DATA COMMUNICATION (Basics of data communication, OSI layers.)
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1 DATA COMMUNICATION (Basics of data communication, OSI layers.) K.K.DHUPAR SDE (NP-II) ALTTC ALTTC/NP/KKD/Data Communication 1
2 Data Communications History 1838: Samuel Morse & Alfred Veil Invent Morse Code Telegraph System 1876: Alexander Graham Bell invented Telephone 1910:Howard Krum developed Start/Stop Synchronisation ALTTC/NP/KKD/Data Communication 2
3 History of Computing 1930: Development of ASCII Transmission Code 1945: Allied Governments develop the First Large Computer 1950: IBM releases its first computer IBM : IBM releases the First Commercial Computer IBM 360 ALTTC/NP/KKD/Data Communication 3
4 Main Contributors of Data Comm. Transmission Technology Packet Switching Technology Internet 1967: ARPANET by Advanced Research Project Agency (ARPA) of U.S. 1975: TCP/IP protocol LAN Technology DIX-Ethernet & IEEE 802 Networks WAN 1976: ISO releases HDLC & CCITT releases X.25 (PSPDN) ALTTC/NP/KKD/Data Communication 4
5 Various Networks Home Area Network (HAN) Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) Global Area Network (GAN) ALTTC/NP/KKD/Data Communication 5
6 Voice & Data In 70 s & 80 s main thrust in Wide Area Networking (WAN) was to put Data on Voice Circuits using Modem & on ISDN lines In 90 s the trend is reverse. Major Efforts were on putting Voice Over Data using: Voice Over Frame Relay Voice Over Internet Voice Over ATM etc ALTTC/NP/KKD/Data Communication 6
7 What is Data Communications? Exchange of digital information between two digital devices is data communication ALTTC/NP/KKD/Data Communication 7
8 Networking Networking is the convenient way of making information accessible to anyone, anytime & anywhere. Evolution of Networking Computers handling Large Data were developed viz. text, graphics,animation, sound & video Improvements in Personal Computers Small, powerful & cheap user friendly systems. Emergence of Computer Networks Distributed, Intelligent, High-speed LAN/WAN ALTTC/NP/KKD/Data Communication 8
9 What is Interoperability? The Capability of two or more computers of different vendors to transmit & receive data and to carry out processes as expected by the user is called Interoperability. IBM Computer AT &T Computer ALTTC/NP/KKD/Data Communication 9
10 Requirements of Data Communications At least Two Devices ready to communicate A Transmission Medium A set of Rules & Procedure for proper communication (Protocol) Standard Data Representation Transmission of bits either Serial or Parallel Bit synchronisation using Start/stop bits in case of Asynchronous Transmission In Synchronous Transmission the agreed pattern of Flag Signal encoding rules viz. NRZ or RZ And other higher layer protocol ALTTC/NP/KKD/Data Communication 10
11 Data Representations A group of bits are used to represent a character/number/ special symbol/control Characters 5-bit code can represent 32 symbols (2 5 =32) 7-bit code can represent 128 symbols (2 7 =128) & 8-bit code can represent 256 symbols (2 8 =256) ALTTC/NP/KKD/Data Communication 11
12 Code Set A code set is the set of codes representing the symbols Very common code sets are : ASCII : this is ANSI s 7-bit American Standard Code for Information Interchange ASCII code(7-bit) is often used with an 8 th bit known as parity bit used for detecting errors during Data Transmission Parity bit is added to the Most Significant bit (MSB) EBCDIC : this is IBM s 8-bit Extended Binary Coded Decimal Interchange Code ALTTC/NP/KKD/Data Communication 12
13 ASCII Code ASCII is defined in ANSI X3.4 Corresponding CCITT recommendation is IA5 (International Alphabet No.5) ISO specification is ISO 646 Total 128 codes 96 codes are graphic symbols (in Col. 2~7). 94 codes are printable And 2 codes viz. SPACE & DEL characters are non printable 32 codes control symbols (Col. 0 & 1) All are non printable ALTTC/NP/KKD/Data Communication 13
14 EBCDIC Code It is an 8-bit code with 256 symbols No parity bit for error checking The graphic symbols are almost same as ASCII Several differences in Control characters as compared to ASCII ALTTC/NP/KKD/Data Communication 14
15 Baudot Teletype code It is a 5-bit code also known as ITA2 (International Telegraph Alphabet No. 2). 32 codes are possible. With the help of Letter shift & Figure shift key same code is used to represent two symbols. Maximum symbols in this code are 58 Used in Telegraphy/Telex ALTTC/NP/KKD/Data Communication 15
16 Data Transmission Data Transmission means movement of the bits over a transmission medium connecting two devices Two types of Data Transmission are: Parallel Transmission Serial Transmission ALTTC/NP/KKD/Data Communication 16
