comunicatia de date
DESCRIPTION
Comunicatia de DateTRANSCRIPT
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Comunicatia de date
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Sursa Emitator Mediu de transmisie Emitator Receptor
Perturbatii
Canal de comunicatie
Modelul simplificat al comunicatiei de date
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Tx RxCanal unidirectional
Tx TxCanal bidirectional
Rx Rx
Tx RxCanal unidirectional
Rx TxCanal unidirectional
SIMPLEX
SEMIDUPLEX
FULL DUPLEX
Moduri de comunicatie
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Comunicatie paralela / comunicatie seriala
10100101
Emitator n linii de semnal
1 1 1 1 10 0 0 0 0 1
“mark”“Start”
0
8 intervale de timp
1 1/0
8 biti de dateParitateStop
1
Comunicatieparalela
Comunicatieseriala
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Transmisie asincrona: • transmitatorul poate emite oricand un sir de biti• tactul nu se transmite catre receptor• Receptorul isi creaza propriul semnal de tact
Transmisie sincrona• se transmit in plus informatii de sincronizare (tactul)• informatia se transmite in pachete de biti (mai multi octeti / caractere intr-un singur pachet)
Viteza de transmisie• numarul de biti transmisi intr-o secunda (bps)
Rata erorilor (bit error rate BER)• numarul de biti receptionat eronat raportati la numarul total de biti transmisi
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Modelul OSI - ISO
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Modelul OSI - ISOrutarea
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Medii de transmisie(nivelul fizic)
• Legaturi prin fir– perechi torsadate– UTP (unshielded twisted pair) / STP– coaxial– linii private– linii comutate
• fibra optica • radio (curenti purtatori)
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Standarde corespunzatoare nivelului fizic
• RS 232C (EIA 232)• CCITT V.35• RS 422• RS 485
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Margine de zgomot
RS 232C ( V.35)
0
1
+15
+3
-3
-15
• asigura legaturi full duplex punct la punct •Distanta maxima - 30 m (practic 3-5 m)• viteza maxima 56 kbps (112kbps)• Nu asigura separare galvanica• Nu este recomandat in medii poluate electromagnetic• minimum 3 fire + linii de control transfer (handshake)
GND GNDTx
Tx RxRx
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RS 422
• asigura legaturi full duplex punct la punct• Distanta maxima - 1200m• viteza maxima 56 kbps (112kbps)• Nu asigura separare galvanica
Tx
Tx RxRx
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RS 485
• nivele logice asociate tensiunii diferentiale (0,2 V prag)• asigura legaturi semi duplex multipunct• Distanta maxima - 1200m• viteza maxima 56 kbps (112kbps)• Nu asigura separare galvanica
TxRx
TxRx
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TxRx
TxRx
Bucla de curent
• nivele logice asociate sensului curentului in bucla• asigura legaturi semi duplex multipunct• Distanta maxima - depinde de rezistenta totala a circuitului• viteza maxima 56 kbps (112kbps)
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Retele - topologii elementare
• punct la punct• magistrala liniara• inel fizic / logic• stea
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punct la punct
Tx
RxTxRx
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magistrala liniara (bus)
Tx
Rx
Tx
RxTx
Rx
Tx
Rx
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Tx
Rx
Tx
Rx
Inel fizic
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CUPLOR STEA
A B C D E
Stea
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A B C D E
Inel logic
Cale de comunicatie
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Babilonul protocoalelor de comunicatie...
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PROTOCOL
• Un set de reguli dupa care functioneaza un sistem de comunicatie
• Impachetare• controlul erorilor• control secventelor• transparenta
• controlul liniei• controlul intarzierilor• proceduri de initializare• cazuri speciale
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Arhitectura retelelor cu 3 straturi
Aplicatie
Legatura de date
Fizic
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Principalele relatii in comunicatii
Punct la punct• Client / server• Difuzor / abonat
(publisher / subscriber)
Master / slave• Cerere / raspuns• numai raspuns
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