Data Communications & Computer Networks. Data Communications and Networks Overview

Transcription

1 Data Communications & Computer Networks Chapter 1 Data Communications and Networks Overview Fall Agenda Networking history Communications model Data communication networking 1

2 Networking history From a historical perspective, electronic communication has actually been around a long time, beginning with Samuel Morse and the telegraph. first telegraph message sent on May 24, 1844 from Washington DC to Baltimore MD, 37 miles away 23 years later, Alexander Graham Bell invented the telephone This led to the development of the ultimate analog network: the telephone system. The first bit-oriented language device was developed by Emile Baudot the printing telegraph. By bit-oriented we mean the device sends pulses of electricity which were either positive or had no voltage at all. These machines did not use Morse code. Baudot s five-level code sent five pulses down the wire for each character transmitted. The machines did the encoding and decoding, eliminating the need for operators at both ends of the wires. For the first time, electronic messages could be sent by anyone. 3 Telephone Network Telephone network Analog Network 2

3 Early computer networks In the 1960 s and 1970 s, traditional computer communications centered around the mainframe host. The mainframe contained all the applications needed by the users, as well as file management, and even printing. This centralized computing environment used low-speed access lines that tied terminals to the host. These large mainframes used digital signals pulses of electricity or zeros and ones, what is called binary to pass information from the terminals to the host. The information processing in the host was also all digital. Low-Speed Access Lines Mainframe Host Computer Systems Application Programs Database Printing Digital Network 5 Problem Analog Analog Digital Digital This brought about a problem: The telephone industry wanted to use computers to switch calls faster and the computer industry wanted to connect remote users to the mainframe using the telephone service. But the telephone networks speak analog and computers speak digital. 3

4 Analog and Digital Signals Digital Transmission: 1 s and 0 s On or Off Computer-speak 1 bit Analog Transmission: Wires or wireless, Audio tones Info conveyed through signal amplitude, frequency, and phase 0 bit Start Bit Stop Bit Solution Modems Modem (=Modulator/Demodulator) Translates digital computer signals to analog signals which the telephone world can understand and vice versa POTS Modem POTS (Plain Old Telephone Service) Modem Mainframe Host 4

5 Another Solution Multiplexing Multiplexer Allows multiple signals to be carried across a single physical medium Broadband Able to carry multiple signals simultaneously Baseband Carries only one signal at a time Mainframe Host Baseband versus Broadband Baseband Local-Area Network (LAN) Broadband Wide-Area Network (WAN) 5

6 A Communications Model Source generates information (called data) to be sent (i.e.transmitted) Transmitter (tx) Converts the data into transmittable signals Transmission System Carries the information (data) Receiver (rx) Converts received signal into data Destination Takes incoming data 11 Simplified Communications Model - Diagram 12 6

7 Key Communications Tasks Transmission System Utilization Interfacing Signal Generation Synchronization Exchange Management Error detection and correction Addressing and routing Recovery Message formatting Security Network Management 13 Key Communications Tasks (1/3) Transmission System Utilization Efficient use of tx facilities, eg multiplexing, congestion control Interfacing Electromagnetic signals propagated over the tx medium Signal Generation Signal must be capable of being propagated through the tx system Signal must be interpretable as data at the rx Synchronization Between tx and rx Rx should determine when a signal begins to arrive, when it ends, and its duration 14 7

8 Key Communications Tasks (2/3) Exchange Management eg if data are exchanged in both directions Must be decided whether both devices may tx simultaneously or in turns, the amount and format of data, etc Error detection and correction Errors may occur, since tx signals may be distorted before reaching the rx Flow control To assure that the source does not overwhelm the destination by sending data faster than they can be processed by the rx Addressing and routing When more than two devices share a tx facility, a source system must indicate the identity of the intended destination The tx system must assure that only the destination system receives the data 15 Key Communications Tasks (3/3) Recovery To resume activity in case of interruption during info exchange Formatting Agreement to the form of data to be exchanged Security The sender of data wants to be assured that only the intended receiver actually receives the data Network Management For system configuration Monitoring the system status Planning Alarm reporting and troubleshooting 16 8

9 Simplified Data Communications Model 17 Data Communication Networking Point-to-point communication not usually practical Devices are too far apart Large set of devices would need impractical number of connections Solution is a communications network Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) 18 9

10 Simplified Network Model MAN or WAN LAN 19 Local Area Network LAN What is a LAN? A collection of computers, printers, modems, and other devices that can communicate with each other in a small area (< ~ 1km) What are the components? Computers Operating System (OS) Network Interface Card (NIC) Hubs/Switches/Routers How is a LAN controlled? Protocols Formal descriptions of sets of rules and conventions that govern how devices on a network exchange information Standards Sets of rules or procedures that are either widely used or officially specified 10

11 Local-Area Networks LANs are designed to: Operate within a limited geographic area Allow multi-access to high-bandwidth media Control the network privately under local administration Provide full-time connectivity to local services Connect physically adjacent devices Network Operating System (OS) Software that allows communicating and sharing of data and network resources PC or Workstation Loaded with network OS 11

