Chapter 1 Introduction. COSC 3213 Summer 2003

Transcription

1 Chapter 1 Introduction COSC 3213 Summer /7/2003

2 Course Objectives Introduce communication networks and understand how different components work including the underlying technology including hardware and software used. 3 components: Data communications: Review basic concepts of digital communications including signal transmission, signal encoding, multiplexing, error detection and error correction schemes. Networking: Understand different topologies used in local area networks (LAN) Comprehend how an internet (wide area network or WAN) is formed Communication protocols: Present various networking standards Transmission Control Protocol / Internet Protocol (TCP/IP) Open Systems Interconnection Reference model (OSI) 2

3 Data Communications Signal transmission, signal encoding, multiplexing, data link control (flow control, error detection and error correction) 3

4 What is a Communication Network? Communication network is a set of equipment, software, and facilities that allows transfer of information between geographically distant users Why Communication networks: Resource Sharing High Reliability: Provide alternative routes and sources of data Parallel Processing: Use multiple computers for a single application Scalability: Ability to increase system size based on demand without degrading system performance 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 4

5 What is a Communication Network? (cont.) 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 Streamed audiovisual services: real-time Audio and video conferencing: real-time 5

6 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 Tx: Tranmistter Rx: Receiver 6

7 Communication Tasks Tx. System Utilization: Interfacing: Message formatting: Signal Generation: Synchronization: Exchange Management: Error detection / correction: Addressing : Routing: Recovery: Security: Flow control: Heterogeneous networks: Firewall: Network Management: Efficient sharing of tx. resources between multiple users Interface info. source / destination to communication network Decide on the format of data that is transmitted Generate tx. signals that are correctly interpretable by rx. Synchronize the start and end of transmission btw. tx. & rx. Manage conventions of data transmissions Detect and eliminate errors resulting from channel distortions Indicate the transmitting source and the intended destination Specify a route between source and destination Resume activity after an interruption Guarantee communication, prevent eavesdropping Control rate of transfer constrained by network conditions Provide connectivity across different networks Prevent trouble makers from accessing the network Configure system, respond to failures, and plan for future 7

8 Network (1) Is a medium (channel) to transfer information between a source and destination. Network From S1 t 0 t 1 Network To D1 S2 S3 S4 S5 D2 D3 D4 D5 Depending upon the route, propagation delay varies in between different packets. 8

9 Network (2) To transfer information, networks must provide connectivity between source and destination. Two possible way of connectivity: Pairwise interconnection and Access network Switch Pairwise Interconnection Access network Pairwise interconnection is not sustainable with increasing number of users (N) Number of lines = N x (N - 1) / 2; N = 5, no. of lines = 10; N = 100, no. of lines =

10 Network (3) Different access networks can be connected via multiplexers and demultiplexers. Mux Trunk Demux Access network a Access network b The concept can be extended to form a metropolitan area network (MAN). 1* a b A β c d A α γ MAN A consists of access subnetworks a, b, c, and d. National network (WAN) consists of Several regional subnetworks α, β, γ. 10

11 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 11

12 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 12

13 Types of Networks (3) 3. Wide area networks (WAN) Spans a continent (<= km), Example: Internet Interconnects various LAN using switches (routers) and transmission lines Uses either: (1) circuit switching to establish a route between the tx. and rx. or (2) packet switching in conjunction with the store and forward technology 13

14 Switching Switching refers to the technology used for transmission of data between two hosts 1. Circuit switching (based on 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 (based on 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. 14

15 Circuit Switching (Connection-oriented Service) Dedicated communication path is established between the source and receiver communication follows the same path. Example is the telephone network Telephone Office Telephone Office Telephone Office The caller picks up the phone triggering the flow of current in wires that connect to the telephone office. The current is detected and a dial tone is transmitted by the telephone office to indicate that it is ready to receive the destination number. The caller sends the number by pushing keys on the telephone set. Each key generates a pair of tones that specify a number. (In older sets the user dials a number which generates corresponding number of pulses.) 4. Telephone Office 5. Telephone Office The equipment in the telephone office uses the telephone network to attempt a connection. If the destination telephone is busy, then a busy tone is returned. If the destination telephone is idle, then ringing signals are sent to both the originating and destination telephones. The ringing signals are discontinued when the destination phone is picked up and communication can then proceed. 6. Telephone Office Either of the users terminate the call by putting down a receiver. 15

