Ch. 6 Digital Data Communication Techniques

6.1Asynchronous & Synchronous Transmission

• Asynchronous Transmission: transmission in which each information character is individually synchronized (usually by the use of start and stop elements).

• Synchronous Transmission: transmission in which the time of occurrence of each signal representing a bit is related to a fixed time frame.

6.1 Asynchronous Transmission• Also known as character transmission or "start-

stop" transmission.

• One character at a time is transmitted.– The line usually idles at a logic 1– Each character has a start bit (logic 0) .– The start bit is followed by 5-8 data bits.– A single party bit can be generated, but it is optional.– 1, 1.5, or 2 stop bits (logic 1) finish the "framing" of

the character.

6.1 Asynchronous Transmission (Fig. 6.1)

• The efficiency E = # of inf. bits/ total # of bits.

• Example: ASCII code, odd parity, 2 stop bits.– # of inf. bits= 7– Total =1 start + 7 data + 1 parity + 2 stop = 11– Efficiency = 7/11= .64 or 64%.

• Transmitter and receiver have a "shift-register" structure.

• A separate clock exists at each end.

• UART--Integrated circuit implementation.

6.1 Asynchronous Transmission

• Timing Requirements (Fig. 6.1)– Consider a 10 kbps transmitter clock.– Each bit will be 100 microseconds.– Assume the receiver is faster by 6%, or 6

microseconds during each bit time.– The transmitter sends 1 start bit and 7 data bits

in 800 microseconds.– The receiver looks for the 8th data bit after

8.5x94=799 microseconds.

6.1 Synchronous Transmission (Fig. 6-2)

• Also known as block transmission.

• Clock is transmitted along with the info. bits.

• Higher data rates can be obtained.

• Overhead bytes are transmitted.

• Can be character-oriented or bit-oriented.

• Large information fields relative to total overhead can provide high throughput (sometimes.)

6.2 Types of Errors• Probabilities of Error Types

– Pb: Probability of a single bit error (BER).

– P1: A frame arrives with no bit errors

– P2: A frame arrives with some undetected bit errors.

– P3: A frame arrives with some detected bit errors.

• Assume that no parity is sent (P3=0): – P1 = (1-Pb) ^F, where F is the frame size.

– P2 = 1 - P1

• Example: If Pb or F is "large" then P2 could become a problem.

6.2 Error Detection • Fig. 6.3--The error detection process.

• Parity Check– The simplest scheme.– Even number of errors is undetectable.

• Cyclic Redundancy Check (CRC)– Very powerful and often used.– Given k message bits, n "check" bits are

generated.– k+n bits are transmitted together.

6.2 Error Detection--CRC

• Modulo 2 Arithmetic– Binary addition without carries (exclusive or

operation.)

• The simple view is to consider variable, binary strings:– D-- data or message (k-bits)– F--frame check sequence (n-k bits)– P--pattern of n-k+1 bits– T--transmitted frame (n bits)

6.2 Error Detection--CRC (p.2)• Consider the following three step process:

– STEP 1: Multiply D by 2n-k.• n-k 0's will be shifted in, from the right.

– STEP 2: Divide by P• This will produce a quotient and a remainder.

• The quotient (Q) will not be used, but the remainder (R) is used as F.

– STEP 3: Add R to the shifted version of D to produce T.

• R replaces the n 0's that were added to D.

• T will be transmitted.

6.2 Error Detection--CRC (p.3)

• Why does this work?– We are looking for something that is evenly

divisible by P.– T/P =(D2n-k +R)/P = (D2n-k/P) + R/P

=Q+R/P+ R/P.– But R/P + R/P =0, because of modulo 2

arithmetic.– Hence T/P=Q which is evenly divisible by P

(remainder is 0).– Example 6.6--page 191.

6.2 Error Detection--CRC(p.4)• Suppose that bit errors are possible during

transmission.– Model the error pattern (E) as a binary string with a

1 in the position of a bit that has been received incorrectly.

– Received frame: Tr = T + E.

– Divide by P: Tr/P = T/P + E/P= Q + E/P

– If the remainder is 0, then assume E=0.– If P is chosen properly, very few error patterns will

be evenly divisible by P (i.e. have a remainder of 0) and very few undetected errors will occur.

