The physical layout of devices on a network and how the cables and wires connect these devices.
Introduction
The topology of a network describes the way in which the devices (nodes) are connected.
A node is any device that is connected to the network, including computers, printers, hubs, switches and routers. All nodes must be able to communicate using the suite of protocols defined for the particular network. In general, all nodes are able to both receive and transmit using the defined network protocols.
Nodes are connected via Transmission Media, either wired cable or wireless.
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The word topology comes from the Greek words topos meaning place and logos meaning study. It is a description of any locality in terms of its layout. Topology is a branch of mathematics concerned with properties of geometric figures that are distorted without tearing or bonding together.
The topology of a network describes these connections in terms of their physical layout and also in terms of how data is logically transferred between nodes.
The physical connections between devices determine the physical topology. The logical topology describes how nodes communicate with each other rather than how they are physically connected.
Physical
The physical layout of devices on a network and how the cables and wires connect these devices
Logical
How the data is transmitted and received between devices on a network regardless of their physical connections.
There are three basic topologies - bus, star and ring.
In addition, two other topologies, hybrid and mesh, are common in larger networks.
Each of these topologies can describe the physical or the logical topology of the network. Often the Logical Topology is different from the Physical Topology. For example, a physical star topology has all the nodes on the LAN connected by individual cables back to a central node - often a hub or a switch. This same network can have a different logical topology, either a logical bus or perhaps a logical ring topology.
Physical Topologies
A physical star topology has a central node with each device connected directly to it.
Today the physical star topology is used on almost all LANs, including wireless LANs.
The central computer serves as a switch.
In most cases the central node is a switch that includes multiple ports.
It receives messages and sends them to the destination device.
Star topology requires extra cabling because each device needs a cable to the central computer rather than to the nearest device.
If one device or cable is broken, the network can still operate.
However, if the central computer fails, then the network fails.
It is also limited by the processing power of the central computer.
Star networks use a time-sharing system that allocates a certain amount of CPU time for each user.
It is the most common topology for a mainframe or wifi.
A physical bus topology is an arrangement where all the devices are attached to a direct line called the bus.
Each device has a unique identity and can only recognise those signals intended for it.
Devices check the bus and retrieve their messages as data travels along the bus.
Each device is considered to be connected to every other device and can communicate directly along the network to any other device.
Bus topology is one of the easiest to set up and can still operate if one node fails.
In the past physical bus topologies were used for most LANs –
in particular Thicknet and Thinnet Ethernet LANs that use coaxial cable as the transmission media.
Although these networks require less cable than current star wired topologies they are unable to accommodate the large number of nodes present on many of today’s LANs.
Furthermore a single break in the backbone disables the entire network.
A physical ring topology is an arrangement where all devices are attached so that the path is in the shape of a continuous circle.
Each device in the ring has a unique address.
Data flow is in one direction, moving from device to device until the data arrives at its destination.
The token ring network is the most common form of access for ring topology.
If the cable is broken at any point then the entire network is disabled.
Therefore removing a node or adding a new node requires the network to be stopped.
Furthermore in most implementations each data packet is received and then retransmitted by each node, hence all nodes must be powered at all times if the network is to operate.
For these reasons physical ring topologies are seldom used for LANs today.
Physical Hybrid Topology
Hybrid or tree topologies use a combination of connected bus, star and ring topologies.
Commonly a physical bus topology forms the backbone, with multiple physical star topologies branching off this backbone
All hybrid topologies have a single transmission path between any two nodes.
This is one reason the name ‘tree’ is used; consider the leaves on a tree, there is one and only one path from one leaf to another – the same is true for nodes in a physical hybrid or tree network.
Hybrid topologies are the primary topology of most organisations’ networks.
They allow for expansion – new branches can be added by simply connecting central nodes and branching out to the new workstations.
It is common practice to install cabling that supports two or more times the anticipated transmission speed so that future expansion can easily and economically be accomplished.
The extra cost of better quality higher-speed cabling being relatively insignificant compared to the installation costs.
Physical Mesh Topology
Mesh topologies include more than one physical path between pairs of nodes.
This is the primary topology of the Internet, where IP datagrams can travel different paths from the transmitter to the receiver.
Mesh topologies require routers to direct each packet over a particular path.
Without routers data packets can loop endlessly or they can be reproduced such that two or more copies arrive at the final destination.
Commonly the nodes on a mesh network are all routers, and each router connects to further routers or a LAN. Mesh networks provide excellent fault tolerance, as packets are automatically routed around faults.
A full mesh topology exists when all nodes are connected to all other nodes.
Full mesh topologies are used in high-speed long distance connections where there are relatively few nodes and network performance and quality of service is absolutely critical.
When a full mesh is used messages can be rerouted along any other path and hence fault tolerance is maximised.
Network Access Methods
There are a number of ways of dealing with multiple users wanting to access the network at the same time.
The two most common access methods are Ethernet and token ring:
Ethernet—the first industry-standard LAN access method, or protocol, based on a bus topology.
Ethernet allows data to be transmitted simultaneously to all nodes on the network in both directions.
Addressing information allows each node to recognise and receive individual data packets intended for it.
With data packets travelling simultaneously, collisions will occur and will cause errors.
To overcome this problem, Ethernet uses a method called Carrier Sense Multiple Access and Collision Detection (CSMA/CD).
In CSMA/CD, all nodes have the ability to sense signals on the network. When a node wishes to transmit, it ‘listens’ to the bus for signals. When there is no signals on the bus, it transmits.
However, occasionally a collision will occur if two nodes sense a clear bus at the same.
When a collision is detected, each device stops transmitting and then retransmits at another time. Ethernet is defined in a standard called IEEE 802.3.
csma/cd & csma/ca made simple
Token ring—a LAN access method, or protocol, based on a ring topology.
The token ring operates by continually passing special data packets called tokens between nodes on the network.
Workstations with data to send capture a free token and attach data along with addressing information.
A busy token with data cannot be used by other nodes.
When the data arrives at the destination, the data is replaced with an acknowledgment and sent back to original sending node.
Network Basics
Network Basics
Topologies 1: Point-to-Point, Bus & Ring Topologies
Point-to-Point, Bus & Ring Topologies
Topologies 2: Star, Tree & Mesh Topologies
Star, Tree & Mesh Topologies
Network Topologies
Network Topologies
Network Topologies - SUMMARY and CONCLUSION
Network Topologies - Conclusion
LAN topologies2.mp4
FAQ
CSMA/CD Overview
What sort of Topology does CSMA/CD usually work with? *
Carrier sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in local area networking using early Ethernet technology. Eg. Bus or Star Topology.
Node senses if message on Line - then waits a random amt of time.
