Transcript www.computing.northampton.ac.uk

Layer 1: Physical Layer
Network 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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Message
Encoder
Message
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Transmitter
Decoder
Transmission
Media
Receiver
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?
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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
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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
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Signals on the physical media are influenced by noise
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Physical layer is guided Media and Unguided Media
Physical Media
Guided
Media
Types
Copper
Twisted Pair
Unshielded
Twisted Pair
Shielded
Twisted
Pair
Twisted Pair cable
Coaxial Cable
Screened
Twisted Pair
Coaxial cable
Unguided Media
Glass
( Wireless)
Fibre Optic
Mono mode
Multi mode
Fibre optic cable
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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
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◦ 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
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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
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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
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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
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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 path
Uses 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 colourcoded 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.
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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) 100m
(twist pair
cable)
10Mbps
Easy to
install
Cheap
RJ45 --- to hubs
Fiber – single
2Km
1G
No noise
Sc connector
Multimode
1km
10G
Wireless
90 to 150
m
2 to
Allows
50Mbps mobility
Access point
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Five new things you have learnt in this session:
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Layer 2- Data Link Layer
 Media Access Protocols
◦ Ethernet
◦ Token Ring
◦ FDDI
 Layer 2 Devices
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