1. 3.3) Transmitting and Receiving

2. Different concepts covered include: transmission of data protocols handshaking networks and their topologies network access methods Transmission of data occurs in two ways: parallel transmission serial transmission

3. Parallel transmission is the transmission of data simultaneously using separate channels. Serial transmission transmits the data one bit after the other along one channel. Parallel is faster, but can only be used over shorter distances. The direction of data flow can be: simplex half-duplex full-duplex

4. Simplex mode allows transmission in one direction only. Half-duplex allows transmission in either direction, but only in one direction at a time. Full duplex allows transmission in both directions simultaneously.

5. Serial transmission can be either asynchronous or synchronous. Synchronous transmission requires that all of the data is sent at the same transfer rate. The same number of bits is sent each second. Both computers are synchronized through the use of an internal clock. It is faster and more efficient, but quite expensive as it requires very stable connections.

6. Asynchronous transmission is sent at varying rates and the flow is regulated through the use of start and stop bits. One of the protocols agreed upon during handshaking is how many start and stop bits will be used. [Diagram 3.10, p.88] Protocols are a set of rules that govern the transfer of data between two computers.

7. They define: how a link is established how data is transmitted how errors are detected and corrected Numerous protocols have been developed for different technologies. [Table 3.1, p.89 – Do the following: IPX, X.25, ATM, Kermit, TCP/IP, HTTP, FTP, POP, MIME]

8. A network is any number of computers and their peripheral devices connected together in some way. Each device in a network is called a node. Terminals are devices that send data to and receive data from another computer. If the terminal has memory and processing capabilities it is called an intelligent terminal. Personal computers are often called workstations on a network.

9. Networks can be either LAN’s, WAN’s or very rarely, GAN’s Local Area Networks (LAN’s) – are linked computers at one physical location Wide Area Network (WAN’s) – connect computers over several geographical locations Global Area Networks (GAN’s) – computers and devices connected all over the world

10. Network topology is the physical arrangement of the devices in a network. Three common types of topologies are: bus star ring

11. Bus Network

12. Star Network

13. Ring Network

14. The bus topology is where all devices are attached to a direct line called a bus. Each devices checks that the line is free and sends its signal. If there is a data collision then the sending terminal waits a random time then attempts resending. If one node fails the system will still work. A star topology has a central computer with each device connected directly to it.

15. The central computer receives messages and sends them to the destination device. It requires extra cabling because each device must be connected to the central computer. If one devices fails, the network will still function unless it is the central computer that is broken. Its speed is determined by the processing power of the central computer.

16. A ring topology is an arrangement where all devices are attached so that the connection is a continuous loop. Data flow is one direction moving from device to device. A ‘token’ is passed along the network and data packets are attached to the token and released when they reach the appropriate node.

17. There are a number of ways to deal with multiple users wanting to access the network. The two most common methods are: Ethernet token ring Ethernet is based upon a bus network. Ethernet allows data to be sent in both directions simultaneously. This means there will be data collisions and errors

18. To overcome this problem, Ethernet uses a system called Carrier Sense Multiple Access and Collision Detection (CSMA/CD) In CSMA/CD all nodes have the ability to sense signals on a network. When a node wishes to transmit, it ‘listens’ for a signal on the bus. When no signals are detected, it transmits. If two nodes transmit at the same time then a data collision can occur.

19. When a collision is detected, each device stops transmitting, waits a randomly determined time, then transmits again. Token ring is a LAN access protocol based upon ring topologies. Special data packets, called tokens, are continually passed between nodes on the network. Workstations with data to send attach it to a free token and transmit it. A busy token cannot be used by other nodes.

20. When the data is received it is replaced with an acknowledgement and sent back to the original sending node. There are several special purpose hardware devices used in a successful network. They include: a network interface card servers routers and switches bridges and gateways hubs

21. A network interface card (NIC) is an expansion card that links the network to the computer motherboard. The type of NIC depends upon the type of network. A server is a dedicated computer that provides services to other computers. Workstations log on to the server to give them access to files, applications or peripheral devices.

22. Types of servers include: file server print server mail server Internet server Networks are often linked to other networks and this requires paths to be established and signal strength to be boosted. Devices used to determine the path between networks include routers and switches.

23. Routers maintain a table of available routes and their conditions. It uses this table to determine the best route for any given data packet. Data packets often travel through several networks and routers before they reach their destination. A switch is similar, but it does not maintain a table, therefore making it a simpler and faster mechanism for passing the message along.

24. However, they do not always choose the faster route for the packet to travel along. The actual connection between networks is achieved through bridges and gateways. A bridge is a combination of hardware and software used to link two similar networks. Bridges examine each data packet and forward any that are addressed to a connected LAN. Gateways are combinations of hardware and software used to link two different kinds of network.

25. This usually involves converting different protocols. A repeater may also be used to increase a diminishing signal. Repeaters are less intelligent than bridges or gateways. A hub is a central connecting device within a network. A hub can be considered passive if it just channels data or it can contain a router to improve traffic flow.

26. As well as the devices connected to a network, we need to consider the transmission medium. Data is transferred along a transmission medium. The capacity of a transmission medium is called the bandwidth. Bandwidth is generally measured in bps They can be categorised as wire or wireless. Wire transmission involves wires and cables.

27. Twisted pair cable consists of two insulated copper wires twisted around each other to form a spiral. There are two types: shielded twisted pair (STP) unshielded twisted pair (UTP) STP is more often used in ‘noisy’ environments. Twisted pair is the slowest medium with transmission speeds up to 60 Kbps.

28. Coaxial cable consists of a single copper wire surrounded by insulation, shielding, then more insulation. It allows transmission with little interference. Generally used for under 2kms. Bandwidth is around 10Mbps. Fibre-optic cables are made from glass fibres the width of a human hair, that are often bundled together.

29. They are: free from interference very secure able to transmit at high speeds (> 400 Mbps) Most LAN’s use twisted pair or coaxial cable as fibre-optic is too expensive but this is changing and fibre-optic is becoming much more common.

30. There are two types of transmission used: baseband broadband Baseband uses the entire capacity of the cable to transmit one signal. Broadband networks divide the cable so that several signals can be transmitted simultaneously. Most LAN’s are baseband. Wireless transmission moves data without a physical connection.

31. Wireless includes: microwaves radio frequencies (RF) infra-red (IR) satellites A microwave is a high frequency radio signal that requires antennae no further than 50 km apart. It is faster than coaxial or regular telephone lines and relatively error-free.

32. Radio frequencies are used for many home and business wireless networks, usually in the 2.4 GHz range. Can have an access point (or hot-spot) or be done in an ad-hoc manner. Satellites can transmit large amounts of data over large distances. Used by major corporations, television networks and telecommunications. IR is used by some devices in LAN’s

33. Each computer in a network must have appropriate network software. This software is usually organised by a network administrator. Their responsibilities include: network security software installations and updates monitoring daily activity enforcing licensing agreements developing a storage solution performing routine backups

34. A network operating system (NOS) is an operating system designed to support networked computers. One part resides on each node with another part on the server. Tasks carried out by a network OS include: administration file management applications resource management security

35. An intranet is a private network that uses a similar interface to the Web. It consists of many interlinked LANs. It provides access similar to the Internet, but only to a select group of users (e.g. employees, students) Firewalls monitor the flow of data in both directions to maintain the security of the organisation.

36. An extranet is an intranet that is accessible to customers, suppliers or others outside the organisation. It provides such information as product descriptions, answers to frequently asked questions, warranties and how to contact customer service. Extranets are still limited to accessing select computer paths and files.