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 Digital Information 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) (b) Shielded twisted pair (d) Coaxial cable
6.2.4 Optical fiber Transmission media (1) Cable structures
Transmission modes 6.2.4 Optical fiber Transmission media (2)
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 Synchronous Asynchronous
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 단류와복류는전압의 +/-까지의폭에있음. 비트값동안전압값을유지한다. 단점. 신호의동기에문제가있슴 0 1 0 0 1 1 1 0 NRZ-L NRZ-I
복류 RZ Digital-to-Digital Encoding (5) 전압 0 값을중심으로 +/- 를가짐 ( 복류 ) 데이터신호비트중간에서 0 으로전이되는신호화방식 + V 0 1 0 0 1 1 1 0 0 V -V
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 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 +v이면다음 1은 -v를가짐 + V 0 1 0 0 1 1 1 0 AMI 0 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 Types of errors Data link layer Transport layer 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: x 7 + x 5 + x 2 + x + 1 standard polynomials CRC-12: x 12 + x 11 + x 3 + x + 1 CRC-ITU: x 16 + x 12 + x 5 + 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) Go-back-N Selective repeat (Selective-reject)
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
Max. number for each protocol Sequence numbers in sliding window 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