I

I can't get to the payroll server.

I could have walked to Finance by now.

Collisions are by-products of the CSMA/CD method used by Ethernet. In a shared-bandwidth Ethernet network, when using hubs, many devices will share the same physical segment. Despite listening first, before they transmit, to see whether the media is free, multiple stations might still transmit simultaneously. If two or more stations on a shared media segment do transmit at the same time, a collision results, and the frames are destroyed. When the sending stations involved with the collision recognize the collision event, they will transmit a special "jam" signal, for a predetermined time, so that all devices on the shared segment will know that the frame has been corrupted, that a collision has occurred, and that all devices on the segment must stop communicating. The sending stations involved with the collision will then begin a random countdown timer that must be completed before attempting to retransmit the data.

As networks become larger, and devices each try to use more bandwidth, it becomes more likely that end devices will each attempt to transmit data simultaneously, and that will ultimately cause more collisions to occur. The more collisions that occur, the worse the congestion becomes, and the effective network throughput of actual data can become slow. Eventually, with sufficient collisions, the total throughput of actual "data" frames becomes almost nonexistent.

Adding a hub to an Ethernet LAN can overcome the segment length limits and the distances that a frame can travel over a single segment before the signal degrades, but Ethernet hubs cannot improve collision issues.

Collision Domains

In expanding an Ethernet LAN, to accommodate more devices with more bandwidth requirements, you can create separate physical network segments called collision domains so that collisions are limited to a single collision domain, rather than the entire network.

In traditional Ethernet segments, the network devices compete and contend for the same shared bandwidth, with all devices sharing a command media connection, only one single device is able to transmit data at a time. The network segments that share the same bandwidth are known as collision domains, because when two or more devices within that segment try to communicate at the same time, collisions can occur.

You can, however, use other network devices, operating at Layer 2 and above of the OSI model, to divide a network into segments and reduce the number of devices that are competing for bandwidth. Each new segment, then, results in a new collision domain. More bandwidth is available to the devices on a segment, and collisions in one collision domain do not interfere with the operation of the other segments. Figure 2-3 shows how a switch has been used to isolate each user and device into its own collision domain.

Figure 2-3 Creating Multiple Collision Domains Using a Switch

Figure 2-3 Creating Multiple Collision Domains Using a Switch

Collision Domain 3

Summary of Ethernet Local-Area Networks

The key points that were discussed in the previous sections are as follows:

■ A segment is a network connection made by a single unbroken network cable. Ethernet cables and segments can only span a limited physical distance, after which the transmissions become degraded.

■ A hub works like a multiport repeater and can effectively extend a network segment by receiving the incoming frames, amplifying the electrical signals, and transmitting these frames back out through all ports to all devices that are connected to segments on the same Ethernet hub.

■ If two or more stations connected to the same hub transmit at the same time, a collision results because of the half-duplex nature of the Ethernet 802.3 CSMA/CD specification.

■ The network segments that share the same bandwidth are called collision domains because when two or more devices on the same segment both communicate and send data at the same time, collisions can occur within that shared segment.

■ It is possible to use other network devices, operating at Layer 2 (or above) of the OSI model, to divide network segments and reduce the number of devices that are competing or contending for bandwidth on any given segment and to provide better throughput for end users and devices on each separate segment.

Was this article helpful?

0 0
The Ultimate Computer Repair Guide

The Ultimate Computer Repair Guide

Read how to maintain and repair any desktop and laptop computer. This Ebook has articles with photos and videos that show detailed step by step pc repair and maintenance procedures. There are many links to online videos that explain how you can build, maintain, speed up, clean, and repair your computer yourself. Put the money that you were going to pay the PC Tech in your own pocket.

Get My Free Ebook


Post a comment