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CCNA RnS #13: CSMA/CD

June 26, 2023 , ,

CCNA RnS – CSMA/CD

In this article let’s talk about CSMA/CD and before we directly jump to the topic let’s go back and understand duplex communication. CSMA/CD is directly linked to half-duplex communication.

Before the modern LAN switches, there used to be a device called a LAN hub to build the Ethernet Network. Just like modern switches, these hubs provide multiple RJ-45 ports for connection. However, devices connected to these ports either can send or receive traffic at a given point in time. This is called the half-duplex. In contrast to half-duplex, full-duplex allows a device to send and receive traffic simultaneously.

Also, LAN hubs were not equipped with the capability to take frame forwarding based on the addresses. I mean when an electrical signal comes on one port, the hub repeats that signal out of all other ports (except the port it received the signal). This flooding reaches all the other devices including the receiving device.

Figure 1: Half Duplex and Full Duplex Connection
Figure 1: Half Duplex and Full Duplex Connection

What happens when two or more devices connected to LAN hub send traffic at same time? A collision! To prevent this collision, half-duplex tells devices that if someone else is sending, wait before sending. This mechanism is called the Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

With CSMA/CD:

  • A device that wants to send frame listens until the Ethernet is not busy
  • When Ethernet is not busy, it begins sending the frame.
  • The sender listens while sending to discover whether a collision occurs. If there is a collision, all sending devices – send a jamming signal that tells all other devices that a collision has happened.
  • All other devices choose a random time to wait before they retry to send.

Modern networks with switches use full-duplex communication. Each port and NIC has a duplex setting. All links between the PCs and switches or between switches are now full-duplex. However, if the network still uses Hub, the connection to Hub should use half duplex.

Figure 2: Half Duplex and Full Duplex Connection

Ethernet shared media is another term that is used when building a network using Hub as this requires CSMA/CD and the bandwidth is shared. The idea is that CSMA/CD implements rules that allow only one device to send the traffic over a connection at any point in time. Hope you find this informative!

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