Data Transmission

Year 12 Lesson

Transcript

1. DATA TRANSMISSION
2. Speed ▪ Bandwidth – Amount of data that can be transmitted across a communications channel. Represented by frequencies – range of frequencies. ▪ Bitrate – The speed at which a communication is taking place, bit rate is limited by the bandwidth available Bitrate measured in bits per second ▪ Baudrate – Another term to describe the speed at which data can be transmitted. One baud = one electronic state change per second
3. Multiplexing ▪ Also known as muxing - a means of combining signals and sending them at the same time across the same wire ▪ Telecommunications (where it originated) used it to send several phone calls across one wire
4. Switches and Packet Switching ▪ A switch connects segments of a network together. ▪ Packets of data received by the switch from one network are transmitted to the other connecting network ▪ In a packet switching system messages are split into packets of a fixed size, a packet contains: – Data being sent – Address of source and destination – Checksum for purposes of checking data has not been corrupted – Data that enables the data to be re-assembled
5. Switches and Packet Switching ▪ Each packet is re-directed along the network by each computer until it arrives at its destination ▪ Data packets may not necessarily take the same route to the destination ▪ Internet is an example of a packet switching network ▪ Advantages : – Not affected by a failed link, many routes packets can take – Good level of security, difficult to intercept a transmission
6. Error checking ▪ Echoing back: – Data is transmitted to destination and sent back to source, the two are compared to check they are the same – If two sets of data are not the same, it is sent again – Process repeats until the echo matches the original
7. Checksum ▪ Data being sent is made up of binary numbers - Numbers can be added together ▪ The original bytes are added together, any carries over a byte are ignored so the answer is itself a byte ▪ Answer is sent with the data ▪ When data arrives at its destination, bytes are added together and their ’checksum’ is worked out ▪ If checksum is the same, it is assumed the data has not being corrupted ▪ If it is different, the data needs to be re-sent
8. Parity Check ▪ Data is sent in fixed byte sizes (normally 8 bits) ▪ Data being sent would be in the first 7 bits of the byte, eighth bit being reserved as the parity bit ▪ Imagine data being sent is 0110110 – there is one bit left over which is used to check ▪ Parity check works by making the number of 1’s in a byte either always odd or always even, this has to be sorted out during the first contact between devices to agree odd/even ▪ The final bit (farthest right) is the parity bit
9. Network collisions ▪ Protocol called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) was developed to enable devices to transmit data at high speeds without interfering with each other ▪ When data is sent, it’s sent in frames, with all frames being re-assembled zt the receiving end ▪ Any device on a wireless network can attempt to send frames ▪ Frames of data can be picked up by any devices in range ▪ Before data is sent, the device uses protocol to see if transmission medium is idle, if so the data is sent
10. Network collisions ▪ If it is busy, the device waits and tries again later (waiting a random amount of time) ▪ This is known as back-off mechanism and is random to reduce the chances of both devices trying to send simultaneously again ▪ On receipt of the data an acknowledgement is sent to confirm that data has been received and not corrupted ▪ If no confirmation is received, a random amount of time is waited and the data is re- sent ▪ Request to Send/Clear to Send (RTS/CTS)