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Ethernet Networking
Ethernet is a contention media access method that allows all hosts on a network to share the
same bandwidth of a link. Ethernet is popular because it’s readily scalable, meaning that it’s
comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet,
into an existing network infrastructure. It’s also relatively simple to implement in the first
place, and with it, troubleshooting is reasonably straightforward. Ethernet uses both Data
Link and Physical layer specifications, and this section of the chapter will give you both the
Data Link layer and Physical layer information you need to effectively implement, troubleshoot,
and maintain an Ethernet network.
Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD),
a protocol that helps devices share the bandwidth evenly without having two devices transmit at
the same time on the network medium. CSMA/CD was created to overcome the problem of those
collisions that occur when packets are transmitted simultaneously from different nodes. And trust
me—good collision management 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 bridges and
routers can effectively prevent a transmission from propagating throughout the entire network.
So, how does the CSMA/CD protocol work? 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 (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 while
before attempting to transmit again. Backoff algorithms determine when the colliding
stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting
to transmit will then timeout.
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