Star Topologies

In the past, there were several different ways of connecting computers and devices together, two of which were known as known as ring and bus topologies. These are rarely used today and by far the most common network topology in use today is the star topology.

A star topology connects each computer or device to a central hub or switch. You can connect as many devices as you like, but due to practical limits imposed by the size of the RJ45 connectors, this is usually limited to 24 or 30 devices. When you need to connect more workstations than that, you need to connect two or more hubs together which is usually done by simply plugging them into each other, which is referred to as daisy-chaining or piggy-backing.

When you use wireless connections instead of physical wires, mesh topologies become a lot more practical. It is also possible to set up a partial mesh topology where the computers are only connected to a few other computers. This is how the Internet is configured, with large central "nodes" having many connections to each other, and many of the connected computers only having one network connection.

Advantages of a mesh topology

• Multiple devices can transmit data at the same time, which may travel via a different route to its destination and means that a mesh network can handle high amounts of traffic.
• If one device on the network fails, the network can still function because alternative connections are available.
• Adding additional devices does not disrupt data transmission between other devices.

Disadvantages of a mesh topology

• The cost to implement is higher than other network topologies, making it a less desirable option. (this is especially true in a wired mesh network
• Building and maintaining the topology is difficult and time consuming.
• The chance of redundant connections is high, which adds to the high costs and potential for reduced efficiency.

The standard that Wi-Fi uses is known as the 802.11 protocols. When you buy a new piece of wireless equipment, it will be compatible with one or more of the 802.11 standards - the latest ones are known as 802.11n, although 802.11ac has just been agreed and promises a three fold increase in speed.

Wi-Fi uses radio waves with a frequency around 2.4 gigahertz and also a second area around 5 gigahertz. Each device uses a channel which allows more than one device to transmit at the same time without interfering with each other. In the diagram below, the three channels shown in blue (channels 1, 6 and 11) can all be used without interfering with each other because their ranges do not overlap.

If you try to use a channel that is the same or close to another one being used nearby, you may get interference which will make the Wi-Fi slower or unreliable.

Encryption is used to scramble information so that it can be sent safely without anyone else being able to read it. The information is encrypted with a password or key that is needed to read the information again. If you visit a website on the internet that starts with 'https://' then this means that all of the information you are looking at or sending is being securely encrypted.

Because the Internet is a mesh network, any data that is sent via the internet (email, photos, data entered using web page forms etc.) can be intercepted and the data could be misused. By using encryption, the data will not make sense unless it can be unscrambled.

The method of encryption is complex but given enough time, a powerful computer could still be used to "crack the code". In practice, this will not be routinely done and you can consider that your data is safe.

A simple way of encrypting a message is to use a long number known as a key. The key can be very long, but in this example we will use an eight digit key (01234567). Each letter in the original message is "moved" by the corresponding number in the key:

The first letter (The red T), is shifted 0 places, which means that it stays the same. The next letter, the red h, is shifted one place to give the blue letter i. This is repeated for each letter until the whole message is encrypted.

To decrypt the message, the process is reversed. The blue letter T is shifted backward zero places to give the red letter T. The next letter, the blue letter i, is shifted back 1 place to give the red letter h.

Of course, when we encrypt messages using a computer, we can use a lot more complicated method than this!

Ethernet is a standardised family of computer networking technologies commonly used in local area networks (LAN), and wide area networks (WAN). It was first introduced in 1983 and has since been refined to support higher bit rates and longer link distances.

The original 10BASE5 Ethernet used a thick coaxial cable to connect devices, while the newer Ethernet variants use twisted pair and fiber optic links in conjunction with switches. Ethernet data transfer rates can be as high as 400 gigabits per second (Gbit/s), but the common speed of connection used in a typical network is 1 gigabit per second.

When data is transmitted over an ethernet network, it is split into short frames or packets. Each packet also contains a "header" which has additional information such as:

• The destination address for the whole message.
• The address that the message was sent from.
• How many packets make up the whole message.
• The actual packet number of the particular packet.
• A checksum, automatically calculated for each packet.