17 Parallel Transmission In this all the bits of a byte are transmitted simultaneously on separate wires. Practicable if two devices are close to each other e.g. Computer to Printer, Communication within the Computer Transmitter Receiver ALTTC/NP/KKD/Data Communication 17
18 Serial Transmission Bits are transmitted one after the other Usually the Least Significant Bit (LSB) has been transmitted first Serial Transmission requires only one circuit interconnecting two devices Suitable for Transmission over Long distance MSB LSB Transmitter Receiver ALTTC/NP/KKD/Data Communication 18
19 What is a Bit Rate? Number of bits that can be transmitted in 1 second If t p is the duration of the bit then the Bit rate R= 1/t p Bit duration need not be same as the pulse duration ALTTC/NP/KKD/Data Communication 19
20 Receiving Data bits Received Signal is never same as transmitted Clock signal samples & regenerates the original bits as it was transmitted Received Signal should be sampled at right instant. Otherwise it will cause bit error Transmitted Signal Received Signal Clock Signal Sampled Signal Recovered Signal ALTTC/NP/KKD/Data Communication 20
21 Modes of Transmission Two methods for Timing control for receiving bits Asynchronous Transmission Sending end commences the Transmission of bits at any instant of time No time relation between the consecutive bits During idle condition Signal 1 is transmitted Start bit before the byte and Stop bit at the end of the byte for Start/Stop synchronisation Synchronous Transmission is carried out under the control of the timing source No Start/Stop bits Continuous block of Data are encapsulated with Header & Trailer along with Flags ALTTC/NP/KKD/Data Communication 21
22 Asynchronous Transmission Start- Stop Synchronisation Direction of Transmission Idle Stop bit Start bit Idle Stop bit Start bit Note: Start bit is always 1 bit duration Start bit is always equal to 0 Stop bit may be 1 or 1.5 or 2 bits duration Stop bit is always equal to 1 Idle period time is arbitrary (variable) ALTTC/NP/KKD/Data Communication 22
23 Synchronous Transmission Flag identifies the Start and End of the block Receiver first detects the Flag (usually a fixed pattern) and then detects the other bits/bytes in the data field Complete Block along with the Flags is called a FRAME Direction of Transmission Flag Block Of bytes Flag Idle data Flag Block Of bytes Flag Block 2 Block 1 ALTTC/NP/KKD/Data Communication 23
24 Signal Encoding For transmission of bits into electrical signals for two binary states simple +ve and ve voltages are not sufficient. Sufficient Signal transition should be present to recover the clock properly at the receiving end Bandwidth of the signal should match with transmission medium Several ways to represent the bits as electrical signals Two broad classes are: Non-Return to Zero (NRZ) and Return to Zero (RZ) ALTTC/NP/KKD/Data Communication 24
25 Non Return to Zero (NRZ) Codes NRZ-L Coding (Data Bits) Clock Signal NRZ-M Coding NRZ-S Coding NRZ-L : Non-Return to Zero Level (coded according to binary values of the Data bits) NRZ-M : Non-Return to Zero on Mark (Voltage Transition takes place on Mark (1) NRZ-S : Non-Return to Zero on Space (Voltage Transition takes place on Space (0) ALTTC/NP/KKD/Data Communication 25
26 Return to Zero (RZ) Codes If there is continuous string of 0 s or 1 s in NRZ code it is very difficult to recover the clock signal Hence Return to Zero code (RZ) was implemented Clock can be extracted from the Return to Zero code by the receiver using lot of transitions RZ signals are the combination of NRZ-L Signal + Clock Signal Various RZ codes are: Manchester Code Bi-phase-M Code Bi-phase-S Code Differential Manchester Code ALTTC/NP/KKD/Data Communication 26
27 Return to Zero (RZ) Codes NRZ-L Coding (Data Bits) Clock Signal Manchester Coding Bi-phase-M Coding Bi-phase-S Coding Differential Manchester Coding Manchester Code: Binary 1 is logical AND of 1 & Clock & binary 0 is logical AND of 0 & Clock. Also known as Bi-phase-L code Bi-Phase-M Code: Transition always at the beginning of bit interval and Binary 1 has another transition in the middle of the bit interval Bi-Phase-S Code: Transition at the beginning of bit interval as in Bi-Phase-M code and Binary 0 has another transition in the middle of the bit interval Differential Manchester Code:Transition always at the Middle of bit interval and Binary 0 has another transition in the beginning of the bit interval ALTTC/NP/KKD/Data Communication 27
