Communication Networks - Computer Science Notes

UNIT 2: Communication **Networking** ARPAnet -- Advanced Research Projects Agency Network. WAN -- wide area network (network covering a very large geographical area). LAN -- local area network (network covering a small area such as a single building). MAN -- metropolitan area network (a network...

UNIT 2: Communication **Networking** ARPAnet -- Advanced Research Projects Agency Network. WAN -- wide area network (network covering a very large geographical area). LAN -- local area network (network covering a small area such as a single building). MAN -- metropolitan area network (a network which is larger than a LAN but smaller than a WAN, which can cover several buildings in a single city, such as a university campus). File server -- a server on a network where central files and other data are stored. They can be accessed by a user logged onto the network. Hub -- hardware used to connect several devices to form a LAN that directs incoming data packets to all devices on the network (LAN). Switch -- hardware used to connect several devices to form a LAN that directs incoming data packets to a specific destination address only. Router -- a device which enables data packets to be routed between different networks (for example, can join LANs to form a WAN). Modem -- modulator demodulator. A device that converts digital data to analogue data (to be sent down a telephone wire); conversely it also converts analogue data to digital data (which a computer can process). WLAN -- wireless LAN. (W)AP -- (wireless) access point which allows a device to access a LAN without a wired connection. PAN -- network that is centred around a person or their workspace. Client-server -- network that uses separate dedicated servers and specific client workstations. All client computers are connected to the dedicated servers. Spread spectrum technology -- wideband radio frequency with a range of 30 to 50 metres. Node -- device connected to a network (it can be a computer, storage device or peripheral device). Peer-to-peer -- network in which each node can share its files with all the other nodes. Each node has its own data and there is no central server. Thin client -- device that needs access to the internet for it to work and depends on a more powerful computer for processing. Thick client -- device which can work both off line and on line and is able to do some processing even if not connected to a network/internet. Bus network topology -- network using single central cable in which all devices are connected to this cable so data can only travel in one direction and only one device is allowed to transmit at a time. Packet -- message/data sent over a network from node to node (packets include the address of the node sending the packet, the address of the packet recipient and the actual data. Star network topology -- a network that uses a central hub/switch with all devices connected to this central hub/switch so all data packets are directed through this central hub/switch. Mesh network topology -- interlinked computers/ devices, which use routing logic so data packets are sent from sending stations to receiving stations only by the shortest route. Hybrid network -- network made up of a combination of other network topologies. Cloud storage -- method of data storage where data is stored on off-site servers. Data redundancy -- situation in which the same data is stored on several servers in case of maintenance or repair. Wi-Fi -- wireless connectivity that uses radio waves, microwaves. Implements IEEE 802.11 protocols. Bluetooth -- wireless connectivity that uses radio waves in the 2.45GHz frequency band. Spread spectrum frequency hopping -- a method of transmitting radio signals in which a device picks one of 79 channels at random. If the chosen channel is already in use, it randomly chooses another channel. It has a range up to 100 metres. WPAN -- wireless personal area network. A local wireless network which connects together devices in very close proximity (such as in a user's house); typical devices would be a laptop, smartphone, tablet and printer. Twisted pair cable -- type of cable in which two wires of a single circuit are twisted together. Several twisted pairs make up a single cable. Coaxial cable -- cable made up of central copper core, insulation, copper mesh and outer insulation. Fibre optic cable -- cable made up of glass fibre wires which use pulses of light (rather than electricity) to transmit data. Gateway -- device that connects LANs which use different protocols. Repeater -- device used to boost a signal on both wired and wireless networks. Repeating hubs -- network devices which are a hybrid of hub and repeater unit. Bridge -- device that connects LANs which use the same protocols. Softmodem -- abbreviation for software modem; a software-based modem that uses minimal hardware. NIC -- network interface card. These cards allow devices to connect to a network/internet (usually associated with a MAC address set at the factory). WNIC -- wireless network interface cards/controllers. Ethernet -- protocol IEEE 802.3 used by many wired LANs. Conflict -- situation in which two devices have the same IP address. Broadcast -- communication where pieces of data are sent from sender to receiver. Collision -- situation in which two messages/data from different sources are trying to transmit along the same data channel. CSMA/CD -- carrier sense multiple access with collision detection -- a method used to detect collisions and resolve the issue. Bit streaming -- contiguous sequence of digital bits sent over a network/internet. Buffering -- store which holds data temporarily. Bit rate -- number of bits per second that can be transmitted over a network. It is a measure of the data transfer rate over a digital telecoms network. On demand (bit streaming) -- system that allows users to stream video or music files from a central server as and when required without having to save the files on their own computer/ tablet/phone. Real-time (bit streaming) -- system in which an event is captured by camera (and microphone) connected to a computer and sent to a server where the data is encoded. The user can access the data 'as it happens' **[Networking Devices]** Here are some of the main benefits of networking computers and devices (rather than using several stand-alone computers): - - - - - - - There are also several drawbacks: - - - - *Networked computers* - Hardware: **»** LAN cards **»** routers **»** switches **»** wireless routers **»** cabling Software: **»** operation and management of the network **»** operation of firewalls **»** security applications/utilities Services: **»** DSL **»** satellite communication channels **»** wireless protocols **»** IP addressing. Networks can be categorised as **private or public**: - - **[Client-server and peer-to-peer networking models]** - - - - - - - - - On a peer-to-peer network, each node joins the network to allow: - - - - - The peer-to-peer model does not have a central server. Each of the nodes y (workstations) on the network can share its files with all the other nodes, and each of the nodes will have its own data. Because there is no central storage, there is no requirement to authenticate users. *Thick and thin clients*  **[Network Topologies]** Types: - - - - - *Bus networks:* A bus network topology uses a single central cable to which all computers and devices are connected.  Advantages: - - Disadvantages: - - - *Star Network:* A star network topology uses a central hub/switch and each computer/device is connected to the hub/switch. Data going from host to host is directed through the central hub/switch. Each computer/device has its own dedicated connection to the central node. Advantages: - - - - Disadvantages: - - *Mesh Networks:* There are two types of mesh network topologies: **routing** and **flooding**. **Routing** works by giving the nodes routing logic (in other words, they act like a router) so that data is directed to its destination by the shortest route and can be re-routed if one of the nodes in the route has failed. **Flooding** simply sends the data via all the nodes and uses no routing logic, which can lead to unnecessary loading on the network.  Advantages: - - - - Disadvantages: - - *Hybrid Networks:* A hybrid network is a mixture of two or more different topologies (bus and star, bus and mesh, and so on). The main advantages and disadvantages depend on which types of network are used to make up the hybrid network. Advantages: - - - Disadvantages: - *Ring Topolgy:* In a ring network all computers have two network connectors and each connector runs either clockwise or anticlockwise onto the next node  Advantages: - - Disadvantages: - **[Public and private cloud computing]** - - Systems: - - - *Data security when using cloud storage:* Companies that transfer vast amounts of confidential data from their own systems to a cloud service provider are effectively relinquishing control of their own data security. This raises a number of questions: - - - *Cloud software:* Software applications can be delivered to a user's computer on demand using cloud computing services. The cloud provider will both host and manage software applications -- this will include maintenance, software upgrades and security for a monthly fee. A user will simply connect to the internet and contact their cloud services supplier. The cloud services supplier will connect them to the software application they require. The main advantages are that the software will be fully tested and it does not need to reside on the user's device. However, the user can still use the software even if the internet connection is lost. Data will simply be stored on the local device and then data will be uploaded or downloaded once the internet connection is restored. **[Wired and wireless connectivity]** *Wireless* - - - -  *Wired* - - - +-----------------------------------+-----------------------------------+ | **Wired** | **Wireless** | +===================================+===================================+ | » More reliable and stable | » It is easier to expand networks | | network (wireless connectivity is | and is not necessary to connect | | often subjected to interference). | devices using cables. | | | | | » Data transfer rates tend to be | » Devices have increased | | faster with no 'dead spots'. | mobility, provided they are | | | within range of the WAPs. | | » Tends to be cheaper overall, in | | | spite of the need to buy and | » Increased chance of | | install cable. | interference from external | | | sources. | | » Devices are not mobile; they | | | must be close enough to allow for | » Data is less secure than with | | cable connections. | wired systems; it is easier to | | | intercept radio waves and | | » Lots of wires can lead to | microwaves than cables so it is | | tripping hazards, overheating of | essential to protect data | | connections (potential fire risk) | transmissions using encryption | | and disconnection of cables | (such as WEP, WPA2). | | during routine office cleaning. | | | | » Data transmission rate is | | | slower than wired networks | | | (although it is improving). | | | | | | » Signals can be stopped by thick | | | walls (in old houses, for | | | example) and signal strength can | | | vary, or 'drop out'. | +-----------------------------------+-----------------------------------+ **[Hardware requirements of networks]** 1. 