12 Network Interface Card Amplifies electronic signals Packages data for transmission Physically connects computer to transmission media (cable) Connector Port Network Interface Card (NIC) PC or Workstation Loaded with network OS Wiring Hub Printer (Also has a NIC) Wiring Hub PC or Workstation Loaded with NOS NIC Serves as center of network Contains multiple independent but connected modules where network equipment can be connected 12

13 Cables or Transmission Media Printer (Also has a NIC) Connectors PC or Workstation Loaded with NOS Wiring Hub NIC Cable Physical environments through which transmission signals pass Twisted pair Coaxial cable Fiber-optic cable Connectors (RJ-11, RJ-45, etc.) RJ-45 Connector Network Cabling Cable is the actual physical path upon which an electrical signal travels as it moves from one component to another. Transmission protocols determine how NIC cards take turns transmitting data onto the cable. Media connecting network components NIC cards take turns transmitting on the cable LAN cables only carry one signal at a time WAN cables can carry multiple signals simultaneously Three primary types of cabling Twisted-pair (or copper) Coaxial cable Fiber-optic cable 13

14 Twisted-Pair (UTP and STP) STP only: Shielded Insulation to Reduce EMI Outer Jacket Twisted-Pair Color-Coded Plastic Insulation Speed : 10/100 Mbps Relative cost: Least costly Media and connector size: Small Maximum cable length: 100 m RJ-45 Connector Coaxial Cable OuterJacket Braided Copper Shielding Plastic Insulation Copper Conductor Speed : Relative cost: Media and connector size: Maximum cable length: BNC Connector 10/100 Mbps More than UTP, but still low Medium 200/500 m 14

15 Fiber-Optic Cable Outer Jacket Kevlar Reinforcing Material Plastic Shield Glass Fiber and Cladding Single mode: One stream of laser-generated light (100 km) Multimode: Multiple streams of LED-generated light (2 km) Speed : 100+ Mbps Average cost per node: Most expensive Media and connector size: Small Maximum cable length: Up to 2 km Multimode Connector Throughput Needs!! 841,000 bits 202,000,000 bits 100,000 bits 64,000 bps 2,457,000 bits/screen 30 screens/second 73,728,000 bps 7,300,000 bits/screen 30 pictures/second 224,000,000 bps!!! 15

16 Throughput Rate and Bandwidth Throughput rate The rate of information arriving at, and possibly passing through, a particular point in a network Bandwidth The total capacity of a given network medium (twisted pair, coaxial, or fiber-optic cable) or protocol Bandwidth is also used to describe the difference between the highest and the lowest frequencies available for network signals. This quantity is measured in hertz (Hz). The bandwidth of a given network medium or protocol is measured in bits per second (bps). Some of the available bandwidth specified for a given medium or protocol is used up in overhead, including control characters. This overhead reduces the capacity available for transmitting data. THROUGHPUT = BANDWIDTH - OVERHEAD Throughput Rate Networking Made Easy Speed Transmit Time 9,600 bps = hrs 10,000 pages = 53 MB (Megabytes) 24,000 bps = 4.91 hrs 56 Kbps = 2.1 hrs 1 Mbps = 7.1 min 10 Mbps = 42.4 sec 1 Byte = 8 bits 1 Megabyte = 1MB 1 Megabit = 1Mb 100 Mbps = 4.24 sec 1 Gbps = 0.42 sec This slide shows the tremendous variation in transmission time with different throughput rates. In years past, megabit (Mb) rates were considered fast. In today s modern networks, gigabit (Gb) rates are possible. Nevertheless, there continues to be a focus on greater throughput rates. As seen in the last slide, throughput is dependent on the bandwidth of the medium and the transmission protocol. 16

17 Metropolitan Area Networks (MANs) Middle-ground between LAN and WAN Private or public network High speed Large area 33 Wide Area Networks (WANs) Large geographical area Consists of a number of interconnected switching nodes Rely in part on common carrier circuits Alternative technologies Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM) IP 34 17

18 Circuit Switching Dedicated communications path established for the duration of the conversation Path is a sequence of connected physical links between nodes Example: POTS telephone network 35 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 Examples Frame Relay ATM IP 36 18

19 Frame Relay Layer 2 technology (data link layer) Packet switching systems have large overheads to compensate for errors Data rates of 64kbps up to 2Mbps Modern systems are more reliable Errors can be caught in end system Most overhead for error control is stripped out 37 Asynchronous Transfer Mode - ATM Layer 2 technology (data link layer) Evolution of frame relay Little overhead for error control Fixed packet (called cell) length of 53bytes Anything from 2Mbps to 2,5Gbps Constant data rate using packet switching technique ATM (Asynchronous Transfer Mode) was originally implemented to meet the transport requirements for local, metropolitan and widearea network applications

20 Internet Protocol - IP Layer 3 technology (network or internet layer) IP allows voice, data, fax and video signals to share a common networking infrastructure. IP can run on top of ATM or Ethernet Data rates from as low as 64kbps up to 10Gbps or more 39 Networking Configuration Example 40 20

21 Further Reading W. Stallings, Data and Computer Communications (7th edition), Prentice Hall, Chapter