Data Communications Systems
l Computer systems that transmit data over communications lines such as telephone lines or cables
l History
l Centralized data processing in early days
l Distributed data processing began in late 1960s
l Networks of personal computers began in 1980s
Centralized Data Processing
l Places all hardware, software, and processing in one location
l Very inconvenient and inefficient
l Input data had to be physically transported to computer
l Processed material had to be delivered to users
Distributed Data Processing
l Uses computers that are at a distance from central computer
l Local computers had access to central computers
l Some processing done on local computers, some on central computers
Network
l Uses communications equipment to connect two or more computers and resources
l Distributed data processing systems are networks
l Local area network (LAN) designed to share data and resources among several users in office or building
Putting Together a Network
l Basic Components
l Sending device
l Communications link
l Receiving device
Digital and Analog Transmission
l Digital transmission
l Analog transmission
l Modem
Digital Transmission
l Sends data as distinct pulses, either on or off
l Similar to how data travels through computer
Analog Transmission
l Continuous electrical signal in the form of a wave
l Called carrier wave
l Many communications media already in place for analog (voice) transmission
l Phone lines are most common
l Digital signal from computer must be converted to analog form to be transmitted over analog lines
Converting Digital to Analog
l Carrier wave can be altered
l Amplitude (height) of wave
l Frequency (number of times a wave repeats during a cycle) of wave
l Conversion from digital to analog called modulation
l Conversion from analog to digital called demodulation
Modem
l Short for modulator/demodulator
l Converts digital signal to analog and vice versa
Types of Modems
l External modem separate from computer
l Internal modem inserted into computer
l Standard on most computers today
l PC Card modem slides into slot on laptop
l Roughly credit card size
l Cable connects modem to standard phone jack
Other Communications Devices
l ISDN
l DSL
l Cable modems
l Cellular modems
Integrated Systems Digital Network (ISDN)
l Special type of telephone circuit
l Can move data at 128,000 bps
l Includes two phone lines, so you can talk on the phone while online
l Drawbacks
l Expensive, especially at installation
l Not available in all areas
Digital Subscriber Line (DSL)
l Uses advanced electronics to send data over telephone line at very high speeds
l Always on – no need to dial a connection
l Can use phone while online
l Drawbacks
l You must be within three miles of telephone company’s switching office
l That office must have DSL equipment
Cable Modems
l Uses coaxial cable already in place for your TV
l Very fast transmission speed, especially for downloading
l Always on: no need to dial a connection
l Drawbacks
l All users share a cable segment’s capacity
l As more users in neighborhood go online, speed decreases
l No security for individual users or data
l Purchase a firewall program for security
Cellular Modems
l Transmit data over the cellular telephone system
l Roughly half the speed of a regular telephone network
Coordinating Sender and Receiver
l Sending data to remote location only works if receiving device is ready to accept it
l Two approaches to keeping devices in step:
l Asynchronous transmission
l Synchronous transmission
Asynchronous Transmission
l Also called start/stop transmission
l Start bit transmitted at the beginning of each group of bits
l Stop bit sent at end of each group
l Each group typically consists of one character
l Receiving device gets start signal and sets up mechanism to accept the group
l Used for low-speed communications
Synchronous Transmission
l Large block of characters transmitted
l Internal clocks of devices synchronized
l Error-check bits make sure all characters received
l Much faster, but equipment is more expensive
Simplex, Half-Duplex, and Full-Duplex Transmission
l Simplex transmission sends data in one direction only
l Example: television broadcasting
l Half-duplex transmission sends data in both directions, but only one way at a time
l Example: bank teller sends data about a deposit; after data received, a confirmation returns
l Full-duplex transmission allows transmission in both directions at same time
l Example: a conversation
l Typically used for high-speed data communication
Communications Media
l Physical means of data transmission
l Bandwidth is measure of the capacity of the communications link
Types of Communications Media
l Wire pairs
l Coaxial cables
l Fiber optics
l Microwave transmission
l Satellite transmission
l Wireless transmission
Wire Pairs
