Thursday, December 11, 2008

NETWORK TOPOLOGIES


In networking, the term topology refers to the layout of connected devices on a network.
Network topologies are categorized into the following basic types:
bus
ring
star
tree
mesh

BUS
More complex networks can be built as hybrids of two or more of the above basic topologies.

Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium, those devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.


Bus consists of a single linear cable called a trunk.
Data is sent to all computers on the trunk. Each computer examines EVERY packet on the wire to determine who the packet is for and accepts only messages addressed to them.

Bus is a passive topology.
Performance degrades as more computers are added to the bus.
Signal bounce is eliminated by a terminator at each end of the bus.
Barrel connectors can be used to lengthen cable.
Usually uses Thin net or Thicket
both of these require 50 ohm terminator
good for a temporary, small (fewer than 10 people) network
But it’s difficult to isolate malfunctions and if the backbone goes down, the entire network goes down.
Repeaters can be used to regenerate signals.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options years ago. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

Ring
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (effectively either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.


To implement a ring network, one typically uses FDDI, Sonet , or Token Ring tech Ring Topology

Computers are connected on a single circle of cable.
usually seen in a Token Ring or FDDI (fiber optic) network
Each computer acts as a repeater and keeps the signal strong => no need for repeaters on a ring topology
No termination required => because its a ring
Token passing is used in Token Ring networks. The token is passed from one computer to the next, only the computer with the token can transmit. The receiving computer strips the data from the token and sends the token back to the sending computer with an acknowledgment. After verification, the token is regenerated.
relatively easy to install, requiring ;minimal hardware


Star
Many home networks use the star topology. A star network features a central connection point called a "hub" that may be an actual hub or a switch. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.


Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)
Computers are connected by cable segments to a centralized hub.

Signal travels through the hub to all other computers.
Requires more cable.
If hub goes down, entire network goes down.
If a computer goes down, the network functions normally.
most scalable and reconfigurable of all topologies


Mesh
The mesh topology connects each computer on the network to the others
Meshes use a significantly larger amount of network cabling than do the other network topologies, which makes it more expensive.
The mesh topology is highly fault tolerant.
Every computer has multiple possible connection paths to the other com-putters on the network, so a single cable break will not stop network communications between any two computers.

Star Bus Topology
Several star topologies linked with a linear bus.
No single computer can take the whole network down. If a single hub fails, only the computers and hubs connected to that hub are affected.


Star Ring Topology
Also known as star wired ring because the hub itself is wired as a ring. This means it's a physical star, but a logical ring.
This topology is popular for Token Ring networks because it is easier to implement than a physical ring, but it still provides the token passing capabilities of a physical ring inside the hub.
Just like in the ring topology, computers are given equal access to the network media

through
The passing of the token.
A single computer failure cannot stop the entire network, but if the hub fails, the ring that the hub controls also fails.


Hybrid Mesh
most important aspect is that a mesh is fault tolerant
a true mesh is expensive because of all the wire needed
Another option is to mesh only the servers that contain information that everyone has to get to. This way the servers (not all the workstations) have fault tolerance at the cabling level.






2 Comments:

iCalvyn.com said...

i am a bit confuse on mesh topology

mahasiswa teladan said...

hi..Im student from Informatics engineering, this article is very informative, thanks for sharing :)

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