Chapter 3.8 Common Network Environments, Connectivity and Security Issues

Transcription

1 Chapter 3.8 Common Network Environments, Connectivity and Security Issues 3.8 (a) Different media for transmitting data and their carrying capabilities. Most of this is covered in Chapter 1.5 in the AS text. Remember that questions may be asked on any part of the A Level Computing syllabus in the exam for module 3. In computer networking and computer science, bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it (kilobits/s, megabits/s etc.) or simply saying bandwidth is the amount of data that can be transmitted in a fixed amount of time. Bandwidth is a fundamental measure of performance within any communication network, whether it is an analogue system such as radio or a digital system such as a network. Nowadays we are getting used to with video conferencing services like Skype, we want to watch movies over Internet at YouTube and download bigger files and for that we opt and pay more to get higher bandwidth at home and office for Internet connectivity. Supposedly the fastest link is within the telecom network itself; however, the slowest link is the low bandwidth link between the local ISP and the home router/modem. Sometimes it is limited by your choice of the opted bandwidth and sometimes by the type of transmission media between your computer and the provider. This chapter will take you through the most important points you need to know about and the kind of network media (wired and wireless) used in modern digital networks and make you able to compare their performances. Page 1 of 14

2 Physical media; wires: One of the key elements that determine bandwidth is the physical nature of the cable being used. A signal becomes weaker and dies down eventually the longer it travels along a cable. Therefore the length of cable determines the bandwidth of the link. For instance the bandwidth of a broadband DSL connection to the home is determined by the length of copper cable between the house and the nearest telephone exchange. Coaxial cable consists of a solid copper core surrounded by insulation which is then surrounded by a copper shielding and finally covered with a plastic sheath. Coaxial cable is widely used for television wiring as it has enough bandwidth to handle a television signal over a typical run from antenna to television. Computer networks also use coaxial cable with a bandwidth of 10Mbps. But for high speed networks (100 Mbps and above) coax cable is no longer sufficient. Copper screen Central conductor Plastic insulators Coaxial Cable Twisted pair cabling is a type of wiring in which two conductors (wires) are twisted together for the purposes of cancelling out electromagnetic interference from external sources or other twisted pairs. It was invented by Alexander Graham Bell. Twisted pair is used to gain enough bandwidth higher than coax cable. Unshielded twisted pair (UTP) cable Unshielded twisted pair or UTP cables are found in many local area networks and telephone systems. A typical subset of these colours (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables as shown above. UTP cable is the most common cable used in computer networking and is often used in LAN because of its relatively lower costs compared to optical fibre and coaxial cable. UTP is also finding increasing use in video applications, primarily in security cameras. Page 2 of 14

3 Shielded twisted pair (STP) cables are often copper shielded in an attempt to prevent electromagnetic interference and so allow bandwidth to be higher for any given length. This shielding can be applied to individual pairs, to the collection of pairs or both. Shielded twis ted pair or STP Copper cable is adequate for network cable runs for up to 100 meters, but above that the signal becomes too weak, therefore an alternative technology is needed. Fibre optics is a technology that uses glass (or plastic) threads (fibres) to transmit data. A fibre optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Fibre optic cable. Fibre optics has several advantages over traditional metal communications lines: Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data. Fiber optic cables are less susceptible than metal cables to interference. Fiber optic cables are much thinner and lighter than metal wires. Data can be transmitted digitally (the natural form for computer data) rather than analogically. The main disadvantage of fibre optics is that the cables are expensive to install. In addition, they are more fragile than wire and are difficult to splice. Fibre optics is a particularly popular technology for local-area networks. In addition, telephone companies are steadily replacing traditional telephone lines with fibre optic cables. In the future, almost all communications will employ fibre optics. Page 3 of 14

4 Wireless networks: Wireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise (business) installations avoid the costly process of introducing cables into a building, or as a connection between various equipment locations. Wireless telecommunications networks are generally implemented and administered using a transmission system called radio waves. Wireless Network If you've been in an airport, coffee shop, library or hotel recently, chances are you've been right in the middle of a wireless network. Many people also use wireless networking, also called WiFi or networking, to connect their computers at home, and some cities are trying to use the technology to provide free or low-cost Internet access to residents. In the near future, wireless networking may become so widespread that you can access the Internet just about anywhere at any time, without using wires. WiFi has a lot of advantages. Wireless networks are easy to set up and inexpensive. They're also unobtrusive -- unless you're on the lookout for a place to use your laptop, you may not even notice when you're in a WiFi hotspot. Page 4 of 14

