A short talk on benefits and disadvantages of two of the most known network topologies.
A bus network topology refers to the pattern in which the machines are connected and is one of the simplest ways to connect multiple clients to a network. A bus topology is typically comprised of a pattern consisting of a single link/cable capable of connecting all the network nodes together and thus forming a continual communication line. This is also the reason we call it a bus topology, as it’s not that different from the bus architecture used in computer motherboards.
Diagram 1 illustrates a typical configuration of a bus topology, in which a set of computers and other network-enabled devices will form a network enabled by a single network cable. In a bus topology, the network communication is achieved by broadcasting packets to all nodes at the same time.
This is usually done by using a half-duplex twisted-wire that enables communication between computers in both directions, but not simultaneously, only one direction at a time. “Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying.” (Frequencies, 2016). When two or more linked computers attempt to communicate on the network simultaneously, it can a cause of one of the biggest problems in a bus topology, the data packet collisions (as shown in Diagram 2). “All bus topologies have this issue. The nodes must employ Medium Access Protocols, which function in conjunction with other nodes, to permit access only at times when the medium us free.” (Topologies, routing and deadlock, 2016)
To overcome and avoid the problem of data transmission collisions, a bus network architecture employs a variety of methods that allow an uninterrupted communication on the bus. One of the ways to prevent the transmission collisions is to use protocols that can control the traffic, one of the most popular is called a CSMA (Carrier Sense Multiple Access) protocol. The main idea behind the CSMA is that protocol controls the traffic and only initiates the data broadcast from each of the nodes, when no other transmission is in progress. If that is not possible, the protocol will wait for a random period ( a typically couple of milliseconds) to try again. “In other words, CSMA is based on the principle “sense before transmit” or “listen before talk”. Multiple access means that multiple nodes may send and receive on the medium. Transmissions by one node are generally received by all other nodes connected to the medium.” (Carrier sense multiple access, 2016). Carrier Sense Multiple Access protocol also helps to alleviate a problem that can occur during data transmission; a situation called a deadlock. This is the state where two or more computers connected to a bus are jammed from further progressing because each is waiting for a resource that is allocated for one another. A good example is a situation in which a computer and printer are connected to a bus topology network and where the computer is trying to access printer at the same time as the printer is trying to access the computer.
As we’ve learned, in a bus topology all machines are connected to a shared communication line called a bus. This is different from a star topology in which “one computer serves as a central focal point to which all the others are connected.” (Brookshear, 2011).
Diagram 3 demonstrates a star topology. As we can see, a central node (typically a hub or switch) provides a common connection point for all nodes. The primary node is acting as a server, whereas all the computer devices are considered clients. “The hub and leaf nodes, and the transmission lines between them form a graph with the topology of a star.” (Network Topologies, 2012)
Bus vs. Star Topology
How do bus and star topology differ? The simplest form of explanation is that in a bus topology all communication happens on a single uninterrupted path (network cable), to which all network devices are connected. “One good way to visualize this is to imagine a wide road with cars on it. Exits on this road lead to various computers.” (Bus and Star Topology, 2006). It is important to note that a bus topology is no longer very popular, and while there are still valid scenarios for using it, it has been superseded by the star topology, in which all network devices connect directly to a central controlling device (switch, hub, etc.). Meaning, that any and every communication between computers connected using start topology, has to pass through the switch. The main advantage of the star topology is that deadlocks and bottlenecks do not occur, simply due to a fact that all transmission goes through the hub, which controls the flow of traffic. On the other hand, “the primary disadvantage of a star topology is the high dependence of the system on the functioning of the central hub. While the failure of an individual link only results in the isolation of a single node, the failure of the central hub renders the network inoperable, immediately isolating all nodes.” (Network Troubleshooting and resource site for school IT staff, 2016).
Following are the two scenarios in which bus or star topology can be used to connect network machines together.
A home network is a good use case for using a bus topology. This is because a bus topology is easy to implement and quick to setup, and also simple to later extend. That makes it an appropriate choice for small home networks. Due to a nature of a bus network where all computers share a single cable, a bus network is also cheaper to deploy than other network topologies. In a bus topology, it’s also easier to debug issues, such as faulty network cables, etc.
A small business office network is an excellent use case for a star network topology. In this type of networking, non-centralized network catastrophes will pose a tiny negative impact on the system and thus a little effect on business functioning. This topology is easy to understand, and the isolation and centralization allow for a simple detection of errors. One of the major benefits for a business implementation is a fact that new nodes can be easily added to a network, without any impact to other devices. As far as cons go, following drawbacks of star topology are crucial to mention. First, it’s more expensive to implement, as each machine needs its own dedicated line to a central switch. Secondly, the central node is a single point of failure (Star network, 2016), if the central switch fails, the whole network goes down. Most businesses alleviated the problem of a central failure by keeping the replacement switch.
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