Data Communications Systems

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


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


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


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


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