Ccna routing and Switching Complete Study Guide

S2

S1

R1

And key to carefully planning your network design is never to allow broadcast domains 

to grow too large and get out of control. Both collision and broadcast domains can easily 

be controlled with routers and VLANs, so there’s just no excuse to allow user bandwidth 

to slow to a painful crawl when there are plenty of tools in your arsenal to prevent the 

suffering!

An important reason for this book’s existence is to ensure that you really get the foun-

dational basics of Cisco networks nailed down so you can effectively design, implement, 

configure, troubleshoot, and even dazzle colleagues and superiors with elegant designs that 

lavish your users with all the bandwidth their hearts could possibly desire.

To make it to the top of that mountain, you need more than just the basic story, so let’s 

move on to explore the collision detection mechanism used in half-duplex Ethernet.

CSMA/CD

Ethernet networking uses a protocol called Carrier Sense Multiple Access with Collision 

Detection (CSMA/CD), which helps devices share the bandwidth evenly while preventing 

two devices from transmitting simultaneously on the same network medium. CSMA/CD 

was actually created to overcome the problem of the collisions that occur when packets 

are transmitted from different nodes at the same time. And trust me—good collision man-

agement is crucial, because when a node transmits in a CSMA/CD network, all the other 

nodes on the network receive and examine that transmission. Only switches and routers 

can effectively prevent a transmission from propagating throughout the entire network!

So, how does the CSMA/CD protocol work? Let’s start by taking a look at Figure 2.4.

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Chapter 2 

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  Ethernet Networking and Data Encapsulation 

f i g u r E   2 . 4    CSMA/CD

Collision

Jam Jam Jam Jam Jam Jam Jam Jam

A

B

C

D

A

B

C

D

A

B

C

D

A

B

C

D

When a host wants to transmit over the network, it first checks for the presence of a 

digital signal on the wire. If all is clear and no other host is transmitting, the host will then 

proceed with its transmission.

But it doesn’t stop there. The transmitting host constantly monitors the wire to make 

sure no other hosts begin transmitting. If the host detects another signal on the wire, it 

sends out an extended jam signal that causes all nodes on the segment to stop sending 

data—think busy signal.

The nodes respond to that jam signal by waiting a bit before attempting to trans-

mit again. Backoff algorithms determine when the colliding stations can retransmit. If 

collisions keep occurring after 15 tries, the nodes attempting to transmit will then time out. 

Half-duplex can be pretty messy!

When a collision occurs on an Ethernet LAN, the following happens: