Layer 1 – Physical Layer Transmission Media Twisted Pair

Transcrição

Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Layer 1 – Physical Layer
Chapter 2: Computer Networks
• 2.1: Physical Layer: representation of
digital signals
• 2.2: Data Link Layer: error protection and
access control
• 2.3: Network infrastructure
• 2.4 – 2.5: Local Area Network examples
• 2.6: Wide Area Network examples
• 2.7 – 2.8: Wireless networks
OSI Reference Model
Application Layer
Presentation Layer
Session Layer
Chapter 4: Application Protocols
Transport Layer
Network Layer
Chapter 3: Internet Protocols
Data Link Layer
Computer Networks
Physical Layer
Page 1
Chapter 2.1: Physical Layer
More detailed:
→ Physical transmission medium (Copper cable, optical fiber, radio, ...)
→ Pin usage in network connectors
→ Representation of raw bits (Code, voltage, etc.)
→ Data rate
→ Control of bit flow:
• serial or parallel transmission of bits
• synchronous or asynchronous transmission
• simplex, half-duplex or full-duplex transmission mode
Page 2
Transmission Media
Twisted Pair
Braided outer conductor
Coaxial cable
Copper conductor
Interior insulation
Twisted Pair
Connection parameters
→ mechanical
→ electric and electronic
→ functional and procedural
Several media, varying in
transmission technology,
capacity, and bit error rate (BER)
Protective outer insulation
Optical fiber
Glass core
Glass cladding
Satellites
Plastic
Characteristics:
• Data transmission through electrical signals
• Problem: electromagnetic signals of the environment
can disturb the transmission within copper cables
• Solution: two insulated, twisted copper cables
• Twisting reduces electromagnetic interference with
environmental disturbances
• Simple principle (costs and maintenance)
• Well known (e.g. telephony)
• Can be used for digital as well as analogous signals
• Bit error rate ~ 10-5
Copper core
Radio connections
Insulation
Page 3
Page 4
Types of Twisted Pair
Coaxial Cable
Category 3
• Two insulated, twisted copper cables
• Shared protective plastic covering for four twisted cable pairs
Category
Category 5
• Similar to Cat 3, but more windings/cm
• Covering is made of Teflon (better insulation, resulting in better
signal quality on long distances)
Category 6,7
• Each cable pair is covered with an additional silver foil
UTP (Unshielded Twisted Pair)
• No additional shielding
STP (Shielded Twisted Pair)
• Each cable pair is shielded separately to avoid interferences
between the cable pairs
• Nevertheless, mostly UTP is used.
Braided outer conductor
Copper conductor
Interior insulation
Protective outer insulation
Characteristics:
• Higher data rates over larger distances than twisted pair: 1-2 GBit/sec up to 1 km
• Better shielding than for twisted pair, resulting in better signal quality
• Bit Error Rate ~ 10-9
Used today mostly is Cat 5.
Shielding
Structure
• Insulated copper cable as center
conductor
• Braided outer conductor reduces
environmental disturbances
• Interior insulation seperates center
and outer conductor
Page 5
Early networks were build with coaxial cable, in the last ten years however it was
more and more replaced by twisted pair.
Page 6
Optical Fiber
Optical Transmission
Characteristics:
• Nearly unlimited data rate (theoretically up to 50.000 GBit/s) over very large
distances
• Wavelength in the range of microns (determined by availability of light emitters
and attenuation of electromagnetic waves: range of the wavelength around
0.85µm, 1.3µm and 1.55µm are used)
• Insensitive to electromagnetic disturbances
• Good signal-to-noise-ratio
• Bit Error Rate: ~ 10-12
Structure of an optical transmission system
• Optical source (converts electrical into optical signals;
normally in the form „1 – light pulse “ ; „0 – no light pulse “)
• Communication medium (optical fiber)
• Detector (converts optical into electrical signals)
electrical signal
electrical signal
optical signal
optical fiber
optical source
optical detector
Physical principle: Total reflection of light at another medium
Medium 2
Medium 1
Page 7
Refractive index:
Indicates refraction
effect relatively to
air
Page 8
Optical Fiber
Problems with Optical Fiber
Structure of a fiber
• Core: optical glass (extremely thin)
• Internal glass cladding
• Protective plastic covering
• The transmission takes place in the core of the cable:
Core has higher refractive index, therefore the
light remains in the core
Ray of light is reflected instead of transiting from
