ICND1 break down – Operation of CSMA/CD

Posted on July 21, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , , |


CSMA/CD – Stands for carrier-sense multiple access with collision detection, which describe how the Ethernet protocol regulates communication among nodes. In the earlier days, all the Ethernet components were the network Interface cards (NIC) and coaxial cable connecting them. In case two NICs are sending message at the same time, the message will be corrupted. CSMS/CD avoided this.

When a NIC has data to transmit, the NIC first listens to the cable to see if a carrier signal is being transmitted by another node. A carrier signal in its physical form is a current flowing in the cable (each bit corresponds to 18-20 mA). Data is only sent when no carrier is observed — the so called carrier-sense multiple access (CSMA).

CSMA have a weak point — If two NICs simultaneously try transmit, then both could see no carrier signal in the medium, thus decide to transmit and a collision will occur.  Collision Detection (CD) resolved this problem. When there is data waiting to be sent, each transmitting NIC will observe if a collision occurs (excess current i.e. >24mA for coaxial Ethernet). If  collision is detected, it stops transmission and sends a 32-bit jam signal instead. The receiving NIC will discard the corrupted frame due to the jam signal. After sending the jam signal, each of the NICs will wait a random amount of time before beginning transmit again. The random time make sure the two NICs will not transmit simultaneously again.

ICND1 and ICND2 break down

Advertisements

Make a Comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

4 Responses to “ICND1 break down – Operation of CSMA/CD”

RSS Feed for IT Certifications Comments RSS Feed

[…] The mallest packet size is defined in RFC document (RFC894), I don’t know why IEEE like the number 64. As far as I concern, physics set a low bound for the packet size. Ethernet LANs use CSMA/CD, all nodes shared a single coaxial cable and only one speaker is allowed at a time. Packets had to be of a minimum size to guarantee that the leading edge of the propagating wave of the message got to all parts of the medium before the transmitter could stop transmitting, thus guaranteeing that collisions would be discovered. If you don’t understand the short answer, let’s review what we have learned in operation of CSMA/CD : […]

please also add Carrier sense multiple access collision avoidance

[…] What is a LAN * Ethernet Types And Standards * The Need For And Operation of CSMA/CD * Ethernet Frame * Ethernet Addressing * Host-to-Host Communication * Ethernet Connectors and Cable […]


Where's The Comment Form?

Liked it here?
Why not try sites on the blogroll...

%d bloggers like this: