Hubs and Switches in Star Topologies
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The Ethernet Hub
In the previous step you learned that bus topology was phased out in favour of star topology, which has a central connection point. Initially this was a hub. Each device connected to it using a dedicated UTP cable that linked into one of the hubs ports.On its way to the hub, a signal will have picked up some noise. As long as the noise does not cause the signal to cross a threshold level, the hub can recreate each bit, effectively removing the noise. This process is called regeneration. If there is too much noise, errors occur during regeneration: a binary 1 may be regenerated as a binary 0 or vice versa.CSMA/CD
Unlike the wireless Aloha network, devices in a wired network can detect whether another device is transmitting data.Want to keep
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Fragmentation
To stop any one device hogging the wire by sending a large amount of data as one block, there is a rule that a device cannot send more that 1500 bytes of data in one go. Data larger than this must be split into smaller chunks that are sent separately this is called fragmentation. Fragmentation gives the other devices a chance to slot in their transmissions in between, thus ensuring fair access to all devices.Ethernet protocol, MAC addresses and frames
A set of conventions and rules that govern the communications between devices is called a protocol. The 1500 byte limit, the CSMA/CD rules, and the need to send destination and recipient addresses are all part of the Ethernet protocol.- One field is for the data that is being transmitted.
- Two fields contain the destination and source addresses, which are called MAC addresses [for Media Access Control]. They are 48 bits long, and are usually referred to in hexadecimal notation, e.g. cd:f1:24:e4:89:a1.
- The Type field is used to indicate what type of data the frame is carrying. You will learn more about this in a later step.
- The Frame Check Sequence [FCS] is used to detect whether errors have occurred during transmission.
The four non-data fields together wrap around the data as the Ethernet Frame, in what is called encapsulation.The actual process used to generate the FCS is called Cyclic Redundancy Check [CRC] and is a complex mathematical process beyond the scope of this course. However, it is similar in concept to a checksum: the sender adds the data values together as it sends, and appends this sum to the end of the data in the FCS. The receiver adds the data values together as they arrive and compares this sum to the appended one; a difference in the sums indicates an error. Try this class activity to see a checksum in action.
Switch-based LANs
Unlike an Ethernet hub, a switch learns the address of each device connected to its ports by looking at the source addresses in the Ethernet frames it receives. It then builds a table, mapping each device to the port it is connected to, so that it can switch incoming frames to the correct device by looking at the destination address in the incoming frame. If the destination address is not in the switchs table, the switch defaults to a hub behaviour and sends the frame out on all ports. Modern switches are capable of switching traffic between multiple ports simultaneously.Next up
In the next section, you will take a quiz to review what youve learned about connections, hubs, switches, and Ethernet frames.Questions
- What happens to an electrical signal when the interference is so bad that the noise takes the signal over the threshold?
- What happens when you plug a new device into a switch?