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Compute Networks: 01 - Introduction to CN

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Computer Networks Introduction

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CSE/T/324A Computer Networks Topic 1- Introduction Sarbani Roy sarbani.roy@jadavpuruniversity.in Office: CC-5-7 Cell: 9051639328
Syllabus • Introduction: Uses of Computer Networks, Types of Computer Networks, OSI Reference Model, TCP/IP model [4L] • Review of Physical Layer [4L] • Data Link Control and Protocols: Link Layer Services, Error detection and Correction Techniques, Multi Access Protocols, Link Layer Addressing, Ethernet, Hubs, Switches and Switches, Point to Point Protocol, Asynchronous Transfer Mode, Multiprotocol Label Switching [6L] • Network Layer: Introduction, Virtual Circuit and Datagram Networks, IP Addressing, Subnetting, Routing Algorithms (Link State, Distance Vector, Hierarchical), Routing in the Internet (RIP, OSPF, BGP), Broadcast and Multicast Routing Algorithms, Routers, ICMP, IPv6 [8L] • Transport Layer: Introduction to Transport Layer Services, Multiplexing and Demultiplexing, Connectionless Transport: UDP, Principles of Reliable Data Transfer, Connection Oriented Transport: TCP, Principles of Congestion Control, TCP Congestion Control, Sockets and Socket Programming, Quality of services (QOS) [8L] • Application Layer: Web and HTTP, Domain Name Space (DNS), Electronic Mail (SMTP, MIME, IMAP, POP3), File Transfer Protocol, Cryptography [6L] • Introduction to Wireless and Mobile Networks [4L]
COs of Theory • CO1: Understand the layered architecture and explain the contemporary issues and importance of MAC sublayer of the Data Link Layer, network layer, transport layer and application layer of TCP-IP model, and how they can be used to assist in network design and implementation. • CO2: Understand the protocols of MAC sublayer of the Data Link Layer and describe the IEEE standards for Ethernet (802.3) and wireless LAN (802.11). • CO3: Understand internetworking principles and explain algorithms for multiple access, routing and different networking techniques and able to analyze the performance of these algorithms. • CO4: Explain the protocols in Transport Layer and able to design network applications using these protocols. • CO5: Explain the protocols in application layer and how they can be used to design popular internet applications.
COs of Lab • CO1: Design and implement error detection techniques within a simulated network environment. • CO2: Design and implement flow control mechanisms of Logical Link Control of Data Link Layer within a simulated network environment. • CO3: Design and implement medium access control mechanisms within a simulated network environment using IEEE 802 standards. • CO4: Design and implement network layer protocols within a simulated network environment or network tools using IEEE 802 standards. • CO5: Design and implement various applications using Transport layer protocols and Application layer protocols for its implementation in client/server environment and analyze the performance.
Suggested Readings 1. Data Communications and Networking, Behrouz A Forouzan, McGraw Hill 2. Computer Networks, by Andrew S. Tanenbaum, Prentice Hall India 3. Computer Networking: A Top-Down Approach Featuring the Internet, by James F. Kurose and Keith W. Ross, Pearson Education Slides are mainly prepared using online documents available of the above books and course materials of different institutes
REVIEW OF ERROR DETECTION IN DATA LINK LAYER
Computer Networks Lab CO1: Design and implement error detection techniques within a simulated network environment.
