Pengantar – Komunikasi Data (1) TUJUAN • Berevolusi dan selalu berkembang sejak pertama kali adanya Computing • Mempelajari konsep-konsep dasar dan terminologi yang berhubungan dengan komunikasi data dan jaringan komputer • Mempelajari beberapa teknik yang dipergunakan untuk melakukan transfer data antara dua computer e.g. dua komputer dalam satu ruangan atau dibedakan dengan jarak dan terhubung melalui jaringan telepon • Mempelajari bermacam-macam fungsi dan protokol jaringan berdasarkan referensi dari ISO Reference Model • Mempelajari dasar-dasar teori yang berhubungan dengan transmisi digital 2 Pengantar – Komunikasi Data (2) KOMUNIKASI DATA • Pertukaran data antara dua perangkat (komputer) • Terhubung melalui beberapa macam media transmisi SISTEM KOMUNIKASI DATA • Kombinasi dari hardware dan software • Memungkinkan terjadinya pertukaran komunikasi data KOMPONEN-KOMPONEN: 3 Pengantar – Komunikasi Data (3) Komponen - komponen sistem komunikasi data • Pesan (Message): Informasi (data) yang akan dikirim Dapat berupa teks, gambar, suara dll • Pengirim/Penerima (Sender/Receiver): Perangkat untuk mengirim/menerima pesan data (data message) Dapat berupa komputer, kamera video dll Membuat istilah “end system atau host” yang mengacu pada keduanya • Medium: Lintasan fisik di mana data/pesan bergerak dari pengirim menuju penerima Dapat berupa kabel, gelombang radio dll 4 Pengantar – Komunikasi Data (4) Komponen - komponen sistem komunikasi data • Protokol: Peraturan-peraturan yang mengatur komunikasi data Menggambarkan kesepakatan antara perangkat-perangkat komunikasi Tanpa protokol, dua perangkat mungkin mungkin untuk saling terhubung, tetapi tidak saling berkomunikasi Sebagai contoh adalah dua orang yang berbicara dengan bahasa yang berbeda, tidak dapat mengerti satu dengan yang lainnya Hubungan antara perangkat - perangkat komunikasi • Konfigurasi sambungan: Mendefinisikan bagaimana cara dua atau beberapa perangkat komunikasi tersambung pada link • Mode transmisi: Mendefinisikan arah alur sinyal antara dua perangkat yang terhubung 5 Materi Kuliah (1) 2. Pengantar – Jaringan komunikasi data: Beberapa tipe dari jaringan komunikasi data – Standar-standar: Evolusi, Beberapa badan international 3. Pengantar – OSI model arsitektur: Lapisan dan stuktur model, Fungsi setiap lapisan 4. Komunikasi Data Interface – Media transmisi: Beberapa macam tipe fisik media transmisi yang dipergunakan untuk data transmisi, Karakteristik dan limitasi dari setiap tipe – Tipe-tipe sinyal: Beberapa tipe sinyal yang digunakan dalam komunikasi data, Sumber sinyal – Sumber redaman dan distorsi: Beberapa tipe redaman dan distorsi, pengaruh redaman dan distorsi pada sinyal – Delay propagasi: Pengaruhnya pada sinyal – Standar interface lapisan fisik: Standar yang digunakan untuk menghubungkan peralatan komunikasi 6 Materi Kuliah (2) 5. Transmisi Data – Dasar transmisi data: Beberapa tipe mode transmisi – Transmisi sinkron dan asinkron: Perbedaan dua tipe kontrol transmisi – Teknik deteksi kesalahan – Kompresi data: Beberapa tipe algoritma kompresi data – Kendali transmisi: Perbedaan dua tipe kontrol transmisi – Kendali komunikasi: Beberapa tipe perangkat komunikasi kendali 6. Dasar-dasar Protokol – Kendali kesalahan: pengertian-pengertian dasar dalam kendali kesalahan – Idle RQ: prinsip kerja, arsitektur, spesifikasi, penggunaan – Continuous RQ: prinsip kerja, arsitektur, penggunaan – Manajemen Link 7 Materi Kuliah (3) 7. Protocol TCP/IP – Karakteristik – Fungsi tiap lapisan 8. Data Link Control – Protokol character-oriented – Protokol bit-oriented – Standar interface lapisan kendali link 9. Multiplexing – FDM - Frequency-Division Multiplexing – Synchronous TDM (Time-Division Multiplexing) – Statistical TDM 10. Multiple Access – FDMA – Frequency Division Multiple Access – TDMA – Time Division Multiple Access – CDMA – Code Division Multiple Access – Random Access 8 Jaringan Komunikasi Data (1) Point-to-point • wire link • other link: PSTN (Public Switched Telephone Network) & Modem A B A B PSTN modem modem 9 Jaringan Komunikasi Data (2) LAN (Local Area (data) Network) • More than 2 computers involved in an application • A switched communication network facility used to enable all the computers to communicate with one another at different times • If all the computers are distributed around single office/building, it is possible to install one single LAN • Otherwise, a few LAN is connected through a link (site-wide (backbone) LAN) 10 Jaringan Komunikasi Data (3) LAN (Local Area (data) Network) • More than 2 computers involved in an application A LAN 1 B Technical and office automation LAN 2 C LAN 3 Site-wide (backbone) LAN Mainframe or minicomputer, advanced workstation or personal computer Bridge 11 Jaringan Komunikasi Data (4) LAN (Local Area (data) Network) Manufactoring cells Factorywide (backbone) LAN Intracell LAN Manufactoring automation 12 Jaringan Komunikasi Data (5) WAN (Wide Area (data) Network) • when computers are located in different establishments (sites), and then used public carrier facilities • Different types depend on the nature of applications e.g.: • Enterprisewide Private Network: when computers belong to the same enterprise and having a requirement to transfer some amounts of data between sites by leasing transmission lines (circuits) from the public carriers and install a private switching system at each site • PSDN – Public Switched Data Network: similar as above, where public carrier network is used • ISDN – Integrated Services Digital Networks: similar, with ability to transmit data without modems 13 Jaringan Komunikasi Data (6) • Enterprisewide Private Network: 14 Jaringan Komunikasi Data (7) computer • PSDN: computer TC PSDN computer computer PSDN = Public Switched Data Network TC TC = Terminal Controller = Interface standards = Communication subsystem (hardware and software) 15 Jaringan Komunikasi Data (8) • ISDN: voice handset voice handset NTE NTE computer computer ISDN voice handset NTE NTE computer voice handset computer NTE = Network termination equipment ISDN = Integrated Services Digital Network 16 Jaringan Komunikasi Data (9) Internetwork or Internet; where communication facility include multiple networks such as LAN-WAN-LAN Broadband Multiservice Networks • Connect workstations that support desktop video telephony, videoconferencing and more general multimedia services • ‘Broadband’ means of its high transmission bit rates ATM (Asynchronous Transfer Mode) • An approach for transmission and switching within the networks • Used to distinguish data communication & speech and video communication 17 Jaringan Komunikasi Data (10) • Worldwide interwork: Site-wide LAN Site-wide LAN National PSDN Ground station Satellite Site-wide LAN National PSDN Ground station = Gateway Site-wide LAN 18 Jaringan Komunikasi Data (11) MAN (Metropolitan Area Network) • A new network with the interconnection of ATM LANs; and data-only LANs are distributed around a town or city ATM LAN Data-only LAN ATM MAN ATM WAN ATM MAN Data-only LAN ATM LAN Data-only workstation / server Multimedia workstation / server 19 Standards (1) Evolution: • 1970s – Mainframe, PSTN • 1980s – Network computing, separate data and voice networks • 1990s – Personal computing, workstations, LAN, ISDN, Internet • 2000s – Global integrated multimedia infomedia network Provides a model for development Allows a product to work regardless of the individual manufacturer Developed by cooperation among standards organizations, e.g. ISO, ITU-T, IEEE, IETF 20 Standards (2) Documented agreements containing technical specifications • To be used consistently as rules, guidelines, or definitions of characteristics of a system • To ensure that adhered systems are fit for their purposes Standards contribute to • Making life simpler • Open systems • Ensure compatibility • Bring costs down by encouraging competition Most telecom related standards are international 21 Standards (3) Enhanced product quality and reliability Simplification for improved usability Increased distribution efficiency, and ease of maintenance Ensure compatibility and interoperability Encourage competition • Bring costs down • Ensure stable supply Developed by cooperation among standards organizations, e.g. ISO, ITU-T, IEEE, IETF 22 Standards (4) ISO: International Standards Organization • A worldwide federation of national standards bodies from some 100 countries, one from each country • A multinational organization dedicated to worldwide agreement on international standards in a variety of fields • Established in 1947 • Example of standards: ISO OSI Reference Model 23 Standards (5) ITU-T: International Telecommunications Union – Telecommunication Standards Sector • United Nations organization that develops standards for telecommunication • Headquartered in Geneva, Switzerland • International organization whithin which governments & private sectors coordinate global telecom networks and services • Activities include: ‣ Coordination, development, regulation and standardization of telecommunication ‣ Organization of regional & world telecom events 24 Standards (6) ITU-T: International Telecommunications Union – Telecommunication Standards Sector (cont) • Example of standards: ‣ V-series: define data transmission over phone line ‣ X-series: define transmission over public digital networks IEEE: Institute of Electrical and Electronics Engineers • A non-profit, technical professional association of more than 377,000 individual members in 150 countries. • Largest national profession group • Example of standards: IEEE 802 series 25 Standards (7) IETF: Internet Engineering Task Force • Standards body for the Internet itself • A large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet • Develops and reviews specification intended as Internet Standards • The actual technical work is done in its working groups, which are organized by topic into several areas (e.g. routing, network management, security, etc.) • Most of the work is handled via mailing list, however, IETF also holds meetings three times per year • Example of standards: RFC 2616 – HTTP protocol 26 Standards (8) ISOC: Internet Society • International organization for global cooperation and coordination for the Internet and its internetworking technologies and applications • Its members reflect the breadth of the entire Internet community and consist of individuals, corporations, non-profit organizations, and government agencies • Its principal purpose is to maintain and extend the development and availability of the Internet and its associated technologies and applications – both as an end in itself, and as a means of enabling organizations, professions, and individuals worldwide to more effective collaborate, cooperate, and innovate in their respective fields and interests 27 Komunikasi Data Pengantar – kuliah ke 2 • OSI model arsitektur Lapisan dan struktur model Fungsi setiap lapisan 1 ISO Reference Model • ISO has taken a model to overcome difficulties to test and modify softwares; since the software for a communication subsystem, were often based on a single, complex, unstructured program, (written in assembly language) with many interacting components The communication subsystem here consists of a complex piece of hardware and software • ISO Reference Model has taken a layered approach, where the complete communication subsystem is broken down into a number of layers, each of which performs a well-defined function • Conceptually, each layers can be considered as performing generic functions: Network-dependent functions Application-oriented functions 2 ISO Reference Model • Three different operational environments: Network environment: concern with the protocols and standards relating to the different types of underlying data communication networks OSI environment; include the network environment and adds additional application-oriented protocols and standards to allow end systems (computers) to communicate with one another in an open way Real systems environment; build on the OSI environment and concerned with a manufacturer’s proprietary software and services; have been developed to perform a particular distributed information processing task 3 ISO Reference Model Diagram environment: Computer A Computer B AP AP Application-oriented functions Application-oriented functions Network-dependent functions Network-dependent functions Data network Network environment OSI environment Real systems environment 4 ISO Reference Model • Network-dependent and application-oriented (network-independent) components of the OSI model are implemented as a number of layers • Each layer performs a well-defined function in the context of the overall communication subsystems; and has a well-defined interface with the layers immediately above and below • The function of each layer is specified formally as a protocol that defines the set of rules and conventions used by the layer to communicate (exchanging messages, both user data and additional control information), with a corresponding (similar) peer layer in another (remote) system • The implementation of a particular protocol layer is independent of all other layers 5 ISO Reference Model The logical structure of ISO Reference Model: Computer A Computer B AP AP Application layer AL (7) Presentation layer PL (6) Session layer SL (5) Transport layer TL (4) Network layer NL (3) Link layer LL (2) Physical layer PL (1) Data network Network environment OSI environment Real systems environment 6 ISO Reference Model • The