COSC 3213: Communication Networks

Transcription

1 COSC 3213: Communication Networks Course Instructor: Marvin Mandelbaum Contact Information: Instructor: Section E Teaching Assistant: Office: CSE 3012 TBA mandel@cse.yorku.ca (416) X40630 URL: Text: A. Leon-Garcia and I. Widjaja, Communication Networks: Fundamentals Concepts and Key Architectures, McGraw Hill, 2 nd edition Class Schedule: T &TR 11:30 13:00 a.m., Room TEL 1005 Assessment: 15% Assignment / Quiz; 25% Mid-term Exam; 60% Final Exam Office Hours: Instructor: CSE 3012, T &TR 13:00 14:00 TA: TBA

2 3000 level General Prerequisites (as of 2008/09) cse Data Structures Grade point average of >= 4.5 in CS & CSE1019 Math1300 and 1310 (Calculus) [One of cse2001,cse2021,cse2031 (Theory Computation, Comp Org., Software Tools)] [One of Math1090,1025, (Logic for COSC, Applied Linear Algebra)] 2

3 Course Objectives Introduce communication networks and understand how different components work including the underlying technology including hardware and software used. Study basic concepts of digital communications including channel encoding, modulation, error detection and error correction schemes. Understand different topologies used in local area networks (LAN) Comprehend how an internet (wide area network or WAN) is formed Introduce the concept of networking standards Transmission Control Protocol / Internet Protocol (TCP/IP) Open Systems Interconnection Reference model (OSI) Demystify terminology!!!!!! 3

4 Course Outline Circuit Switching, Message Switching, and Packet Switching. OSI and TCP/IP Reference Model. Theoretical limits (Nyquist Signaling rate and Shannon Channel Capacity theorems). Time and Frequency Representations of Signals. Intro to Fourier Analysis. Line Coding (Unipolar NRZ, Polar NRZ, Bipolar, Manchester, Differential Manchester) Digital Communication (ASK, PSK, FSK). Analogue Communication (AM, FM, PM). Modems (V.90) and Transmission Media (Microwave, Twisted pair, Coaxial, Optical Fiber). Error Detection (Parity Bit Checking, 2D Parity Codes, Checksum, Polynomial Codes, CRC). Flow Control (ARQ, Sliding-window, Go-back-N, Selective Repeat). Multiplexing: FDM and TDM; Group, Supergroup, and Mastergroup configurations in FDM; DS1, DS2, and DS3 configurations in TDM. Telephone Network and Circuit Switches: Space Division Switches, Time-Division Switches. LAN: MAC, Data Link Layer and Logical Link Control (LLC). LAN: Medium Access Control I ALOHA, Slotted ALOHA, CSMA, CSMA/CD LAN Technologies: Ethernet (802.3), Token Ring (802.5), Wireless LANs (802.11) TCP/IP: Network and transport layers (TCP, UDP and IP Protocols) 4

5 What is a Communication Network (1)? Communication network is a set of equipment, software, and facilities that allows transfer of information between geographically distant users. Why Communication networks: Resource Sharing: Data sharing between distant sites. High Reliability: Provide alternative sources of data. Parallel Processing: Use multiple computers for a single application. Scalability: Ability to increase system size based on demand. Applications Examples: Radio and TV broadcasting: Single source transmits to multiple users; Real-time (low latency), Unidirectional (simplex). Telephone connection: One to one connection (connection-oriented), Real-time (low latency), Bidirectional (Duplex), Fixed location Cellular Telephone: One to one connection through microwave (roaming), similar in service to telephone connection 5

6 What is a Communication Network (2)? Examples (contd): Electronic mail ( ): Not real-time, Not connection-oriented, Reliable. World wide web (WWW): accessed through a uniform resource locator (URL) identifying a home page developed using a hypertext language. Video-on-demand: Not real-time, Unreliable. Streamed audiovisual services: Audio and video conferencing: Real-time, Unreliable. Peer-to-peer applications such as Napster, Gnutella, Kazaa file exchange Searching for ExtraTerrestrial Intelligence (SETI) 6

