NETWORK TOPOLOGY

Network topology is the framework of any network. It describes the physical and logical design of the network starting from the way the devices are physically connected, the cabling layout, to the routing path and the way which data is being transmitted over the network.

Network topology is basically divided in to two types;

•  Physical topology

•   Logical topology

 Although a network can have both its physical and logical topology to be the same, it is also possible that you can have different logical working on a different physical topology on the same network. In this article we will examine these topologies, and sort out the types, major advantages, and major differences between them.

PHYSICAL TOPOLOGY

Like I mentioned in the previous article physical topology has much to do with the physical design of the network. It includes the physical devices and most importantly the cabling, and the type of cable used, is it twisted pair, coaxial cable or even fiber optics? Cabling on any network must be understood because you identify your network physical topology type depending on the type of cabling.

Types of Physical Topology

Physical topology is of three types; bus, ring and star topology.

Bus Topology:  this topology in the early days was supported by the use of coaxial cables to connect devices in lines.  However in modern days bus topology is built in the bus hardware devices and the host devices is connected to the bus using a twisted pair cable.

Star Topology: This is the most commonly and also widely used topology. Just like the name, this topology takes the form of a star. In this topology, the entire host and other network devices on the network is connected to a central network device e.g. the switch, hub or router, using a twisted pair cable. They are categorized into two; star, and extended star. We will learn more about this in the logical topology.

Ring Topology: This topology takes the shape of a ring. The network devices are wired in from one device to another until the last device and this device is also directly connected to the first device that was wired. Here the devices are connected on the network by connecting to the nearest network device to it. They are of two categories; Ring and dual Ring.

LOGICAL TOPOLOGY

Logical topology like I already said has to do with the structure used to transmit data frames and Packets within and across the network. It is like a data transmission map, used to determine how best to send signal to hosts and other devices across the network.

Categories of Logical Topology:

Bus Topology: Imagine a city bus picking you up for work from a particular bus stop every day. It then moves over to the next stop to pick more people. This continues for every bus stop until you finally drop off in front of your work place. This is how the bus topology works; data is transmitted and made to move along the same line until it reaches the destination host. All of the devices on the bus topology are effectively connected to a single cable in a linear form and all of them receive the signal sent but only the host or group of hosts that the data is being directed to will own up the data. The data signal proceeds from one computer to the next in a single line until the entire hosts receive the signal; this is called a linear bus. A terminator which terminates electrical signals must be used on the main cable to absorb electrical signals when it reaches the end of the wire, to avoid those signals from bouncing back into the network thereby causing network errors.  An example of a bus device is a hub. An Ethernet hub implements physical star on a logical bus topology.

Star Topology

• Star Topology: This is the most commonly used topology in LAN. Signals generated by any host on the network passes through a central device E.g. switch, hub, or router this device then distributes the signal to other hosts on the network. If the central device fails, the entire network shuts down too. But if an individual host device or cable fails, the network will still function as normal.

• Extended Star:Although very expensive to implement but it’s worth the cost, this is when a network implementing a star topology is expanded by including additional network device that is connected to the main (central) network device. The additional device, acts as an extension to the network and it allows you to add more hosts to the network. The advantage to this is that if one central device fails, others can still work although it may leave the entire or a large portion of the network completely cut off from the network if the main device fails. 

Ring Topology: I don’t think that you are wrong if you say that a ring topology has no beginning or end. It keeps going round in a circular form. Here, a token travels round the ring, stopping by each host device. You may wonder how the network knows which device data is being directed to, well I’ll tell you this; if a host device wants to transmit data, it adds the data and the destination host address which includes the MAC and IP address of the device token. The data passes round stopping by each host until the host with a matching destination address owns up the data. Ring topology is of two categories;

·         Single Ring: here the token connection is only on directional therefor if a cable or host is faulty, this may affect network continuity on some host devices that depends on the faulty host for signal. Sometimes the entire network stops working on a single fault since they all depend on each other for data transmission.

·         Dual Ring: unlike the single ring where hosts are connected on a single cable and is only one directional, dual ring is a two directional ring topology. It is designed in such a way that hosts are connected to two cables, the first acts as the main cable while the other may act as a backup. It is two directional, each cable transmits on opposite direction to each other. It allows data transmission from both sides therefor if there is fault on a host, a reverse direction can be used to transmit data to its destination. See illustration below.

Mesh Topology:  In this topology, your network devices are connected to each other in such a way that at least one host device is connected to all the host devices in the entire network. Since it is a two-directional network topology so you don’t have to worry about which host is up and which one is down, to keep your network fully operational. This is very similar to the star topology, it tolerates more faults and also very expensive to implement. It is categorized into two forms;

• ·         Partial Mesh: Here at least one of the devices is connected to the entire host devices on the network.

• ·         Full Mesh:  All the devices on the network is fully connected to each other for redundancy with full error tolerance. It is more expensive to implement compared to partial mesh, but it is worth the cost.