Physical Layer

• Data Encoding

• Transmission media

• Signals

• Bits to signal transformation Timing (bit rate) Synchronization

Data Encoding

• We represent information such as readable characters by coding them into sequences of zeros and ones

• There are a number of accepted code sets in use

• The most commonly used standards are ASCII, EBCDIC, and UNICODE

Physical Layer

Transmission Media (chapter 4)Copper mediaOptical fiberMicrowaveSatelliteRadio

Signals and encoding Signals, timing (chapter 5)Digital Transmission (chapter 5)Analog Transmission (chapter6)Multiplexing (chapter 6)

Copper Media

• Twisted pairs (UTP) Minimum 2 twisted, insulated copper wires Multiple twisted pairs may be required for

certain applications Each set (1, 2, or more pairs) form a link May be packaged in large bundles Used for telephone Digital data transmitted as digital signals or

analog signals

Copper media

Telephones connect to central office with copper pairs

Primary intra-building wiring media Cheapest in building distribution Limited distance for data Limited data rates Data rates of up to 1 gigabit/sec possible

depending on distance and quality of wire. Work ongoing for 10 gigabits/sec standard

Twisted Pair Categories

Level Application/bit rate

Category 1 voiceCategory 2 voice, data to 4 MbpsCategory 3 10 MbpsCategory 4 16 MbpsCategory 5 100 Mbps

Cat 5E 1000 Mbps with 4 pairsCategory 6 1000 Mbps with 4 pairsCategory 7 10 Gbps with 4 pairs

Distance for data limited to 100 meters

Twisted Pair

(a) Category 3 UTP.(b) Category 5 UTP.

Twisted Pair Problems

• Attenuation

• Near End Cross Talk (NEXT) Problem with unshielded twisted pairs No shielding of pairs Signals from one pair cause electrical

interference with signals of another pair in the cable

EIA 568 Standard

• Standards for UTP cabling

• Different versions EIA 568A EIA 568B

• Termination in special connectors RJ 11 (2 pairs) RJ 12 (3 pairs) RJ 45 (4 pairs)

EIA 568

                                                         

EIA 568A

Pin #

Ethernet 10BASE-T100BASE-

TX

EIA/TIA 568AEIA/TIA 568B or AT&T

258A

1 Transmit + White with green strip White with orange stripe

2 Transmit - Green with white stripe or solid green

Orange with white stripe or solid orange

3 Receive + White with orange stripe

White with green stripe

4 N/A Blue with white stripe or solid blue

Blue with white stripe or solid blue

5 N/A White with blue stripe White with blue stripe

6 Receive - Orange with white stripe or solid

orange

Green with white stripe or solid green

7 N/A White with brown strip or solid brown

White with brown stripe

8 N/A Brown with white stripe or solid

brown.

Brown with white stripe or solid brown

EIA 568 Cabling Rules

• Avoid running near power cables

• Do not make sharp bends in cable

• Do not tie too tightly or crimp

• Keep cabling away from devices that introduce noise (fluorescent lights)

• Do not staple

• Never run outside buildings

Shielded Twisted Pairs (STP)

• Similar to UTP except a metal shield is wrapped around each pair

• Used in some networking Technologies IBM Token Ring

• No longer in favor

STP

Coaxial Cable

• Two conductors One inner conductor Outer conduction shield

• Size of inner conductor may be as much as 50 time larger than conductor in a twisted pair cable

• Has been used for Long distance telephone and television Local Area Networks (LANs) CATV cable television

• Can achieve very high data rates• Less susceptible to interference and crosstalk• Long distance amplifiers needed at various distances

Coaxial Cable

Ethernet Cable

• Based on a 1973 paper by Robert Metcalfe at MIT

• He developed Ethernet about 1975 while at Xerox Corporation

• Originally based on a specific coaxial cable• Basically a ‘bus’ with multiple attachments• Very successful• IEEE develop a standard based on this called

IEEE 802.3

Optical fiber

• Thin cable of glass which transmits light• Not affected by electromagnetic interference• Now used for most long distance

communications Voice, video, data

• Data rates of 10 Gbps now common• 40 and 100 Gbps under development• Distances of 35-40 miles between repeaters

possible• Many fibers can be bundled into a single cable

Light Characteristics

angle of reflection

angle of refraction

If > second medium has higher optical density

If < first medium has higher optical density

Index of refraction = Cos()/Cos()

Light Characteristics• If IR < 1

• Second medium less optically dense• With a < some critical angle

NO refracted light ALL light reflected

• Optical fiber Core - very pure glass or plastic Cladding - surrounds core

- also glass or plastic - but optically less dense than core

Protective cover

Optical fiber

Optical fiberMultimode and Single Mode

• Multimode – multiple light beams at various angles

• Single light source • Various connector standards (ST, SC, MT-RJ)

Wave Division Multiplexing (WDM)

Each color (wave length) can carry same data rate as regular fiber

Up to 40 Gbps

Microwave

• Terrestrial – ‘tower to tower’

– ‘mountain to mountain’

• Line of site – 40-50 miles apart

• Alternative to coax for long distance

• Used for telephone, television, data

• High data rates possible

Microwave

Satellite

• Space station used as relay

• Uses different frequencies for transmit

receive

• Used for voice, television, data

• Broadcast mediumSingle transmission can be received by many

stations

Geosynchronous Satellite

• Remains in a fixed position relative to earth• Distance from earth to satellite?

22,236 miles or 35,785 Km• Delay characteristics?

How long does it take for a bit to go

from one host to another?

How long does it take send a message from

host A to host B and get a response?

Satellite

Radio

• Broadcast medium

• Usually not point to point like microwave

• Range can be 25 – 30 miles

• An predecessor of Ethernet was based on radio

University of Hawaii - ALOHA

Wireless LAN

• Operate on radio frequencies• A number of standards now in effect

802.11(b) 2.4 Ghz band11 Mbps

802.11(g) 2.4 Ghz band54 Mbps

802.11(a) 5 Ghz band54 Mbps

802.11(n) (draft) 2.4 & 5 Ghzup to 300 Mbps

• Many other devices use these frequencies

Other physical layer media

• Broadband over Power Lines

• Infrared

• Lasers

• Low Earth Orbit Satellites