Error Detection • Data transmission can contain errors – Single-bit – Burst errors of length n (n: distance between the first and last errors in data block) • How to detect errors – If only data is transmitted, errors cannot be detected  Send more information with data that satisfies a special relationship  Add redundancy
Error Detection Methods • Vertical Redundancy Check (VRC) – Append a single bit at the end of data block such that the number of ones is even  Even Parity (odd parity is similar) 0110011  01100110 0110001  01100011 – VRC is also known as Parity Check – Performance: • Detects all odd-number errors in a data block
Error Detection Methods • Longitudinal Redundancy Check (LRC) – Organize data into a table and create a parity for each column 11100111 11011101 00111001 10101001 11100111 11011101 00111001 10101001 10101010 11100111 11011101 00111001 10101001 10101010 Original Data LRC
Error Detection Methods – Performance: • Detects all burst errors up to length n (number of columns) • Misses burst errors of length n+1 if there are n-1 uninverted bits between the first and last bit • Checksum – Used by upper layer protocols – Similar to LRC, uses one’s complement arithmetic
Cyclic Redundancy Check • Powerful error detection scheme • Rather than addition, binary division is used  Finite Algebra Theory (Galois Fields) • Can be easily implemented with small amount of hardware – Shift registers – XOR (for addition and subtraction)
Cyclic Redundancy Check • Let us assume k message bits and n bits of redundancy • Associate bits with coefficients of a polynomial 1 0 1 1 0 1 1 1x6 +0x5 +1x4 +1x3 +0x2 +1x+1 = x6 +x4 +x3 +x+1 xxxxxxxxxx yyyy k bits n bits Block of length k+n
Example • Send – M(x) = 110011  x5 +x4 +x+1 (6 bits) – P(x) = 11001  x4 +x3 +1 (5 bits, n = 4)  4 bits of redundancy – Form xn M(x)  110011 0000  x9 +x8 +x5 +x4 – Divide xn M(x) by P(x) to find C(x) = C(x) Send the block 110011 1001 • Receive No remainder  Accept 100001 1001 11001 10000 11001 1100110000 11001 00000 11001 11001 11001 1100111001 11001
Cyclic Redundancy Check • Let M(x) be the message polynomial • Let P(x) be the generator polynomial – P(x) is fixed for a given CRC scheme – P(x) is known both by sender and receiver • Create a block polynomial F(x) based on M(x) and P(x) such that F(x) is divisible by P(x) ) ( 0 ) ( ) ( ) ( x P x Q x P x F  
Cyclic Redundancy Check • Sending 1. Multiply M(x) by xn 2. Divide xn M(x) by P(x) 3. Ignore the quotient and keep the reminder C(x) 4. Form and send F(x) = xn M(x)+C(x) • Receiving 1. Receive F’(x) 2. Divide F’(x) by P(x) 3. Accept if remainder is 0, reject otherwise
Assignment 1: Design and implement an error detection module. Due on: 19-23 October 2020 (in your respective lab classes) • Design and implement an error detection module which has four schemes namely LRC, VRC, Checksum and CRC. Please note that you may need to use these schemes separately for other applications (assignments). You can write the program in any language. The program should accept the name of a test file (contains a sequence of frames) from the command line and then print the input data frame, signature block and the output data frame on console standard output. Next, test the same program to produce a PASS/FAIL result for following cases. (a) Error is detected by all four schemes. Use a suitable CRC polynomial (list is given in next page). (b) Error is detected by checksum but not by CRC. (c) Error is detected by VRC but not by CRC. [Note: Inject error in random positions in the input data frame. Write a separate method for that] • Please note that your design and implementation of these schemes can help you to make your assignment different from your classmates. Write the code by yourself and protect your code. The assignment is simple, but we encourage you to do improvisation with this.
Compute Networks: 01 - Introduction to CN
Report • Title: (Basic information like name, class, group, assignment number, problem statement and date (deadline and submission)). • Design: (short paragraph describing the purpose of the program, draw a structure diagram to reflect the procedural organization of the program as you have designed it in the previous step, input and output format) • Implementation: (code snippet, method description, interfaces etc.) • Test cases: (Describe the tests you will perform to verify the correctness of your program. This should be a thorough and exhaustive list of test cases designed to show that your program does everything it is supposed to do. The combined tests should exercise every line of code in your program. For each test case, give sample test data and state what you are checking. • Results: (Figures or graphs, tables ) • Analysis: (discussion on results considering different test cases, error, correctness, known bugs, possible improvements) • Comments: Evaluate the lab assignment. What did you learn from it? Was it too hard? (explain why?) Too easy? (explain why?) Suggest improvements if you can.
Networks • Network is a set of devices (often referred to as nodes) connected by communication links. • A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. A link can be a cable, air, optical fiber, or any medium which can transport a signal carrying information.
Computer Networks • Computer network connects two or more autonomous computers. • The computers can be geographically located anywhere.
Applications of Networks • Resource Sharing – Hardware (computing resources, disks, printers) – Software (application software) • Information Sharing – Easy accessibility from anywhere (files, databases) – Search Capability (WWW) • Communication – Email – Message broadcast • Remote computing • Distributed processing (Cloud Computing)
Some terms..CPS, IoT, Cloud
Network Components • Physical Media • Interconnecting Devices • Computers • Networking Software • Applications
Network Criteria • Performance – Depends on Network Elements – Measured in terms of Delay and Throughput • Reliability – Failure rate of network components – Measured in terms of availability/robustness • Security – Data protection against corruption/loss of data due to: – Errors – Malicious users
Physical Structures • Type of Connection – Point to Point - single transmitter and receiver – Multipoint - multiple recipients of single transmission • Physical Topology – Connection of devices – Type of transmission - unicast, mulitcast, broadcast
Types of Connections
Categories of Topology
Network Topology • The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions.
A fully connected mesh topology (five devices)
Star Topology • The star topology is the most commonly used architecture in Ethernet LANs. • Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.
A star topology connecting four stations
Bus Topology • Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.
A bus topology connecting three stations
Ring Topology • A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. • The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. – Single ring – All the devices on the network share a single cable – Dual ring – The dual ring topology allows data to be sent in both directions.
A ring topology connecting six stations
A hybrid topology: a star backbone with three bus networks
Mesh Topology • The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance. • It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions. • Implementing the mesh topology is expensive and difficult.
Categories of Networks • Local Area Networks (LANs) – Short distances – Designed to provide local interconnectivity • Wide Area Networks (WANs) – Long distances – Provide connectivity over large areas • Metropolitan Area Networks (MANs) – Provide connectivity over areas such as a city, a campus
LAN, MAN and WAN • Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network) • Network in a City is call MAN (Metropolitan Area Network) • Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)
LAN
WANs: a switched WAN and a point-to-point WAN
WAN technologies • There are two types of long distance communication technologies that are used for WANs. – Dedicated connection • A dedicated line is a full-time point-to-point connection provided by a communication carrier that lasts for the length of the lease period. – Switched connection • There are several types of switched connections: circuit switched, packet switched, and cell switched
Circuit Switching • In circuit switching, circuits are established prior to the transmission of data and torn down at the end of the transmission. • During transmission of data, all of the packets take the same path. • The Public Switch Telephone Network (PSTN) is an example of a circuit switch system. – A call is placed and a circuit established when the other end of the circuit is answered. – Modems that operate between computer systems are a specific example.
Packet Switching • In packet switching, circuits may be selected on a packet-by-packet basis. • The Internet widely uses packet switch networks, for individual packets in the same transmission may take different routes through the network to the same destination. • Upper layers of the OSI model place the packets into the correct order.
Cell Switching • Cell switching is associated with Asynchronous Transmission Mode (ATM) which is considered to be a high speed switching technology that attempted to overcome the speed problems faced by the shared media like Ethernet. • Cell switching uses a connection-oriented packet-switched network. – It is called cell switching because this methodology uses a fixed length of packets of 53 bytes out of which 5 bytes are reserved for header. – Unlike cell technology, packet switching technology uses variable length packets. – Even though cell switching closely resembles packet switching because cell switching also breaks the information into smaller packets of fixed length.
LANMAN WAN
INTERNET Big and getting bigger….
Hierarchical organization of the Internet
Protocols • A protocol is synonymous with rule. It consists of a set of rules that govern data communications. It determines - – what is communicated – how it is communicated and – when it is communicated. • The key elements of a protocol are syntax, semantics and timing
Elements of a Protocol • Syntax – Structure or format of the data – Indicates how to read the bits - field delineation • Semantics – Interprets the meaning of the bits – Knows which fields define what action • Timing – When data should be sent and – What speed at which data should be sent or speed at which it is being received.
NETWORK MODELS
Communication Architecture • Strategy for connecting host computers and other communicating equipment. • Defines necessary elements for data communication between devices. • A communication architecture, therefore, defines a standard for the communicating hosts. • A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software. • Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.
Example • We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.
Tasks involved in sending a letter
Layer Architecture • Layer architecture simplifies the network design. • It is easy to debug network applications in a layered architecture network. • The network management is easier due to the layered architecture. • Network layers follow a set of rules, called protocol. • The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.
ISO or OSI • ISO (International Organization for Standardization) – is the organization. • OSI (Open Systems Interconnection ) – is the model.
Open Systems Interconnection (OSI) Model • International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication. • Open Systems Interconnection (OSI) reference model is the result of this effort. • In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. • Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.
OSI Reference Model • The OSI model is now considered the primary Architectural model for inter-computer communications. • The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network. • The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems . • This separation into smaller more manageable functions is known as layering.
OSI Model
OSI Reference Model: 7 Layers
The interaction between layers in the OSI model
OSI: A Layered Network Model • The process of breaking up the functions or tasks of networking into layers reduces complexity. • Each layer provides a service to the layer above it in the protocol specification. • Each layer communicates with the same layer’s software or hardware on other computers. • The lower 4 layers (transport, network, data link and physical — Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. • The upper three layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. • Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.
