Chap-1 (MM Comm)

Chapter 6

Digital Communications Basics6.1 Introduction6.2 Transmission media6.3 Source of signal impairment6.4 Asynchronous Transmission6.5 Synchronous Transmission6.6 Error Detection Methods6.7 Protocol basics6.8 The HDLC protocol

Chap-1 (MM Comm)

Transmission SIGNALs

• Physical layer– manage the moving information in the form of

electromagnetic across network connection• information

– voice, image, numeric data, characters, or video• collecting information from computer• sending information from computer• use encoder/decoder to create/reconstruct a stream of 1s and 0s

– converts a form to transfer via transmission media» the form of electromagnetic signals

Chap-1 (MM Comm)

Analog and Digital Signal

• Analog– refers to something that is continuous

• Digital– refers to something that is discrete

Text,Voice,VideoImage,

etc

Encoder

Information

Digital

Analog

Chap-1 (MM Comm)

6.1 Introduction• Baseband transmission : digital interface via NIC in LAN,ISDN

Chap-1 (MM Comm)

Modulated transmission• Modulated transmission : analog transmission in PSTN

Chap-1 (MM Comm)

Effect of attenuation, distortion, and noise on transmitted signal

Attenuated: decreased in amplitudedistorted: misshappennoise

Chap-1 (MM Comm)

6.2 Transmission media (1)

(a) Two-wire and multiwire open line

(b) Unshielded twisted pair

Chap-1 (MM Comm)

6.2 Transmission media (2)

(d) Coaxial cable

(b) Shielded twisted pair

Chap-1 (MM Comm)

6.2.4 Optical fiber Transmission media (1)

Cable structures

Chap-1 (MM Comm)

6.2.4 Optical fiber Transmission media (2)Transmissionmodes

Chap-1 (MM Comm)

6.2.5 Satellites

Broadcast

television

Data

communications

Chap-1 (MM Comm)

6.2.7 Ground-based Radio transmission

Single

cell

Multiple

cells

Chap-1 (MM Comm)

6.3 Sources of signal impairment

Chap-1 (MM Comm)

6.4 Digital Data Transmission

Digital dataTransmission

Parallel Serial

AsynchronousSynchronous

Chap-1 (MM Comm)

Parallel and Serial Transmission

Chap-1 (MM Comm)

Parallel and Serial Transmission (2)• All transfer that are external to the system

» are carried out bit-serially

• NIC must perform the following functions– parallel-to-serial conversion of each character– serial-to-parallel conversion of each received character– achieve bit, character, and frame synchronization in

receiver– generate a suitable error check digits

Chap-1 (MM Comm)

Asynchronous and Synchronous transmissionAsynchronous transmission

Synchronous transmission

Chap-1 (MM Comm)

Asynchronous transmission• Send one start bit (0) at beginning and one or more stop

bits (1s) at the end of each byte• may be a gap between each byte

– means “asynchronous at the byte level”– but the bits are still synchronized

Chap-1 (MM Comm)

Synchronous transmission

• Send bits one after another without start/stop bits or gaps• is the responsibility of the receiver to group the bits

Chap-1 (MM Comm)

6.4 Asynchronous Transmission (1)Principle ofoperation

Timingprinciple

Chap-1 (MM Comm)

6.4 Asynchronous Transmission (2)• Bit synchronization

– one start bit and two stop bits– clock cycle (Figure 6.12)

• Character synchronization– buffer register using one start bit and two stop bits

• Frame synchronization– start-of-text (STX) character– end-of-text (ETX) character– data link escape (DLE) character

• to overcome an abnormally termination by an ETX character in receive processing

Chap-1 (MM Comm)

Figure 6.12 Examples of three different receiver clock rate ratios(a) x1, (b) x4, © x16

Chap-1 (MM Comm)

Chap-1 (MM Comm)

6.5 Synchronous Transmission (1)• Two Synchronous Transmission

– Character-oriented– Bit-oriented

Chap-1 (MM Comm)

