.

Since breaking up its segment lengths.


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Instead, to add a segment to an existing line

anywhere along the cable (ie: if a

workstation needed to be added near an

existing Thicknet line), administrators would instead use "vampire"

connectors, which had barbs to puncture the

Thicknet in order to "tap in" to it


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Coaxial Cable Connectors

The most common type of connector used with coaxialcables is the Bayone-Neill-Concelman (BNC) connector.

Different types of adapters are available for BNC

connectors, including a T-connector, barrel connector,

and terminator.


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Thin Ethernet (10base2), sometimes also called

"Cheapernet", is based on using a coax-cable, which isspecified as RG58 cable (we call it 50ohm cable) and which

runs from system to system.

Coaxial cables are not used much in today’s computer

networks but it has a good immunity to externalinterferences.

The cable is connected via BNC-T-connectors to the network

card installed in the PC


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It is NOT allowed to put in any extension/cable between

the T-connector and the BNC-connector on the networkcard !

If you do that, your network will either NOT work or

becomes unreliable, working slow or sometimes failing !


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Each connection to an Thin-Ethernet RG-58 cable is called

a "node", which can be a system like a PC, a UNIX-

workstation or a Fileserver.

Anything else connecting via a BNC-connector also counts

as a node (network printers, repeaters,....).


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Limitations:

maximum 30 nodes on one Thin-Ethernet segment.

minimum 0.5 meter distance between nodes. maximum total cable-length of 185 meters.

If more than 30 nodes need to be connected or if the

total cable-length needs to be longer than 185 meters, a

repeater is the solution.


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Twisted Pair Cabling

Twisted pair cabling is the mostly preferred way ofEthernet cabling today.

There are two kinds of cabling. UTP – Unshielded Twisted

pair and STP – shielded twisted pair.


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The quality of UTP may vary from telephone-grade wire

to extremely high-speed cable. The cable has four pairs of wires inside the jacket.

Each pair is twisted with a different number of twists per

inch to help eliminate interference from adjacent pairs

and other electrical devices.


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The tighter the twisting, the higher the supported

transmission rate and the greater the cost per foot.


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The EIA/TIA (Electronic Industry Association

/Telecommunication Industry Association) has establishedstandards of UTP and rated six categories of wire (additional

categories are emerging).


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POTS – Plain old Telephone system

Traditional Telephone system over two pair copper alsobeing used to communicate the data of networks when

Wide Area Networks are used.

E1 and T1 Network standards that are used by many

telecommunications operators who are operating intraditional copper cabling.


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Fiber – Fiber Optics

Fiber optic cabling consists of a center glass coresurrounded by several layers of protective materials.

It transmits light rather than electronic signals eliminating

the problem of electrical interference.

This makes it ideal for certain environments that contain

a large amount of electrical interference.

Fiber optic cable has the ability to transmit signals over

much longer distances than coaxial and twisted pair.

It also has the capability to carry information at vastly

greater speeds.


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This capacity broadens communication possibilities to include

services such as video conferencing.

The cost of fiber optic cabling is higher comparable to coppercabling.

The center core of fiber cables is made from glass or plastic

fibers. A plastic coating then cushions the fiber center, and kevlar

fibers help to strengthen the cables and prevent breakage.

The outer insulating jacket made of teflon or PVC.


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There are two kinds of fibers which are called indoor fiber

which is more manageable and Outdoor fiber which ismeant for harder use as the name stands for.


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Power Line – Data transmission

through power lines

Almost every house/office has

electricity now.

The electrical wiring that is

used to distribute electricity is

can be reutilized for

transferring data.

There are several networking

devices are in current market

which are designed to workwith existing power lines

forming a local area network

without laying additional wires.


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As a summary we can summarize the specifications of

network cabling according to following table.


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Wireless Media

Mostly the wireless means sending data through radiofrequency signals.

Have to select a frequency that is un-congested or not

used by other services such as radio / television

broadcasts.

There are several frequencies that cannot be used due to

interference of equipment and the climate interference.

Some frequencies are susceptible to interference of

climate such as storms and lightening.

Some frequencies are susceptible to interference from

lights and electronic devices.


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The security is another issue that a person can intercept

the radio signals and decode and obtain the data which istransmitted if those are not scrambled or encrypted.

Various Wireless networking technologies are evolved

and in use today. For the indoor and outdoor wireless the

WIFI standards which operates in 2.4 Ghz and 5 Ghzrange.


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The mobile operators are using the

GSM frequencies to deliver theWireless internet using 3G and

3.5G Technology.

In addition there is another

technology evolved to do the small

range personal networking using

wireless that is called Bluetooth

technology.


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Though the data rate is bit low comparing with wired

technologies.wireless networks are preferred because of

Mobility - With a laptop computer or mobile device,

access can be available throughout a school, at the mall,

on an airplane, etc. More and more businesses are alsooffering free WiFi access.

Fast setup - If your computer has a wireless adapter,

locating a wireless network can be as simple as clicking

"Connect to a Network" -- in some cases, you will connectautomatically to networks within range.


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Cost - Setting up a wireless network can be much morecost effective than buying and installing cables.

Expandability - Adding new computers to a wirelessnetwork is as easy as turning the computer on (as long asyou do not exceed the maximum number of devices).

There are several disadvantages in Wireless Networkingtoo.

Security - Wireless networks are much more susceptibleto unauthorized use. If you set up a wireless network, besure to include maximum security. You should always

enable WEP (Wired Equivalent Privacy) or WPA (Wi-FiProtected Access), which will improve security and helpto prevent virtual intruders and freeloaders.


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Interference - Because wireless networks use radio signals

and similar techniques for transmission, they are susceptible

to interference from lights and electronic devices.

Inconsistent connections - How many times have you hears"Wait a minute, I just lost my connection?" Because of the

interference caused by electrical devices and/or items

blocking the path of transmission, wireless connections are

not nearly as stable as those through a dedicated cable.


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Power consumption - The wireless transmitter in a laptop

requires a significant amount of power; therefore, thebattery life of laptops can be adversely impacted. If you

are planning a laptop project in your classroom with

wireless connectivity, be sure to have power plugs and/or

additional batteries available.

Speed - The transmission speed of wireless networks is

improving; however, faster options (such as gigabit

Ethernet) are available via cables.

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