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Layer 1: Physical LayerNetwork Media

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What did we do last week?◦ Reference Models and Standards OSI – 7 Layer Model TCP/IP Model

◦Essential building blocks of Computer Communications Sender/Receiver Message Rules Confirmation

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Communication begins with a message, or information Message are sent across a data network by breaking into

small chunks called packets. Communication uses a medium to transfer

information. Which layer of OSI model it refers to?

Message Encoder Transmitter

Decoder ReceiverMessage

Transmission Media

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Objectives◦Introduction to different cable types. Unshielded twisted-pair cable (UTP).

straight-through, crossover, and rollover cables

Fiber-optic cable Wireless

The essential service provided by the physical layer consists of an unstructured bit stream, which can be used by higher layers to provide the basis for higher layer communication services

Signals on the physical media are influenced by noise

Physical layer is guided Media and Unguided Media

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Guided Media Types

Mono mode

Coaxial Cable

Glass

Screened Twisted Pair

Copper

Fibre Optic

Multi mode

Twisted Pair

Shielded Twisted Pair

Unshielded Twisted Pair

Fibre optic cableTwisted Pair cable Coaxial cable

Physical Media

Unguided Media

( Wireless)

Distance over which signal can be transported without being amplified due to signal distortion and attenuation

Speed at which data transfer would happen Cost of media Construction and installation◦Heavy, flexible and installation

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Data rate ◦transferring data per unit of time.◦Data rate determines how long it will take to transfer a file. The larger the file, the longer it takes, as there is more information to transfer.

What is the basic unit of data/data transfer? Bit/bits per second

Bandwidth is the amount of information that can flow through a network connection in a given period of time.

Analogy (road traffic): Wider the road, more the number of lanes possible and more the number of cars, it can handle

Measured in bits per second (bps). e.g. 100 Kilo bits per second or

100Kbps Faster data rate =>quicker

information travel Which units are more commonly used

than others ?

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Roughly hierarchical◦ Divided into tiers◦ Tier-1 ISPs are also called

backbone providers, e.g., AT&T, Verizon, Sprint, Level 3, Qwest

An ISP runs (private) ◦ Points of Presence (PoP) where

its customers and other ISPs connect to it

ISPs also connect at (public) Internet Exchange Point (IXP)◦ public peering

http://en.wikipedia.org/wiki/List_of_Internet_exchange_points_by_size

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Fiber node: 500 - 1K homes Distribution hub: 20K - 40 K homes Regional headend: 200 K - 400 K homes

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Asymmetrical Digital Subscriber Loop (ADSL) Telephone company’s solution to “last mile problem”

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Light is used to carry data using◦ light / no light or ◦ increasing/decreasing

light intensity Electronic circuits are used

to convert light back to data

Light travels through the optical media by the way of total internal reflection.

Modulation scheme used is intensity modulation.

Two types of Fiber media :Multimode

Many pulses of light generated by LED travel at different angles

Single mode Carries light pulses along single pathUses Laser Light Source

Multimode Fiber can support less bandwidth than Singlemode Fiber.

Singlemode Fiber has a very small core and carry only one beam of light. It can support Gbps data rates over > 100 Km without using repeaters.

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Highest theoretical capacity per user: 1 Gbps◦ currently 5 to 30 Mbps

downstream, 2 to 5 Mbps upstream

Services◦ analog and digital video◦ video viewable with and

without set-top converter ◦ voice◦ interfaces with existing phone,

data, video cabling

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Twisted-pair is a type of cabling that is used for most modern Ethernet networks.

A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.There are two basic types,

shielded twisted-pair STP)

unshielded twisted-pair (UTP).

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Unshielded twisted-pair ◦ Consists of four pairs of colour-

coded wires that have been twisted together and then encased in a flexible plastic sheath.

Used for Local Area Networks (LAN)

IEEE is the standards body that classifies data cables according to their bandwidth◦ e.g., Category 5 (Cat5) cable

Other categories include Enhanced Category 5 (Cat5e) cable and Category 6 (Cat6), Category 6e (Cat6)

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Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires.◦ Each pair of wires is

wrapped in metallic foil.◦ The four pairs of wires are

wrapped in an overall metallic braid or foil.

Cat 3 UTP◦ Category 3 UTP is rated to carry data up to 10Mbit/s

Cat 5 UTP◦ Category 5 UTP is rated to carry Ethernet up to 100Mbit/s and ATM up

to 155Mbit/s Cat 5e UTP◦ Category 5e UTP is an enhanced version of Cat 5 UTP.◦ Rated to carry data up to 1000Mbit/s.◦ Standard cable for use with Ethernet 1000Base-T.◦ Can also be used to extend the distance of 100Base-TX cable runs up to

350 meters. Cat 6 and 7 UTP

◦ Emerging standards rated for 250 and 600 MHz respectively

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No physical wire Differences from wired link ….◦decreased signal strength: radio signal attenuates as it

propagates through matter (path loss)◦ interference from other sources: standardized wireless

network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere

◦multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

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Type Distance BW Features Installation comment

10BaseT (UTP)

(twist pair cable)

100m 10Mbps Easy to install

Cheap

RJ45 --- to hubs

Fiber – single

Multimode

2Km

1km

1G

10G

No noise Sc connector

Wireless 90 to 150 m

2 to 50Mbps

Allows mobility

Access point

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Five new things you have learnt in this session:◦ ?◦ ?◦ ?◦ ?◦ ?

Layer 2- Data Link Layer Media Access Protocols◦Ethernet◦ Token Ring◦FDDI

Layer 2 Devices

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