When a packet is received by a computer, the checksum is recalculated. If it does not match the one sent with the message, then the whole packet contains a faulty value, and is not valid. If it has been possible to read the number of packets and the packet number for this individual packet, then the destination computer may request that the sending computer re-sends the packet.

The destination computer may also request that a packet is re sent if it doesn't receive all the packets in a certain time frame.

In the case of this protocol, it would be possible to still come into a lesson without following the protocol, but if you didn't follow the green cross code (protocol), then you could be injured or die, and if you didn't follow the bank protocol when you use an ATM machine to withdraw money from a "hole in the wall", then you will lose your bank card and you will not be able to get your money.

MAC (Media Access Control) Address

Every single device that connects to a network, such as network interface cards and wireless interface cards, have a unique identifying number which is given to the device by the manufacturer and cannot be changed.

A MAC address is a 48 or 64 bit number and is used by a network to identify the computer. Every time a computer needs to send or receive data, the MAC address is used to direct the traffic.

IP (Internet Protocol) address

An Internet Protocol address is a string of four numbers which is assigned to every computer when it joins a network. An IP address looks like this: 192.168.1.14

Each of the four numbers is stored in one eight bit byte, so can be any number between 0 and 255. This gives 256 x 256 x 256 x 256 possible numbers which is approximately 4.3 billion unique numbers. When this protocol was created, no one ever expected there to be more than four billion computers, but that number was passed in 2017. The old IPv4 addresses (using 4 bytes or 32 bits) has been extended to a much large IPv6 address range. The IPv6 range uses 128-bit addressing to support approximately 340 trillion trillion (or 2 to the 128th power). Instead of the IPv4 address method of four sets of one- to three-digit numbers, IPv6 uses eight groups of four hexadecimal digits, separated by colons.

At the moment, many computers stick to the older IPv4 protocol but all new computers are "IPv6 ready" which means that they will be able to move over to the larger range at any time.

Click on this link to visit the What is my IP address website. It will show you the address that is currently assigned to your computer. If you are using a school network, you may find that every computer on the network has the same IP address, which appears to break the rules around IP protocols. This is because each computer has one IP address on the school network, and the school's Internet router is given an address that is shared with everyone on the network.

Differences between IP and MAC address

This is essential information, you will be asked about this in your exam!

• IP addresses are used when sending data on a network, including the Internet
• MAC addresses are linked to the hardware, whereas IP addresses are assigned to the device by the network and can be changed
• An IP address can be fixed so it does not change. This is known as a static IP address. You will have to use a static IP address if you connect a device to the Internet and want other computers to be able to reach it all the time.

Communication between devices connected together in networks can be very complex, but all devices work with each other because they use standard protocols. Communication is organised into "layers" and each device/or program communicates between these layers using standardized protocols. We use layers because:

• Reduces the complexity of the problem into manageable sub-problems
• Devices can be manufactured to operate at a particular layer
• Products from different manufacturers will work together.

HTTP

The Hypertext Transfer Protocol is used by web servers and clients and is used to serve HTML web pages. A web server will store the web pages and other files such as images and these will be sent to a web browser when requested. Web pages are written in Hypertext Markup Language, and this can contain hyperlinks which allow access to other pages. HTTP was originally proposed by Tim Berners Lee and is the basis behind the World Wide Web. the WWW is a service that runs on the Internet. The Internet refers to the hardware that makes up the actually network.

HTPPS

HTTP documents are sent as "plain text" which means that any computer that intercepts the signal has full access to the contents of the pages. This could mean that usernames, passwords and other sensitive information can be easily intercepted and used fraudulently.

HTTPS (Hypertext Transfer Protocol - Secure) uses encryption to secure all the transmitted data. The data will be encrypted to make it illegible which prevents anyone from accessing information that was not meant for public consumption.

FTP

File Transfer Protocol is used when files (of any type) need to be transmitted over a network. It is possible to receive a list of files that are stored in a certain directory, and then move to a different directory, send, receive, delete and rename a file. This Protocol is used in networks when files need to be transfered. There is also a secure version of this protocol , called FTPS which uses a similar method of encryption as HTTPS