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collision domains

By rocenosek ·
What happens when two or more computers on the same segment send out a request at the same time?

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by Dumphrey In reply to collision domains

They shouldn't ethernet uses a formula and "hello" packet to check if the line is clear to send info. If there is a collision, then both sending devices receive a report of this based on line noise, and start a hold-down timer which is a random amout of time. At the end of the hold dow timer, the devices will send again. But in Gbit ethernet its a whole nother game.

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by CG IT In reply to collision domains

humm I think your mistaking Cisco's term Collision Domain to describe a single network segment and packet collisions on the wire.

In the old days with old technology, packets did tend to collide with each other. This resulted in both layer 1 devices stopping their transmit, wait a predetermined amount of time and retransmit. In the early days, packet collisions was such a problem on some networks that IBM came up with Token Ring to combat it.

Todays technology, packet collisions are near non-existant as the first answer said.

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by dpahari23 In reply to collision domains

Must Ethernet media use CSMA/CD as an access method to access the network.
In CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode (carrier sense). With shared ethernet, this means that when a device wants to send data, it first must check to see whether the networking medium is busy. The device must check whether there are any signals on the networking media. After the device determines that the networking media is not busy, the device begins to transmit its data. While transmitting its data in the form of signals, the device also listens, to ensure that no other stations are transmitting data to the networking medium at the same time. Thus avoid the collisions.

I hope this helps.

[Ref: Cisco Networking Academic Programme,Cisco press, CCNA1 page 262]

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by lamczyknic3000 In reply to collision domains

A collision domain can be a single segment of Ethernet cable in shared-media Ethernet, or a single Ethernet hub in twisted-pair Ethernet, or even a whole network of hubs and repeaters. The OSI model dictates which devices extend or compound collision domains. Carrier Sense Multiple Access (CSMA)

Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performance when there is a higher medium utilisation. When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring whether a current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are sent by encoding them with a 10 (or 100 MHz for Fast Ethernet) clock using Manchester encoding. Data is only sent when no carrier is observed (i.e. no current present) and the physical medium is therefore idle. Any NIC which does not need to transmit, listens to see if other NICs have started to transmit information to it.

However, this alone is unable to prevent two NICs transmitting at the same time. If two NICs simultaneously try transmit, then both could see an idle physical medium (i.e. neither will see the other's carrier signal), and both will conclude that no other NIC is currently using the medium. In this case, both will then decide to transmit and a collision will occur. The collision will result in the corruption of the frame being sent, which will subsequently be discarded by the receiver since a corrupted Ethernet frame will (with a very high probability) not have a valid 32-bit MAC CRC at the end.
Collision Detection (CD)

A second element to the Ethernet access protocol is used to detect when a collision occurs. When there is data waiting to be sent, each transmitting NIC also monitors its own transmission. If it observes a collision (excess current above what it is generating, i.e. > 24 mA for co

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Good

by mohanrav In reply to

So the thing is when the packets are sent at the same time in one physical medium that leads to collsion
So this is called collison domain.
Am i rite??
what abt collison doamin in switches,hubs.bridges,routers??
can u explain this too??
Thanks
Mohan R

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by lamczyknic3000 In reply to collision domains

A collision domain can be a single segment of Ethernet cable in shared-media Ethernet, or a single Ethernet hub in twisted-pair Ethernet, or even a whole network of hubs and repeaters. The OSI model dictates which devices extend or compound collision domains. Carrier Sense Multiple Access (CSMA)

Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performance when there is a higher medium utilisation. When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring whether a current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are sent by encoding them with a 10 or 100 MHz for Fast Ethernet clock using Manchester encoding. Data is only sent when no carrier is observed and the physical medium is therefore idle. Any NIC which does not need to transmit, listens to see if other NICs have started to transmit.

However, this alone is unable to prevent two NICs transmitting at the same time. If two NICs simultaneously try transmit, then both could see an idle physical medium, and both will conclude that no other NIC is currently using the medium. In this case, both will then decide to transmit and a collision will occur. The collision will result in the corruption of the frame being sent, which will subsequently be discarded by the receiver since a corrupted Ethernet frame will not have a valid 32-bit MAC CRC at the end.
Collision Detection (CD)

A second element to the Ethernet access protocol is used to detect when a collision occurs. When there is data waiting to be sent, each transmitting NIC also monitors its own transmission. If it observes a collision, it stops transmission immediately and instead transmits a 32-bit jam sequence. The purpose of this sequence is to ensure that any other node which may currently be receiving this frame will

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by lamczyknic3000 In reply to collision domains

A collision domain can be a single segment of Ethernet cable in shared-media Ethernet, or a single Ethernet hub in twisted-pair Ethernet, or even a whole network of hubs and repeaters.

Carrier Sense Multiple Access (CSMA)

Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performance when there is a higher medium utilisation. When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring whether a current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are sent by encoding them with a 10 or 100 MHz for Fast Ethernet clock using Manchester encoding. Data is only sent when no carrier is observed and the physical medium is therefore idle. Any NIC which does not need to transmit, listens to see if other NICs have started to transmit.

However, this alone is unable to prevent two NICs transmitting at the same time. If two NICs simultaneously try transmit, then both could see an idle physical medium, and both will conclude that no other NIC is currently using the medium. In this case, both will then decide to transmit and a collision will occur. The collision will result in the corruption of the frame being sent, which will subsequently be discarded.

Collision Detection (CD)

A second element to the Ethernet access protocol is used to detect when a collision occurs. When there is data waiting to be sent, each transmitting NIC also monitors its own transmission. If it observes a collision, it stops transmission immediately and instead transmits a 32-bit jam sequence. The purpose of this sequence is to ensure that any other node which may currently be receiving this frame will receive the jam signal in place of the correct 32-bit MAC CRC, this causes the other receivers to discard the frame due to a CRC error.

The NIC then waits a period

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by lamczyknic3000 In reply to collision domains

A collision domain can be a single segment of Ethernet cable in shared-media Ethernet, or a single Ethernet hub in twisted-pair Ethernet, or even a whole network of hubs and repeaters.

Also check this web site to learn more about collisions and how they occur and what protocols are used IE. CMSA/CD.
http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html

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