6.5 Synchronous Transmission (2)• Character-oriented sync

– SYN character– STX, ETX, DLE character

• Bit-oriented sync– an unique 8-bit pattern

• flag byte or flag pattern– idle byte

Chap-1 (MM Comm)

Character-oriented sync

Chap-1 (MM Comm)

Bit-oriented sync

Chap-1 (MM Comm)

Encoding

Chap-1 (MM Comm)

Digital-to-Digital Encoding (1)• Encoding the transmitted data into the binary 1s

and 0s– a sequence of voltage pulses

01011101 Digital/digitalEncoding

• Types of digital-to-digital encoding– Uni-Polar: use only one technique– Polar: use two of which have multiple variations

» NRZ, RZ, biphase– Bi-Polar: use three vairations

» AMI, B8ZS, HDB3

Chap-1 (MM Comm)

Digital-to-Digital Encoding (2)• Uni-polar encoding

– uses only one level of value (= one polarity)– the polarity of a pulse : positive and negative– one voltage level for binary 0 and 1

» a valued voltage is 1, zero voltage is 0– two problems

• DC (Direct Current) Component• Synchronization

– cannot change in voltage level to indicate the bit type

1 1 11110 00

Chap-1 (MM Comm)

Digital-to-Digital Encoding (3)

• Polar– uses two voltage levels (one positive and negative) of amplitude

– DC Component is eliminated by Manchester» binary 1: a negative-to-positive transition» binary 0: a positive-to-negative transition

– Types of polar encoding• Non-Return to zero (NRZ)

– Non-Return to zero , Level (NRZ-L) – Non-Return to zero , Invert (NRZ-L)

• Return to Zero (RZ) : Sync• Biphasea: Sync

– Manchester– Differential Manchester

Chap-1 (MM Comm)

Digital-to-Digital Encoding (4)

• 단류 NRZ방식– 단류와복류는전압의 +/-까지의폭에있음.– 비트값동안전압값을유지한다.

– 단점. 신호의동기에문제가있슴0 1 0 0 1 1 1 0

NRZ-L

NRZ-I

Chap-1 (MM Comm)

Digital-to-Digital Encoding (5)• 복류 RZ

– 전압 0값을중심으로 +/-를가짐(복류)– 데이터신호비트중간에서 0으로전이되는신호화방식

0 1 0 0 1 1 1 0

0 V

+ V

- V

Chap-1 (MM Comm)

Digital-to-Digital Encoding (6)• Manchester

– +v/-v로끝나며, 신호의중간에서전이동기에유리– 1은 -v 에서 +v 로끝나며, 0은 +v 에서 -v 로끝난다.

• Differential Manchester– 1은 ‘No transition’, 0은 ‘transition’

0 1 0 0 1 1 1 0

Manchester

DifferentialManchester

Chap-1 (MM Comm)

Digital-to-Digital Encoding (7)• Bi-Polar

– like RZ; uses three voltage levels: + , -, zero– unlike RZ

• zero level is binary 0

– Three type• Alternate Mark Inversion (AMI)

– the simplest type of bipolar encoding• Bipolar 8-Zero Substitution (B8ZS)

– adopted in Noth America– forces artificial changes, called violations within the 0

string• High-Density Bipolar 3 (HDB3)

– used in Europe and Japan– every time four consecutive 0’s

Chap-1 (MM Comm)

Digital-to-Digital Encoding (8)

• AMI (Alternate mark Inversion)– zero voltage is binary 0– alternate is 1 inversion– 앞의 1이 +v이면다음 1은 -v를가짐

0 1 0 0 1 1 1 0

AMI 0 V

+ V

- V

Chap-1 (MM Comm)

6.6 Error detection methods• Error Detection and Correction

– For reliable communication• Data can be corrupted during transmission• Errors must be detected and corrected

– Data link layer– Transport layer

• Types of errors– Single-Bit Error

• means that only one bit of a given data unit is changed– Multiple-Bir Error

• means that two or or more nonconsecutive bits in a data unit have changed

– Burst Error• means that two or more consecutive bits in a data unit have

changed

Chap-1 (MM Comm)

