Tuesday, June 21, 2011

What are collision Domains and Broadcast domains and Why are they so Important?

Collision Domain

A Collision Domain is defined as all the Ethernet* segments between a pair of bridges or other layer 2 devices. The reason for this is that all traffic must appear on all the cables between bridges. Thus if a frame is transmitted from a station on a concentrator, all the stations on that concentrator will see the frame at nearly the same time.
A collision occurs when a station begins transmission and then receives the beginning of a frame from another station. The station will immediately stop transmission and issue a JAM signal onto the wire. This will indicate to the other transmitting station that a collision has occurred and both stations will back off for a random amount of time and try to re-transmit.
This back-off time is dependent on the number of consecutive collisions that were issued before a successful transmission. The more collisions, the longer the maximum back-off time.
This mechanism requires that stations be close enough together for each station to see any possible attempted transmission before the first 64 bytes of its frame have been transmitted. This is because 64 bytes is the minimum frame size for an Ethernet network.

Broadcast Domains?

A broadcast domain is a logical network segment in which any computer connected to the network can directly transmit to any other in the domain without having to go through a routing device, providing they share the same subnet and gateway address and are in the same VLAN, (default or installed).
More specifically it is an area of the computer network made up of all the computers and networking devices able to be reached by sending a frame to the data link layer broadcast address.
A very basic network that uses hubs rather than switches or routers is like a post office clerk checking the mail. One clerk looks at each letter to confirm that the mail is not for himself or herself. When one letter (signal) is sent from one point, all the other points in the network will have to check in order to confirm that the letter (signal) is not for themselves.
Routers and Layer-3 Switches are used to segment broadcast domains

I hope you get it.
If not here is a simple formula devised by me
 
Collision Domains:
 
Hubs – All Ports: Single Collision Domain
Switch –  Each port: 1 Collision Domain
Router – Each port: 1 Collision Domain
 
Broadcast Domains:
 
Hub: Single broadcast Domain
Switch: Single broadcast Domain
Router: Broadcast domain = Number of Router interface used
 
Now here is an Example to make the topic “What are collision domains and What are Broadcast domain?” Crystal Clear
How may collision and Broadcast domain does this below network have?
One, two, three, ten, hundred?
 
 
Seven. This is a subjective view and may not agree with the authors of the
question. Read the justification below to understand the technology.  A collision domain is a logical network where data packets can collide with
one another when sent on a shared medium.
A broadcast domain is a logical network where any host can send broadcast
which will reach all other hosts in a domain.
A hub does not affect seperation of collision or broadcast domains.
An ethernet bridge seperates collision domains. The same can be said for
each standard interface of an ethernet switch.
A router seperates broadcast domains. A router also happens to seperate
collision domains, but this is hardly ever mentioned.
Examples of broadcasts include DHCP requests, ARP requests, and some other
forms of network traffic such as a Windows host broadcasting a NetBIOS node
name in order to elicit an IP response from the named host. These
broadcasts are blocked by routers from progressing further into the network.
Exceptions to this border of a broadcast domain are not in the default
configuration of routers and should not be taken into consideration, such as
using a router as a bridge or implementing technology such as aloowing a
router to forward DHCP requests.

Referecing the diagram:
(0) Hub1 does not seperate collision domains.
(3) Each connection out of Switch1 is a collision domain.
(2) Each connection out of Switch2 is a collision domain.
(2) Each connection out of Bridge1 is a collision domain.
Total = 7
Another way to state this exact same result:

Broadcast Domain #1
(1) Hub1 to Switch1 is part of the same collision domain as the other
connections out of Hub1
(1) Switch1 has a collision domain on its connection out the top of the
device in the diagram
(1) Switch1 to Router1 is a collision domain

Broadcast Domain #2
(1) Router1 to Switch2 is a collision domain
(1) Switch2 to Router2 is a collision domain
Broadcast Domain #3
(1) Router2 to Bridge1 is a collision domain
(1) Bridge1 has a collision domain on its connection out the right of the
device in the diagram
Total Broadcast Domains = 3
Total Collision Domains = 7
Remember: Bridge = Switch
The terms are interchangable. We only traditionally see a bridge as a two
interface device (like a highway road bridge with two ends) and a switch as
a multi-interface device. Technically a switch functions like a hub with a
mini-bridge on every port. If you hear of a Brouter, which is a router and
a switch in the same device, just consider it a layer 3 routing capable
switch.


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