ECEN 5032 Data Networks Physical Layer Peter Mathys - - PowerPoint PPT Presentation

ECEN 5032 Data Networks

Physical Layer

Peter Mathys

mathys@colorado.edu

University of Colorado, Boulder

Data Networks, Physical Layer, c

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Maximum Data Rate

Nyquist’s result (1924): The maximum data rate of a noiseless signal with

• levels and bandwidth

is maximum data rate

• bits/sec

Shannon’s result (1948): Channel Coding Theorem. For a given noisy channel there exists a code that will permit error-free transmission at rate

, provided

, where

is the channel capacity.

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Binary Symmetric Channel

Transition probability

• (probability of error).
• ✂

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Channel Capacity of BSC

The capacity

• f the binary symmetric channel (BSC)

is

• ✁

• ✄

bits/use

• ✄

• ✆✞✝

• ✂

• ✄

• ✄

where

• is the probability of error and

• ✄

is the binary entropy function for

• .

Examples:

• ✂

bits/use.

• ✂

bits/use.

• ✂

bits/use.

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BSC Capacity

−3 −2.5 −2 −1.5 −1 −0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Transition Probability log

10 p

Capacity C [bits/use] Capacity C = 1 − H(p) of Binary Symmetric Channel p=0.1, C=0.531 p=0.01, C=0.919 p=0.001, C=0.989

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Gaussian Channel Capacity

Shannon, 1948: The capacity

• f a noisy channel

with signal-to-noise ratio

• ✁

and bandwidth

in Hz is given by

• ✁

bits/sec

Example: Telephone channel has

• ✁

dB (

• ✁

) and

• Hz. Thus

bits/sec

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Gaussian Channel Capacity

−10 10 20 30 40 50 2 4 6 8 10 12 14 16 18 S/N [dB] C/W [bits/sec/Hz] Capacity C/W = log2(1+S/N) of Gaussian Channel

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Guided Transmission Media

Magnetic media. Twisted pair wire. UTP (unshielded twisted pair): CAT 3 (16 MHz BW), CAT 5 (100 MHz BW), CAT 6 (250 HMz BW), CAT 7 (600 MHz BW). Coaxial cable. 50-ohm, 75-ohm. Bandwidth

GHz. Step-index multimode (“multimode”) optical fiber. 62.5/125

• m (core/cladding) or 50/125
• m are

common (

m at 1 Gb/s, using 1300 nm LED). Graded-index multimode (“laser optimized”) optical

• fiber. 50/125
• m (

m at 10 Gb/s, using 850 nm laser). Single-index monomode (“single-mode”) optical fiber. 8.3

• m core (up to 40 km at 10 Gb/s, using 1550 nm

laser).

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Optical Fiber Types

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Optical Fibers

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Why Different Fibers?

The costs associated with single-mode systems are about 25–30% higher than multimode systems

• perated at the same speed.

Long wavelength (1310 nm, 1550 nm) electronics are more expensive. Single mode fiber requires significantly tighter alignment tolerances to couple light into its tiny core, necessitating transceivers and connector using high precision mechanics.

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Optical Fiber Types

Attenuation Fiber Type

• m

MHz

km MHz

km dB/km dB/km 850 nm 1300 nm 850 nm 1300 nm Multimode Standard 62.5/125

• m

62.5 200 500 3.5 1.0 Standard 50/125

• m

50 500 500 3.5 1.5 Laser-Opt 50/125

• m

50 1500 500 3.5 1.5 Attenuation Fiber Type

• m

MHz

km MHz

km dB/km dB/km 850 nm 1300 nm 1310 nm 1550 nm Single-Mode Standard SMF 8.2 n/a n/a 0.4 0.3

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Optical Fiber IR Attenuation

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Ethernet Wiring

Standard Year Rate Topo Medium Max Len 10Base5 1983 10 Mb/s Bus 50

• Thick Coax

500 m 10Base2 1985 10 Mb/s Bus 50

• Thin Coax

185 m 10BaseT 1990 10 Mb/s Star 100

• Cat.3 UTP

100 m 100BaseTX 1995 100 Mb/s Star 100

• Cat.5 UTP

100 m 100BaseFX 1995 100 Mb/s Star 2 optical fibers 2000 m 100BaseT4 1995 100 Mb/s Star 100

