6 Chapter 1 Introduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7 1-1 DATA COMMUNICATIONS The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. Components Data Representation Data Flow Topics discussed in this section:

8 Figure 1.1 Five components of data communication

9 Data Representation TEXT – unicode (32 bits to represent character)- ASCII NUMBERS IMAGES-composed of a matrix of pixel AUDIO- recording or broadcasting of sound or music. VIDEO- recording or broadcasting of a picture or movie

10 Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

NETWORKS A 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. Distributed Processing Network Criteria Physical Structures Network Models Categories of Networks Interconnection of Networks: Internetwork Topics discussed in this section:

12 NETWORKS Set of nodes connected via physical links –1) Distributing Processing –2) Sharing Data and centralization –3) Security and robustness

13 Network Criteria 1) Performance : Transmission and response times - throughput and delay. 2) Reliability: How often the networks fail 3) Security: Privacy and integrity of communication Data; recovery from breaches and data loss.

14 Physical Structures: Types of connections: 1) Point to Point: (P to P) Dedicated link to be utilized only by end devices. 2) Multipoint (Multidrop): Many end devices share the link capacity.

15 Figure 1.3 Types of connections: point-to-point and multipoint

16 Physical Topology

17 Figure 1.4 Categories of topology

18 Physical Topology A) Mesh: Each device has dedicated point-to-point link to other devices. Fully connected mesh will have n( n-1)/ 2 F.D. links –Where n = number of nodes Advantages: Fast communication, Robust and Privacy (Security) Disadvantages: Cabling Space and cost

19 Figure 1.5 A fully connected mesh topology (five devices)

20 Physical Topology (cont..) B) Star: Devices are connected Point to Point to a central “Hub” (Controller Exchanger) Advantages: Less cabling and H/W ports, two hops only. Disadvantage: Not robust

21 Figure 1.6 A star topology connecting four stations

22 Figure 1.7 A bus topology connecting three stations

23 –C) Bus: Multipoint link as “backbone” for a network where devices have drop line to tap into the bus Advantage: Less Cabling Disadvantages: Topology dependent, limit number of nodes on the bus due to signal power loss with distance, not so robust

24 Figure 1.8 A ring topology connecting six stations

25 C) Ring: Each device connects Point to Point with only two other devices in a left and right neighbors arrangement via a repeater. Advantages : easy installation, better fault isolation and robustness. Disadvantages: N/2 hops communication

26 Figure 1.9 A hybrid topology: a star backbone with three bus networks

27 D) Hybrid: Pizza “star” inside a “ ring” Pros: Better robustness, still low cost, Delay is 1 hop (until the star is not with a ring cut, max 2. Star of busses

28 Categories of Networks: Local Area Networks (LAN): Few Kilometers Connects devices (PCs, printers, servers.) within the same room, building, company, and campus. Topologies most used are bus, ring and star.

29 Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet

30 Categories of Networks(cont..) Wide Area Networks (WAN): span a large geographical area about 100’s – 1000’s of Km 1) Switched: End users connected via a cloud of switches (subnet). 2) Point-to Point: Line leased from telephone company/ TV connecting users to the ISP for Internet access.

31 alternative technologies used include: –circuit switching –packet switching –frame relay –Asynchronous Transfer Mode (ATM)

32 Circuit Switching uses a dedicated communications path established for duration of conversation comprising a sequence of physical links with a dedicated logical channel eg. telephone network

33 Packet Switching data sent out of sequence small chunks (packets) of data at a time packets passed from node to node between source and destination used for terminal to computer and computer to computer communications

34 Frame Relay packet switching systems have large overheads to compensate for errors modern systems are more reliable errors can be caught in end system Frame Relay provides higher speeds with most error control overhead removed

35 Asynchronous Transfer Mode (ATM) evolution of frame relay fixed packet (called cell) length with little overhead for error control anything from 10Mbps to Gbps constant data rate using packet switching technique with multiple virtual circuits

36 Figure 1.11 WANs: a switched WAN and a point-to-point WAN

37 Figure 1.12 A heterogeneous network made of four WANs and two LANs

38 Categories of Networks(cont..) Metropolitan Area Networks: MAN town/city High-speed backbone linking multiple LAN’s, DSL, TV cables.

THE INTERNET The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use. A Brief History The Internet Today (ISPs) Topics discussed in this section:

40 The Internet Internet evolved from ARPANET –first operational packet network –applied to tactical radio & satellite nets also –had a need for interoperability –lead to standardized TCP/IP protocols

41 The Internet: Huge number of interconnected Networks (100000’s) private organized, e.g.’ government, schools, research facilities, in many countries Collection of LAN’S, MAN’S and WAN’s. – The internet protocol stack is the TCP/IP – End users use the internet via Internet Service providers (ISPs) which are of the following hierarchies:

42 Figure 1.13 Hierarchical organization of the Internet

43 Internet Elements

44 Internet Architecture

45 Example Configuration

PROTOCOLS AND STANDARDS In this section, we define two widely used terms: protocols and standards. First, we define protocol, which is synonymous with rule. Then we discuss standards, which are agreed-upon rules. Protocols Standards Standards Organizations Internet Standards Topics discussed in this section:

47 What is a protocol?  Communications between computers requires very specific unambiguous rules  A protocol is a set of rules that governs how two or more communicating parties are to interact Examples  Internet Protocol (IP)  Transmission Control Protocol (TCP)  HyperText Transfer Protocol (HTTP)  Simple Mail Transfer Protocol (SMTP)

48 Elements: 1) Syntax: PDU format –2) Semantics: The meaning of each PDU’s field –3) Timing: Synchronization of communication when PDU is to be transferred and its data rate.

49 What is a standard? – A “ standard is set of guidelines” to users and manufactures to ensure interconnectivity. –1) “Defacto” not approved but widely used (TCP/IP) –2) “Dejure”: approved by recognized body (IEEE 802.X).

50 Summary introduced data communications needs communications model defined data communications overview of networks introduce Internet