Access Methods

Access Methods:

Introduction:
In this blog post I will be explaining about the different types of access methods by stating what they are for and why they are used. Additionally, I will include diagrams and examples to help enhance my explanations and help validate my points. Take into account that these access methods are all working within a LAN. The access methods are:

• CSMA/CD (Carrier Sense Multiple Access/Collision Detection).
• CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance).
• Token Passing

Overall, access methods are required to notify which node or device will then send its data in turn. This is to avoid data collision, however if it does occur then the device will be using collision detection method and thus attempts to solve it by using collision avoidance method. These are mainly used because collisions are unpredictable due to the file sizes the data carry within the network and are vulnerable wherever; they reduce the changes of failure.

CSMA/CD:

CSMA/CD is the abbreviation for Carrier Sense Multiple Access/Collision Detection. This access method constantly checks whether the traffic of data in the data line is free in all devices. Furthermore, CSMA/CD is created to tackle problems created mainly by Ethernet. This is because it allows any device on the same network to send data whenever and this ends up with a collision when two or more devices’ data collide. CSMA/CD functions in logical steps to intercept this right after the data collision:

1. It firstly checks whether or not the data line has any traffic (if any data is currently being sent across the network by a device).
2. Assuming there is no traffic; CSMA/CD will allow the device to send data.
3. It will then wait for the recipient to send an acknowledgement (confirmation) that the data has been successfully transmitted across.
4. However, if a data collision has occurred then no acknowledgement will be sent back. This triggers the CSMA/CD to wait a random amount of time (within a second).
5. It will return to step one where it checks for the data line to see if it has any traffic. This will repeat until the data is successfully sent – normally the first reattempt works.

I have created a flowchart to represent this, see figure 1.

Figure 1: CSMA/CD Flowchart

CSMA/CA:

CSMA/CA is the abbreviation for Carrier Sense Multiple Access/Collision Avoidance. This access method/protocol commonly takes place before CSMA/CD to avoid collision data as much as possible by informing the other nodes to not transmit data, unlike CSMA/CD where it only performs when CSMA/CA fails.

1. Before any data is sent, is the connection line available to prevent data collision?
2. If other devices are transmitting data across the network, then wait a random amount of time and go back to step 1.
3. If the connection line is free, the data packet will be sent.
4. If the recipient device receives the file, acknowledgment and confirmation takes place. Then it returns to step 1 and repeats until the file is sent.
5. However, if CSMA/CA fails then CSMA/CD will take place.

Token Passing:

For devices to communicate, an access control method has been designed to control which device communicates (sends data). This is where token passing mainly takes place in the ring network/topology to allow each node to communicate in turn, (see figure 2). However, as the ring network expands this can cause an issue especially when big talkers are present.

Big talkers are usually servers that communicates more (sends more data) than others, causing traffic. This links to CSMA/CA and CSMA/CD as it can help this problem.

Figure 2: Token Passing in Ring Topology

Harvard Referencing: (In order of reference)
- Anderson, K. Atkinson-Beaumont, D.Kaye, A. Lawson, J. McGill, R. Phillips, J and Richardson, D. 2011. Information Technology Level 3 Book 1 BTEC National. Harlow: Pearson Education Limited.
- Figure 1: Chan, D. 2015. CSMA/CD Flowchart by Danny Chan. [Online] [Accessed 20 January 2015].
- Figure 2: manishasanyal.blog, 2012. Token Passing Ring Topology. [Online] Available at: <http://manishasanyal.blog.com/files/2012/08/ring_topology.gif> [Accessed 20 January 2015].

Bibliography: (In alphabetical order)

- webopedia, 2015. CSMA/CA. [Online] Available at: <http://www.webopedia.com/TERM/C/CSMA_CA.html> [Accessed 20 January 2015].