The physical layout, which describes how a LAN is constructed, is called the topology.
Bus Topology:
A bus topology is one in which all devices on the LAN are attached to a linear networking medium. This linear networking medium is often referred to as the trunk line, bus, or highway. Workstations and servers, is independently attached to the common bus wire through some kind of connection. The bus wire must end in a terminating resistance, or terminator, which absorbs electrical signals so they do not bounce, or reflect, back and forth on the bus.
Signal Transmission over a Bus Topology
The signal travels in both directions from the source when a
source transmits data over the networking media in a bus topology. These
signals are made available to all devices on the LAN. If the destination MAC
address/destination IP address, carried by the data does not match that of a
device, the device ignores the data. However, if the destination MAC address/destination
IP address carried by the data does match that of a device, the device copies
the data and passes it up to the data link and network layers of the OSI reference model.
Star Topology:
In LANs where the star topology is used, the networking
media run from a central hub out to each device attached to the network. The physical layout of the star topology resembles spokes
radiating from the hub of a wheel. As the figure shows, a central point of
control is used in a star topology. When a star topology is used, communication
between devices attached to the LAN is via point-to-point wiring to the central
link or hub.
All network traffic in a star topology passes through the
hub. Data is first sent to the hub. and then the hub directs data to the
pathway of the device associated with the destination address carried by the
data.
In a star topology. the hub can be either active or passive.
If it is active, the hub not only connects the networking media, but it
regenerates the signal and acts as a mutiport repeater. which is sometimes
referred to as a concentrator. By regenerating the signal, such active hubs
enable data to travel over greater distances. By contrast, a passive hub simply
connects networking media.
Ring Topology:
A ring topology consists of a set of stations connected serially by cable. In other words, it Is a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle.
In most instances data flow is in one direction only, with one single node receiving the transmission and relaying it to the next node in the ring. The ring topology is attractive because it is rarely subjected to the bottlenecks associated with hierarchical and star configurations. Moreover, the logic to implement a ring network is relatively simple.
Note that while this topology functions logically as ring, it is physically wired as a star. The central connector is not called a hub but a Multistation Access Unit or MAU.
Under the ring concept, a signal is transferred sequentially via a “token” from one station to the next. When a station wants to transmit, it grabs” the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.
Mesh Topology:
Mesh is a network topology in which devices are connected with many redundant interconnections between network nodes.
There is two types of mesh topologies:
1. Full Mesh and
2. Partial Mesh
Full mesh topology occurs when every node has circuit other node in a network. Full mesh is very expensive to implement but yields the greatest amount of redundancy, so in the event that on of those nodes fails, network traffic can be directed to any of the other nodes. Full mesh is usually reserved for backbone networks.
Partial mesh topology is less expensive to implement and yields less redundancy than full mesh topology. With partial mesh, some nodes are organized in a full mesh scheme but others are only connected to one or two in the network. Partial mesh topology is commonly found in peripheral networks connected to a full meshed backbone.
Tree Topology:
A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.
