Chapter 2 Network Classifications (Cont.)
2.3 Topological Network Classification
Examining the Basics of a Network Layout To implement a network, you must first decide what topology will best meet your needs. Topology refers to the physical layout of the computers, cables, and other network resources Also how those components communicate The arrangement of cabling is the physical topology The path that data travels between computers on a network is the logical topology Topology has a significant effect on the network s performance and growth, and equipment decisions 3
Understanding Standard Topologies Networks are based on three physical topologies A bus consists of a series of computers connected along a single cable segment A star topology has computers connected via a central concentration point; a hub or switch. A ring topology forms a loop from of the computers. Physical topologies describe the cable arrangement Logical topologies describe how the data travels And include bus, ring, and switching, all of which are usually implemented as a physical star 4
2.3.1 Physical Bus Topology
Physical Bus Topology 7
Signal Propagation Computers communicate by sending information across the media as a series of signals In a typical (copper wire) physical bus, those signals are sent as electrical pulses that travel along the length of the cable in all directions The signals continue to travel until they weaken enough so as not to be detectable or until they encounter a device that absorbs them This traveling across the medium is called signal propagation At the end of a cable, the signal bounces back 8
Signal Bounce 9
Cable Termination 10
Cable Failure 11
Logical Bus Topology Logical topologies describe the path that data travels from computer to computer A physical bus topology is almost always implemented as a logical bus as well Technology has moved past the physical bus, but a logical bus topology is still in use on some physical topologies, in particular a star All computers communicate in the same way They address data to one or more computers and then transmit that data across the cable in the form of electronic signals 12
Sending the Signal When a computer has data to send, it addresses that data, breaks it into manageable chunks, and sends it across the network as electronic signals All computers on a logical bus receive those signals Only the destination accepts the data All users must share the available amount of transmission time Thus, network performance is reduced A bus topology is a passive topology In an active topology network, computers and other devices regenerate signals and are responsible for moving data through the network 13
Logical Bus Topology (cont.) Advantages: easy to connect a node to a linear bus less expensive, require less cable Disadvantages: entire network shuts down if there is a break in the main cable difficult to identify the problem if the entire network shuts down
2.3.2 Physical Ring Topology
Physical Ring Topology 16
Logical Ring Topology Data in a logical ring topology travels from one device, or node, on the network to the next device until the data reaches its destination Token passing is one method for sending data around a ring Modern logical ring topologies use smart hubs that recognize a computer s failure and remove the computer from the ring automatically An advantage of the ring topology lies in its capability to share network resources fairly 17
2.3.3 Physical Star Topology
Physical Star Topology 19
A Logical Bus Implemented as a Physical Star 20
A Logical Ring Implemented as a Physical Star 21
Switching Implemented as a Physical Star Switching is neither a bus nor a ring logically, but is always implemented as a physical star A switch takes a signal coming from a device connected and builds a circuit on the fly to forward the signal to the intended destination computer Superior to other logical topologies because, unlike bus and ring, multiple computers can communicate simultaneously without affecting each other Dominant method used in almost every LAN design 22
Physical Star Topology (cont.) Advantages: a separate cable connects to each computer, and if one cable breaks, only a single computer should be affected the hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow easy to install and wire no disruptions to the network when connecting or removing devices easy to detect network problems
Disadvantages: more expensive, requires a hub and more cable length than a linear bus topology all nodes shut down if the hub or concentrator fails Guide to Networking Essentials, Fifth Edition 24
Ring and Star Topologies Ring Star very high speed links over long distances potential of providing best throughput single link or repeater failure disables network uses natural layout of wiring in building best for short distances high data rates for small number of devices
Examining Variations of Physical Topologies The major physical topologies have three typical variations or combinations Extended star Mesh Combination star and bus These combinations can be used to get the most from any network 26
Extended Star Topology 27
Mesh Topology 28
Combination Star Bus Topology 29