Types of Errors

Chap-1 (MM Comm)

Detection (1)• Error Detection

– uses the concept of redundancy, which means adding extra bits for detecting errors at the destination

• Detection Methods– Vertical Redundancy Check (VRC) : called Parity check– Longitudinal Redundancy Check (LRC)

• two dimension of VRC– Cyclic Redundancy Check (CRC)– Checksum

– VRC, LRC, CRC : are implemented in the physical layer for use in the data link layer

– Checksum: is implemented in the transport layer

Chap-1 (MM Comm)

Detection (2)

Redundancy

Chap-1 (MM Comm)

VRC• Called Parity Check

– a parity bit : a redundant bit• is appended to every data unit so that the total number of 1s in

the unit becomes either even or odd– even parity : even– odd parity: odd

• Reliability– can detect all single-bit errors– can detect multiple-bit or burst errors only if the total number of

errors is odd– ex: 6: 1000111011 --> 1111111011:9 , 0110111011:7,

1100010011:5» 1’s are odd ---> rejected by VRC check

– ex: 6: 1000111011 --> 1110111011: 8, 1100011011: 6 , 1000011010: 4

» 1’s are even ----> accepted by VRC check

Chap-1 (MM Comm)

LRC• To increase the detecting of multiple-bit and burst errors

– groups a predetermined number of data units, each already containing a VRC parity bit

• A redundant unit is added after a number of data units– The bits in the redundant unit are calculated from the

corresponding bits in the data units using VRC

• Reliability– increases the detecting of multiple-bit and burst errors– exist one pattern of errors

• if two bits in exactly the same positions • ex two data units: 11110000 and 11000011

» 01110001 and 01000010 (00110011)

Chap-1 (MM Comm)

6.6.2 Block sum check (1)

Chap-1 (MM Comm)

6.6.3 CRC (1)• Most powerful redundancy checking technique

– based on binary division (no bit addition)

– a sequence of redundant bits, called the CRC or the CRC remainder

• is appended to the end of a data unit • the resulting data unit becomes exactly divisible by a

predetermined binary number.• At its destination,the incoming data unit is divided by the same

number.• If at this step

– no remainder,the data unit is assumed to be intact and is therefore accepted.

– A remainder indicates that the data unit has been damaged in transit and therefore must be rejected.

Chap-1 (MM Comm)

6.6.3 CRC (2)• The redundancy bits used by CRC

– are derived by dividing the data unit by the pre-determined divisor• binary division

– the remainder is the CRC. – appending it to the end of the data

Chap-1 (MM Comm)

6.6.3 CRC (3)• Reliability

– CRC will detect all possible errors – except those that change the bit value of a block of code by exactly

the value of the divisor.– Popular CRC divisors,

• use I3,l7,and 33 bits, » the likelihood of an undetected error almost to zero.

• The CRC Generator– uses modulo-2 division.or– uses an algebraic polynomial

• ex: x7 + x5 + x2 + x + 1 • standard polynomials

– CRC-12: x12 + x11 + x3 + x + 1 – CRC-ITU: x16 + x12 + x5 + 1

Chap-1 (MM Comm)

6.6.3 CRC (4)

Chap-1 (MM Comm)

Ref: Checksum (1)• Checksum Generator

– subdivides the data unit into equal segments of n bits (usually l6) in the sender

– These segments are added together using one’s complement arithmetic

» the total is also n bits long

• Checksum – That total(sum) appended to the end of the original data unit as

redundancy bits,called the checksum

• The extended data unit is transmitted across the network so if the sum of the data segment is T,the checksum will be-T

Chap-1 (MM Comm)

Ref: Checksum (2)

Chap-1 (MM Comm)

Ref: Checksum (3)• Checksum checker

• Reliability– Checksum detects all errors involving odd numbers of bits,as well

as most errors involving even numbers of bits.– However, if one or more bits of a segment are damaged and the

corresponding bit or bits of opposite value in a second segment are also damaged,

