Communication Communication SystemsSystems

3/5 Transmitting And

receiving

Transmitting and receiving

In this unit you will:• learn about network topologies

• see how networks manage user access

Transmitting and receiving

• Refers to the transfer of data and information.

• It is carried out using numerous communication concepts and

completed by network hardware and

software.

Communication concepts• Protocols• Handshaking• Error detection and error correction• Network topologies:

• StarStar• BusBus• RingRing

• How each of the network topologies deal with multiple users wanting to access the network at the same time.

Data transmission methods• Serial transmission:

• Data transmitted one bit after another• Asynchronous transmission(typical PC) is

untimed, uses special start and stop bits.• Synchronous transmission is timed, sent at the

same rate, no extra bits, used on larger systems.

• Slower than parallel transmission.• Requires fewer wires for each signal.

• Parallel transmission:• Data transmitted simultaneously along separate

channels.

• Synchronisation of signal can be difficult

• Not suited to long distances.

• Expensive because of extra wiring and circuitry

• Parallel transmission is faster than serial transmission.

Data transmission methods

Parallel transmission

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CAT in parallel transmission

T A C

Transmitted simultaneously using separate lines (wires)

Parallel transmission

Parallel connectors for printers

• Used within computers

• from computers to disk drives

Serial transmission

0 0 1 0 1 0 1 1 0 0 0 0 0 1 1 1 0 0 0 0 1

C A T

CAT in serial transmissionData transmitted one bit after another using one line

Used for long distance communication

Serial transmission

P/S 2 type keyboard connector

USB connector

Serial ports at the back of PC

Keyboards, Modems and mouse

Ports and connectors

Data transmission methodsSynchronous and Asynchronous

• Synchronous transmission- relies on coordination of computers’ internal clocks.

• Asynchronous transmission -allows computers with different clock speeds to be linked. The most common type of serial transmission

0 1 1 0 0 0 1 1 1 0 0 1 0 1 1 0

1 0 0 1 0 1 1 1 1 0 1 1 0 1 0 1

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Data transmission methodsModes:- the direction of the flow of data:• Simplex: one direction (e.g. radio,

television)

• Half duplex:- both directions but not at same time. (intercom, 2way radio, disk drive)

• Full duplex:- both directions at the same time; most common. (telephone, email)

Simplex Mode

• one direction at a time (e.g. radio, television)

Half duplexBoth directions but not at same time.

(intercom, 2way radio, disk drive)

Both directions but not at same timeBoth directions but not at same time

Full Duplex

• Full duplex:- both directions at the same time; most common.

(telephone)

Both directions at the same timeBoth directions at the same time

Protocols and Handshaking

Handshaking involves sending signals to indicate the type of protocol to be used for an agreement to be reachedProtocols is the set of rules that governs the transfer of data between computers.

Protocols and Handshaking

• Some data transfer protocols are:• X.25:- standard packet switching protocol

• TCP/IP:- rules for data transmission and error detection across the Internet (Transmission Control Protocol/Internet Protocol)

• HTTP:- enables the user to send and retrieve files using the Internet (Hypertext Transfer Protocol)

• FTP:- used to enable the client computer(user) to log on to the server. (File Transfer Protocol)

Networks• A network is a number of computers and their

peripheral devices connected together.

• A node is a device in a network (printers, storage devices, terminals and workstations)

• A terminal is a device used to send data to and receive data from another computer system.

• A intelligent terminal has both memory and processing capabilities.

• Intelligent terminals are called workstations on a network.

Network topologies

Bus topology Ring topology Star topology

Refers to the physical layout of the devices in a network

A bus topologyAll the devices are connected to a direct line called the bus.

Each node has a unique identityNodes check bus and retrieve their messages.Ethernet and PowerTalk use a bus topology Each node communicates directly to any other

device Easy and cheap to set up Easy to add and remove nodes Can still operate if one node failso Collisions are more commono Difficult to locate faulty cables

A star topology

Central computer serves as a switch Each node has its own cable Can easily add new links Easy to identify faulty cables Use a time-sharing systemo If central node fails, entire network failso Requires more cablingo Limited by the processing power of the central

computer

Has central computer with each device connected to it.