medium 1 to medium 2
• Refractive index is material dependent
• A cable consists of many fibers
• The ray of light is increasingly weakened by the medium!
Absorption can weaken a ray of light gradually
Impurities in the medium can deflect individual rays
• Dispersion (less bad, but transmission range is limited)
Rays of light are spreading in the medium with different speed:
- Ways (modes) of the rays of light have different length (depending on the
angle of incidence)
- Rays have slightly different wavelengths (and propagation speed)
Refractive index in the medium is not constant (effect on speed)
Here only a better quality of radiation source and/or optical fiber helps!
Medium 2
optical source
(LED, Laser)
Medium 1 (core)
Glasfaser
Optical
Fiber
kurzes,
starkes
Signal
Electrical
input signal
Medium 2
Page 9
Page 10
Types of Fiber
The profile characterizes the fiber type:
• X axis: Size of refractive index
• Y axis: Thickness of core and cladding
langes,
schwaches
Signal
Electrical
output signal
Optical Fiber Types
Note: Single mode does not mean
that only one wave is
simultaneous on the way. It
means that all waves take „the
same way“. Thus dispersion is
prevented.
Simple multimode fiber
• Core diameter: 50 µm
• Different used wavelengths
• Different signal delays
• High dispersion
r
n2 n1
Single mode fiber
• Core diameter: 8 - 10 µm
• All rays can only take one way
• No dispersion (homogeneous
signal delay)
• Expensive due to the small core
diameter
r
n2 n1
Multimode fiber with gradient index
• Core diameter: 50 µm
• Different used wavelengths
• Refractive index changes continuously
• Low dispersion
r
n2
Page 11
n1
Page 12
Radiation Sources and Detectors
Encoding of Information
Shannon:
Radiation sources
Light emitting diodes (LED)
• cheap and reliable (e.g. regarding variations in temperature)
• broad wavelength spectrum, i.e. high dispersion and thus small range
• capacity is not very high
„The fundamental problem of communication consists of reproducing on one side
exactly or approximated a message selected on the other side.“
Objective: useful representation (encoding) of the information to be transmitted
Laser diodes
• expensive and sensitive
• high capacity
• small wavelength spectrum and thus high range
Encoding categories
• Source encoding
(Layer 6 and 7)
Photon detector
Photodiodes
• differ in particular within signal-to-noise ratio
Page 13
E.g. ASCII-Code (text), tiff (pictures), PCM
(speech), MPEG (video),…
• Channel encoding
(Layer 2 and 4)
Representation of the transmitted data in code
words, which are adapted to the characteristics
of the transmission channel (redundancy).
• Cable encoding
(Layer 1)
Protection against transmission errors through
error-detecting and/or -correcting codes
Through the usage of improved material properties of the fibers, more precise sources of light and thus
reduction of the distances between the utilizable frequency bands, the amount of available channels
constantly increases.
Encoding of the original message
Physical representation of digital signals
Page 14
Baseband and Broadband
Continuous vs. Discrete Transmission
The transmission of information can take place either on the baseband or on
broadband. This means:
• On baseband, discrete (digital) signals are transmitted.
• On broadband, continuous (analogous) signals are transmitted
Baseband
The digital information is transmitted over the medium as it is.
For this, encoding procedures are necessary, which specify the representation of
“0” resp. “1” (cable codes).
Signal theory: each periodical function (with period T) can be represented as a sum of
• weighted sinus functions and
∞
• weighted cosinus functions: s (t ) = a0 +
an cos 2πnFt + bn sin 2πnFt
∑[
n =1
(
)
)]
(
Broadband
The information is transmitted analogous (thereby: larger range), by modulating it
onto a carrier signal. By the use of different carrier signals (frequencies), several
information can be transferred at the same time.
While having some advantages in data communications, broadband networks are
rarely used – baseband networks are easier to realize. But in optical networks and
radio networks as well as for Cable TV this technology is used.
Page 15
F = 1/T is base frequency
Meaning: a series of digital signals can be interpreted as such a periodical function.
Using Fourier Analysis: split up the digital representation in a set of analogous
signals transported over the cable.
Page 16
Analogous Representation of Digital Signals
