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09: Data-link Layer - Logical Vs physical topologies part 4

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Logical Versus Physical Topologies

A network topology means the way devices are linked or connected to each other. Nodes at the Data-link Layer might be arranged in different ways. Network topologies can be viewed at two levels: logical level and physical level, or they may be depicted physically or logically. Thus, drawing a parallel between these two levels of topology would be indispensably crucial.

Before delving into elucidating these topologies, I find it of prime importance to provide a brief definition of each.  As for physical topology, its name represent its nature. It is the actual connection between devices in a given network. Besides, at this topology level,  media used in nodes’ connections are all considered (cables, switches, routers, bridges, etc). In contrast, logical topology is used to represent how networks transfers frames from one device (node) to another regardless of its physical design,That is, it is only a created circuit that shows the way nodes are connected. To get a complete idea, see the following figures:

To illustrate, for the Data-link Layer to determine how  nodes  access the media, it relies on the logical topology not on the physical one. Because the physical topology may include several devices and then get a new structure ; consequently, it is difficult for the Data-link Layer to recognize each added device to the network. In addition, both topologies may be identical even if the physical topology has different characteristics, such as  distances between nodes, signal types, transmission rates, physical connectors. As you notice on the above figures, both topologies are considered Point-to-point altough the presence of the intermediary devices in figure 2. Therefore, logical topology is how the networks are virtually seen, and the physical topology is how the networks are actually seen.

There are a variety of network topologies. Bus Topology, Star Topology, Hybrid Topology,  Extended star Topology, Distributed Topology, Mesh Topology, Point-to-point Topology, Multi-access Topology, and Ring Topology are all network topologies. Some of which are hugely used today, some are used in specific networks, others are extinct for the reason that they serve no utilitarian purpose, expensive, or inefficient.

Given the abundance of network topologies, we are going to cover only the last three topologies in view of their simplicity to pave the way for other topologies in another lesson.

Point-to-point Topology

By reading the previous lesson, you must have now a clear idea of what point-to-point topology is ; however, another dissected explanation is given in what follows :

This is the simplest type of topologies, for it is composed only of two nodes that are directly connected. Nodes connected by means of intermediary devices are still regarded directly connected on the grounds that topologies are viewed at their logical levels. In this topology, frames are sent from one node, put on media, then received at the other node without any other nodes’ interference. In other words, media is devoted to two nodes merely.

Nodes can transmit data frames in two ways. The first one, a node can send frames to the receiver but no frames can arrive from the other direction ; this operation is called Half Duplex. Conversely, the second one, a node can send frames  to the receiver and receive frames concurrently, this operation is referred to as Full Duplex.

The connection that is established between two nodes, notwithstanding the presence of intermediary devices, is known as virtual circuit. Simply put, a virtual circuit is the establishement of a logical connection within a network between two network devices. Devices on either end of the virtual circuit exchange data frames with each other.

Multi-access Topology

In multi-access topology, nodes are arranged in a way that each one has access to a medium when it is free. All devices share the same medium. A frame sent from one device is seen by all other devices connected to the same medium. Each node has a physical address (MAC address) through which the sender can identify the intended receiver. When a node sends a data frame, all devices will receive the frame and read its content, but only the concerned node that is going to respond. The structure of multi-access topology looks like this:
The problem that this topology faces is collisions. However, it is solved by means of CSMA/CD or CSMA/CA.  In addition, token-passing method can be also used    for media access (you will learn more about this method in the next lessons).

Ring Topology

This topology may seem facile for you to grasp because its structure is its name. Nodes are attached to each other in a circular way. A device is connected to the next device. When a device receives a frame, it checks it to see if the frame is destined  to it or not. In case the frame is not destined to the device that checked it, it passes it on to the next node in the network. Take a look at the following figure:


There are multiple of MAC techniques used in this topology. For instance, token-passing is a method that is commonly used with a logical ring. Only one frame at a time can be transmitted on the media. When no device is transmitting, a signal known as token is placed the media to indicate that the media is ready for use.
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09: Data-link Layer - Logical Vs physical topologies part 4 Reviewed by BOUFTIRA on 5:39:00 PM Rating: 5

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