Ring topology

Each node has a unique address. Data flow in one direction, moving from node to node until the data arrives at its destination

Single cable is used to connect all nodes Easily expandable Suited to fibre optics

o If ring breaks, entire network fails

o Ring must be broken (network stops)

to add new nodes or remove existing ones.

o If one w/station fails, the whole network fails.

Each node is part of a circle linking all nodes together

Network access methods

The main aims of a network are to allow the users to:

• Communicate with each other.

• Share documents, data and software.

• Share expensive resources such as high quality printers.

Network access methodsCarrier Sense Multiple Access with Collision Detection

(CSMA/CD)

• Used in Ethernet network using bus or star topologies.

• Allows data to be transmitted at the same time to all nodes in both directions.

CSMA/CD: All nodes have the ability to sense signals.

• Each node ‘listens’ to the bus for signals and if there’s no signal it transmits.

• CD: When collision occurs each node stops transmitting and wait random time to transmit.wait random time to transmit.

Network access methodsToken ring- also called ‘token passing’

A LAN access method, or protocol, based on a ring topology.

• An electronic ‘token’ is passed around the ring from node to node.

• When node wants to send data it must ‘capture’ the token.

• The node adds its message along with the address of the destination and its own address.

• The receiver node copies data packet and retransmits frame to sender node which detaches data packet and sends on empty token.

Network hardware

Network Interface Card: NIC

• each computer connected to the network requires a NIC.

• installed in computer, printer, servers on a network with driver software.

• has a unique identifying code for the network. (MAC address Machine access code)

ServersA server is a computer that provides services to

other computers on the network.

• A file server

• A print server

• A mail server

• A Web server

• Proxy Server

Servers

• File server: stores data (documents), controls login protocols, shared programs used by network workstations (nodes).

• Print server: manages printing jobs for network.

• Mail server: manages e-mail for users of a network.

• Web server: manages Internet access on the way in, stores web pages and may cache web pages for network.

• Proxy server: acts as a cache (temporary store) for frequently accessed data. Node on network or remote. Data access is faster from proxy server than Internet.

Connecting to the Internet

Dial-up-connection:- use phone line for Dial-up-connection:- use phone line for connection.connection.

Dial into with your computer.

Least expensive type of Internet connection.

Use the phone line (copper twisted wire).

• Slow access and download speed. (56K)

• Can’t use phone while on the Net.

• Requires a modem to modulate and demodulate.

Connecting to the Internet…

ISDNISDN: : IIntegrated ntegrated SServices ervices DDigital igital NNetworketwork Very high speed connection (1.5Mpbs vs. 56Kbps)

Uses the same wiring as phone line- no new wiring

Surf the Net and use the phone line at the same time.

• Works better when close to telephone company’s central

office

• Not available everywhere

• Requires ISDN modem

• ISP must support ISDN technology

Connecting to the Internet

Cable modem- Cable modem- coaxial TV cable coaxial TV cable connectionconnection

Use coaxial TV cable to connect to internet

High speed connection to internet:- 1.5Mbps

• Requires cable modem.

• Company must support technology.

ADSL: Asymmetric Digital Subscriber LineADSL: Asymmetric Digital Subscriber Line

• Sends data faster in one direction:- 1Mbps up and

8Mbps down

• Very high speed access

• Using the existing phone lines

• ISP have to support technology

• Requires ADSL modem

Connecting to the Internet

Routers• A Router is a device that determines where to

send a data packet between at least two networks. A router maintains a table of available routes and their conditions.

• They make sure that packets always arrive at the proper destination

A Switch

A switch is a device that directs data packets along a path.

• It is simpler and faster than a router as it does not maintain knowledge.

A Switch

Bridges and gateswaysThe actual interconnection between networks.

• A Bridge is a combination of hardware and software to link two similar networks using the same protocols.(e.g. two Ethernet LAN’s)

Bridges and gateways• A Gateway is a combination of hardware

and software to link two different types of networks using different protocols.

(e.g. Ethernet and token ring)Ethernet protocol Token ring protocol

Hubs• A hub is a central connecting device in a network.

• They linked groups of computers to one another and let

computers communicate with one another.