An exchange using the OSI model
Physical Layer • Provides physical interface for transmission of information. • Defines rules by which bits are passed from one system to another on a physical communication medium. • Covers all - mechanical, electrical, functional and procedural - aspects for physical communication. • Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications. OSI Model
Physical Layer The physical layer is responsible for movements of individual bits from one hop (node) to the next.
Data Link Layer • Data link layer attempts to provide reliable communication over the physical layer interface. • Breaks the outgoing data into frames and reassemble the received frames. • Create and detect frame boundaries. • Handle errors by implementing an acknowledgement and retransmission scheme. • Implement flow control. • Supports points-to-point as well as broadcast communication. • Supports simplex, half-duplex or full-duplex communication. OSI Model
Data Link Layer The data link layer is responsible for moving frames from one hop (node) to the next.
Hop to Hop Delivery
Network Layer • Implements routing of frames (packets) through the network. • Defines the most optimum path the packet should take from the source to the destination • Defines logical addressing so that any endpoint can be identified. • Handles congestion in the network. • Facilitates interconnection between heterogeneous networks (Internetworking). • The network layer also defines how to fragment a packet into smaller packets to accommodate different media.
Network Layer The network layer is responsible for the delivery of individual packets from the source host to the destination host.
Source to destination delivery
Transport Layer • Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. • Ensures that the data units are delivered error free. • Ensures that data units are delivered in sequence. • Ensures that there is no loss or duplication of data units. • Provides connectionless or connection oriented service. • Provides for the connection management. • Multiplex multiple connection over a single channel. OSI Model
Transport Layer • The transport layer is responsible for the delivery of a message from one process to another.
Reliable process-to-process delivery of a message
Session Layer • Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. • This layer requests for a logical connection to be established on an end-user’s request. • Any necessary log-on or password validation is also handled by this layer. • Session layer is also responsible for terminating the connection. • This layer provides services like dialogue discipline which can be full duplex or half duplex. • Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint. OSI Model
Session Layer • The session layer is responsible for dialog control and synchronization.
Presentation Layer • Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. • Also handles data compression and data encryption (cryptography). OSI Model
Presentation layer • The presentation layer is responsible for translation, compression, and encryption.
Application Layer • Application layer interacts with application programs and is the highest level of OSI model. • Application layer contains management functions to support distributed applications. • Examples of application layer are applications such as file transfer, electronic mail, remote login etc. OSI Model
Application layer • The application layer is responsible for providing services to the user.
OSI in Action • A message begins at the top application layer and moves down the OSI layers to the bottom physical layer. • As the message descends, each successive OSI model layer adds a header to it. • A header is layer-specific information that basically explains what functions the layer carried out. • Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers. OSI Model
Summary of layers
TCP/IP Model TCP/IP MODEL
TCP/IP protocol suite • The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.
OSI & TCP/IP Models TCP/IP Model
TCP/IP and OSI model
Overview of the Internet: TCP/IP reference model
TCP/IP Model TCP/IP Model Application Layer Application programs using the network Transport Layer (TCP/UDP) Management of end-to-end message transmission, error detection and error correction Network Layer (IP) Handling of datagrams : routing and congestion Data Link Layer Management of cost effective and reliable data delivery, access to physical networks Physical Layer Physical Media
Addressing • Physical Addresses • Logical Addresses • Port Addresses • Specific Addresses
Addresses in TCP/IP
Relationship of layers and addresses in TCP/IP
Physical Addresses • Here a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.
• Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.
IP Addresses: A part of an internet with two routers connecting three LANs • Each device (computer or router) has a pair of addresses (logical and physical) for each connection. • In this case, each computer is connected to only one link and therefore has only one pair of addresses. • Each router, however, is connected to three networks (only two are shown in the figure). • So each router has three pairs of addresses, one for each connection.
Port addresses • The sending computer is running three processes at this time with port addresses a, b, and c. • The receiving computer is running two processes at this time with port addresses j and k. • Process a in the sending computer needs to communicate with process j in the receiving computer. • Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. The physical addresses will change from hop to hop, but the logical addresses usually remain the same.
• A port address is a 16-bit address represented by one decimal number as shown. 753 • A 16-bit port address represented as one single number.

