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31 Days Before Your CCENT Exam: Ethernet and Media Access Control

Chapter Description

To help you prepare for your CCENT exam. this chapter reviews Ethernet technologies and operation at both the data link and physical layers.

Benefits of Using Switches

A collision domain is a set of devices whose frames could collide. All devices on a 10BASE2, 10BASE5, or any network using a hub risk collisions between the frames that they send, so all devices on one of these types of Ethernet networks are in the same collision domain and use CSMA/CD to detect and resolve collisions.

LAN switches significantly reduce, or even eliminate, the number of collisions on a LAN. Unlike hubs, switches do not create a single shared bus. Instead, switches do the following:

  • They interpret the bits in the received frame so that they can typically send the frame out the one required port, rather than all other ports.
  • If a switch needs to forward multiple frames out the same port, the switch buffers the frames in memory, sending one at a time, thereby avoiding collisions.

In addition, switches with only one device cabled to each port of the switch allow the use of full-duplex operation. Full-duplex means that the NIC can send and receive concurrently, effectively doubling the bandwidth of a 100Mbps link to 200Mbps—100Mbps for sending and 100Mbps for receiving.

These seemingly simple switch features provide significant performance improvements as compared with using hubs. In particular:

  • If only one device is cabled to each port of a switch, no collisions can occur.
  • Devices connected to one switch port do not share their bandwidth with devices connected to another switch port. Each has its own separate bandwidth, meaning that a switch with 100Mbps ports has 100Mbps of bandwidth per port.
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