6.3 Error Detection--CRC (p.5)

• Polynomials (Example 6.7 and Fig. 6.4)– The CRC process can be viewed as binary

polynomials of X.– The coefficients of the polynomials correspond to

the previously defined bit-string variables.– Arithmetic operations use modulo-2 (this makes

it an "algebra.")

• Generating Polynomials– Carefully chosen polynomials can allow the

detection of many types of errors(p.193).– Ex: CRC-12, CRC-16, CRC-CCITT, CRC-32.

6.3 Error Detection--CRC (p.6)

• CRCs can be generated using hardware (See Fig. 6.5 and 6.6 and Example 6.8).– Basic components:

• D Type flip-flops (or J-K Equivalents)-- one for each CRC bit.

• Exclusive OR gates (or equivalents)– Operation:

• Flip-Flops are initialized to 0.• Message bits are clocked into the circuit.

6.4 Error Correction• Fig. 6.7 Error Correction Process

– k bits are “mapped” into n bit block.

– Result is called a codeword.

– Example 6.7--Forward Error Correction

– (n-k)/k is called the redundancy.

– k/n is called the code rate.

• Fig. 6.8 How Coding Improves System Performance

6.5 Line Configurations• Topology: refers to the physical arrangement

of stations on a link.– Point-to-point– Multipoint--saves you money!

• Duplexity: refers to the direction and timing of signal flow.– Full-duplex digital lines generally require 4-wires.– Half-duplex digital lines require 2 wires

• Fig. 6-9 Traditional Computer/Terminal Configurations.

6.5 Line Discipline• Point-to-Point: Three Phases (not in 8th Edition)

– Establishment

– Data Transfer

– Termination

• Multipoint Links– Poll--the primary requests data from secondary

– Select--the primary has data to send and informs secondary that data are coming.

– Contention--no primary; a station can transmit when the line is free used in LANs and satellite systems.

Appendix G: Interfacing (Fig.G.1)

• DTE--Data Terminal Equipment (not in 8th Edition)– Equipment consisting of digital end instruments that

convert the user information into data signals for transmission, or reconvert the received data signals into user information.

• DCE--Data Circuit-terminating Equipment– In a data station, the equipment that provides the signal

conversion and coding between the data terminal equipment (DTE) and the line.

– DCE may be separate equipment or an integral part of the DTE or intermediate equipment.

G.1 Interfacing (cont.)• Interchange Circuits

– The connection between the DTE and DCE.

• Standards--Physical Layer of the OSI Model– V.24/EIA-232-F (RS-232--1962)– X.21--15 wire interface for public switched

network interfacing.– ISDN Physical Interface (8 wire interface).

G.1 Four Characteristics• Mechanical

– Pertain to the actual physical connection of the DTE and DCE (the terminator plugs and sockets).

• Electrical– The voltage levels and timing of voltage changes.

• Functional– The functions performed by various interchange

circuits: data, control, timing and ground.

• Procedural– The sequence of events for transmitting data.

G.1 EIA-232-F

• Mechanical (ISO 2110)– DB-25 connector (a 25 pin connector)– Fig. G.2.

• Electrical(V.28)– Digital signaling; up to 20 kbps; up to 15m.– Logic 1 and OFF : less than -3 volts– Logic 0 and ON : greater than +3 volts– And more (C, R, short circuit current, max

voltages, slew rate, etc.)

G.1 EIA-232-F (p.2)

• Functional (V.24)– Table G-1--Interchange Circuits

• Procedural (V.24) – Fig. G.4

G.1 Loopback Testing

• EIA-232-F control circuits assist in loopback testing and fault isolation.– Local loopback tests are used to check the

functioning of the local interface and the local DCE.

– Remote loopback tests are used to check the transmission channel and the remote DCE.

• Figure G.3 Local and remote loopback.

G.1 The Null Modem

• Used to connect two DTEs directly (no DCEs used).

• It is not a real modem, but simply a cable that rewires the circuits to trick the DTEs into thinking that they are talking with DCEs.

• Fig. G.5 illustrates the null modem wiring.

G.2 ISDN Physical Interface• X.21--15 pin connection for digital

interface to public switched networks.• ISDN--ISO 8877 specifies an 8 pin

connector.• The reduction of interface circuits forced

greater complexity in the logic circuits at each end of the cable, but integrated circuits have become cheap whereas wire remains relatively expensive.

• Fig. G.6 shows the ISDN Interface.