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Compute Networks: 01 - Introduction to CN

  • 1. CSE/T/324A Computer Networks Topic 1- Introduction Sarbani Roy [email protected] Office: CC-5-7 Cell: 9051639328
  • 2. Syllabus • Introduction: Uses of Computer Networks, Types of Computer Networks, OSI Reference Model, TCP/IP model [4L] • Review of Physical Layer [4L] • Data Link Control and Protocols: Link Layer Services, Error detection and Correction Techniques, Multi Access Protocols, Link Layer Addressing, Ethernet, Hubs, Switches and Switches, Point to Point Protocol, Asynchronous Transfer Mode, Multiprotocol Label Switching [6L] • Network Layer: Introduction, Virtual Circuit and Datagram Networks, IP Addressing, Subnetting, Routing Algorithms (Link State, Distance Vector, Hierarchical), Routing in the Internet (RIP, OSPF, BGP), Broadcast and Multicast Routing Algorithms, Routers, ICMP, IPv6 [8L] • Transport Layer: Introduction to Transport Layer Services, Multiplexing and Demultiplexing, Connectionless Transport: UDP, Principles of Reliable Data Transfer, Connection Oriented Transport: TCP, Principles of Congestion Control, TCP Congestion Control, Sockets and Socket Programming, Quality of services (QOS) [8L] • Application Layer: Web and HTTP, Domain Name Space (DNS), Electronic Mail (SMTP, MIME, IMAP, POP3), File Transfer Protocol, Cryptography [6L] • Introduction to Wireless and Mobile Networks [4L]
  • 3. COs of Theory • CO1: Understand the layered architecture and explain the contemporary issues and importance of MAC sublayer of the Data Link Layer, network layer, transport layer and application layer of TCP-IP model, and how they can be used to assist in network design and implementation. • CO2: Understand the protocols of MAC sublayer of the Data Link Layer and describe the IEEE standards for Ethernet (802.3) and wireless LAN (802.11). • CO3: Understand internetworking principles and explain algorithms for multiple access, routing and different networking techniques and able to analyze the performance of these algorithms. • CO4: Explain the protocols in Transport Layer and able to design network applications using these protocols. • CO5: Explain the protocols in application layer and how they can be used to design popular internet applications.
  • 4. COs of Lab • CO1: Design and implement error detection techniques within a simulated network environment. • CO2: Design and implement flow control mechanisms of Logical Link Control of Data Link Layer within a simulated network environment. • CO3: Design and implement medium access control mechanisms within a simulated network environment using IEEE 802 standards. • CO4: Design and implement network layer protocols within a simulated network environment or network tools using IEEE 802 standards. • CO5: Design and implement various applications using Transport layer protocols and Application layer protocols for its implementation in client/server environment and analyze the performance.
  • 5. Suggested Readings 1. Data Communications and Networking, Behrouz A Forouzan, McGraw Hill 2. Computer Networks, by Andrew S. Tanenbaum, Prentice Hall India 3. Computer Networking: A Top-Down Approach Featuring the Internet, by James F. Kurose and Keith W. Ross, Pearson Education Slides are mainly prepared using online documents available of the above books and course materials of different institutes
  • 6. REVIEW OF ERROR DETECTION IN DATA LINK LAYER
  • 7. Computer Networks Lab CO1: Design and implement error detection techniques within a simulated network environment.
  • 8. Error Detection • Data transmission can contain errors – Single-bit – Burst errors of length n (n: distance between the first and last errors in data block) • How to detect errors – If only data is transmitted, errors cannot be detected  Send more information with data that satisfies a special relationship  Add redundancy
  • 9. Error Detection Methods • Vertical Redundancy Check (VRC) – Append a single bit at the end of data block such that the number of ones is even  Even Parity (odd parity is similar) 0110011  01100110 0110001  01100011 – VRC is also known as Parity Check – Performance: • Detects all odd-number errors in a data block
  • 10. Error Detection Methods • Longitudinal Redundancy Check (LRC) – Organize data into a table and create a parity for each column 11100111 11011101 00111001 10101001 11100111 11011101 00111001 10101001 10101010 11100111 11011101 00111001 10101001 10101010 Original Data LRC
  • 11. Error Detection Methods – Performance: • Detects all burst errors up to length n (number of columns) • Misses burst errors of length n+1 if there are n-1 uninverted bits between the first and last bit • Checksum – Used by upper layer protocols – Similar to LRC, uses one’s complement arithmetic
  • 12. Cyclic Redundancy Check • Powerful error detection scheme • Rather than addition, binary division is used  Finite Algebra Theory (Galois Fields) • Can be easily implemented with small amount of hardware – Shift registers – XOR (for addition and subtraction)
