Post on 05-Jan-2016
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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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