» the sums of those columns will not change and the receiver will not detect a problem

Chap-1 (MM Comm)

Ref: Error Correction• Two ways

– have the sender retransmit the entire data unit– use an error-correcting code

• more sophisticated • require more redundancy bits

» limited to one, two, three-bit errors

• A single-bit error correction – redundancy bits to indicate the location of the error bit

• data bits (m) + redundancy bits ( r) --> m + r bits• different states : 2r

• see table 9.1 : relationship between data and redundancy bits

– Hamming code

Chap-1 (MM Comm)

Ref: Hamming code• Redundancy bits in Hamming code

– r1 : bit 1, 3, 4, 5, 9, 11– r2: bit 2, 3, 6, 7, 10, 11– r4: bit 4, 5, 6,7– r8: bit 8, 9, 10, 11

Chap-1 (MM Comm)

Ref: Redundancy bits calculation

Chap-1 (MM Comm)

Ref: Example of Redundancy bits calculation

Chap-1 (MM Comm)

Ref: Example

Chap-1 (MM Comm)

6.7 Protocol basics

Chap-1 (MM Comm)

6.7.1 Error Control• Automatic Repeat Request (ARQ)

– Error control mechanism in data link layer– Basic concepts

• anytime an error is detected in an exchange• a negative acknowledgment (NAK) is returned• the specified frames are retransmitted

Error Control

Idle RQ(Stop-and-wait ARQ)

Continuous RQ(Sliding window ARQ)

Selective repeat(Selective-reject)Go-back-N

Chap-1 (MM Comm)

Stop-and-wait ARQ, damaged frame

Chap-1 (MM Comm)

Stop-and-wait ARQ, lost ACK frame

Chap-1 (MM Comm)

Go-back-n, damaged data frame

Chap-1 (MM Comm)

Go-back-n, lost data frame

Chap-1 (MM Comm)

Go-back-n, lost ACK

Chap-1 (MM Comm)

Selective-reject, damaged data frame

Chap-1 (MM Comm)

6.7.4 Flow Control• Flow Control

– refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment

– Two ways• Stop-and-Wait

– Send one frame at a time– the sender sends one frame and waits for an

acknowledgement before sending the next frame

• Sliding Window– Send several frames at a time– several frames can be in transit at a time

Chap-1 (MM Comm)

Stop-and -Wait

Chap-1 (MM Comm)

Flow control principle via sliding window

Chap-1 (MM Comm)

Sequence numbers in sliding window

Max. number foreach protocol

Example assuming 8 sequence numbers

Chap-1 (MM Comm)

Example of Sliding Windows

Chap-1 (MM Comm)

6.7.6 Layered architecture

Chap-1 (MM Comm)

6.8 The HDLC protocol• High-level Data Link Control (HDLC) protocol

– logical link layer protocol in data link protocol

• A data link protocol– a set of specifications used to implement the data link layer

• Two categories– Asynchronous protocol

• treats each character in a bit stream independently

– Synchronous protocol• takes the whole bit stream and chop it into characters of equal

size

Chap-1 (MM Comm)

Asynchronous Protocols in DLL• Protocols

– have been developed over the last several decades– are employed mainly in modems

• are not complex and are inexpensive to implement• are accomplished by using extra bits (start and stop bits) to

frame• a receiver does not need to know exactly when a data unit is

sent– its inherent slowness

• stemming from the required additions of start and stop bits

– is being replaced by higher-speed synchronous mechanisms

Chap-1 (MM Comm)

Modem• Zmodem

– a file transfer protocol for telephone line communication between PCs

– a half-duplex stop-and-wait ARQ protocol• 1st field : one-byte start of header (SOH)• 2nd field: two-byte header

– one: sequence number, carries the frame number– the other: used to check the validity of the sequence

number• last field: CRC-16

Chap-1 (MM Comm)

Synchronous Protocols in DLL• The better choice for LAN, WAN technology

– High speed over asynchronous transmission

• Two types– Character oriented protocol

• interpret a transmission frame or packet as a succession of characters

» composed of byte, called byte-oriented protocol• all information is encoded to ASCII characters