• Cat.3 UTP

100 m 100BaseT2 1997 100 Mb/s Star 100

• Cat.3 UTP

100 m 1000BaseLX 1998 1 Gb/s Star Multi-mode fiber 550 m 1000BaseLX 1998 1 Gb/s Star Single-mode fiber 5000 m 1000BaseT 1999 1 Gb/s Star 100

• Cat.5 UTP

100 m 10GBaseLX4 2002 10 Gb/s Star MMF 1310 nm WDM 300 m 10GBaseLX4 2002 10 Gb/s Star SMF 1310 nm WDM 10 km 10GBaseSR 2002 10 Gb/s Star MMF 850 nm 26-300 m 10GBaseEW 2002 10 Gb/s Star SMF 1550 nm 40 km

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Media Independent Interface

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Ethernet Wiring

Standard Cable Data Rate Coding Baud Rate Traffic 10BaseT 2p Cat.3 UTP 10 Mb/s Manch. 10 Mbd Full-Dup 100BaseTX 2p Cat.5 UTP 100 Mb/s 4B/5B 125 Mbd Full-Dup 100BaseFX 2 fibers 100 Mb/s 4B/5B 125 Mbd Full-Dup 100BaseT4 4p Cat.3 UTP 100 Mb/s 8B6T 25 Mbd Half-Dup 100BaseT2 4p Cat.3 UTP 100 Mb/s PAM5x5 25 Mbd Full-Dup 1000BaseLX 2 MMF/SMF 1 Gb/s 8B/10B 1.25 Gbd Full-Dup 1000BaseT 4p Cat.5 UTP 1 Gb/s 4D-PAM5 125 Mbd Full-Dup 10GBaseLX4 MM/SM 4WDM 10 Gb/s 8B/10B 3.125 Gbd Full-Dup 10GBaseSR 2 MMF/SMF 10 Gb/s 64B/66B 10.3 Gbd Full-Dup

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Timing Extraction

Suppose you need to transmit the message “I am a ...”. In ASCII (LSB first) this becomes

The straightforward way to send this over a wire or fiber is to use NRZ (non-return to zero) encoding: Problem: Long runs of zeros and/or ones make it difficult to extract timing information at the receiver.

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Coding for Timing Extraction

Manchester coding: 1 is low

• high and 0 is high
• low

transition in middle of symbol interval. Doubles bandwidth required. Has no dc component. NRZI (NRZ inverted) coding: 1 is encoded as transition (alternating low

• high and high
• low), 0 is encoded as

absence of transition. Only solves problem of consecutive 1’s.

Runlength-limited coding: At least

0’s must follow each 1, at most

0’s in sequence. Together with NRZI encoding this ensures transitions are not too close together and not too far apart.

Runlength-limited coding: At most

consecutive 0’s and at most

consecutive 1’s are allowed.

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Coding for Timing Extraction

Bipolar or AMI (alternate mark inversion) coding: Uses three levels, 0 and

• ✁

. 0’s are coded as 0 level and 1’s are coded as alternating

and

• ✁

. Has no dc component, but long strings of 0’s yield no timing information. BNZS (bipolar

• zero substitution) coding: All strings
• f

zeros are replaced with special

symbol sequence containing at least one bipolar violation. Examples are

• ,

• ,

• . For

• substitutions are

• ✄

• after
• r

• ✡
• ✄

after

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Coding for Timing Extraction

• ✁✄✂

coding: Converts blocks of

• input bits into block of

code bits. Often

• ✆

, e.g., in

. Manchester is

code with

and

• disallowed. General goal is to

limit runs of 0’s and/or 1’s and to reduce dc component. Unused

sequences can be used for control purposes.

coding:

bits are converted to

ternary (0,

)

• symbols. Also called pseudoternary codes. Not all ternary

code combinations are used

redundancy can be used to create desirable code properties. Examples:

and

. Scrambling: “Whitening” of binary data sequence by passing it through binary LFSR (linear feedback shift register) or by XORing with

• sequence. First method is

called self-synchronous scrambing, second method is called side-stream scrambling.

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Line Codes

NRZI (inverted NRZ): Transition for each 1, no transition for 0’s. Original Ethernet uses Manchester, SONET uses NRZ, FDDI uses

and NRZI.

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4B/5B Code

Each code has maximum of one leading zero and two trailing zeros.