7 Block Representation: Communication Network Source: generates data Tx: Converts data into tx signals Transmission System: Carries signals Rx: Converts rx signal back into data Destination: Accepts incoming data 7

8 Representation of a Communication Network Host Communication Network Server Equipment (hardware & software) and facilities that provide the basic communication service is represented by a white cloud. The equipment inside a communication network is virtually invisible to the user. Equipment includes repeaters, routers, bridges, servers, switches, multiplexers, hubs, modems, interconnecting wires, Network architecture specifies the plan used to built and operate a communication network. TCP/IP and OSI are example of network architectures. Since the overall communication process is complex, network architecture partitions the overall communication process into separate functional areas called layers. TCP/IP consists of five layers, while OSI has seven layers. 8

9 Services versus Applications Service: the basic information transfer capability of a communication network. Internet transfer of individual block of information. Internet reliable transfer of a stream of bytes. Real-time transfer of a voice signal. Applications use communication services to built a complete feature, hiding most of the technical details from the users. and web built on reliable stream service. Fax and modems built on basic telephone service. Newer applications use multiple (sometimes heterogeneous) networks. SMS builds on Internet reliable stream service and cellular telephone text messaging 9

10 Evolution of Network Architectures (1) Information transfer rate in bits per second (bps) 1.0E E E E E E E E ? Telegraph networks Telephone networks Internet, Optical & Wireless networks Next Generation Internet 10

11 Evolution of Network Architectures (2) Communication networks can basically be classified in the following categories: 1. Telegraph Networks 2. Telephone Networks 3. Internet Used for digital transmission. Uses message switching to transfer data from one node to the next one. Used for transmission of both analog and digital information. Uses circuit switching to transfer data. Primarily used for computer applications. Uses packet switching to transfer data. Analogous to the postal mail service. Provides best-effort service with equal priority to every user. 4. Next-Generation Internet Planned to provide prioritized communication instead of best effort service. Will be based on multiservice packet switching network technology. 11

12 Telegraph Networks and Message Switching (1) Pre-telegraph services used physical transport of the message using messenger pigeons, pony express, or ship service. In telegraphs, the message is typically converted into a digital signal and then transmitted across a network. In the eighteen century, optical telegraphs were initially used. 1. Claude Chappe invented the first optical telegraph in the 1790 s. 2. Semaphore mimicked a person with outstretched arms with flags in each hand. 3. Different angle combinations of arms and hands generated hundreds of possible signals. 4. Code for enciphering messages kept secret. Later, electrical telegraphs using electrical signals were invented. 1. William Sturgeon Electro-magnet (1825): used electrical current in a wire wrapped around a piece of iron to generate a magnetic force. 2. Joseph Henry (1830): transmitted current over 1 mile of wire to ring a bell. 3. Samuel Morse (1835) used pulses of current to deflect electromagnet to generate dots & dashes. Optical Telegraph Electric Telegraph 12

13 Telegraph Networks and Message Switching (2) In electric telegraphs, the message is typically converted into a digital signal and then transmitted across a network. Initially, Morse code was used to encode the alphanumeric message. Electrical signals were used to transmit the alphanumeric message. Later on 7-bit ASCII code was used to encode the messages. A B C D E F G H I Morse Code J K L M N O P Q R Morse Code S T U V W X Y Z 1 Morse Code Morse Code 13

14 Telegraph Networks and Message Switching (3) Since point-to-point communications between every pair of subscriber is physically unrealizable, network nodes were created where several telegraph lines met. Each node operated on the principle of the store-and-forward operation. 1. Message arriving on each line was decoded to extract the destination address. (addressing) 2. Next-hop in route determined by the destination address of the message (routing). 3. Each message was stored until the next-hop is available for transmission (store). 4. On availability of the next-hop, entire message is transmitted to the next node (forwarding). Message Switching Message Message Message Source Message Nodes/Switches Destination 14

