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

Hello Guys,

Here I am again with an interesting and the most annoying(to some) topic about basic network.

What are Collision Domains and What are broadcast domain and why are they so important.

What are collision Domains?

As defined on Computer repairs Logan Home page, “A collision domain is a logical network segment where data packets can ‘collide’ with one another for being sent on a shared medium, in particular in the Ethernet networking protocol.”

This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it.

Colloquial usage
The term collision domain may also be used to refer to a system in which a single name or identifier is open to multiple interpretations by different layers or processing. The notion of a namespace has been widely adopted as a software programming practice to avert undesired clashes.

The analogy to ethernet collision domains is rather direct. In many cases, a small software system that begins life with little concern for namespace semantics later grows to become a large software system with undesirable and problematic collision domains (concerning names used within the system) that a mature architectural approach would have eliminated in the design phase. It is no longer considered acceptable practice to design a system without considering namespace semantics. The C preprocessor is a good example of a language facility that remains pervasive, which is no longer accepted as good practice. [1] The same evolution has taken place from the older generation of broadcast ethernet hubs which functioned as collision domains, but have now been replaced by superior switched devices which relegate this problem to the dustbin of rotary dial telephones and vinyl recordings.

Conceptually, however, the metaphor is indirect because broadcast ethernet involves collisions in time concerning a shared resource (the transmission line), rather than collisions in the naming of objects or services

To put it simpler

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.


What are 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

Broadcast on a network means one node transmits packetsto all other nodes in the same domain simultaneously.Multicast is a special form of broadcast, in which aselected group of users, who requested to listen to thebroadcast, will receive the broadcasts. Broadcast transmission is supported on most LANs (e.g. Ethernet).Broadcast can also happen in a virtual LAN (VLAN). IPv6 has unicast, multicast and anycast. Broadcast has disappearedas a term, but is considered one form of multicast.
A broadcast domain is a restricted area in which information can be transmitted forall devices in the domain to receive. More specifically, Ethernet LANs are broadcastdomains. Any devices attached to the LAN can transmit frames to any other device because the medium is a shared transmission system. Frames are normallyaddressed to a specific destination device on the network. While all devices detectthe frame transmission on the network, only the device to which the frame isaddressed actually receives it. A special broadcast address consisting of all 1s isused to send frames to all devices on the network.
A repeater is a device that joins two LANs to extend the distance of the LAN.All network traffic is sent across the repeater unaltered.A bridge is a device that joins two LANs into a single broadcast domain, butisolates them so that problems on one LAN do not propagate to the otherLAN. In addition, bridges maintain separate collision domains, so thatcomputers on each segment only contend with other computers on the samesegment for access.If multiple LANs are connected with routers , the router forms the boundaryof the broadcast domain. Broadcast traffic and collision signals do not cross  routers, although most routers can be configured to forward specific broadcast traffic.

To put it in Simpler terms

Broadcast domains are exactly what they imply: they are network segments that allow broadcasts to be sent across them. Since switches and bridges allow for broadcast traffic to go unswitched, broadcasts can traverse collision domains freely. Routers, however, don’t allow broadcasts through by default, so when a broadcast hits a router (or the perimeter of a VLAN), it doesn’t get forwarded. The simple way to look at it is this way: switches break up collision domains, while routers (and VLANs) break up collision domains  and broadcast domains. Also, a broadcast domain can contain multiple collision domains, but a collision domain can never have more than one broadcast domain associated with it.
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

Got it, If you still don’t get it.

Reply back and I will make sure you get it.


Dhawal D

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