28 Data Transmission & Data Communication Data Transmission deals with: the physical movement of information. polarity, synchronisation, clock, electrical characteristics, modulation, demodulation etc. Data Communication deals with: Meaningful exchange of information between the communication entities e.g. dialogue discipline, interpretation of messages, acknowledgement etc. ALTTC/NP/KKD/Data Communication 28
29 Synchronous & Asynchronous Communication Synchronous Communication is: more disciplined information exchange entities send a message only when it is permitted to do so Asynchronous Communication is: less disciplined can send a message whenever it wishes to ALTTC/NP/KKD/Data Communication 29
30 Directional Capabilities for Data Exchange Three possibilities are: Transfer in one direction only Transfer in either direction but one at a time Transfer in both the direction simultaneously Direction Capability One Direction only Transmission Simplex (SX) Communication One Way (OW) One direction at a time Both directions at the same time Half Duplex (HDX) Full Duplex (FDX) Two-Way Alternate (TWA) Two-Way Simultaneous (TWS) ALTTC/NP/KKD/Data Communication 30
31 OSI Layers The OSI model is built of seven ordered layers: Layer-1: Physical Layer-2: Data Link Layer-3: Network Layer-4: Transport Layer-5: Session Layer-6: Presentation Layer-7: Application Application Presentation Session Transport Network Data Link Physical ALTTC/NP/KKD/Data Communication 31
32 OSI Layers The seven layers can be thought of as belonging to three sub groups Network Support Layers (Layers 1-3) Deal with the physical aspects of moving data from one device to another User Support Layers (Layers 5-7) Allow interoperability among unrelated software systems Layer-4 ensures end to end reliable data transmission ALTTC/NP/KKD/Data Communication 32
33 Hierarchical communication. Within a single machine, each layer calls upon the services of the layer just below it The passing of data and network information is made possible by an interface between each pair of adjacent layers The messages exchanged between the adjacent layers, to obtain the required services, are called Interface Control Information (ICI) ALTTC/NP/KKD/Data Communication 33
34 Data Units Protocol Control Information (PCI) (N)-PCI is the protocol control information exchanged between the (N)- entities to coordinate their functions Service Data Units (SDU) (N)-SDU is the data unit transferred between the ends of a (N)-connection whose identity is preserved during the transfer Protocol Data Unit (PDU) (N)-PDU is the combination of (N)-PCI and (N)-SDU ALTTC/NP/KKD/Data Communication 34
35 Layer-1(Physical) A P S T N D P First of three network support layers Concerned with physical transmission of data bits and ensures that a bit entering at one end of the transmission media reaches the other end Deals with the mechanical and electrical specifications of the interface and transmission medium e.g. Optical, coax, RF, twisted pair etc. Defines the type of encoding i.e. how 0s and 1s are changed to signals Defines data rate / transmission rate i.e. defines the duration of a bit ALTTC/NP/KKD/Data Communication 35
36 Layer-1(Physical) A P S T N D P Responsible for synchronisation of sender and the receiver clocks Concerned with the connection of the devices to the medium Point-to-point configuration Multipoint configuration Physical topology Mesh; Star; Ring; Bus Transmission Mode Simplex; Half-Duplex; Full-Duplex ALTTC/NP/KKD/Data Communication 36
37 Layer-1(Physical) From Data Link Layer L2 Data To Data Link Layer L2 Data L Physical Layer L Transmission Medium ALTTC/NP/KKD/Data Communication 37
38 Layer-2 (Data Link) A P S T N D P Second of three network support layers Divides the bit stream received from network layer into manageable data units called frames Transforms the physical layer to a reliable link by adding mechanism to detect and retransmit damaged frames Responsible for physical addressing of the devices Responsible for link-by-link flow control and error free delivery of data Responsible for Media Access Control ALTTC/NP/KKD/Data Communication 38
39 Layer-2 (Data Link) From Network Layer L3 Data To Network Layer L3 Data T2 H2 Data Link Layer T2 H2 L L To Physical Layer From Physical Layer ALTTC/NP/KKD/Data Communication 39
40 Layer-3 (Network) A P S T N D P Last of the three network support layers Responsible for Source-to-Destination delivery of individual packets across multiple links If two systems are connected to the same link there is usually no need for a network layer Responsible for the unique logical addressing of the sender and the receiver Responsible for routing of packets ALTTC/NP/KKD/Data Communication 40
41 Layer-3(Network) From Transport Layer L4 Data To Transport Layer L4 Data H3 Network Layer H3 L3 Data To Data Link Layer L3 Data From Data Link Layer ALTTC/NP/KKD/Data Communication 41
