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  1. Member
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    #1

    Default Collision domain vs. Broadcast domain

    So I started my first chapter in the Sybex book today. I seemed to get everything pretty well except for the collision domains and broadcast domains. I understand how they get broken up depending on the hardware, but I don't really understand the difference between the two. Seems like the broadcast domain is a larger collision domain? I cant really see the difference from the definitions that I have read. If anyone can give me a better description I would really appreciate it. Thanks.
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  3. Questionably Benevolent Moderator Slowhand's Avatar
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    #2
    Basically, a collision domain is a network segment that allows normal network traffic to flow back and forth. In the old days of hubs, this meant you had a lot of collisions, and the old CSMA/CD would be working overtime to try to get those packets re-sent every time there was a collision on the wire (since ethernet allows only one host to be transmitting at once without there being a traffic jam). With switches, you break up collision domains by switching packets bound for other collision domains. These days, since we mostly use switches to connect computers to the network, you generally have one collision domain to a PC.

    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.

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  4. Member
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    #3
    Great thanks for the detailed description. That cleared up a lot.
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  5. Junior Member
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    #4
    in short collision domain is number of devices that can sent/receive data at a time.....
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  6. Senior Member SdotLow's Avatar
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    #5
    Hell of a necro there.
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  7. Junior Member Registered Member
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    #6
    that was hell of an answer, very useful clears a lot of things up
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  8. Senior Member
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    #7
    A collision domain is a segment where all devices share the same line. Think of your average home phone. All the phones in the house are on a single collision domain so to speak. If your teenage daughter is up in her room talking to her BFF on the phone in her room you can't make or receive a call on the phone in the living room because they share the same line. A switch breaks up collision domains. Switches used to be very expensive - too expensive for every computer to have its own switchport. So, what was commonly done was that a number of computers would be connected to a Hub and all the Hubs connected to a switch. Because a hub lacked intelligence (the ability to track MAC/IP Addresses), it was alot cheaper than a switch. A Hub is simply a multiport repeater. Anything received on any port is broadcast out all the ports. This meant that when anyone was transmitting, noone else could. Prior to Hubs, there was something called ThinNet/ThickNet. My HS Computer Lab used ThinNet. Every computer was the connected to a single coaxial cable. Both Hubs and ThinNet/ThickNet created a single collision domain.

    BTW, since the air is a single collision domain, Wireless has to deal with some of the same issues as a Hub. Something similar to CSMA/CD called CSMA/CA (Collision Avoidanced instead of Collision Detection) was developed for Wi-Fi to deal with these issues. This is also one reason a wireless network might run slow.

    CSMA/CD was a mechanism created to prevent and detect collisions. Carrier Sense Multiple Access (CSMA) refers to the fact that before transmitting it would listen to the line to make sure noone else was transmitting. Sort of like listening for a second make sure nobody's already using the line before trying to dial a phone number. Those of us who remember the days before everyone had cellphones know how annoying it is to be in the middle of a conversation and hear the sound of numbers being dialed [on another extension,] because a family member didn't bother to put the phone to their ear first. Collision Detection (CD) refers to the fact that if a Collision is detected that both computers will stop and wait a random amount of time before attempting to transmit again. This would be like if two people in the house decided to make a call at the exact same time. You both listen to the line and don't hear anyone talking so you both try to dial at the same instant. The equivalent of CD would be for both parties to hang up and wait a random amount of time, then recheck the line. If noone is using it they can make their call. When the other person picks up the line, they'll hear a call in progress and wait to make their call.

    Broadcast Domains are all devices that will receive a Broadcast packet. Since Routers do not forward broadcasts, they breakup Broadcast Domains. This is why some protocols, such as DHCP will not work beyond the local subnet without assistance (a DHCP Relay for example converts a DHCP Broadcast to Unicast so that it can be forwarded outside the local subnet). If a broadcast domain grows too large the amount of broadcast traffic (ARP, DHCP, etc...) can slow down the network.
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