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09: Data-link Layer - MAC technniques for shared and non-shared media part 3

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MAC technniques for shared and non-shared media

In real life, cars need roads furnished with stop signs, signal lights or traffic cops so that their movements are well organized.  Analogously, frames also need something to regulate their placement on the media. The absence of a regulator will result in chaos which is equivalent of vehicles not obeying traffic signals. For frames to be orderly placed on the media, some methods, called MAC methods, were devised. These methods are defined by protocols at the Data-link Layer. Thus, the primary aim of these methods is to safely place frames on different media.

For these MAC methods to perform their job efficiently concerning the proper placement of frames onto the media, they  need to know the structure of the network (topology), or rather the connection between nodes (hosts are named nodes as long as tackling Data-link Layer), and the media sharing ; in other words, how nodes share media. Nodes can be connected to each other in two ways. The first one is when nodes share  a common medium, referred to as shared media, whereas in the second way the nodes are independent, that is, they do not share a common medium, known as non-shared media.

MAC for shared media

MAC methods used in this type of connection     are distinctive because nodes here share the same medium. To put in another way, computers of a given LAN network send their frames on the same cable, let it be an Ethernet cable, or all of the computers are attached to the same cable.

Shared media encompasses two methods of accessing or placing frames on the media, Controlled Access  and Contention-Based Access.

Controlled Access method

In Controlled Access method all nodes in a Local Area Network use the same medium and take turns, in sequence, to access it. When it is a device’s turn to access the media but the device has nothing to send, the opportunity to utilize the medium passes to the next device in line. The disadvantage of this media access method is that only one station can transmit at a time. If the other devices connected to the same medium wish to transmit, they have to wait the station that is transmitting until it finishes. This method is also referred to as scheduled access or deterministic. The following figure is an example of Controlled Access method :

 The above figure represent the Controlled Access method. It is called ‘Controlled’ because frames’ transmission is organized. Two computer cannot send their frames at the same time. This way has one advantage which is no collision will occur. Conversely, suppose that PC-1 and PC-2 are allowed to send their frames simultaneously. The result will be a collision. The frames will collide with each other, then discarded. Token ring and FDDI are also examples of this method (they are explained in network topologies’ lesson).

Contention-Based Access method

In this method, two computers can send and receive frames at the same time without any problem. Nodes are allowed to access the medium whenever they have frames to transmit. This method is sometimes referred to as non-deterministic. See the following figure to have an idea of Contention-Based Access method :

This figure may seem like the previous one in terms of nodes connection. And you might be wondering how come collisions do not occur using this method. To prevent collisions in this method, something called Carrier Sense Multiple Access (CSMA) is used. The idea of this process is that cables are checked to see if they are carrying  signals. if it detects a signal on the medium, it means that a device is transmitting, so waiting until it finishes is required, but this  is not like the Controlled Access. The device that wishes to transmit waits only a very short period of time.

It is important to point out that there is a possibility for collisions to occur, but they are solved by means of a method known as Collision detect or CD. So, CSMA works in conjunction with CD to form CSMA/CD. They are implemented together for the purpose of resolving media contention. Traditional forms of Ethernet use this method. In wireless technologies, these is also a method to avoid collisions, called CSMA/CA (Collision Avoid). (CSMA/CD is explained thoroughly in the forthcoming lesson).

MAC for non-shared media

MAC for non-shared media is different because only little or no control is needed to organize frame transmission on the media. The most prominent example of non-shared mdia is Point-to-Point topology or PPP. In this topology there are only two nodes connected to each other with no other devices sharing the used medium. Think of it as a road  with only two entrances at each end as shown in the following figure :

The above figure shows two houses connected by means of a road with two lanes, which means that cars may go in both paths. This is called Full Duplex in networking. In contrast, if we remove a lane and left only with one lane, in this case, it is referred to as Half Duplex in networking.

In networking terms, Point-to-Point connection may use two types of connection, either Half Duplex or Full Duplex. As for Half duplex,  the nodes (analogous to houses) can either send or receive, they cannot transmit and receive at the same time, which means that data frames can flow merely in one direction and one node should wait until the data transmission is finished so that the medium is empty of signals. Take a look at the figure below:

Conversely, Full Duplex is another type of connection which allows two nodes to transmit and receive frames simultaneously. Referring to the road analogy, cars can flow in both lanes, going and coming. In networks, data frames can also flow in two directions. The sender can send a set of data frames and receive a bunch of data frames at the same times, and the same is true for the receiver. The following figure clarifies Full Duplex connection:


All in all, Half Duplex and Full Duplex connections are both used in Point-to-Point non-shared media.
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09: Data-link Layer - MAC technniques for shared and non-shared media part 3 Reviewed by BOUFTIRA on 9:47:00 AM Rating: 5

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