  • 13. Cyclic Redundancy Check • Let us assume k message bits and n bits of redundancy • Associate bits with coefficients of a polynomial 1 0 1 1 0 1 1 1x6 +0x5 +1x4 +1x3 +0x2 +1x+1 = x6 +x4 +x3 +x+1 xxxxxxxxxx yyyy k bits n bits Block of length k+n
  • 14. Example • Send – M(x) = 110011  x5 +x4 +x+1 (6 bits) – P(x) = 11001  x4 +x3 +1 (5 bits, n = 4)  4 bits of redundancy – Form xn M(x)  110011 0000  x9 +x8 +x5 +x4 – Divide xn M(x) by P(x) to find C(x) = C(x) Send the block 110011 1001 • Receive No remainder  Accept 100001 1001 11001 10000 11001 1100110000 11001 00000 11001 11001 11001 1100111001 11001
  • 15. Cyclic Redundancy Check • Let M(x) be the message polynomial • Let P(x) be the generator polynomial – P(x) is fixed for a given CRC scheme – P(x) is known both by sender and receiver • Create a block polynomial F(x) based on M(x) and P(x) such that F(x) is divisible by P(x) ) ( 0 ) ( ) ( ) ( x P x Q x P x F  
  • 16. Cyclic Redundancy Check • Sending 1. Multiply M(x) by xn 2. Divide xn M(x) by P(x) 3. Ignore the quotient and keep the reminder C(x) 4. Form and send F(x) = xn M(x)+C(x) • Receiving 1. Receive F’(x) 2. Divide F’(x) by P(x) 3. Accept if remainder is 0, reject otherwise
  • 17. Assignment 1: Design and implement an error detection module. Due on: 19-23 October 2020 (in your respective lab classes) • Design and implement an error detection module which has four schemes namely LRC, VRC, Checksum and CRC. Please note that you may need to use these schemes separately for other applications (assignments). You can write the program in any language. The program should accept the name of a test file (contains a sequence of frames) from the command line and then print the input data frame, signature block and the output data frame on console standard output. Next, test the same program to produce a PASS/FAIL result for following cases. (a) Error is detected by all four schemes. Use a suitable CRC polynomial (list is given in next page). (b) Error is detected by checksum but not by CRC. (c) Error is detected by VRC but not by CRC. [Note: Inject error in random positions in the input data frame. Write a separate method for that] • Please note that your design and implementation of these schemes can help you to make your assignment different from your classmates. Write the code by yourself and protect your code. The assignment is simple, but we encourage you to do improvisation with this.
  • 19. Report • Title: (Basic information like name, class, group, assignment number, problem statement and date (deadline and submission)). • Design: (short paragraph describing the purpose of the program, draw a structure diagram to reflect the procedural organization of the program as you have designed it in the previous step, input and output format) • Implementation: (code snippet, method description, interfaces etc.) • Test cases: (Describe the tests you will perform to verify the correctness of your program. This should be a thorough and exhaustive list of test cases designed to show that your program does everything it is supposed to do. The combined tests should exercise every line of code in your program. For each test case, give sample test data and state what you are checking. • Results: (Figures or graphs, tables ) • Analysis: (discussion on results considering different test cases, error, correctness, known bugs, possible improvements) • Comments: Evaluate the lab assignment. What did you learn from it? Was it too hard? (explain why?) Too easy? (explain why?) Suggest improvements if you can.
  • 20. Networks • Network is a set of devices (often referred to as nodes) connected by communication links. • A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. A link can be a cable, air, optical fiber, or any medium which can transport a signal carrying information.
  • 21. Computer Networks • Computer network connects two or more autonomous computers. • The computers can be geographically located anywhere.
  • 22. Applications of Networks • Resource Sharing – Hardware (computing resources, disks, printers) – Software (application software) • Information Sharing – Easy accessibility from anywhere (files, databases) – Search Capability (WWW) • Communication – Email – Message broadcast • Remote computing • Distributed processing (Cloud Computing)
  • 24. Network Components • Physical Media • Interconnecting Devices • Computers • Networking Software • Applications
  • 25. Network Criteria • Performance – Depends on Network Elements – Measured in terms of Delay and Throughput • Reliability – Failure rate of network components – Measured in terms of availability/robustness • Security – Data protection against corruption/loss of data due to: – Errors – Malicious users
  • 26. Physical Structures • Type of Connection – Point to Point - single transmitter and receiver – Multipoint - multiple recipients of single transmission • Physical Topology – Connection of devices – Type of transmission - unicast, mulitcast, broadcast
  • 29. Network Topology • The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions.
