Data Communications Basics 2 - Data transmission
Learning Outcomes:
- Identify and describe the three communication models: simplex, half-duplex and duplex.
- Compare asynchronous and synchronous transmission in terms of timing and overhead.
- Know the simple concept of multiplexing and its function in sharing the capacity of data transmission media.
- Explain briefly the mechanism used to co-ordinate data transmission in Ethernet and wireless networks.
- Knowledge of both Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is required.
- Understand the simple concepts of circuit switching and packet switching.
- The basic function of each component of a packet including header, data and trailer should be known.
Communication models:
- Simplex: communication is unidirectional. One of the devices sends data only, while another one receives data only. E.g. Radio. Advantage: simple and easier setup.
- Half-duplex: Both devices can send and receive data, but not simultaneously. E.g. Walkie-talkie.
- Duplex: Both devices can send and receive data simultaneously. E.g. Telephone, interactive TV programmes.
Relevant past paper:
DSE ICT Elect B(SP-2017):
2012 3c. 2013 2bi, cii. 2015 1ei. 2017 3ai.
CE CIT Elec C(2005-2011): 2005 2b. 2007 2e. 2010 2ai,iii.
Asynchronous and synchronous transmission
- Synchronous transmission: Transmitter and receiver are synchronized with timing signals. Data transfer with continuous stream of data. Therefore, it is suitable for large amount of data.
- Advantage: faster transmission time c.f. asynchronous transmission
- Disadvantages: costly
- Asynchronous transmission: sender sends 1 character each time. Time gap exists between data.
- Advantage: economical (cheaper hardware) c.f. synchronous transmission
- Disadvantages: slower transmission, larger overhead (larger amount of control bits)
Relevant past paper:
DSE ICT Elect B(SP-2017): 2013 2bii. 2017 3aii.
Multiplexing
- When multiple data signals use a single line of transmission medium, a device called multiplexor combines the data signals into a 1 single signal for transmission through the transmission medium. At the other end of the transmission medium, a demultiplexor will break down the single signal into the original multiple data signals.
Relevant past paper:
DSE ICT Elect B(SP-2017): 2012 3b.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
- CSMA/CD:
- Used in shared medium of a Ethernet
- The sender senses whether the transmission medium is in use or not. In case of a collision, the sender defers data transmission for a random period of time before retransmission.
- CSMA/CA:
- Mainly used in wireless medium as collision detection is not possible
- Aim at preventing collision
- The sender checks if the channel is in use or not. If the channel is in use, the sender waits for random time before resending checking signal.
Relevant past paper:
DSE ICT Elect B(SP-2017):
PP 1ci. 2012 1d. 2015 1eii. 2016 1d.
CE CIT Elec C(2005-2011): 2005 1a.
Circuit switching and packet switching
- Circuit switching:
- A dedicated connection is established. E.g. telephone system.
- Packet switching:
- A dedicated connection between two parties is not required.
- The data is divided into small parts known as packets. The packets are sent out and go through any network pathway that is readily available. The packets may then arrive at the receiving end in any possible orders.
- The receiving end will reassemble the packets in correct order and perform error checking.
DSE ICT Elect B(SP-2017):
2013 4c. 2017 3aiii.
AS CA(2000-2013): 2010 1ab.
Packet
- A data packet typically has 3 parts: Header, Body and Trailer.
- Examples of contents of each part:
- Header: packet length, source address, destination address, Header checksum
- Body: the actual data
- Trailer: bits that signify end of the packet
DSE ICT Elect B(SP-2017):
2015 1di. 2017 3aiii.
AS CA(2000-2013): 2011 4a.
References:
https://www.geeksforgeeks.org/transmission-modes-computer-networks/
https://www.guru99.com/difference-between-synchronous-and-asynchronous-transmission.html
https://www.tutorialspoint.com/difference-between-csma-ca-and-csma-cd
https://www.tutorialspoint.com/circuit-switching
No comments:
Post a Comment