• Data arrives at the hub from one or more devices and is

forwarded out using just one cable.

Transmission mediaThe capacity of the transmission medium is called the

bandwidth. WIRE TRANSMISSION

CharacteristicCharacteristic Twisted pairTwisted pair CoaxialCoaxial Fibre opticFibre opticCostCost Cheap Moderate Expensive

StructureStructure Pair copper wires Core with mesh sheath

Thin glass fibre.

Used inUsed in LAN LAN Joins of LAN’s

TopologiesTopologies Star, ring Bus Star, ring

Max. lengthMax. length 100m 185m 2000m

Max. bandwth.Max. bandwth. 60 Mbps 10 Mbps >400 Mbps

SpeedSpeed 10Mbps,100Mbps 10 Mbps 10Mbps, 1Gbps

StandardsStandards 10Base-T 100B-T 10Base-2 10Base-F,100B-F

Wireless transmissionmicrowaves, satellites, wireless networks and mobile

phones

MicrowaveMicrowaveHigh frequency,line-of-sight through space.• Faster than wire transmission.• Transmit over long distances.• Can transmit both voice and data Affected by weather conditions, trees and

buildings.• Examples: pay TV, data services

Wireless transmission: Satellites

SatellitesSpecial transmitter and receiver that orbits the earth.

Satellites in geosynchronous orbits which receive and transmits to ground.

• Can transmit large amounts of data over long distances at great speeds.

• Use microwaves signals Can be affected by solar storms. High cost of launching and production.Examples: TV, telephone, radio, internet

Wireless transmission: Radio waves

Radio wavesUse radio waves as transmission medium.

Line-of-sight but may be wide area

Low security, limited range and speed.

Lower reliability than cable.

• Offer cheaper connections

Examples: mobile phones (GSM standard)

wireless laptops

Wireless transmission: wireless networks

• Wireless LAN :-use radio waves as transmission medium.

• Problem with signal quality.• Concerns about radiation.

Wireless transmission

• Microwaves, satellites, radio waves, wireless networks and mobile phones

• Mobile phones transmit data to a grid of cellular stations.

• Mobile phones use radio waves to communicate with cellular station.

Network software

Network operating system(NOS) is designed to support computers connected on a LAN. (Novell Netware and Windows NT)

NOS tasks:• Administration- adds, removes and organises

users; installs hardware devices and software applications also- backups

• Applications- handles requests from users to share data and applications.

• Resource management- allows devices such as printers and modems to be shared; assigns users to printers .

• Security- monitors and restricts access to network resources

• File management-gives users access to remote hard disks on the server and provides a file system and the ability to manage a network directory.

Network software

Logon and logoff procedures• Logon is the procedure used to get access to the

network using a user ID (username) and a password.

• The password must not be readily available or an easily guessed word.

• Network must keep passwords secure: user account.

• Password file must be encrypted and protected.

• Users must frequently change their passwords

• Correct logging off procedure should be carried out:-to clear the communications line for other users

Internet, Intranet and Extranet

• Internet not individually owned, publicly accessed, linking of LAN’s and WAN’s.

• Intranet privately owned network, has connection to Internet.

• Extranet is an intranet that is accessible to customers and suppliers: (FAQ’s, warranties…)

• A firewall a software barrier around the network. Decides which data are allowed in and out of the network.

Intranets• An internal internet designed to be used

within the confines of a company, university or organisation.

• Intranets are privately own networks.

• Makes communication much simpler for companies.

• Its purpose is to share information and computing resources among employees: (staff news, product info., policies)

Extranet

• A network that allows companies to share information with other businesses and customers.

• Transmit information over the Internet.

• Users require a password.

• Data is accessed through company servers.

Firewall• A program or hardware device that filters the

information coming through the Internet connection into your private network or computer system.

• Protects the internal networks from intruders.

Peer-to-peer and Client servernetworks

Peer-to- peer• No controlling computer, each workstation

shares its resources with all others.

Client server• Stores data on server which manages

resources• Thin client are workstations without hard

drives -running as slaves of a network.

The EndThe End3/5 Transmitting and receiving3/5 Transmitting and receiving

in

Communication Systems