network-dependent layers (three lowest layers) are concerned with the protocols associated with the data communication network being used to link the two communicating computers • The application-oriented layers (three upper layers) are concerned with the protocols that allow two end-user application processes to interact with each other, normally through a range of services offered by the local operating system • The intermediate transport layer (4) masks the upper application-oriented layers from the detailed operation of the lower network-dependent layers; it develops the services provided by providing the application-oriented layers with a network-independent message interchange service 7 Function of each layer of the OSI model: End-user application process Distributed information services File transfer, access and management, document and message interchange, job transfer and manipulation Application layer Syntax-independent message interchange services Transfer syntax negotiation, data representation transformations Dialog and synchronization control for application entities Presentation layer Session layer Network-independent message interchange services End-to-end message transfer (connection management, error control, fragmentation, flow control) Network routing, addressing, call set-up, and clearing Data link control (framing, data transparency, error control) Mechanical and electrical network interface definitions Transport layer Network layer Link layer Physical layer Physical connection to network termination equipment Data communication network 8 ISO Reference Model • Each layer provides a defined set of services to the layer immediately above; also uses the services provided by the layer immediately below it to transport the message units associated with the protocol to the remote peer layer example: transport layer provides a network-independent message transport service to the session layer above it and uses the service provided by network layer below it to transfer a set of message units associated with the transport protocol to a peer transport layer in another system 9 ISO Reference Model • Network-dependent layers Physical layer Link layer Network layer • Application-oriented layers Transport layer Session layer Presentation layer Application layer 10 ISO Reference Model Physical layer • Concern with the physical and electrical interfaces between the user equipment and the network terminating equipment • Provide the link layer with a means of transmitting a serial bit stream between two equipments • Examples: wires, connectors, voltages, data rates 11 ISO Reference Model Link layer • Develop a physical connection provided by the particular network to provide network layer with a reliable information transfer facility • Responsible for functions such as error detection and retransmission messages (if there is a transmission error) • Examples: physical addressing, network topology, error notification, flow control • Two types of services: Connectionless, treats each information frame as a self-contained entity that is transferred using a best-try approach. If errors are detected in a frame, then the frame is simply discarded Connection oriented, try to provide an error-free information transfer facility 12 ISO Reference Model Network layer • Responsible for establishing and clearing a network wide connection between two transport layer protocol entities • It includes functionality as network routing (addressing) and, flow control across the computer-to-network interface • In internetworking, it provides various harmonizing functions between the inter-connected networks 13 ISO Reference Model Transport layer • Interface between the higher application-oriented layers and the underlying network-dependent protocol layers • Provide the session layer (one layer above) with a message transfer facility that is independent of the underlying network type; therefore transport layer hides the detailed operation of the underlying network from the session layer • It offers a number of classes of service to compensate for the varying quality of service (QOS) provided by the network layers associated with the different types of network • Five classes of services ranging from class 0 (provides only basic functions needed for connection establishment and data transfer) to class 4 (provides full error control and flow control procedures 14 ISO Reference Model Session layer • Interhost communication: establishes, manages and terminates sessions between applications • Allows two application layer protocol entities to organize and synchronize their dialog and manage their data exchange • Responsible for setting up (& clearing) a communication (dialog) channel between two communicating application layer protocol entities (presentation layer protocol entities in practice) for the duration of the complete network transaction • A number of optional services provided: Interaction management; data exchange associated with a dialog may be duplex or half-duplex (where each protocol here provides facilities for controlling the exchange of data/dialog units in a synchronized way) 15 ISO Reference Model Session layer • A number of optional services provided: Synchronization; for lengthy network transactions, the user (through the services provided by the session layer) may choose periodically to establish synchronization points associated with the transfer. If a fault develops during a transaction, the dialog may be restarted at an earlier synchronization point Exception reporting; non-recoverable exceptions arising during a transaction can be signaled to the application layer by the session layer 16 ISO Reference Model Presentation layer • Concerned with the representation (syntax) of data during transfer between two communicating application processes • Data security; encrypted/enciphered (using a key) data sent is (hopefully) known only by the intended recipient presentation layer, and later the key is used to decrypts (deciphers) the received data before passing it onto the intended recipient 17 ISO Reference Model Application layer • Provides the user interface (an application program/process) a range of network wide distributed information services examples: file transfer access and management, general document and message interchange services (e-mail) • The layer provides other services as: Identification of the intended communication partner by name or by address Determination of the current availability of an intended communication partner Establishment of authority to communicate Agreement on privacy (encryption) mechanism 18 ISO Reference Model Application layer • The layer provides other services as: Authentication of an intended communication partner Selection of the