Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Physical and Datalink Layer and LANS: Part I
Document related concepts
Transcript
Digital Interfaces • An interface links two devices • Interface Standards define: – mechanical specifications - how many wires & connector type – electrical specifications - frequency, amplitude and phase of signal – functional - what does each wire do? – Procedural – how & when to perform functions Serial vs. Parallel • In serial transmission one bit is sent with each clock pulse. • Two types of serial transmission: – asynchronous – synchronous • In parallel transmission multiple bits are sent with each clock pulse. Faster/more expensive upated 1/2002 2 Serial Transmission 0110 0010 Parallel Transmission 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 Serial Transmission • Asynchronous & Synchronous • Concerned with timing issues • How does the receiver know when the bit period begins and ends? • Small timing difference become more significant over time if no synchronization takes place between sender and receiver Sender clock Synchronizations Receiver clock time 1 0 0 1 0 0 1 0 0 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 0 0 Sender data time Received data Asynchronous Transmission • Serial communication • Data transmitted 1 character at a time • Character format is 1 start & 1 or more stop bits, plus data of 5-8 bits • Character may include parity bit • Timing needed only within each character • Resynchronization each start bit • Uses simple, cheap technology • Wastes 20-30% of bandwidth Asynchronous Transmission START BIT 0110 0010 STOP BIT 0110 0010 Gaps of indeterminate size 0110 0010 Synchronous Transmission • Serial communication • Large blocks of bits transmitted without start/stop codes • Synchronized by clock signal or clocking data • Data framed by preamble/post amble bit patterns • More efficient than asynchronous • Overhead typically below 5% • Used at higher speeds than asynchronous Synchronous Transmission 0110 0010 0110 0010 0110 0010 0110 0010 0010 Synchronization Choices • Low-speed terminals and PCs commonly use asynchronous transmission – inexpensive – “burst” tendency of communication reduces impact of inefficiency • Large systems and networks commonly use synchronous transmission – overhead too expensive; efficiency necessary – error-checking more important Generic Communications Interface Illustration DTE Generates Data DCE Converts to transmission media/ converts to generated data (Network) DCE Converts to transmission media/ converts to generated data DTE Receives Data RS-232C (EIA 232C) • EIA’s “Recommended Standard” (RS) • Specifies mechanical, electrical, functional, and procedural aspects of the interface • Used for connections between DTEs and voice-grade modems, and many other applications Mechanical Specifications • 25-pin connector with a specific arrangement of leads • DTE devices usually have male DB25 connectors while DCE devices have female • In practice, fewer than 25 wires are generally used in applications RS-232 DB-25 Connectors DB-25 Female DB-25 Male RS-232 DB-9 Connectors Limited RS-232 Electrical Specifications • Specifies signaling between DTE and DCE • Uses NRZ-L encoding – Voltage -15V to -3V = binary 1 – Voltage +15V to +3V = binary 0 • Rated for < 20Kbps and < 15Mts – greater distances and rates are theoretically possible, but not necessarily wise Functional Specification • Specifies the role of the individual circuits • Data circuits in both directions allow fullduplex communication • Timing signals allow for synchronous transmission (although asynchronous transmission is more common) Functional Specification DB-25 Pinouts Procedural Specification • Multiple procedures are specified • Provides means of attachment between computer and modem – Specifies method of transmitting data between devices – Specifies method of cooperation for exchange of data between devices – Handshaking between the two equipment Null Modem Cable • Allows DTE to DTE direct communication SG DTR DSR RTS CTS CD TD RD SG DTR DSR RTS CTS CD TD RD Modems • Modulator - converts digital signal to analog signal • Demodulator - converts analog signal to digital signal upated 1/2002 22 Data Rate • Encoding Technique • Physical properties of medium • Can increase speed by increasing frequency but every line has upper and lower limits • frequency range = bandwidth upated 1/2002 23 Bit Rate Summary Encoding ASK, FSK, 2-PSK 4-PSK, 4-QAM 8-PSK, 8-QAM 16-QAM 64-QAM 256-QAM HDX 2400 4800 7200 9600 14,400 19,200 upated 1/2002 FDX 1200 2400 3600 4800 7200 9600 24