Cable Code: Requirements
The original signal is approximated
by continuously considering higher
frequencies.
How can digital signals be represented electrically?
• As high robustness against distortion as possible
1
But:
• Attenuation – weakening of the
signal
• Distortion – the signal is going out of
shape
Reasons:
• The higher frequencies are
attenuated more than lower
frequencies.
• Speed in the medium depends on
frequency
• Distortion from the environment
Page 17
1
Transmission
0
0
1
0
T
2T 3T 4T 5T 6T 7T
t
binary code:
+5V/- 5V?
ternary code:
+5 V/0V/- 5V?
quaternary code: 4 states (coding of 2 bits at the same time)
• Synchronization with the receiver, achieved by frequent changes of voltage
level regarding to a fixed cycle
• Avoiding direct current: positive and negative signals should alternatively arise
Page 18
Differential NRZ
Simple approach:
• Encode „1“ as positive tension (+5V)
• Encode „0“ as negative tension (- 5V)
0
0
t
NRZ: Non Return to Zero
1
2T 3T 4T 5T 6T 7T
• Efficiency: as high data transmission rates as possible by using code words
0
T
Differential NRZ: similar principle to NRZ
• Encode „1“ as tension level change
• Encode „0“ as missing tension level change
1
0
0
1
0
+5V
+5V
- 5V
- 5V
Advantage:
• Very simple principle
• The smaller the clock pulse period, the higher the data rate
1
0
1
1
0
0
1
Very similar to NRZ, but disadvantages only hold for sequences of zeros.
Disadvantage:
• Loss of clock synchronization as well as direct current within
long sequences of 0 or 1
Page 19
Page 20
Manchester Code
Differential Manchester Code
For automatic synchronization, with each code element the clock pulse is
transferred. Used is a tension level change in the middle of each bit:
• encode „0“ as tension level change of positive (+5V) to negative (-5V)
• encode „1“ as tension level change of negative (- 5V) to positive (+5V)
0
1
0
1
1
0
0
Variant of the Manchester Code. Similar as it is the case for the Manchester code,
a tension level change takes place in the bit center, additionally a second change
is made:
• Encode „1“ as missing level change between two bits
• Encode „0“ as level change between two bits
1
+5V
0
1
0
1
1
0
0
1
+5V
- 5V
- 5V
Advantages
• Clock synchronization with each bit, no direct current
• End of the transmission easily recognizable
Disadvantage
• Capacity is used only half!
Page 21
4B/5B Code
4B/5B Code Table (FDDI)
Disadvantage of the Manchester code:
→ 50% efficiency, i.e. 1B/2B Code (one bit is coded into two bits)
An improvement is given with the 4B/5B Code:
→ four bits are coded in five bits: 80% efficiency
Functionality:
→ Level change with 1, no level change with 0 (differential NRZ code)
→ Coding of hexadecimal characters: 0, 1,…, 9, A, B,…, F (4 bits)
in 5 bits, so that long zero blocks are avoided
→ Selection of the most favorable 16 of the possible 32 code words
(maximally 3 zeros in sequence)
→ Further 5 bit combinations for control information
→ Expandable to 1000B/1001B Codes?
Page 22
Worst case:
11100|01110
3 Zeros
Page 23
D e c im a l
D a ta
T ra n s m itte d
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
S y m b o l A s s ig n m e n t
Q u ie t -lin e s ta te
In v a lid
In v a lid
In v a lid
H a lt -lin e s ta te
In v a lid
In v a lid
R -R e s e t (lo g ic a l 0 )-c o n tro l
In v a lid
D a ta
D a ta
D a ta
In v a lid
T -E n d in g d e lim ite r
D a ta
D a ta
In v a lid
K -s ta rtin g d e lim ite r
D a ta
D a ta
D a ta
D a ta
D a ta
D a ta
J -s ta rtin g d e lim ite r
S - s e t (lo g ic a l 1 ) - c o n tro l
D a ta
D a ta
D a ta
D a ta
D a ta
Id le -lin e s ta te
(s ta tu s )
(s ta tu s )
(c o n tro l)
(c o n tro l)
(c o n tro l)
(c o n tro l)
(c o n tro l)
(s ta tu s )
Page 24
Modes of Operation:
Simplex, Half-duplex & Full-duplex
Sender
Sendeeinrichtung
Simplex
S
Receiver
Empfangseinrichtung
Leitung
Medium
• Transmission in only one direction
• Distribution of information (broadcast, television)
E
R
S
E
R
Half-duplex
“single-railed railroad line”
E
R
• Bi-directional operable transmission medium
• Transmission of the communication partners takes place mutually
• Communication partners must agree on who may send
S
Full-duplex
• simultaneous transmission in both directions
• realizable through:
two cables
a cable with two channels
simultaneous sending with filtering
S
E
R
E
R
S
Page 25

Layer 1 – Physical Layer Transmission Media Twisted Pair

Transcrição

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