dialog discipline, including the initiation and release procedures Agreement on responsibility for error recovery Identification of constraints on data syntax (character sets, data structures, etc.) 19 Data Encapsulation 20 Data Encapsulation 21 ISO vs TCP/IP Reference Model 22 TCP/IP Reference Model The importance of TCP/IP Although the OSI model is universally recognized, the historical and technical open standard of the Internet is TCP/IP (Transmission Control Protocol/Internet Protocol) The TCP/IP model and the TCP/IP protocol stack make data communication possible between any computers, anywhere in the world Four layers in TCP/IP model: Application; Transport; Internet; Network 23 TCP/IP Reference Model Application layer • The designer felt that the higher level protocols should include the session and presentation layer details • They created this layer that handles high-level protocols, issues of representation, encoding and dialog control • The TCP/IP combines all application-related issues into one layer, and assumes this data is properly packaged for the next layer 24 TCP/IP Reference Model Transport layer • It deals with the quality of service issues of reliability, flow control and error correction • One of its protocols, the transmission control protocol (TCP), is a connection-oriented protocol that provides excellent and flexible ways to create reliable, well-flowing, low-error network communications • It dialogues between source and destination while packaging application layer information into units called segments 25 TCP/IP Reference Model Internet layer • It governs by the Internet protocol (IP) • Send source packets from any network on the internetwork and have them arrive at the destination independent of the path and networks they took to get there • Best path determination and packet switching • Example: when we mail a letter, we do not know how it gets there (various possible routes), but we do care that it arrives 26 TCP/IP Reference Model Physical layer • It also called the host-to-network layer • Concerned with all issues that an IP packet requires to actually make a physical link • This includes a LAN and WAN technology details, and all the details in the OSI physical and data link layers 27 TCP/IP Reference Model 28 Comparing TCP/IP with OSI • TCP/IP combines the presentation and session layer issues into its application layer • TCP/IP combines the OSI data link and physical layers into one layer • TCP/IP appears simpler because it has fewer layers • TCP/IP protocols are the standards around which the internet developed, so the TCP/IP model gains credibility just because of its protocols. 29 TRANSMISI DATA & MEDIA TRANSMISI TERMINOLOGI (1) Network Devices Terminologi (2) Terminologi (3) Frekuensi, Spektrum dan Bandwidth Sinyal-Sinyal Kontinu dan Diskrit Sinyal Periodik Gelombang Sinus Keragaman Gelombang Sinus Panjang Gelombang (Wavelength) Frequency Domain Concepts Add Frequency Component Frequency Domain Analog And Digital Data Transmission Data Acoustic Spectrum Signal Data & Signal Media on PC Ethernet Modem PC Card (WLAN) DVB Ethernet - Architecture Ethernet – IEEE 802.3 10Base5 – Thick wire coaxial 10Base2 – thin wire coaxial / cheaper net 10BaseT – Twisted Pair 10BaseF – Fiber Optics 100BaseT – Fast Ethernet Ethernet – 10Base5 Ethernet – 10Base2 Ethernet – 10BaseT NIC Connector type Coaxial Cable Bayonet Nut Connector (BNC) Twisted Pair Cable RJ-45 (8 wire) Fiber Optics SMA connectors Modem Allows modems of different vendors to operate together Define How modems operate: Modulation techniques Data compression technique Error detection strategy Modem – Analog vs Digital Analog Infinite number of levels Conform to voice pattern Times from highest to lowest and back to the highest point in one second is the frequency Can be transmitted over long distance Digital Only two levels (high and low) Conforms to how computers operate Cannot transmitted over long distance Modem - Connection Modem – Internal/External PC Card – WLAN (802.11) Bypassing last mile PC Card (WLAN) - 1 PC Card (WLAN) - 2 PC Card (WLAN) – Indoor Antena DVB – DVB/IP Tech The Digital Video Broadcast over Internet Protocol (DVB/IP) system is functionally an IP-over-Ethernet simplex satellite service that incorporates frame-relay type traffic management. At its core, DVB is a modemon-a-chip, plus an intelligent multiplexer. The antenna is typically a CATV dish, 2.4 3.8 meters in diameter (for C-band), or 0.6 1.8 meters (for Ku-band). DVB – DVB/IP Routing DVB - Adaptability DVB – DVB Card Media Transmisi Wired Twisted Pair – UTP Coaxial Fiber Wireless Microwave Satellite Twisted Pair Two wired wrapped in a twisted fashion Designed to reduce cross-talk due inductance Still subject to interference from stray signal Primarily used for local loop connections and LANs Comparatively narrow bandwidth UTP – Unshielded Twisted Pair Pair 1 White/blue Blue Pair 2 W h i t e / O r a n g e Orange Pair 3 Pair 4 White/Green Green White/Brown Brown RJ45 Connector UTP – Straight Cable Wo/O Wg/B Wb/G Wbr/Br UTP – Cross Cable Wg/G Wo/B Wb/O Wbr/Br UTP - Category Category CAT 1 Maximum Data Rate Less than 1 Mbps Usual Application Analog voice (POTS), Integrated Services Digital Network Basic Rate Interface in ISDN Doorbell wiring CAT 2 4 Mbps Mainly used in the IBM Cabling System for token ring networks CAT 3 16 Mbps Voice and data on 10BASE-T Ethernet CAT 4 20 Mbps Used in 16 Mbps Token Ring Otherwise not used much CAT 5 100 Mbps 100 Mbps TPDDI (100BASE-T or Fast Ethernet) 1000 Mbps (4 pair) 155 Mbps ATM Gigabit Ethernet CAT 5E 100 Mbps 100 Mbps TPDDI (100BASE-T or Fast Ethernet) 155 Mbps ATM CAT 6 200-250 MHz Super-fast Broadband Applications. Coaxial Center lead conducts the signal Protected by insulation and braded wire Used mostly for television and connections to antenna Larger Bandwidth but large in size Fiber Optical Signal transmitted by photos rather than electrons Dramatically higher bandwidth Used mostly for backbone communication connections, very high speed LANs and fast network connections Fiber Optical Wireless Transmission Microwave – Antenna (1) Omni 2.4GHz 8dBi Microwave – Antenna (2) Sectoral 14dBi 180deg Microwave – Antenna (3) Sectoral 17dBi 90deg Microwave – Antenna (4) Direct (Grid) 2.4 GHz 24dB Microwave – Power AMP Satellite - Broadband Media Comparison Multiplexer HUB Switch Bridge Router Hub versus Switch Hub provide connection to all ports (i.e. in one port and out all other ports). Passive hub – no signal regeneration Active hub – provide signal regeneration Switch direct the message from appropriate port (directs a message from the input port to the desired output port). utilization More expensive but better bandwidth Hub versus Switch LAN Hub device Hub