5 Choosing the appropriate network medium is a must knowledge for answering questions for the given scenario in CIE exams. Following points must be considered: Speed (Bandwidth): Speed or bandwidth is especially important when transmitting large files. Industrial standards specify the performance/bandwidth of the medium. Fiber Optic up to 2.5 Gbps Twisted pair Cat 6 up to 1000 Mbps UTP Coaxial cable 1000 Mbps g up to 54Mbps n Greater than 100Mbps Bluetooth 700 kbps - 1 Mbps Distance: The further that data travels the signal becomes attenuated (become weaker). Distances may be affected by the environment and the amount of network noise (interference). Twisted pair Cat 5e/6 distances of up to 100m Fiber Optic 2 Kilometers Coaxial cable 500 m WiFi n Up to 100m Bluetooth 10 m Infrared IrDa 1 m Environment: Interference (static) that destroys the integrity of signals on a medium line. Noise can come from a variety of sources, including radio waves, nearby electrical wires, lightning, and bad connections. One of the major advantages of fibre optic cables over metal cables, light and radio waves is that they are much less susceptible to noise. Security: The less susceptible to electromagnetic interference (EMI) the less prone to eavesdropping. Fibre Optic Cables are not susceptible to EMI. Twisted Pair Less Susceptible UTP Very Susceptible STP Good resistance Coaxial Good resistance WiFi Very Susceptible Cost: You pay for speed, distance and security. The most expensive and difficult cable to install is Fibre Optic. The cheapest and easiest cable to install is twisted pair. Wireless networking is relatively inexpensive and very easy to install but there is a greater need for router encryption. Page 5 of 14

6 What is attenuation? Attenuation is the data loss experienced in a transmission media as a function of length. Attenuation increases as the distance a signal increases. Attenuation will eventually prevent a signal reaching its intended destination. Page 6 of 14

7 3.8 (b) Network Components A possible use of modem and switch/router in a home or office. Switches use the same type of wiring as hubs. However, each connector has full network speed. A typical layout is shown in Fig. 3.8 (b)1. Here, each station has full speed access to the server. However, if any of these stations wish to access the main network, they would have to share the connection to the main network. Stations S W I T C H To main network Server Page 7 of 14 Fig. 3.8 (b)1 If the number of stations is increased and they all want to access the main network, the increased local speed would be less useful because of sharing access to the main

8 network. In a case like this, it may be necessary to upgrade the link to the main network. A router is used to connect different types of network together. A router can alter packets of data so that two connected networks (LANs or WANs) need not be the same. Routers use network addresses and addresses of other routers to create a route between two networks. This means that routers must keep tables of addresses. These tables are often copied between routers using routing information protocol (RIP). Routers enable public networks to act as connections between private networks as shown in Fig. 3.8 (b)2. LAN Router Public network Router LAN Fig. 3.8 (b)2 In order to route data round a network, a router takes the following steps. 1. Router receives a network level packet. 2. Network destination address is checked. 3. Packet is sent to the appropriate network. Note that, in the case of the Internet, the destination address is the IP address. Basically, a router determines the fastest way possible, which is also usually the shortest way possible, in a particular network. It has the capability to route the packets through the most effective determined route. Routers have the ability to allow hosts that aren t practically on the same logical network, to be able to communicate with each other. Every router can receive chunks of data, which are called packets, on an interface. It will then forward the data packets to the intended location in the most efficient manner. The directing, or routing, of packets is based on the routing table, by allowing routers to know where a particular network is found. Aside from being a device, a router can be software in a computer. Routers should be, at least, connected to two networks. It is sort of a gateway to another network. Functionally, it is capable of generating traffic between logically separated networks. The third layer, which is the network layer of the OSI model, is where routers operate. Understanding the OSI model is the key to figuring out differences between routers, Page 8 of 14

9 gateways and bridges. The network layer is responsible for moving packets from a particular port to another. A bridge, or network bridge, is a device that can also connect networks, but unlike a router, its operation does not include the network layer of the OSI model. Only the one and two layers are included in a bridge s operation the physical layer and the data link layer respectively. It can only connect existing networks that you can access. It is basically not concerned with, and is unable to distinguish networks, unlike a router. They can only be used when you intend to connect networks of same type. In bridging mode, the process does not bother to understand network communications protocol, such as IP addresses. It only recognizes and considers the physical means, like the Media Access Control (MAC) address, which is usually an Ethernet. Thus, traffic will only exist in a bridged network if the networks concerned are logically the same. In terms of practicality, routers are more favoured, because routing is more efficient and offers easier call management. Bridging is required for cases where you cannot subnet an IP network and for cases where you need to utilize non-routable protocols. A router, or routing, is considered more intelligent than a bridge, or bridging, because they make smarter decisions. It only sends a packet to its intended destination, eliminating unnecessary traffic. With regards to routers, there is improved call management, while for bridging, call management and performance is sacrificed, as packets are automatically broadcasted to all the computers on a network. Summary: 1. Routers are more intelligent than bridges. 2. Routers allow hosts that aren t practically on the same logical network to be able to communicate with each other, while bridges can only connect networks that are logically the same. 3. Routers operate at the layer 3 (network layer) of the OSI model, while bridges are only at the layer 2 (Data link layer). 4. Routers understand and consider IP and IPX addresses, while bridges do not, and instead they recognize MAC addresses. 5. Routing is more efficient, and has better call management, than bridging. Modems are needed to convert analogue data to digital data and vice versa. A modem combines the data with a carrier to provide an analogue signal. This means that ordinary telephone lines can be used to carry data from one computer to another. Page 9 of 14