42 Layer-4 (Transport) A P S T N D P Responsible for Source-to-Destination delivery of the entire message Uses service-point address (port address) for end-to-end delivery Network layer gets each packet to correct computer, transport layer gets the entire message to the correct process Responsible for segmenting a message into transmittable segments At the destination the message is correctly reassembled ALTTC/NP/KKD/Data Communication 42
43 Segmentation and Reassembly From Session Layer To Session Layer L5 Data L5 Data H4 H4 Transport Layer H4 H4 L3Data L3Data L3Data To Network Layer L3Data From Network Layer ALTTC/NP/KKD/Data Communication 43
44 Layer-4 (Transport) A P S T N D P Utilises network layer to ensure reliable, sequenced data exchange Transport layer can be connectionless or connection oriented A connectionless transport layer treats each segment as an independent packet A connection oriented transport layer makes a connection with the transport layer at the destination machine before delivering the packets After all the data is transmitted, the connection is terminated ALTTC/NP/KKD/Data Communication 44
45 Layer-4 (Transport) A P S T N D P Responsible for end-to-end flow control of data Responsible for end-to-end error control of data Error correction is usually achieved through retransmission ALTTC/NP/KKD/Data Communication 45
46 Layer-5(Session) A P S T N D P First of the three user support layers It is the network dialog controller It establishes, maintains, and synchronises the interaction between communicating systems It allows the communication between two processes to take place either in half-duplex or full-duplex Allows a process to add checkpoints (synchronisation points) into a stream of data ALTTC/NP/KKD/Data Communication 46
47 Layer-5(Session) From Presentation Layer L6 Data To Presentation Layer L6 Data H5 Session Layer SYN H5 L5 Data To Transport Layer L5 Data From Transport Layer ALTTC/NP/KKD/Data Communication 47
48 Layer-6 (Presentation) A P S T N D P Second of the three user support layers Concerned with the syntax and semantics of the information exchanged between two systems At sender end, changes the information from sender dependent format into a common format At the receiving end, changes the information from common format into its receiver dependent format ALTTC/NP/KKD/Data Communication 48
49 Layer-6 (Presentation) A P S T N D P Responsible for encryption and decryption of sensitive information Responsible for data compression of the data to be transmitted ALTTC/NP/KKD/Data Communication 49
50 Layer-6(Presentation) From Application Layer L7 Data Encoded, Encrypted and Compressed data To Application Layer L7 Data H6 Presentation Layer H6 To Session Layer L6 Data Decoded, Decrypted and Decompressed data L6 Data From Session Layer ALTTC/NP/KKD/Data Communication 50
51 Layer-7(Application) A P S T N D P Top of the three user support layers Enables the user, human or software, to access the network It provides user interfaces and support for services e.g. electronic mail, remote file access and transfer, shared database management and other types of distributed information services ALTTC/NP/KKD/Data Communication 51
52 Layer-7(Application) A P S T N D P Specific services provided by the application layer include Network Virtual terminal Software version of a physical terminal Allows user to log on to a remote host File Transfer, Access and Management Allows user to access, retrieve, manage and control files in a remote computer Mail Services Provides basis for forwarding and storage Directory Services Provides distributed database sources and access for global information about various services ALTTC/NP/KKD/Data Communication 52
53 An exchange using OSI model A-7 P-6 L7-Data L6-Data H7 H6 S-5 L5-Data H5 T-4 L4-Data H4 N-3 L3-Data H3 D-2 T4 L2-Data H2 P ALTTC/NP/KKD/Data Communication 53
54 An exchange using OSI model L7-Data L6-Data H7 H6 A-7 P-6 L5-Data H5 S-5 L4-Data H4 T-4 L3-Data H3 N-3 T4 L2-Data H2 D P-1 ALTTC/NP/KKD/Data Communication 54
55 Summary of OSI Layers Functions Translate, encrypt and compress data Reliable end to end delivery & error recovery Organise bits into streams; Node to node delivery Application 7-6 Interface Presentation 6-5 Interface Session 5-4 Interface Transport 4-3 Interface Network 3-2 Interface Data Link 2-1 Interface Physical Allow access to network resources Establish, manage and terminate sessions Movement of packets; Provide internetworking Transmit bits; Mechanical and electrical specifications ALTTC/NP/KKD/Data Communication 55
56 ALTTC/NP/KKD/Data Communication 56
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