Data Code 0000 11110 0001 01001 0010 10100 0011 10101 0100 01010 0101 01011 0110 01110 0111 01111 Data Code 1000 10010 1001 10011 1010 10110 1011 10111 1100 11010 1101 11011 1110 11100 1111 11101

Plus 9 control sysmbols (for sync, link idle, etc)

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8B/10B Code

Developed by IBM, Patent 4665517 (filed Dec 1983, granted May 1987) Used in ANSI X3.230-1994 (Fibre Channel Physical and Signaling Interface). Enables clock recovery at receiver. Supports data and control characters. Makes dc-balance possible. Each code has 4, 5, or 6

• nes (and thus 6, 5, or 4 zeros). Depending on running

disparity (difference between number of 1’s and 0’) two forms of codewords can be used. E.g., (using LSB first) 11110100 is encoded either as 0101111001 or 1010001001. Has good error detection properties.

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Scramblers

A scrambler is a randomizing mechanism that is used to eliminate long strings of consecutive identical transmitted symbols and avoid the presence of data dependent spectral lines in the signal spectrum, without changing the data rate. Self-Synchronous Scrambler or Feedthrough

• Scrambler. A scrambler whose current state are the

prior

• utput bits.

Side-Stream Scrambler. A scrambler whose current state only depends on the previous state but not on the data.

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Scramblers

Self-synchronous scrambler,

• Side-stream scrambler,

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Scramblers

100BASE-T2, 1000BASE-T use side-stream scrambling with

• ✂

(master) or

(slave). 10GBASE-R uses 64B/66B code followed by self-synchronous scrambler with

. 10GBASE-W uses side-stream scrambler with

(same as specified in ANSI T1.105-1995 for SONET).

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Copper vs. Fiber

Fiber advantages: High bandwidth, low attenuation, immune to electromagnetic interference, immune to corrosion, thin and lightweight, difficult to tap. Fiber disadvantages: Technology less familiar to engineers, damaged if bent too much, inherently unidirectional, electrical to optical interfaces are expensive. Future of all fixed data communications for distances more than a few meters is clearly with fiber.

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Wireless Transmission

Some people believe the future holds only two kinds of communication: Fiber and wireless. Maxwell predicted electromagnetic waves that can propagate through space (even vacuum) in 1865. Frequency

• in Hz, wavelength

in m, and speed of light

m/s (in vacuum) are related by

• ✂

Thus, 1 MHz waves (AM radio) are about 300 m long, 100 MHz waves (FM radio) are about 3 m long, 1 GHz waves (cell phone) are 30 cm long. Millimeter waves have a frequency of 30 GHz or more.

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Electromagnetic Spectrum

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Radio Transmission

Properties of radio waves are frequency dependent. In the VLF , LF , MF (3 kHz - 3 MHz) bands radio waves follow the ground. Such waves can travel up to a 1000 km, especially at the lower frequencies. They can penetrate walls and buildings easily. In the HF and VHF (3-300 MHz) bands the ground waves tend to be absorbed by the earth. However, these waves are bounced back by the ionosphere (layer of charged particles at 100-300 km above earth). General problem in VLF to HF bands: Low bandwidth for data communication.

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Microwave Transmission

Above 100 MHz radio waves travel mostly using line-of-sight paths. Thus, RF energy can be focused, similar to focusing a light bream. Before optical fibers were used, much of the long distance telphone network used microwave line-of-sight communications. Microwaves do not pass through buildings very well. Fading is created by waves that arrive through paths with different lengths. Microwaves above 4 GHz are severly attenuated by rain, snow and fog. Microwave communication is used very widely

Severe shortage of spectrum has developed.

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ISM Bands in the US

ISM stands for Industrial, Scientific, Medical. ISM band allocations and regulations vary from country to country. How can several users and services coexist?

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Time-Division Multiplexing

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Freq.-Division Multiplexing

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GSM Example

GSM (Global System for Mobile communications) uses 124 pairs of 200 kHz wide simplex channels (890.2-914.8 and 935.2-959.8 MHz). Each 200 kHz channel supports 8 separate connections using TDM.

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Code-Division Multiplexing

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Example

The sequences

• ✄

• ✄
• ✄

• ✄

• ✄
• ✄
• ✄

• ✄
• ✄

• are mutually orthogonal and can be used to

accomodate 4 users simultaneously.

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