  • 30. A fully connected mesh topology (five devices)
  • 31. Star Topology • The star topology is the most commonly used architecture in Ethernet LANs. • Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.
  • 32. A star topology connecting four stations
  • 33. Bus Topology • Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.
  • 34. A bus topology connecting three stations
  • 35. Ring Topology • A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. • The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. – Single ring – All the devices on the network share a single cable – Dual ring – The dual ring topology allows data to be sent in both directions.
  • 36. A ring topology connecting six stations
  • 37. A hybrid topology: a star backbone with three bus networks
  • 38. Mesh Topology • The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance. • It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions. • Implementing the mesh topology is expensive and difficult.
  • 39. Categories of Networks • Local Area Networks (LANs) – Short distances – Designed to provide local interconnectivity • Wide Area Networks (WANs) – Long distances – Provide connectivity over large areas • Metropolitan Area Networks (MANs) – Provide connectivity over areas such as a city, a campus
  • 40. LAN, MAN and WAN • Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network) • Network in a City is call MAN (Metropolitan Area Network) • Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)
  • 41. LAN
  • 42. WANs: a switched WAN and a point-to-point WAN
  • 43. WAN technologies • There are two types of long distance communication technologies that are used for WANs. – Dedicated connection • A dedicated line is a full-time point-to-point connection provided by a communication carrier that lasts for the length of the lease period. – Switched connection • There are several types of switched connections: circuit switched, packet switched, and cell switched
  • 44. Circuit Switching • In circuit switching, circuits are established prior to the transmission of data and torn down at the end of the transmission. • During transmission of data, all of the packets take the same path. • The Public Switch Telephone Network (PSTN) is an example of a circuit switch system. – A call is placed and a circuit established when the other end of the circuit is answered. – Modems that operate between computer systems are a specific example.
  • 45. Packet Switching • In packet switching, circuits may be selected on a packet-by-packet basis. • The Internet widely uses packet switch networks, for individual packets in the same transmission may take different routes through the network to the same destination. • Upper layers of the OSI model place the packets into the correct order.
  • 46. Cell Switching • Cell switching is associated with Asynchronous Transmission Mode (ATM) which is considered to be a high speed switching technology that attempted to overcome the speed problems faced by the shared media like Ethernet. • Cell switching uses a connection-oriented packet-switched network. – It is called cell switching because this methodology uses a fixed length of packets of 53 bytes out of which 5 bytes are reserved for header. – Unlike cell technology, packet switching technology uses variable length packets. – Even though cell switching closely resembles packet switching because cell switching also breaks the information into smaller packets of fixed length.
  • 50. Protocols • A protocol is synonymous with rule. It consists of a set of rules that govern data communications. It determines - – what is communicated – how it is communicated and – when it is communicated. • The key elements of a protocol are syntax, semantics and timing
  • 51. Elements of a Protocol • Syntax – Structure or format of the data – Indicates how to read the bits - field delineation • Semantics – Interprets the meaning of the bits – Knows which fields define what action • Timing – When data should be sent and – What speed at which data should be sent or speed at which it is being received.
  • 53. Communication Architecture • Strategy for connecting host computers and other communicating equipment. • Defines necessary elements for data communication between devices. • A communication architecture, therefore, defines a standard for the communicating hosts. • A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software. • Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.
  • 54. Example • We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.
  • 55. Tasks involved in sending a letter
  • 56. Layer Architecture • Layer architecture simplifies the network design. • It is easy to debug network applications in a layered architecture network. • The network management is easier due to the layered architecture. • Network layers follow a set of rules, called protocol. • The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.
  • 57. ISO or OSI • ISO (International Organization for Standardization) – is the organization. • OSI (Open Systems Interconnection ) – is the model.
  • 58. Open Systems Interconnection (OSI) Model • International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication. • Open Systems Interconnection (OSI) reference model is the result of this effort. • In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. • Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.
  • 59. OSI Reference Model • The OSI model is now considered the primary Architectural model for inter-computer communications. • The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network. • The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems . • This separation into smaller more manageable functions is known as layering.
  • 62. The interaction between layers in the OSI model
  • 63. OSI: A Layered Network Model • The process of breaking up the functions or tasks of networking into layers reduces complexity. • Each layer provides a service to the layer above it in the protocol specification. • Each layer communicates with the same layer’s software or hardware on other computers. • The lower 4 layers (transport, network, data link and physical — Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. • The upper three layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. • Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.