– Bit oriented protocol• interpret a transmission frame or packet as a succession of

individual bits• all information is depended in the bit position or pattern

Chap-1 (MM Comm)

Character-oriented protocol in DLL• Binary Synchronous Communication

– a popular character-oriented data link protocol developed by IBM in 1964

– supports half-duplex transmission using stop-and-wait ARQ– does not support full-duplex transmission or sliding window

protocol

– BASC Frames• Control frames

– connection, flow and error control, and disconnection• Data frames

– transmission of data

Chap-1 (MM Comm)

BSC data frame

Chap-1 (MM Comm)

Bit-Oriented Protocols in DLL• Can pack more information into shorter frames

• are not grouped into predefined patterns forming characters

• Categories– SDLC: synchronous data link protocol

» developed in 1975– HDLC: high-level data link protocol

» based on SDLC, developed in 1979– LAPs: Link Access Protocols

» based on HDLC, developed in 1981» LAPB, LABD, LAPM, LAPX, etc

– LANs: LAN’s access control protocol» Frame relay and PPP are developed by ITU-T and

ANSI» based on HDLC

Chap-1 (MM Comm)

HDLC• HDLC

– a basis for all bit-oriented protocols– supports both half-duplex and full-duplex modes in point-to-point

and multi-point configuration– can be characterized by station types, configurations, and response

modes

• Station types– are of three types: primary, secondary, and combined

• primary: sends commands• secondary: sends responses• combined station: sends commands and responses

• Configurations– refers to the relationship of hardware devices on a link – primary or secondary between peers

Chap-1 (MM Comm)

HDLC Configurations

Chap-1 (MM Comm)

Communication Modes in HDLC• The Relationship between two devices involved in an exchange

– describes who controls the link• Three modes of communication

– NRM: Normal Response Mode• the standard primary-secondary relationship• a secondary device must have permission from the primary

– if granted the permission, then sends the responses– ARM: Asynchronous Response Mode

• a secondary may initiate a transmission without primary’s permission

– ABM: Asynchronous Balanced Mode

Chap-1 (MM Comm)

Frames in HDLC• Three types of frames

– Information frames (I-frames)• are used to transport user data and control information relating

to user data– Supervisory frames (S-frames)

• are used only to transport control information, primary data link layer flow and error controls

– Unnumbered frames (U-frames)• are reserved for system management • are intended for managing the link itself

• Six fields– a beginning flag, an address, a control, an information, a frame

check sequence (FCS), and an ending flag

Chap-1 (MM Comm)

HDLC frame types

Chap-1 (MM Comm)

HDLC control fields

Chap-1 (MM Comm)

Example of polling using HDLC

Chap-1 (MM Comm)

Example of selecting using HDLC

Chap-1 (MM Comm)

Example of peer-to-peer using HDLC

Chap-1 (MM Comm)

Link Access Procedures• LAPB (Link access procedure, for balanced)

• a simplified subset of HDLC used only for connecting a station to a network

• provides the basic control functions required for communication between a DTE and A DCE

• is used only in balanced configurations of two devices» be used in ISDN on B channel

• LAPD (Link access procedure, for D channel)– a simplified subset of HDLC used in ISDN

– uses ABM – is used for out-of-band (control) signaling

• LAPM (Link access procedure, for Modem)– a simplified subset of HDLC for modems

» has been developed to apply HDLC features to modems

– is designed to do asynchronous-synchronous conversion, error detection, and transmission

Chap-1 (MM Comm)

CF: DTE-DCE Interface (1)

• DTE: Data Terminal Equipment– any device that is a source of or destination for binary digital data

• DCE: Data Circuit-Terminating Equipment– any device that transmits or receives data in the form of an analog or

digital signal through a network

Chap-1 (MM Comm)

Example: Modems• Stands for modulator/demodulator• Modulator: Converts a digital signal to an analog signal• Demodulator: Converts a analog signal to digital siganl