Chap5 dB Framing

8/8/2019 Chap5 dB Framing

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ECE-4074Web Site ht t p:/ / www.csc.gat ech.edu/ ~copeland/ 40 74

Prof . John A. Copeland

[email protected] ech.edu

404 894 - 5177

Of f ice: GCATT 5 7 9

For of f ice visit , email or call me

or drop by during of fice hours post ed on t he Web Sit e

Appendices 3 A-3C, pp 11 5-1 2 0

Framing

Rat ios as Decibels

Charact erist ic Impedance

Scrambling

EE40 74 .03 v8


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Charact erist ic Impedance

A unifor m t ransmission line will conduct an elect rical pulse

in one direction at a constant speed ( usually ~ 2 E8 m/ s)

without reflect ion i f t he end is t erminat ed wit h R = Zo

+

-

R

Voltage

Dist ance

TransmitterTransmission Line, Zo = R


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Decibels = 1 0 * log (Rat io)

Loss

facto r

= .05

-13 dB

Gain

facto r

= 4 0

=16 dB

Gain

facto r

= 100

= 20 dB

Loss

facto r

= .01

= -20 dB

Loss

facto r

= .02

= -17 dB

Circuit Gain =

.0 5

x 100

x .02x 4 0

x .01

0.04

Circuit Gain =

-13 dB

+20 dB

-17 dB+16 dB

-20 dB

-14 dB

.0 5 = 1 0 * * -1 .3

10 0 = 1 0 * * +2 .0

.0 2 = 1 0 * * -1 .7 4 0 = 1 0 * * +1 .6

.0 1 = 1 0 * * -2 .0


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Decibel Relat ionships

Rat io dB Quick Calculat ion

1 0 1 = 10 * * 0

1 .2 5 1 1 .2 5 = 5 / 4 = 7 -6 dB

2 3 ( 5 / 4 ) * * 3 = 1 2 5 / 6 4

4 6 2 * 2 = 3 +3 dB

5 7 1 0 / 2 = 1 0 -3 dB

8 9 2 * 2 * 2 = 3 +3 +3 dB1 0 1 0 1 0* * 1

10 0 2 0 1 0* * 2

1 0 00 3 0 1 0* * 3

1 / R -D where R -> D log(1 / x) = -log( x)

1 +0 .2 5* f f f < 1 ( f ract ion)

Memorize t he rat ios corresponding t o 1 , 3, and 10 dB, and you can

quickly calculat e t he rat io for any dB.


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Bit -Orient Framing

1 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0 0 1 1 0 1 0 0 0 1 1 0 1 1 1 1 1 0 0 1 1

1 0 0 1 0 1 1 1 1 1 1 0 0 1 0 0 1 0 0 0 0 1 0 0 0 1 0 1 0 1 0 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 0 1 1 1 1 1 0 0 1 1

S t a r t H E L L O

One technique is t o use a special 8-bit " f lag" code (byt e or oct et) t hat

cont ains six "1" s in a row (01 111 110 ) .

A st ream of 0 's and 1's are received. We know it is t ext data

(characters encoded as 8-bit codes). Where are t he f irst bit s of a

code?

FlagFlag Dat a

The " f lags" " frame" t he " dat a". This ty pe of st ructure is cal led a

" packet" , " cell" or " frame" depending on the prot ocol.


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Bit Stuff ing

To prevent a f lag-like sequence appearing in t he dat a, t he following rule is

applied when t he frame is f ormed:

Aft er f ive "1 " s in a row, an extra zero is "st uff ed" int o the bit stream

(whet her the next bit is a " 1" or a " zero") .

When data is t aken out of a frame at t he receiving end, t he following rulerestores the data:

whenever five " 1" s in a row appear, t he next bit is discarded.

Da t a : 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 1 0 1 1 1 1 1 1 0 0 1 0 1 0 0 0 0 0 0

F r a me d Da t a :

0 1 1 1 1 1 1 0 0 1 1 0 1 1 1 1 1 0 0 1 1 0 0 0 1 0 1 1 1 1 1 0 1 0 0 1 0 1 0 0 0 0 0 0 0 1 1 1 1 1 1 0


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Encoding Rule Violat ion Framing

0 x 1 1 0 1

Man-

chester

Encoding

Bit st ream

In some systems, t he t ransmit t er will send a signal t hat violat es an

encoding rule t o signal t he st art of a f rame. Examples:

0 1 X 1 0 1

Alternate

Mark-

Inversion

AMI

Bit st ream


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Charact er-Orient ed Framing

Some charact er-orient ed prot ocols need t o divide a st ream ofcharact ers (oct et s, byt es) int o frames. Three codes are used to

build fr ames which are designat ed:

" Dat a Link Escape" or DLE

" St art Transmission" or STX

" End Transmission" or ETX

The rules are: Frames are st arted wit h the t wo-oct et s: DLE STX

Frames are ended wit h t he t wo oct et s: DLE ETX

Since all 25 6 po ssible 8- bit codes can appear in t he dat a,

whenever a DLE oct et appears, a second DLE is st uff ed int o t he

charact er stream aft er i t .

Da t a : A B DL E C D S T X F E T X G H

F r a me d Da t a :

D L E S T X A B DLE DL E C D S T X F E T X G H D L E E T X


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Charact er-Orient ed Frame Decode

F r a me d Da t a :

D L E S T X A B DLE DL E S T X D E F E T X G H D L E E T X

Da t a : A B DL E S T X D E F E T X G H

Decoding r ule: whenever a DLE charact er appears in t he incoming

charact er st ream, it is int erpreted in combinat ion wit h t he next charact er

according t o t his t able:

DLE STX = Sta rt o f Fr am e ( d iscar d, st a rt sav ing data )

DLE DLE = Replace wit h single DLE

DLE ETX = End of Frame (d iscard, pass data to higher level)

DLE [ anyt hing else] = coding error.


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Scramblers

There are problems when long st rings of only " 0 " s or

only " 1 " s are t ransmit t ed. To prevent t his a scrambler

can be used at t he t ransmit t er, and a mat ching

descrambler at t he receiver.

Scrambler: B( m) = A( m) B(m-3) B(m-5)

where in t he " exclusive or"

A(m) is t he m' t h bit input

B(m) is t he m' t h b it out put .

Descrambler: C( m) = B( m) B(m-3) B(m-5)

B(m) is the m' t h b it input

C(m) is t he m' t h b it out put .p. 147-149


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Shif t Regist er Scramblers

Scrambler: B( m) = A( m) B(m-3) B(m-5)

Descrambler: C( m) = B( m) B(m-3) B(m-5)

A(m) B ( m )C(m)

C(m) = B(m) B(m-3) B(m-5)

C(m) = {A( m) B(m-3) B(m-5)} B(m-3) B(m-5)

C(m) = A(m) since X X = 0 , Y 0 = Y

B(m-3)

B(m-5)

B(m-1)

B(m-2)

B(m-4)

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Chap5 dB Framing