LAN Switch device Switch Bridge – Access Point (WLAN) Router Connecting different segment Have different interfaces (Ethernet, WAN-Serial, Fiber, etc) Table Routing Router Komunikasi Data Interface Komunikasi & Transmisi Data – kuliah ke 4 Interface komunikasi – Sumber redaman dan distorsi Beberapa tipe redaman dan distorsi Pengaruh redaman dan distorsi pada sinyal – Delay propagasi - pengaruhnya pada sinyal Transmisi Data – Dasar transmisi data: beberapa tipe mode transmisi – Transmisi sinkron dan asinkron: persamaan & perbedaan – Teknik deteksi kesalahan: beberapa metode pendeteksian – Kompresi data: beberapa tipe algoritma kompresi 1 Attenuation & Distortion Various attenuation & distortion effects that affects any signal carried on a transmission medium: • Attenuation • Limited bandwidth • Delay distortion • Noise 2 Attenuation & Distortion Attenuation • Signal attenuation - if a signal that propagates along a transmission medium (line) gets an amplitude decreasing • A limit is set on the length of cable that can be used to ensure that receiver can detect and interpret the received attenuated signal correctly • If longer cable is needed, one or more amplifiers (=repeaters) are inserted at intervals along the cable to restore the received signal to its original level 3 Attenuation & Distortion Attenuation • Signal attenuation increases as a function of frequency; since signal consists of a range of frequencies • Solutions: – By designing amplifiers to amplify different frequency signals by varying amounts – By using equalizers to equalize the attenuation across a defined band of frequencies 4 Attenuation & Distortion Attenuation • Measurement of attenuation and amplification (also known as gain) is in decibels (dB) Attenuation 10 log10 P1 dB P2 Amplification 10 log 10 P2 dB P1 where P1 is the transmitted signal power level & P2 is the received power; both in watts. • Therefore, decibels are dimensionless and simply a measure of the relative magnitude of the two power levels • The use of logarithms means that the overall attenuation/amplification of a multisection transmission channel can be determined by summing together the attenuation/amplification of the individual sections 5 Attenuation & Distortion Attenuation • Each signal element can represent more than a single binary digit; e.g. when binary informations are transmitted over a limited-bandwidth channel such as the PSTN, we often use more than two signal levels. • If the number of signal levels is M, then the number of bits per signal element m, is given by: m log2 M • The rate of change of the signal is known as the signaling rate (Rs); measured in baud. R Rs log2 M 6 Attenuation & Distortion Limited bandwidth • Any communications channel/transmission medium has a defined bandwidth associated with it; which specifies the band of sinusoidal frequency components that will be transmitted by the channel unattenuated • Therefore, when transmitting data over a channel, it is needed to quantify the effect of the channel bandwidth on the transmitted data signal • From the fourier analysis, it is known that any periodic binary sequence is made up of an infinite series of sinusoidal signals including the fundamental frequency component, f0, and its harmonic components. 7 Attenuation & Distortion Limited bandwidth • When a binary signal is transmitted over a channel, only those frequency components that are within the channel bandwidth will be received • From the figure: the larger the channel bandwidth, the higher-frequency components are received and the closer is the received signal to the original (transmitted) signal • The maximum information transfer rate of noiseless transmission channel C, is given by Nyquist expression as: C 2W log2 M Where W is the bandwidth of the channel in Hz, and M is the number of levels per signaling element 8 Attenuation & Distortion 9 Attenuation & Distortion Limited bandwidth • In practice, with binary data, extra bits are added for transmission control purposes; therefore the useful data rate is often less than the actual bit rate • When information are transmitted over a communications channel, three rates are involved: the signaling rate, the bit rate, and the data rate; all of which may be the same or different • Since the duration of each bit, Tb (in sec.), is the reciprocal of the bit rate, R, (in bps), then the bandwidth efficiency of a transmission channel, B, that defined as R/W, can be derived as : m 1 R B bps Hz-1 W WTs WTb 10 Attenuation & Distortion Limited bandwidth • From the expression, it can be assumed that the higher the bit rate, relative to the available bandwidth, the higher bandwidth efficiency • Typical values of B range from 0.25 to 3.0 bps Hz-1 where the first corresponding to a low bit rate relative to the available bandwidth, and the second a high bit rate that requires a relatively high signaling rate • In general, the higher the bandwidth efficiency, the stricter are the design parameters of the associated equipment and hence the higher cost 11 Attenuation & Distortion Delay distortion • When a digital signal is transmitted, the various frequency components making up the signal arrive at the receiver with varying delays. This results in delay distortion of the received signal • The amount of distortion increases as the bit rate of the transmitted data increases, since some of the frequency components associated with each bit transition are delayed and start to interfere with the frequency components associated with a later bit • It also called intersymbol interference, since the received signal, that is normally sampled at the nominal center of each bit cell, can lead to incorrect interpretation of the received signal as the bit rate increases 12 Attenuation & Distortion Noise • Line noise level, a random perturbations that present on the line even when no signal is being transmitted • Signal-to-noise ratio (SNR) is the ratio of the average power in a received signal S, to the power in the noise level N; normally expressed in decibels; given as: SNR 10 log 10 S dB N 13 Attenuation & Distortion Noise • High SNR means a high power signal relative to the existing noise level, resulting a good-quality received signal; on the other hand, a low SNR means a low-quality received signal • Shannon-Hartley law is the expression to determine the theoretical maximum information (data) rate of a transmission channel related to the SNR ratio; given as: S C W log 2 1 bps N where C is the information (data) rate [bps], W is the bandwidth of the line channel [Hz], S is the average signal power [watts] and N is the random noise power [watts] 14 Attenuation & Distortion Noise examples: • Near-end crosstalk (NEXT) or self-crosstalk, caused by the strong signal