10 3.8 (c) Common Network Environments Probably the largest network in use is the Internet. The internet provides facilities to link computers world-wide, usually using telecommunications systems. It allows fast communications between people, the transfer of data between computers and the distribution of information. Messages are passed from the source computer, through other computers, to the destination computer. The Internet provides the World Wide Web, electronic mail, file transfer. In order for this system to work, there are Internet Service Providers (ISP) who connect a subscriber to the backbone of the Internet. These providers then pass data between them and onto their respective clients. Fig. 3.8 (c)1 shows how data, including electronic mail, are passed from one computer to another. Fig. 3.8 (c)1 An intranet is a network offering the same facilities as the Internet but solely within a particular company or organisation. An intranet has to have very good security for confidential information. Sometimes the organisation allows the public to access certain parts of its intranet, allowing it to advertise. This Internet access to an intranet is called an extranet. Suitable software is required to make these systems work. Browsers allow a user to locate information using a universal resource locator (URL). This is the address for data on the Internet. The URL includes the transfer protocol to be used, for example http, the domain name where the data is stored and other information such as an individual filename. Page 10 of 14

11 e.g. will load the British Computer Society's home page. Domain names are held in an hierarchical structure. Each name is for a location on the Internet. Each location has a unique name. The names in the various levels of the hierarchy are assigned by the bodies that have control over that area. Consider the domain name PC195-staff.acadnet.wlv.ac.uk The domain is uk and the ac would be assigned to a particular authority. (In this case UKERNA). This authority would then assign the next part, i.e. wlv. As this is Wolverhampton University, it is responsible for all the parts prior to wlv. Those in charge of acadnet are responsible for PC195-staff. Each computer linked to the Internet has a physical address, a number called its IP (Internet protocol) address. This numeric address uniquely identifies the physical computer linked to the Internet. The domain name server converts the domain name into its corresponding IP address. Page 11 of 14

12 3.8 (d) Confidentiality of Data Once an organisation opens some of its network facilities up, there is a problem of confidentiality of data. An organisation may well wish that potential customers have access to their product database. However, they will not want them to have access to employee files. A first step is to encrypt the confidential data and this is addressed in the next Section. Another solution is to install firewalls. These sit between WANs and LANs. The firewall uses names, Internet Protocol addresses, applications, and so on that are in the incoming message to authenticate the attempt to connect to the LAN. There are two methods of doing this. These are proxies and stateful inspection. Proxies stop the packets of data at the firewall and inspect them before they pass to the other side. Once the packets have been checked and found to be satisfactory, they are passed to the other side. The message does not pass through the firewall but is passed to the proxy. This method tends to degrade network performance but offers better security than stateful inspection. Stateful inspection tracks each packet and identifies it. To do this, the method uses tables to identify all packets that should not pass through the firewall. This is not as secure as the proxy method because some data do pass through the firewall. However, the method uses less network resources. Another way of ensuring privacy of data is to use authorisation and authentication techniques. These are explained in the next Section. Page 12 of 14

13 3.8 (e) Encryption, Authorisation and Authentication Encryption is applying a mathematical function, using a key value, to a message so that it is scrambled in some way. There are many techniques for this. The problem is to make it virtually impossible for someone to unscramble the message. Clearly, whatever function is applied to the original message must be reversible. The problem is to make it very difficult for anyone to find the inverse of the original function. It also means that there is a problem of many people needing to decrypt a message. All these people need the key to unlocking the message. This makes it highly likely that an unauthorised person will get hold of this key. One method of overcoming this is to use Public Private Key technology. This involves the sender having a public key to encrypt the message and only the receiver having the private key to decrypt the message. Authentication is used so that both parties to the message can be certain that the other party is who they say they are. This can be done by using digital signatures and digital certificates. Digital signatures require encryption. Basically, a digital signature is code that is attached to a message. In order to understand how public key cryptography works, suppose Alice and Bob wish to send secure mail to each other: First, both Bob and Alice need to create their public/private key pairs. This is usually done with the help of a Certification Authority (CA). Alice and Bob then exchange their public keys. This is done by exchanging certificates. Bob can then use his private key to digitally sign messages, and Alice can check his signature using his public key. Bob can use Alice's public key to encrypt messages, so that only she can decrypt them. Page 13 of 14

14 A primary advantage of public-key cryptography is the application of digital signatures, which help combat repudiation, i.e. denial of involvement in a transaction. Since the owner keeps their private key secret, anything signed using that key can only have been signed by the owner. The predominant public-key algorithm is RSA, which was developed in 1977 by, and named after, Ron Rivest, Adi Shamir, and Leonard Adleman. The RSA algorithm is included as part of Web browsers from Netscape and Microsoft and also forms the basis for many other products. How https works inside the browser using public private keys and certificates Page 14 of 14