15 Telephone Networks and Circuit Switching (1) Alexander Graham Bell (1875) discovered that voice signals can be transmitted over electrical wires. Microphones used to voice pressure variations (sound) into an analogous electrical signal. Electrical circuits communicated the signals between the communications parties. Loudspeakers converted the electrical signals back into audible sound. Telephone patent was first granted in 1876 with the Bell Telephone Company founded in To provide interconnectivity between multiple users, two types of connections are possible. 1 1 N 2 N Switch 4 3 Pairwise interconnections No. of connections = N(N 1)/2 4 3 Access network No. of connections = N 15

16 Telephone Networks and Circuit Switching (2) 1. Telephone network Pick up phone 2. Telephone network Listen for Dial tone. Connection set up 3. Telephone network Dial number of the destination. 4. Telephone network Information transfer 5. Telephone network Exchange voice signals Connection release 6. Hang up to terminate the connection. 16

17 Computer Networks and Packet Switching (1) The initial computer networks (SAGE, SABRE, etc.) were terminal-oriented networks (main frames) with a single central computer shared by multiple users. Each user had access to a dummy terminal (keyboard with monitor) connected with a dedicated line to the central computer. A user would type instructions using the dummy terminal. The instructions would be passed on to the central computer, where they were executed, and the results, returned to the dummy terminal, were displayed on the terminal. To allow fair access to the central computer, time-sharing techniques were employed. Terminal... Terminal Modem Telephone Network Modem Terminal Central computer 17

18 Computer Networks and Packet Switching (2) Modification # 1: Use a multidrop line to connect terminals with the central computer Dedicated communication lines were expensive so two simplex lines were used to interconnect the dummy terminals with the central computer. Time sharing (medium access control) was provided by polling frames that polled each terminal in a sequential order. Address in the header of the polling frame was used to identify the terminal. If a terminal wanted to communicate to the central computer, it would do so when polled. The central computer would execute the instruction and pass the result back to the terminal. Polling frames & output frames input frames Terminal Terminal... Terminal Host computer Terminals at different locations in a city Must avoid collisions on inbound line 18

19 Computer Networks and Packet Switching (3) Modification # 2: Use multiplexing to (a) transmit multiple messages simultaneously and (b) to detect communication errors. Multiplexers provided a second approach for sharing the communication line. Message from each terminals was encapsulated into a frame with destination address and an error detection code (for example the CRC code). The central computer would sort out the messages from each terminal, perform possible detection of errors, carry out the necessary processing, and return the results inside a frame. The multiplexer would separate the results and direct each result to the appropriate terminal. Frame CRC Information Header Terminal Terminal Header Information CRC... Terminal Central computer Multiplexer 19

20 Computer Networks and Packet Switching (3) The second generation of networks were Computer-to-computer networks. As cost of computers dropped, dumb terminals were replaced by self-computing terminals. Interconnecting such computers was required to support: (1) file transfers; (2) remote telnet to allow execution of a program on another computer; and (3) parallel processing to execute a single program over multiple computers. Circuit or message switching resulted in long communication delays. Packet switching was introduced in which long messages are broken into multiple packets. Each packet has the destination address and is transmitted independently on the network. 20

21 Computer Networks and Packet Switching (4) ARPANET was the first research wide area network (WAN) commecting several universities. ARPANET operated on the principle of packet switching. Host generates message. Each message is converted into several smaller packets. Each packet contains source and destination addresses and is transferred independently. At the destination computer, the packets are combined into a single message. AMES McCLELLAN UTAH BOULDER GWC CASE RADC ILL LINC CARN AMES USC UCSB MIT MITRE STAN SCD ETAC 21 UCLA RAND TINKER BBN HARV NBS

