A Presentation for BSIT - USEP STUDENT : By ND ARQUILLANO

1. The history of how Ethernet handles
collisions and collision domains dates
back to research at the University of
Hawaii in 1970. In its attempts to
develop a wireless communication
system for the islands of Hawaii,
university researchers developed a
protocol called Aloha. The Ethernet
protocol is actually based on the Aloha
protocol.

2. Segmentation
 One important skill for a networking professional is the ability to recognize collision
domains. Connecting several computers to a single shared-access medium that has
no other networking devices attached creates a collision domain. This situation limits
the number of computers that can use the medium, also called a segment. Layer 1
devices extend but do not control collision domains.
 Layer 2 devices segment or divide collision domains. Controlling frame propagation
using the MAC address assigned to every Ethernet device performs this function.
Layer 2 devices, bridges, and switches, keep track of the MAC addresses and which
segment they are on. By doing this these devices can control the flow of traffic at the
Layer 2 level. This function makes networks more efficient by allowing data to be
transmitted on different segments of the LAN at the same time without the frames
colliding. By using bridges and switches, the collision domain is effectively broken
up into smaller parts, each becoming its own collision domain.
 These smaller collision domains will have fewer hosts and less traffic than the
original domain. The fewer hosts that exist in a collision domain, the more likely the
media will be available. As long as the traffic between bridged segments is not too
heavy a bridged network works well. Otherwise, the Layer 2 device can actually slow
down communication and become a bottleneck itself.
 Layer 3 devices, like Layer 2 devices, do not forward collisions. Because of this, the
use of Layer 3 devices in a network has the effect of breaking up collision domains
into smaller domains.
 Layer 3 devices perform more functions than just breaking up a collision domain.
Layer 3 devices and their functions will be covered in more depth in the section on
broadcast domains.

3. Layer 2 broadcasts
To communicate with all
collision domains, protocols
use broadcast and multicast
frames at Layer 2 of the OSI
model. When a node needs
to communicate with all
hosts on the network, it sends
a broadcast frame with a
destination MAC address
0xFFFFFFFFFFFF. This is an
address to which the network
interface card (NIC) of every
host must respond.

4. Layer 2 Broadcast
 Layer 2 devices must flood all broadcast and multicast traffic. The accumulation of broadcast and
multicast traffic from each device in the network is referred to as broadcast radiation. In some cases,
the circulation of broadcast radiation can saturate the network so that there is no bandwidth left for
application data. In this case, new network connections cannot be established, and existing
connections may be dropped, a situation known as a broadcast storm. The probability of broadcast
storms increases as the switched network grows.
 Because the NIC must interrupt the CPU to process each broadcast or multicast group it belongs to,
broadcast radiation affects the performance of hosts in the network. Figure shows the results of tests
that Cisco conducted on the effect of broadcast radiation on the CPU performance of a Sun
SPARCstation 2 with a standard built-in Ethernet card. As indicated by the results shown, an IP
workstation can be effectively shut down by broadcasts flooding the network. Although extreme,
broadcast peaks of thousands of broadcasts per second have been observed during broadcast storms.
Testing in a controlled environment with a range of broadcasts and multicasts on the network shows
measurable system degradation with as few as 100 broadcasts or multicasts per second.
 Most often, the host does not benefit from processing the broadcast, as it is not the destination being
sought. The host does not care about the service that is being advertised, or it already knows about
the service. High levels of broadcast radiation can noticeably degrade host performance. The three
sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast
applications.

5. Broadcast domains A broadcast domain is a grouping of collision
domains that are connected by Layer 2
devices. Breaking up a LAN into multiple
collision domains increases the opportunity for
each host in the network to gain access to the
media. This effectively reduces the chance of
collisions and increases available bandwidth
for every host. But broadcasts are forwarded by
Layer 2 devices and if excessive, can reduce the
efficiency of the entire LAN. Broadcasts have to
be controlled at Layer 3, as Layer 2 and Layer 1
devices have no way of controlling them. The
total size of a broadcast domain can be
identified by looking at all of the collision
domains that the same broadcast frame is
processed by. In other words, all the nodes that
are a part of that network segment bounded by
a layer three device. Broadcast domains are
controlled at Layer 3 because routers do not
forward broadcasts. Routers actually work at
Layers 1, 2, and 3. They, like all Layer 1 devices,
have a physical connection to, and transmit
data onto, the media. They have a Layer 2
encapsulation on all interfaces and perform
just like any other Layer 2 device. It is Layer 3
that allows the router to segment broadcast
domains.

6. Broadcast domains
 In order for a packet to be forwarded through a router
it must have already been processed by a Layer 2
device and the frame information stripped off. Layer 3
forwarding is based on the destination IP address and
not the MAC address. For a packet to be forwarded it
must contain an IP address that is outside of the range
of addresses assigned to the LAN and the router must
have a destination to send the specific packet to in its
routing table.