6.3 Source of signal impairment 6.4 Asynchronous Transmission 6.5 Synchronous Transmission 6.6 Error Detection Methods 6.7 Protocol basics

Chapter 6

Digital Communications Basics 6.1 Introduction

6.2 Transmission media

6.3 Source of signal impairment 6.4 Asynchronous Transmission 6.5 Synchronous Transmission 6.6 Error Detection Methods 6.7 Protocol basics

6.8 The HDLC protocol

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

Analog and Digital Signal

• Analog

– refers to something that is continuous

• Digital

– refers to something that is discrete

Text, Voice,

Video Image,

etc

Encoder

Information

Digital

Analog

6.1 Introduction

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

Modulated transmission

• Modulated transmission : analog transmission in PSTN

Effect of attenuation, distortion, and noise on transmitted signal

Attenuated: decreased in amplitude distorted: misshappen

noise

6.2 Transmission media (1)

(a) Two-wire and multiwire open line

(b) Unshielded twisted pair

6.2 Transmission media (2)

(d) Coaxial cable

(b) Shielded twisted pair

6.2.4 Optical fiber Transmission media (1)

Cable structures

6.2.4 Optical fiber Transmission media (2)

Transmission modes

6.2.5 Satellites

Broadcast television

Data

communications

6.2.7 Ground-based Radio transmission

Single cell

Multiple cells

6.3 Sources of signal impairment

6.4 Digital Data Transmission

Digital data Transmission

Parallel Serial

Asynchronous

Synchronous

Parallel and Serial Transmission

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

Asynchronous and Synchronous transmission

Asynchronous transmission

Synchronous transmission

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

Synchronous transmission

• Send bits one after another without start/stop bits or gaps

• is the responsibility of the receiver to group the bits

6.4 Asynchronous Transmission (1)

Principle of operation

Timing principle

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

Figure 6.12

Examples of three

different receiver clock rate ratios (a) x1, (b) x4, © x16

6.5 Synchronous Transmission (1)

• Two Synchronous Transmission – Character-oriented

– Bit-oriented

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

Character-oriented sync

Bit-oriented sync

Encoding

Digital-to-Digital Encoding (1)

• Encoding the transmitted data into the binary 1s and 0s

– a sequence of voltage pulses

01011101 Digital/digital Encoding

• 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

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 0 1 1 0 0 1 1 1

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

Digital-to-Digital Encoding (4)

• 단류 NRZ방식

– 단류와 복류는 전압의 +/-까지의 폭에 있음.

– 비트값 동안 전압 값을 유지한다.

– 단점. 신호의 동기에 문제가 있슴

0 1 0 0 1 1 1 0

NRZ-L

NRZ-I

Digital-to-Digital Encoding (5)

• 복류 RZ

– 전압 0값을 중심으로 +/-를 가짐(복류)

– 데이터 신호 비트 중간에서 0으로 전이되는 신호화 방식

0 1 0 0 1 1 1 0

0 V + V

- V

Digital-to-Digital Encoding (6)

• Manchester

– +v/-v로 끝나며, 신호의 중간에서 전이 동기에 유리 – 1은 -v 에서 +v 로 끝나며, 0은 +v 에서 -v 로 끝난다.

• Differential Manchester

– 1은 ‘No transition’, 0은 ‘transition’

0 1 0 0 1 1 1 0

Manchester

Differential

Manchester

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

Digital-to-Digital Encoding (8)

• AMI (Alternate mark Inversion) – zero voltage is binary 0

– alternate is 1 inversion

– 앞의 1이 +v이면 다음 1은 -v를 가짐

0 1 0 0 1 1 1 0

AMI 0 V + V

- V

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

Types of Errors

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

Detection (2)

Redundancy

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

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)

6.6.2 Block sum check (1)

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.

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

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

6.6.3 CRC (4)

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

Ref: Checksum (2)

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

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 : 2^r

• see table 9.1 : relationship between data and redundancy bits – Hamming code

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

Ref: Redundancy bits calculation

Ref: Example of Redundancy bits calculation

Ref: Example

6.7 Protocol basics

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

Stop-and-wait ARQ, damaged frame

Stop-and-wait ARQ, lost ACK frame

Go-back-n, damaged data frame

Go-back-n, lost data frame

Go-back-n, lost ACK

Selective-reject, damaged data frame

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

Stop-and -Wait

Flow control principle via sliding window

Sequence numbers in sliding window

Max. number for each protocol

Example assuming 8

sequence numbers

Example of Sliding Windows

6.7.6 Layered architecture

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

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

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

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

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

BSC data frame

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

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

HDLC Configurations

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

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

HDLC frame types

HDLC control fields

Example of polling using HDLC

Example of selecting using HDLC

Example of peer-to-peer using HDLC

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

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

Example: Modems

• Stands for modulator/demodulator

• Modulator: Converts a digital signal to an analog signal

• Demodulator: Converts a analog signal to digital siganl