22 History of Networks 1. SAGE (Semi-Automatic Ground Environment System): First computer network developed in 1950 for air-defense purposes 2. SABRE: an airline reservation system introduced in ARPANET: 4. NSFNET: Developed in mid 1960s at height of cold war to survive a nuclear war. Based on packet switching technology such that part of the network is working under any circumstances. Developed in mid 1970s to connect research institutions and universities in U.S. 5. ARPANET and NSFNET connected in Internet was the gluing technology for (5). Growth continued exponentially with the size of networks doubling almost every year. 7. Other networks connected to Internet include Aurora (MIT, IBM, UPenn, Bell core); Blanca (resulting from XUNET project in AT&T), CASA (a network for supercomputers based in CA), Nectar (CMU to UPitt), Vistanet (Univ. in North Caroliona), and many others 22

23 Comparison of Switching Techniques (1) 1. Circuit switching (designed by telephone companies) End-to-end path is established between the transmitter and receiver. Complete block of data is transmitted and circuit terminated. Tx and Rx are inaccessible for the duration of the connection. 2. Message switching (designed for telegraphic networks): No physical path is established between Tx and Rx. Connection is established between the Tx and first switching office (router). Entire block of data is transmitted to the switching office. Block is forwarded one hop at a time. No limit on block size, switching stations inaccessible for duration of transfer. 3. Packet switching (used in Internet): A tight limit is placed on maximum block size. Data is broken in different sub-blocks and each sub-block is transmitted one hop at a time, one after the other. 23

24 Timing Diagram Start TX TX time Complete TX Propagation time Station A: Source Transmitter Station B: Destination Receiver 24

25 Comparison of Switching Techniques (2) Timing of events for: (a) circuit switched; (b) message switched; and (c) packet switched networks 25

26 Comparison of Switching Techniques (3) Efficiency Routing Synchroniza tion Circuit Switching Message Switching Packet Switching Connection-oriented service. Connectionless service. Connectionless service. Highly inefficient due to long setup time. Complete path is established once at initialization of connection. Improvement over circuit switching because the entire connection is not established. Intelligent adaptive routing algorithms are required at each switching station. Best under normal working conditions. Intelligent adaptive routing algorithms are required at each router for each packet. No problem No problem Each packet may use a different path and hence arrive out of order Congestion No problem Can be an issue. A big issue since numerous packets from different sources are roaming in the network. Data Loss No problem until a connection is dropped in the middle of transfer A router will be accepting data from various sources and may overflow. Highest probability of overflow and data loss. 26

27 Functions required in Communication Networks Basic user services: including smtp, ftp, telnet, http, video conferencing, Switching: transfer information between communication lines. Transmission: ability to transmit (store and forward) information across a medium Addressing: identify communication lines and stations Multiplexing: means for coupling information from different sources together Routing: identify the shortest path between the source and destination Congestion control: identify congestion of data and /or ways to prevent it Flow control: prevent overwhelming of a slower computer Quality of service (QoS): allocate different class of service to different users Compatibility: connect heterogeneous networks Error detection: identify errors and / or correct them Security: prevent eavesdropping Management: monitor and recover from faults, manage bills etc. 27

28 Types of Networks (1) Networks are typically classified in three types: LAN, MAN, and WAN 1. Local area networks (LAN) Small networks confined to a few kilometer (<= 1 km). Speeds confined to 100 Mbps. Newer LANs run at up to 10 Gbps. Uses the principal of broadcasting (one transmits, others listen) Various Topologies including Token bus and Token ring are possible Terminator Token Bus Token Ring 28

29 Types of Networks (2) 2. Metropolitan area networks (MAN) Covers up to a city (<= 10 km), Example: Cable TV network, IEEE Cable network were initially designed for TV and later extended to Internet Internet is fed into a head for subsequent re-distribution 29

30 Types of Networks (3) 3. Wide area networks (MAN) Spans a continent (<= km), Example: Internet Interconnects various LAN using switches (routers) and transmission lines Uses packet switching in conjunction with the store and forward technology 30