  • 64. An exchange using the OSI model
  • 65. Physical Layer • Provides physical interface for transmission of information. • Defines rules by which bits are passed from one system to another on a physical communication medium. • Covers all - mechanical, electrical, functional and procedural - aspects for physical communication. • Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications. OSI Model
  • 66. Physical Layer The physical layer is responsible for movements of individual bits from one hop (node) to the next.
  • 67. Data Link Layer • Data link layer attempts to provide reliable communication over the physical layer interface. • Breaks the outgoing data into frames and reassemble the received frames. • Create and detect frame boundaries. • Handle errors by implementing an acknowledgement and retransmission scheme. • Implement flow control. • Supports points-to-point as well as broadcast communication. • Supports simplex, half-duplex or full-duplex communication. OSI Model
  • 68. Data Link Layer The data link layer is responsible for moving frames from one hop (node) to the next.
  • 69. Hop to Hop Delivery
  • 70. Network Layer • Implements routing of frames (packets) through the network. • Defines the most optimum path the packet should take from the source to the destination • Defines logical addressing so that any endpoint can be identified. • Handles congestion in the network. • Facilitates interconnection between heterogeneous networks (Internetworking). • The network layer also defines how to fragment a packet into smaller packets to accommodate different media.
  • 71. Network Layer The network layer is responsible for the delivery of individual packets from the source host to the destination host.
  • 73. Transport Layer • Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. • Ensures that the data units are delivered error free. • Ensures that data units are delivered in sequence. • Ensures that there is no loss or duplication of data units. • Provides connectionless or connection oriented service. • Provides for the connection management. • Multiplex multiple connection over a single channel. OSI Model
  • 74. Transport Layer • The transport layer is responsible for the delivery of a message from one process to another.
  • 76. Session Layer • Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. • This layer requests for a logical connection to be established on an end-user’s request. • Any necessary log-on or password validation is also handled by this layer. • Session layer is also responsible for terminating the connection. • This layer provides services like dialogue discipline which can be full duplex or half duplex. • Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint. OSI Model
  • 77. Session Layer • The session layer is responsible for dialog control and synchronization.
  • 78. Presentation Layer • Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. • Also handles data compression and data encryption (cryptography). OSI Model
  • 79. Presentation layer • The presentation layer is responsible for translation, compression, and encryption.
  • 80. Application Layer • Application layer interacts with application programs and is the highest level of OSI model. • Application layer contains management functions to support distributed applications. • Examples of application layer are applications such as file transfer, electronic mail, remote login etc. OSI Model
  • 81. Application layer • The application layer is responsible for providing services to the user.
  • 82. OSI in Action • A message begins at the top application layer and moves down the OSI layers to the bottom physical layer. • As the message descends, each successive OSI model layer adds a header to it. • A header is layer-specific information that basically explains what functions the layer carried out. • Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers. OSI Model
  • 85. TCP/IP protocol suite • The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.
  • 86. OSI & TCP/IP Models TCP/IP Model
  • 87. TCP/IP and OSI model
  • 88. Overview of the Internet: TCP/IP reference model
  • 89. TCP/IP Model TCP/IP Model Application Layer Application programs using the network Transport Layer (TCP/UDP) Management of end-to-end message transmission, error detection and error correction Network Layer (IP) Handling of datagrams : routing and congestion Data Link Layer Management of cost effective and reliable data delivery, access to physical networks Physical Layer Physical Media
  • 90. Addressing • Physical Addresses • Logical Addresses • Port Addresses • Specific Addresses
  • 92. Relationship of layers and addresses in TCP/IP
  • 93. Physical Addresses • Here a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.
  • 94. • Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.
  • 95. IP Addresses: A part of an internet with two routers connecting three LANs • Each device (computer or router) has a pair of addresses (logical and physical) for each connection. • In this case, each computer is connected to only one link and therefore has only one pair of addresses. • Each router, however, is connected to three networks (only two are shown in the figure). • So each router has three pairs of addresses, one for each connection.
  • 96. Port addresses • The sending computer is running three processes at this time with port addresses a, b, and c. • The receiving computer is running two processes at this time with port addresses j and k. • Process a in the sending computer needs to communicate with process j in the receiving computer. • Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. The physical addresses will change from hop to hop, but the logical addresses usually remain the same.
  • 97. • A port address is a 16-bit address represented by one decimal number as shown. 753 • A 16-bit port address represented as one single number.