l Also known as twisted pair
l Two wires twisted around each other to reduce electrical interference
l Inexpensive
l Already in place (for telephone systems)
l Susceptible to electrical interference and noise
l Noise – anything that causes signal distortion
Coaxial Cable
l A center conductor wire surrounded by layer of insulation and metallic sheath
l Commonly used to connect to cable TV
l Higher bandwidth and less susceptibility to noise than twisted pair
Fiber Optics
l Use light instead of electricity to send data
l Much higher bandwidth than coaxial cable
l Immune to electrical interference
l Materials cheaper than coaxial, but installation costs high
Microwave Transmission
l Uses line-of-sight transmission of data signals
l Sending microwave must “see” receiver
l Requires relay stations approximately every 30 miles
l Waves are straight, earth is curved
l Offers high speed and cost-effectiveness
l Susceptible to weather conditions
Satellite Transmission
l A form of microwave transmission
l Satellite acts as relay station
l Components
l Earth station sends and receives signal to satellite
l Transponder receives and amplifies signal, changes frequency, and retransmits data
l Useful when signal must travel thousands of miles
Wireless Transmission
l Transmits data over relatively short distances without wires
l Examples
l IrDA – uses infrared line-of-sight
l Bluetooth – uses radio waves to connect mobile devices
l 802.11 standards – govern wireless transmission
Setting Standards
l Protocol – a set of rules for the exchange of data between a terminal and a computer or two computers
l Agreement on how data is to be sent and receipt acknowledged
l Needed to allow computers from different vendors to communicate
l Transmission Control Protocol/Internet Protocol (TCP/IP) permits any computer to communicate with the Internet
Network Topology
l The physical layout of a network
l Node – each computer, printer, or server on network
l Three common topologies
l Star
l Ring
l Bus
Star Topology
l Central (hub) computer manages network
l All messages routed through hub
l Helps prevent collisions among messages
l Connection failure between hub and any node will not affect overall system
l If hub is down, the network fails
Ring Topology
l Links all nodes in a circular chain
l Data messages travel around ring in a single direction
l Each node checks message to see whether that node is addressee
l If not, message passed to next node
l No danger of data collision
l If one node fails, ring is broken and network fails
Bus Topology
l All nodes connected to single line (bus)
l Computers send messages to other computers on network
l If messages collide with other messages, sending node resends message
l Nodes can be added/removed from network without affecting network
l If a node fails, network does not fail
Wide Area Network (WAN)
l Can span the world or link computers across town
l Metropolitan Area Network (MAN) – networks that cover a single city
l Components
l Communications services
l WAN hardware
l WAN software
Local Area Network (LAN)
l A collection of computers that share hardware, software, and data
l Typically personal computers
l Typically within an office or building
LAN Components
l Network cable
l Network interface card (NIC)
l Router
l Gateway
Network Cable
l Provides a way to connect to network
l Low-cost LANs connected with twisted pair wire
l Many connected by coaxial or fiber optic cable
l Wireless access point connects to wired network
l Provides wireless connection to network
Network Interface Card
l Connects each computer to wiring in the network
l Handles sending, receiving, and error checking of transmitted data
l Can be a circuit board or PC card
l Wireless NIC allows user to connect through wireless access point
Routers
l Bridge allows connection of similar networks (those using the same protocol)
l Router directs communications traffic when several networks connected together
l If network traffic clogged, router can redirect traffic to another route
l IP switch used in place of router when networks use the Internet protocol
l Less expensive and faster than routers
Gateway
l Lets a node communicate with a computer on another dissimilar network
l Primary function is converting protocol among networks
Client/Server Network
l Server computer controls network
l Often has several hard drives, fastest printer
l Client computer requests services from server
l Thin client has little or no storage
l Processing approaches
l Client/server
l File server
File Server
l Client requests data from server
l Server sends entire file
l Client performs all data entry and processing
l File retransmitted to server
Peer-to-Peer Networks
l All computers have equal status
l Users share each other’s files, printers, etc. as needed
l Common in small offices
l Networks tend to be slow