2E, F, G, H, I Component 2

Learning Intentions Learners will readdress their understanding of network collision, collision detection and routing Learners will develop their understanding of Data transfer rates and routes on a network Learners will develop an understanding of how the Internet is used as a world-wide communications infrastructure

Assessment Outcomes 2E – Explain network collision, network collision detection and how these collisions are dealt with. 2F – Describe methods of routing traffic on a network. 2G (A2) – Calculate data transfer rates on a network. 2H (A2) – Calculate lowest cost routes on a network. 2I (A2) – Describe the internet in terms of a world-wide communications infrastructure.

Network Collision A network cable must have just one data packet on it at any one time. When a packet is 'sent', it is actually broadcast to all the computers on the network but the computer with the correct IP address 'grabs' the packet, because it sees that it is addressed to them. If two different computers try to place a packet on the network at exactly the same time and try to broadcast their packets to all computers, then a 'collision' happens. 

Collision Detection When a collision has been detected the network protocol is responsible for dealing with it. The network protocol (typically TCP/IP) declares both packets void, and then asks both computers to resend their packets, but this time, ensuring that it is done at slightly different times. A user on the network will not notice this happening and any slowing down of the network on small networks, but if there are thousands of computers on a network, for example, across a university campus, there could be problems with the network slowing down if it has to deal with lots of network collisions.

How can collisions be avoided? Ethernet Ethernet is a widely-used design for LANs. Ethernet networks make use of collision avoidance and detection strategies as described above. As traffic increases on an Ethernet network, the number and frequency of collisions increase. This is because every station is broadcasting to every other one and the more workstations there are broadcasting, the more the communications on the network increase. Ethernet will try its best to prevent data collisions. When it does detect a collision, however, it will have the packets put on the network again but this time with a delay between the events.

How can collisions be avoided? Switches A switch can reduce the number of collisions by being able to make multiple connections at the same time. for example, if Computer #1 wants to send a message to Computer #3, at the same time as Computer #5 wanting to send a message to Computer #2, then the switch can set up links between these two pairs of computers to ensure that they can communicate at the same time, but not interfere with each other. 

Reducing Data Collisions Reducing data collisions on a 'switched Ethernet LAN' There are strategies for reducing the number of collisions on a network. We discussed one of them above using a switch, so that two pairs of computers can be given their own temporary communication link to ensure successful communication. A switch can also be used to split up a large network into separate areas that improve the efficiency of communication. It can be used to ensure each station is given a ‘time-slice’ in which to send data. 

2E, F – Network Collision exercises Complete the 2e, f network collision exercises If you use your notes to help you answer the questions circle an N next to it – this will help you see what you need to focus on in revision. 15 Minutes.

2G – Calculate Data Transfer rates on a network Definition The data transfer rate is the rate that a known amount of data is transferred in a given period of time. It is measured in terms of bits per second. Units of data storage and simplifications The rate can be calculated by dividing the amount of data transfered in bits by the time it took. Before we look at some examples, it is worth reminding ourselves about a few things!  1 Kilobyte (1 Kbyte) is 1 024 bytes exactly, or 210 bytes exactly, or about 1000 bytes, or about a thousand bytes.   1 Megabyte (1 Mbyte) is 1 048 576 bytes exactly, or 220 bytes exactly, or about 1000 000 bytes, or about a million bytes.   1 Gigabyte (1 Gbyte) is 1 073 741 824 bytes exactly, 230 bytes exactly, or about 1000 000 000 bytes, or about a thousand million bytes.   1 Terabyte (1 Tbyte) is 1 099 511 627 776 bytes exactly, 240 bytes exactly, or about 1000 000 000 000 bytes, or about a million million bytes.

Calculating the amount of data So 15 Kbytes is about 15 thousand bytes. 128 Mbytes is about 128 million bytes. 20 GBytes is about 20 thousand million bytes and so on. More often than not, you don't need to know the exact number of bytes, just an approximation! We will also assume a byte has 8 bits of data. Strictly speaking, it may have 8 bits but it can also be other values. However, it is generally considered to have 8 bits so that is the value we will always use. Because a byte is a very small unit, we nearly always end up talking about Megabytes or Gigabytes, for example. We should always try to put a number in the most appropriate units. For example: 32 000 bytes is okay, but better is 32 Kbytes. 4 000 000 bytes is okay, and 4000 Kbytes is okay, but better is 4 Mbytes.