2. 3. 4. 5. - - - - - 6. 7.  8. **[Ethernet]** - 1. 2. 3. ***Conflicts:*** may occur if devices on the same network have been given the same IP address; without a unique IP address it is not possible to connect to a network. This is most likely to occur on a LAN where dynamic IP addresses may have been used. ***Collisions:*** two messages using the same data channel could be sent at the same time, leading to a collision. Carrier sense multiple access with collision detection (CSMA/CD) was developed to try and resolve this issue. Collison detection depends on simple physics: when a frame is sent it causes a voltage change on the Ethernet cable. When a collision is detected, a node stops transmitting a frame and transmits a 'jam' signal and then waits for a random time interval before trying to resend the frame. CSMA/CD protocol will define the random time for a device to wait before trying again. **[Bit streaming]** Bit streaming is a contiguous sequence of digital bits sent over the internet or a network that requires a high-speed data communication link (such as fast broadband). Since bit streaming often involves very large files (such as video) the files must undergo some data compression before transmission. It is also necessary to have some form of **buffering** to ensure smooth playback of the media files. The data transmission rate from the file server to the **buffer** must be greater than the rate at which data is transmitted from the buffer to the media player. The larger the buffer, the better the control over the bit rate being sent to the media player.  ***On demand bit streaming:*** 1. 2. 3. 4. 5. ***Real time bit streaming:*** 1. 2. 3. 4. 5. **The Internet** **Internet** -- massive network of networks, made up of computers and other electronic devices; uses TCP/IP communication protocols. **World Wide Web (WWW)** -- collection of multimedia web pages stored on a website, which uses the internet to access information from servers and other computers. **HyperText Mark-up Language (HTML)** -- used to design web pages and to write http(s) protocols, for example. **Uniform resource locator (URL)** -- specifies location of a web page (for example, www.hoddereducation.co.uk). **Web browser** -- software that connects to DNS to locate IP addresses; interprets web pages sent to a user's computer so that documents and multimedia can be read or watched/listened to. **Internet service provider (ISP)** -- company which allows a user to connect to the internet. They will usually charge a monthly fee for the service they provide. **Public switched telephone network (PSTN)** -- network used by traditional telephones when making calls or when sending faxes. **Voice over Internet Protocol (VoIP)** -- converts voice and webcam images into digital packages to be sent over the internet. **Internet protocol (IP)** -- uses IPv4 or IPv6 to give addresses to devices connected to the internet. **IPv4** -- IP address format which uses 32bits, such as 200.21.100.6. **Classless inter-domain routing (CIDR)** -- increases IPv4 flexibility by adding a suffix to the IP address, such as 200.21.100.6/18. **IPv6** -- newer IP address format which uses 128bits, such as A8F0:7FFF:F0F1:F000:3DD0: 256A:22FF:AA00. **Zero compression** is a way to reduce the length of an IPv6 address by replacing groups of zeroes with a double colon (::); this can only be applied once to an address to avoid ambiguity. **Sub-netting** -- practice of dividing networks into two or more sub-networks. **Private IP address** -- an IP address reserved for internal network use behind a router. **Public IP address** -- an IP address allocated by the user's ISP to identify the location of their device on the internet. **Domain name service (DNS)**---(also known as domain name system) gives domain names for internet hosts and is a system for finding IP addresses for domain names. **JavaScript®** -- object-orientated (or scripting) programming language used mainly on the web to enhance HTML pages. **PHP** -- hypertext processor; an HTML- embedded scripting language used to write web pages. **[The differences between the internet and the World Wide Web]** There are fundamental differences between the internet and the World Wide Web (WWW). **Internet** - - - **World Wide Web** - - - - - **[Hardware and software needed to support the internet]** The fundamental requirements for connecting to the internet are - - - - - **[IP Addresses]** *Internet Protocols* - - - - - - *Zero Compression* IPv6 addresses can be quite long, but there is a way to shorten them using **zero compression.** - Zero compression can only be applied ONCE to an IPv6 address; otherwise, it would be impossible to tell how many zeros were replaced on each occasion where it was applied. *Public and Private IP addresses* - - **[Uniform resource service (URLs)]** Web browsers are software that allow users to access and display web pages on their screens. They interpret HTML sent from websites and display the results. Web browsers use uniform resource locators (URL) to access websites; these are represented by a set of four numbers,but it is much easier to type this into a browser using the following format: protocol://website address/path/filename - - - - **[Domain name service (DNS)]** It gives domain names for internet hosts and is a system for finding IP addresses of a domain name. Domain names eliminate the need for a user to memorise IP addresses.