Communication Systems

1. Communication Systems 3/5 Transmitting And receiving

2. Transmitting and receiving In this unit you will: • learn about network topologies • see how networks manage user access

3. 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.

4. Communication concepts • Protocols • Handshaking • Error detection and error correction • Network topologies: • Star • Bus • Ring • How each of the network topologies deal with multiple users wanting to access the network at the same time.

5. 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.

6. Data transmission methods • 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.

7. Parallel transmission CAT in parallel transmission Transmitted simultaneously using separate lines (wires) T C A

8. Parallel transmission Parallel connectors for printers • Used within computers • from computers to disk drives

9. Serial transmission CAT in serial transmission Data transmitted one bit after anotherusing one line C A T Used for long distance communication

10. Serial transmission Ports and connectors Keyboards, Modems and mouse USB connector P/S 2 type keyboard connector Serial ports at the back of PC

11. Data transmission methodsSynchronous and Asynchronous Character Character • 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 Character

12. Data transmission methods Modes:- 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)

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

14. Half duplex Both directions but not at same time. (intercom, 2way radio, disk drive) Both directions but not at same time

15. Full Duplex • Full duplex:- both directions at the same time; most common. (telephone) Both directions at the same time

16. Protocols and Handshaking Handshaking involves sending signals to indicate the type of protocol to be used for an agreement to be reached Protocolsis the set of rules that governs the transfer of data between computers.

17. 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)

18. Networks • A network is a number of computers and their peripheral devices connected together. • A nodeis a device in a network (printers, storage devices, terminals and workstations) • A terminalis 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 calledworkstations on a network.

19. Network topologies Refers to the physical layout of the devices in a network Bus topology Ring topology Star topology

20. A bus topologyAll the devices are connected to a direct line called the bus. Each node has a unique identity Nodes 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 fails • Collisions are more common • Difficult to locate faulty cables

21. A star topology Hascentral computer with each device connected to it. 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 system • If central node fails, entire network fails • Requires more cabling • Limited by the processing power of the central computer

22. Ring topology Each node is part of a circle linking all nodes together 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 • If ring breaks, entire network fails • Ring must be broken (network stops) to add new nodes or remove existing ones. • If one w/station fails, the whole network fails.

23. 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.

24. 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.

25. 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.

26. Network hardware • 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) Network Interface Card: NIC

27. Servers A 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

28. 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.

29. Connecting to the Internet Dial-up-connection:- use phone line for 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.

30. Connecting to the Internet… ISDN: Integrated Services Digital Network • 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

31. Connecting to the Internet • Cable modem- coaxial TV cable • connection • Use coaxial TV cable to connect to internet • High speed connection to internet:- 1.5Mbps • Requires cable modem. • Company must support technology.

32. Connecting to the Internet ADSL: 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

33. Routers • A Routeris 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

34. 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

35. Bridges and gateswaysThe actual interconnection between networks. • A Bridgeis a combination of hardware and software to link two similar networks using the same protocols.(e.g. two Ethernet LAN’s)

36. 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

37. 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.

38. Transmission mediaThe capacity of the transmission medium is called the bandwidth. WIRE TRANSMISSION

39. Wirelesstransmissionmicrowaves, satellites, wireless networks and mobile phones Microwave High 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

40. Wireless transmission: Satellites Satellites Special 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

41. Wireless transmission:Radio waves Radio waves Use 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

42. Wireless transmission: wirelessnetworks • Wireless LAN :-use radio waves as transmission medium. • Problem with signal quality. • Concerns about radiation.

43. 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.

44. 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

45. Network software • 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 usersaccess to remote hard disks on the server and provides a file system and the ability to manage a network directory.

46. 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

47. 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 firewalla software barrier around the network. Decides which data are allowed in and out of the network.

48. 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)

49. 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.

50. 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.