16 Message Switching A complete communication path is not established between the source and receiver. Message is transmitted one hop at a time. Example is the telegraphic service. 1. Network 2. Network 3. Network The telegram application establishes a connection with the regional office Regional office acknowledges indicating it is ready to accept message Sender (client) sends the complete message with the receiver address. 4. Network 5. Network Regional office forwards message to next hop possibly to city office The message is queued at the city office and forwarded on its turn to the country office. The process is repeated till the destination regional office is reached 6. Network The regional office contacts the telegram application and notifies the destination of reception of a message. 16

17 Packet Switching (Connectionless Service) 1. No dedicated communication path is established between the source and receiver. 2. Data is divided into packets. 3. Each packet contains destination address and reaches rx. Independently. 4. Example is the postal service. Other switching techniques used are Frame relay, ATM, ISDN, and Broadband ISDN (pp ). 17

18 Switching: Comparison Timing of events for: (a) circuit switched; (b) message switched; (c) packet switched networks 18

19 Switching: Comparison (cont.) Circuit Switching Message Switching Packet Switching Efficiency Highly inefficient due to long setup time Improvement over circuit switching by dividing the connection in smaller segments Best under normal working conditions Routing Complete path is established once at initialization of connection Intelligent adaptive routing algorithms are required at each switching station Same as for message switching Synchronization 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 19

20 Protocols: Basic Definitions 1. Protocol: Set of rules and conventions used by two communicating parties. These rules dictate how a communication will be initiated and terminated, how data and control information are arranged in a packet, what control information is included, etc. Examples include Hypertext transfer protocol (HTTP), File transfer protocol (FTP), Simple mail transfer protocol (SMTP), and Transmission control protocol (TCP). 2. Client/Server configuration: A server is a powerful machine that houses data and other shared resources. Clients are smaller machines that connect to the server to retrieve shared information. The two communicate based on the following protocol: Client makes a request over the network to the server Client waits When server gets the request, it performs the requested job and returns a reply 20

21 Protocols: Basic Definitions (cont.) 3. Port (Service Access Points): A server process waits for incoming requests by listening to a port. Analogous to a wall socket in a telephone network. 4. Networks consist of two components: Hardware that forms the infrastructure connecting the computers, e.g., twisted pair wire / optical fibre / cable, routers, switches, servers, etc. Software that forms a cohesive connection such that the user sees the entire network as a single coherent system. The design of software is highly structured typically broken in layers. There are two standards available: OSI (7-layered model) and TCP/IP (5-layered model). 21

22 Example: A Three-Layer Model Transfer of a file between two computers connected via a network using a hypothetical 3-layer model. Computer X Application Layer Transport Layer Network Access Layer Application Layer: 1. Format translation to provide compatibility between different file formats used by 2 computers. 2. Ensure that file transfer application in destination application module is ready to accept data. Transport Layer: 1. Ensure that the destination computer is ready to accept data. 2. Confirms that data is received correctly with no errors. Network Access Layer: 1. Activates a connection between the source and destination computers. 22

23 Example (2) Each module communicates virtually except for the lowest module with its peer module on the other machine. Computer X Application Module Transport Module Application Protocol Transport Protocol Computer Y Application Module Transport Module Network Module Service Access Point (SAP) Network Network Access Protocol Network Module Each layer has specific tasks to do that are accomplished by adding headers Application Module Application PDU Application Data Transport Module Transport PDU TH Application Data I TH Application Data 2 Network Module Network PDU NH CH Application Data I NH CH Application Data 2 23

24 Example (3) Each layer in Source X adds a header while the corresponding layer in destination Y removes it before passing on the PDU to the upper layer. 24

25 Reference Models: OSI vs. TCP/IP 25

26 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 s. t. 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 glueing 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 26

27 ARPANET as of 1972 AMES McCLELLAN UTAH BOULDER GWC CASE RADC ILL LINC CARN AMES USC UCSB MIT MITRE STAN SCD ETAC UCLA RAND TINKER BBN HARV NBS 27