Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Chapter 2: Computer Networks
Local Area Networks
• • • • • •
• Communication infrastructure for a restricted geographical area (10 m up to some km)
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: DSL as “Last Mile” network
• Usually maintained by one local organization OSI Reference Model Application Layer Presentation Layer
• PCs/Workstations/...., are linked for exchange of information and for sharing peripherals and resources • Transmission capacity up to 1,000 Mbit/s • Transmission delay of a message in the range of milliseconds (~10 ms) • Simple connection structures (“Simple is beautiful”)
Session Layer Chapter 4: Application Protocols Transport Layer
Topologies • Bus
Chapter 3: Internet Protocols
Network Layer Data Link Layer
• Star
LAN
• Ring • Tree
Computer Networks
Physical Layer
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Chapter 2.1: Physical Layer
• Meshed network
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Metropolitan Area Network (MAN) • Designed for larger distances than a LAN, usage e.g. in a whole town • Similar technologies as in a LAN • In general, only 1 or 2 cables without additional components • Main difference to LANs: Time slots
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Chapter 2.1: Physical Layer
Wide Area Network (WAN) • • • •
Bridging of any distance Connects LANs and MANs over large distances Irregular topology, based on current needs Consists out of stations (routers) which are connected through point-to-point with each other • Mostly quite complex interconnection of subnetworks which are owned by independent organizations
MAN
WAN
Router Host
Chapter 2.1: Physical Layer
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LAN
Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Layer 1 – Physical Layer
Transmission Media Braided outer conductor
Connection parameters → mechanical → electric and electronic → functional and procedural
Interior insulation
Twisted Pair
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
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Several media, varying in transmission technology, capacity, and bit error rate (BER)
Protective outer insulation
Optical fiber Glass core
Glass cladding
Satellites
Plastic
Radio connections
Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Twisted Pair
Types of Twisted Pair Category 3 • Two insulated, twisted copper cables • Shared protective plastic covering for four twisted cable pairs
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
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 Used today mostly is Cat 5.
Shielding
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.
Copper core
Insulation
Chapter 2.1: Physical Layer
Coaxial cable
Copper conductor
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Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Coaxial Cable
Optical Fiber
Structure • Insulated copper cable as center conductor • Braided outer conductor reduces environmental disturbances • Interior insulation seperates center and outer conductor
Braided outer conductor Copper conductor
Interior insulation
Protective outer insulation
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
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
Early networks were build with coaxial cable, in the last ten years however it was more and more replaced by twisted pair.
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Optical Transmission
Optical Fiber
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)
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
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
Chapter 2.1: Physical Layer
Medium 2
Refractive index: Indicates refraction effect relatively to air
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optical source (LED, Laser)
Chapter 2.1: Physical Layer
Medium 1 (core)
Medium 2
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Problems with Optical Fiber
Types of Fiber The profile characterizes the fiber type: • X axis: Size of refractive index • Y axis: Thickness of core and cladding
• 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!
Glasfaser Optical Fiber kurzes, starkes Signal Electrical input signal
langes, schwaches Signal Electrical output signal
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
n2 n1
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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
r
Laser diodes • expensive and sensitive • high capacity • small wavelength spectrum and thus high range
r
Photon detector Photodiodes • differ in particular within signal-to-noise ratio
n2
Chapter 2.1: Physical Layer
Chapter 2.1: Physical Layer
r
Radiation Sources and Detectors
n2 n1
Multimode fiber with gradient index • Core diameter: 50 µm • Different used wavelengths • Refractive index changes continuously • Low dispersion
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
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Optical Fiber Types Simple multimode fiber • Core diameter: 50 µm • Different used wavelengths • Different signal delays • High dispersion
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.
n1
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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.
Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Encoding of Information
Baseband and Broadband
Shannon: „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 Encoding categories • Source encoding (Layer 6 and 7)
Encoding of the original message E.g. ASCII-Code (text), tiff (pictures), PCM (speech), MPEG (video),…
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). Broadband
• 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
Chapter 2.1: Physical Layer
The transmission of information can take place either on the baseband or on broadband. This means:
Physical representation of digital signals
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
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 this technology is used.
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Continuous vs. Discrete Transmission
Analogous Representation of Digital Signals
• On baseband, discrete (digital) signals are transmitted. • On broadband, continuous (analogous) signals are transmitted
The original signal is approximated by continuously considering higher frequencies.
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
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
∑[ n =1
(
)
)]
(
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.
Chapter 2.1: Physical Layer
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Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Cable Code: Requirements
NRZ: Non Return to Zero
How can digital signals be represented electrically?
Simple approach: • Encode „1“ as positive tension (+1V) • Encode „0“ as negative tension (- 1V)
• As high robustness against distortion as possible 1
1
0
Transmission 0
0
T
2T 3T 4T 5T 6T 7T
t
1
0
1
1
0
0
1
+1V
0 0
T
2T 3T 4T 5T 6T 7T
t
- 1V
• Efficiency: as high data transmission rates as possible by using code words binary code:
+5V/- 5V?
ternary code:
+5 V/0V/- 5V?
Advantage: • Very simple principle • The smaller the clock pulse period, the higher the data rate
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
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Chapter 2.1: Physical Layer
Disadvantage: • Loss of clock synchronization as well as direct current within long sequences of 0 or 1
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Differential NRZ
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 „1“ as tension level change of positive (+1V) to negative (-1V) • encode „0“ as tension level change of negative (- 1V) to positive (+1V)
Differential NRZ: similar principle to NRZ • Encode „1“ as tension level change • Encode „0“ as missing tension level change
0
1
0
1
1
0
0
1
1
+1V
+1V
- 1V
- 1V
0
1
0
0
1
1
0
Advantages • Clock synchronization with each bit, no direct current • End of the transmission easily recognizable Disadvantage • Capacity is used only half!
Very similar to NRZ, but disadvantages only hold for sequences of zeros.
Chapter 2.1: Physical Layer
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Chapter 2.1: Physical Layer
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Chapter 2.1: Physical Layer
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Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
Differential Manchester Code
4B/5B Code
Variant of the Manchester Code: • Similar as it is the case for the Manchester code, a tension level change is done in the bit center • Additionally, a second change is made: To encode „1“, stay on same level as before To encode „0“, do level change between two bits
0
1
0
1
1
0
0
1
+1V - 1V
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Chapter 2.1: Physical Layer
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme
4B/5B Code Table (used in FDDI)
Worst case: 11100|01110 3 Zeros
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
Chapter 2.1: Physical Layer
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 )
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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?
Chapter 2.1: Physical Layer
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