output by a transmitter circuit being coupled and interfered with the much weaker signal at the input to the local receiver input • Impulse noise, caused by impulses of electrical energy associated with external activity or equipment being picked up by a signal line. Ex: switching circuits in old telephone exchanges (loud clicks on the line, etc) • Thermal noise, caused by the thermal agitation of electrons associated with each atom in the device or transmission line material. -present in all electronic devices and transmission media -all transmission media experience this noise at all temperatures above absolute zero 15 Attenuation & Distortion Noise examples: • - this noise is also known as white noise, since it is made up of random frequency components (across the complete frequency spectrum) of continuously varying amplitude In practice, it is important to determine the minimum signal level that must be used (relative to the noise level), to achieve a specific minimum bit error rate ratio bit error rate ratio is an acceptable low probability that a single bit will be misinterpreted by the receiver over a defined period ex: a bit error rate ratio of 10-4 means that, on average, 1 bit in every 104 received will be misinterpreted 16 Attenuation & Distortion Noise • The energy per bit in a signal, Eb is given by: E b [joules] ST b S [watt - second] R where S is the signal power [watts] and Tb is the time period for 1 bit [seconds], R is the data transmission rate that is equal to 1/T • The noise power density [watts Hz-1], the level of (thermal) noise in a bandwidth of 1 Hz in any transmission line is given by: N 0 kT [watt Hz -1 ] where k is Boltzmann’s constant (1.3803 x 10-23 joule K-1) and T is the temperature [K] • From these, to quantify the effect of noise, the energy per bit Eb is expressed as a ratio of the noise energy per Hz, N0 : 17 Attenuation & Distortion Noise • The effect of noise, the energy per bit Eb is expressed as a ratio of the noise energy per Hz, N0 : S S Eb R R N0 N0 kT Or in decibels: Eb S 10 log [dB] 10 log 10 N0 R 10 kT • It can be seen that, to achieve an acceptable Eb / N0 ratio (hence minimum bit error rate), the signal power level S is needed with increasing of the temperature T and bit rate R 18 Attenuation & Distortion Noise • Since N0 is the noise power density [watts Hz-1] and a channel of bandwidth is W Hz, the noise power in a received signal N given by N = W N0 then E S W b N 0 N R • Or in decibels: Eb [dB] 10 log N0 S 10 log 10 N 10 W 10 log 10 R 19 Propagation delay • There is always a short but finite time delay for a signal (electrical, optical or radio) to propagate (travel) from one end of a transmission medium to the other; known as the transmission propagation delay, Tp of the medium • Round-trip delay is the time delay between the first bit of a block being transmitted by the sender and the last bit of its associated acknowledgement being received The function is not only of the time taken to transmit the frame at the link bit rate (known as the transmission delay, Tx) but also of the propagation delay of the link Tp • The relative weighting of the two varies for different types of data link and hence the two times varies for different are often express as a ratio a: 20 Propagation delay • The relative weighting of the two varies for different types of data link and hence the two times varies for different are often express as a ratio T a: a p where Tx Tp physicalseparation S in meters velocity of propagation V in meters per second and Tx number of bits to be transmitted N link bit rate R in bits per second • If a < 1: the round trip delay is determined primarily by the transmission delay • If a = 1: both delays have equal effect • If a > 1: the propagation delay dominates 21 Data Transmission • In data communication, the term ‘data’ is reserved for describing a set/a block of one/more digitally encoded alphabetic and numeric characters that being exchanged between two devices • When a data communication facility is used to transfer this type of data, the two communicating parties (DTEs) must also exchange control messages (i.e. to overcome the effect of transmission errors) within the communication facility Coded bit patterns: codewords Encode Keyboard -EBCDIC (Extended Binary Coded Decimal Interchange Code) -ASCII (American Standards Committee for Information Interchange) Decode Printer 22 Data Transmission • Transfer modes: – Parallel transmission mode: multiple wires connected to each subunit, and separate wire carries each bit of data; results in minimal delays – Serial transmission mode: use a single pair of lines; the data is transmitted in a single bit of time using a fixed time interval for each bit 23 Data Transmission • Communication modes: – Simplex: used when data is to be transmitted in one direction only; i.e. in a data logging system in which a monitoring device returns a reading at regular intervals to the data gathering facility – Half-duplex: used when the two interconnected devices wish to exchange information (data) alternately; i.e. if one of the devices returns some data only in response to a request from the other. The two devices must be able to switch between send and receive modes after each transmission – Duplex/full-duplex: used when data is to be exchanged between the two connected devices in both directions simultaneously; i.e. if for throughput reasons data can flow in each direction independently 24 Data Transmission • For the receiving device to decode and interpret the pattern correctly, it must be able to determine: – The start of each bit cell period (in order to sample the incoming signal in the middle of the bit cell): bit or clock synchronization – The start and end of each element (character or byte): character or byte synchronization – The start and end of each complete message block (frame): block or frame synchronization • Transmission modes: synchronization method determined whether the transmitter and receiver clocks are independent (asynchronous) or synchronised (synchronous) 25 Data Transmission • Two different transmission modes: – Asynchronous transmission: each character (byte) is treated independently for clock (bit) and character (byte) synchronization purposes and the receiver resynchronizes at the start of each character received – Synchronous transmission: the complete frame (block) of characters is transmitted as a contiguous string of bits and the receiver endeavors to keep in synchronism with the incoming bit stream for the duration of the complete frame (block) 26 Data Transmission • Error control: – Need mechanism to detect when a bit is in error (parity bits, block checksums, etc) – parity bit: used in an asynchronous transmission, where a binary digit (bit) is added within each transmitted character – error check sequence: used in a synchronous transmission, where possible transmission errors on the complete frame, as the basic unit of transmission is in frame – Another schemes are exist to enable the receiver to obtain another copy of the original transmission when errors are detected 27 Data Transmission • Flow control: – The receiver may not be able to handle all the information sent due to limited processing power, buffer space, etc. – the control of the flow information between two DTEs when the two devices operate at different data rates; i.e. control of the mean output rate of the faster device, to prevent the communication network becomes congested – need when two devices are communicating through an intermediate network, as often the network will buffer only a limited amount of data – method to control the flow data transfer to ensure that the receiver does not lose any of the transmitted data because the receiving device has insufficient storage 28 Data Transmission • Data link protocols – protocol: a set of conventions or rules that must be adhered to by both communicating parties to ensure that information being exchanges across a serial data link is received and interpreted correctly – defines: error and flow control; data format (number of bits per element and type of encoding scheme being used); type and order of messages that are to be exchanged in order to achieve reliable (error free and no duplicates) information transfer between two communicating parties. 29 Asynchronous Transmission • Use when the data to be transmitted is generated at random intervals; i.e. a user at a keyboard communicating with a computer • Each transmitted character/byte is encapsulated between an additional start bit and one or more stop bits • The polarity of the start and stop bits is different; to ensures that there is always a minimum of one transition ( 1 0 1 ) betweneach successive characters 30 Asynchronous Transmission • Baud: the number of line signal transitions per second; if each transmitted signal can be in one of two states, baud and bits per second (bps) are equivalent. • In general, signaling rate is more general to use, since it avoid confusion. The data or information transfer rate represents the number of data bits per second (bps). I.e. a signaling rate of 300 baud with 4 bits per signaling element would yield a data rate of 1200 bps. • To achieve bit and character synchronization, the receiving transmission control circuit must be set to operate with the same characteristics as the transmitter in terms of the number of bits per character and the bit rate being used 31 Asynchronous Transmission Bit synchronization • The receiver clock runs asynchronously with respect to the incoming signal. In order for the reception process to work reliably, a scheme whereby the local receiver clock samples the incoming signal as near to the center of the bit cell as possible 32 Asynchronous Transmission Bit synchronization • Since the receiver clock (RxC) is running asynchronously with respect to the incoming signal (RxD), the relative positions of the two signals can be anywhere within the single cycle of the receiver clock. • The higher the clock rate ratio, the nearer the sampling instant will be to the nominal bit cell center. • The maximum bit rate normally used with this transmission is 19.2 kbps. 33 Asynchronous Transmission Character synchronization • The receiving transmission control circuit is programmed to operate with the same number of bits per character and the same number of stop bits as the transmitter. • After the start bit has been detected and received, the receiver achieves character synchronization simply by counting the programmed number of bits. • It then transfers the received character (byte) into a local buffer register and signals to the controlling device (e.g. microprocessor) that a new character (byte) has been received. • It then awaits the next line signal transition that indicates a new start bit (and character) is being received 34 Asynchronous Transmission Frame synchronization • Here, the receiver must be able to determine the start and end of each frame • Example of transmitting blocks of printable characters encapsulated by the complete block between two special (nonprintable) transmission control characters: STX (start-of-text; indicates the start of a new frame after an idle period) and ETX (end-of-text; indicates the end of the frame) • In transmitting binary data, the two transmission control characters STX and ETX are each preceded by a third transmission control character DLE (data link escape) 35 Asynchronous Transmission Frame synchronization • Character/byte stuffing: On receipt of each byte after the DLE-STX start-of-frame sequence, the receiver determines the next character/byte. If it is a DLE, the receiver processes the next byte to determine whether that is another DLE or ETX. If it is a DLE, the receiver discards it and awaits the next byte. If it is an ETX, this is taken as the end of the frame. 36 Synchronous Transmission • Use in transmission of large blocks of data at higher bit rates; the complete block or frame of data is transmitted as a contoguous bit stream with no delay between each 8-bit element. • To enable the receiving device to achieve the various level of synchronization: – The transmitted bit stream is suitable encoded so that the receiver can be kept in synchronism – All frames are preceded by one or more reserved bytes/characters for byte/character synchronization (to ensure the receiver reliably interprets the received bit stream on the correct byte/character boundaries – The contents of each frame are encapsulated between a pair of reserved character/byte synchronization 37 Synchronous Transmission • During the period between the transmission of successive frames, either synchronous idle characters/bytes are continuously transmitted to allow the receiver to retain bit and byte synchronism; or each frame is preceded by two or more special synchronizing bytes or characters to allow the receiver to regain synchronism 38 Error Detection • The concept behind error control is the prevention of delivery of incorrect messages (bits) to a higher level in the communication hierarchy • Two factors that determine the type of error-detection techniques: – Bit Error Rate (BER); whether the errors occur as random singlebit errors – Burst Error; whether the errors occur as groups of contiguous strings of errors • The BER is the probability P of a single bit being corrupted in a defined time interval. BER of 10-13 means that, on average, 1 bit in 103 will be corrupted during a defined time period 39 Error Detection • Two approaches to achieve a copy of (hopefully) correct information: – Forward error control; each transmitted character/frame contains additional (redundant) information so that the receiver can not only detect when errors are present but also determine where in the received bit stream the errors are. The correct data is obtained by inverting these bits. In practice, the number of additional bits, required to achieve reliable forward error control, increases rapidly as the number of information bits increases – Feedback (backward) error control; each character/frame includes only sufficient additional information to enable the receiver to detect when errors are present but not their location. A retransmission control scheme is used to request that another copy of information be sent 40 Error Detection • Widely used error detection schemes: parity, check sum, cyclic redundancy check Parity • Extra bit at the end of a character (5-7 bits) specifying how many of the bits are 1’s • The parity bit is said to be even if it is set to make the total number of 1’s even, and oddif it is set to make the total number of 1’s odd. • Can detect all odd numbers of bit errors in the message • Can not detect even numbers of bit errors – error cancellation • Hamming distance: number of bits in which two words differ • Typically used in asynchronous transmission, since timing and spacing between characters is uncertain. 41 Error Detection • Typically used in asynchronous transmission, since timing and spacing between characters is uncertain. 42 Error Detection Parity 43 Error Detection Block Sum Check • Block error rate: the probability of a block containing an error • Block (sum) check character check the error by using a vertical parity check that calculates parity over the same bit of multiple characters. • Used in conjunction with longitudinal parity check. • Overhead is related to number of bits and can be large 44 Error Detection Cyclic Redundancy Check (CRC) • Parity bits still subject to burst noise, uses large overhead (potentially) for improvement of 2-4 orders of magnitude in probability of detection • CRC is based on a mathematical calcilation performed on message. The following terms will be used: – M – message to be sent (k bits) – F – Frane check sequence (FCS) to be appended to message (n bits) – T – Transmitted message includes both M and F (k+n bits) – G – a (n+1) bit pattern (called generator) used to calculate F and check T 45 Data Compression • Mainly done in applications that involve public transmission facilities, such as PSTN, where charges are based on time (duration) and distance. If the time to transmit each block of data can be reduced, it will reduce the call cost • In practice, a range of compression algorithm can be applied, each suited to a particular type of data • Some modems (intelligent modems) now offer an adaptive compression feature which selects a compression algorithm to suit the type of data being transmitted • Some common types of data compression algorithm: 46 Data Compression Packed decimal • Used to compress a frame contain only numeric characters • It reduces the number of bits per character from seven (ASCII) to four by using a binary-coded-decimal (BCD); all have 011 in their three high-order bit positions 47 Data Compression Relative encoding • Send only the magnitude difference between each value together with a known reference value • Effective for data logging applications; ex. monitoring the water level of a river • In general, binary values with a bit-oriented protocol produce the largest saving 48 Data Compression Character suppression • A variation of the relative encoding scheme; use in a more general way to compress other character types • In transmitting frames contain printable characters, there are often sequences in the frame when the same character repeats; e.g. the space character. The control device at the transmitter scans the frame contents prior to transmission and, if a contiguous string of three or more characters is located, replaces these with the three-character seq. 49 Data Compression Huffman coding • Exploits the property that not all symbols in a transmitted frame occur with the same frequency e.g. in a frame comprising strings of characters, certain characters occur more often than others. • Instead of using a fixed number of bits per character, a statistical encoding scheme is used, where the most common characters are encoded using fewer bits than less frequent characters. • The character string to be transmitted is analyzed and the character types and their relative frequency is determined. The coding operation then creates an unbalanced tree with some branches (as codewords) shorter than others. The degree of imbalance is a function of the relative frequency of occurrence of the characters; the larger the spread, the more unbalanced is tree huffman code tree 50 Data Compression Huffman coding • Huffman (code) tree is a binary tree with branches assigned the value 0 or 1. The base of the tree (at geometric top), is known as root node, and the point where a branch divides is a branch node. The termination point of a branch is known as a leaf node, where the symbols being encoded are assigned. • The theoretical minimum average number of bits per codeword to transfer message string is known as the entropy, H, of the message. • The formula to calculate H is derived by Shannon: n H Pi log 2Pi (bits per codeword) i1 Where n is the number of characters in the message and Pi is the probability of character (i) occuring in the message. 51 Data Compression Huffman coding • It is most efficient when frequency distribution of the characters being transmitted is wide and long character strings are involved; it is not suitable for the transmission of binary-coded computer data since the 8-bit bytes generally occur with about the same frequency 52 Data Compression Dynamic Huffman Coding • Problem with basic Huffman encoding is that transmitter and receiver must know the code tree; here, updates Huffman tree dynamically. • With this method, if the character to be transmitted is currently present in the tree, its codeword is determined and sent in the normal way. If the character is not present (its first occurrence), the character I transmitted in its uncompressed form. • The encoder updates its Huffman tree either by incrementing the frequency occurrence of the transmitted character or by introducing the new character into the tree. • This means that the savings in transmission bandwidth start only when characters begin to repeat themselves; I.e. in transmitting the text files 53 Data Compression Fascimile compression • Effective for data logging applications • In general, binary values with a bit-oriented protocol produce the largest saving 54
