The basic types of network topology as shown in fig 12.1. For the physical layer, we confine for to an introduction of the basic LAN topologies. The common topologies for LANs are bus, tree, ring, and star (Figure 12.1). The bus is a special case of the tree, with only one trunk and no branches; we shall use the term bus/tree when the distinction is unimportant.
Fig 12.1: Categories of topology.
1) Bus Topologies :
A bus topology are characterized by the use of a multipoint medium. For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.
Fig 12.2 : Bus Topology
2) Tree Topologies :
The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the head end, where one or more cables start, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts. Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement.
First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended.
Second, a mechanism is needed to regulate transmission. To see the reason for this, consider that if two stations on the bus attempt to transmit at the same time, their signals will overlap and become garbled. Or, consider that one station decides to transmit continuously
for a long period of time.
To solve these problems, stations transmit data in small blocks, known as frames. Each frame consists of a portion of the data that a station wishes to transmit, plus a frame header that contains control information. Each station on the bus is assigned a unique address, or identifier, and the destination address for a frame is included in its header.
Fig 12.3 :Tree Topology
3) Ring Topology :
In the ring topology, the network consists of a set of repeaters joined by point-to-point links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater.
The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or
counterclockwise). Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames.
As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed .Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.
Fig 12.4 : Ring Topology
4) Star Topology:
In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion.
A transmission of a frame from one station to the node is retransmitted on all of the outgoing
links. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.
Fig 12.5 : Star Topology
5) Mesh Topology:
In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects.In fig 12.6 shows the mesh topology configuration.
Fig 12.6: Mesh Topology
To find the number of physical links in a fully connected mesh networks with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n-1 nodes, node 2 must be connected to n-1 nodes, and finally node n must be connected to n-1 nodes.
In others words, we can say that in a mesh topology, we need duplex-mode link.
n (n-1)/2
A mesh offers several advantages over other network topologies.
First, the use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.
Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.
Third, there is the advantage of privacy or security.
Finally, point-to-point links make fault identification and fault isolation easy.
# Bus verses Ring:
For the user with a large number of devices and high-capacity requirements, the bus or tree broadband LAN seems the best suited to the requirements. For more moderate requirements, however, the choice between a baseband bus LAN and a ring LAN is not at all clear-cut. The baseband bus is the simpler system. Passive taps rather than active repeaters are used. There is no need for the complexity of bridges and ring wiring concentrators. The most important benefit of the ring is that it uses point-to-point communication links, and here there are a number of implications. First, because the transmitted signal is regenerated at each node, transmission errors are minimized and greater distances can be covered than with baseband bus. Broadband busltree can cover a similar range, but cascaded amplifiers can result in loss of data integrity at high data rates. Second, the ring can accommodate optical fiber links, which provide very high data rates and excellent electromagnetic interference (EMI) characteristics. Finally, the electronics and maintenance of point-to-point lines are simpler than for multipoint lines.