Baud rate Baud rate Another way of writing down the transfer rate is to use the baud rate. 1 baud = 1 bit per second so 56 kbaud is 56 000 bits per second

Example 1 If you transferred 50 MBytes of data in three minutes, what is the data transfer rate? 50 MBytes is the same as 50 000 000 bytes approximately. There are 8 bits per byte, so we transferred 400 000 000 bits. Three minutes is 3 * 60 seconds = 180 seconds. The data transfer rate is therefore given by 400 000 000 divided by 180 which equals 222 222 bits per second, which we will round to 220 000 bps.  We should always use appropriate units as this is not an exact calculation we are doing. We can put 220,000 bits per second into kilobits per second by dividing 220,000 by 1000. This gives us a final answer of 220 kilo bits per second, or 220 kbits/s, or 220 kbps or 220 kb/s or 220 kbaud.

Example 2 An Internet connection is working at a data transfer rate of 768 kbps. How much data is downloaded in one minute? 768 kbps = 768 000 / 8 bytes per second = 96 000 bytes per second In 60 seconds, you can therefore transfer 96 000 * 60 = 5 760 000 bytes per minute, or just under 6 Mbytes of data can be transferred in one minute. 

2H – Calculate the lowest cost routes on a network The lowest cost route on a network is the shortest path that a packet of data can travel on a network. To determine the shortest path that a packet of data can take each ‘node’ (device) on the network floods the network with data about which other nodes it can connect to. The result is a ‘tree graph’ and the fastest route is usually the route with the least connections (nodes).

Dijkstras algorithm Dijkstras algorithm shows how each node connects itself to every node that it can – these nodes then connect to other nodes until the destination node is reached. The path with the fewest connections is then identified and this can be called the ‘lowest cost route’ between two nodes.

Other factors Other factors such as speed are also taken into account, if a node is known to have a slow transfer rate it may be quicker to send the packet of data along a ‘longer’ route that has a faster rate of transfer.

2G, H - Exercises Complete the 2G, H Exercises. Use the Answers file to self-assess these exercises before moving on to the Grade A extension

2I The Internet as a world-wide communications infrastructure The Internet is a set of networks, both Local Area Networks (LANs) and Wide Area Networks (WANs) which are connected together usually using telecommunications facilities. These include the phone lines but also such communication technologies as satellite. The Internet allows everyone to communicate quickly and allows the transfer of files and data between computers. Data (an email, a file or a web site, for example) starts at one computer. It is broken down into digital 'packets' of information. Each packet is then passed through other computers, with each packet often going by a completely different route to the other packets. When all the packets reach the final destination, they are reassembled into the original communication and can then be read.

The World Wide Web The World Wide Web (WWW) is often used interchangeably with the 'Internet' but it does have a different meaning. The WWW is a collection (a very large collection) of information held on the Internet in multimedia form. This includes text, pictures, video, sound and animations.

What happens when you request a web page? You type the address into a browser (or you click on a link) Your ISP will give your computer some information to tell it which DNS server to look for Your web browser will contact that DNS server first when you want a web page The DNS server will look up the human-readable form of the web page you want and get back the IP address for that web page If that DNS server can't find a particular IP address then it will ask another DNS server that is higher up in importance to it, and then another one even higher up, until it either finds the IP address that corresponds to the web page you want Or it returns a message to you that it doesn't exist or can't be found.

DNS Servers he task of the Domain Name System is to translate a domain name such as www.google.com into the correct IP address for the server or service. If you type www.google.com into your browser, your computer will automatically make use of the Domain Name System. It works as follows

DNS Hierarchy

2I Exercises Complete the 2I Exercises. Use the Answers file to self-assess these exercises before moving on to the Grade A extension

Pre-Reading Recap your noted on: We will be recapping this year 12 theory next week. 3a Explain the terms bit, byte and word.   3b Describe and use the binary number system and the hexadecimal notation as shorthand for binary number patterns. Storage of Characters 3c Describe how characters and numbers are stored in binary form. 3d Describe standardised character sets. Data types 3e Describe the different primitive data types: Boolean, character, string, integer and real. 3f Describe the storage requirements for each data type.