Segments can be divided to reduce the number of
users and increase the bandwidth available to each
user in the segment. Each new segment created
results in a new collision domain. Traffic from one
segment or collision domain does not interfere with
other segments, thereby increasing the available
bandwidth of each segment. In the following figure,
each segment has greater bandwidth, but all segments
are still on a common backbone and must
share the available bandwidth. This approach works
best when care is taken to make sure that the largest
users of bandwidth are placed in separate segments.
There are a few basic methods for segmenting an
Ethernet LAN into more collision domains:
• Use bridges to split collision domains.
• Use switches to provide dedicated domains to
each host.
• Use routers to route traffic between domains
(and to not route traffic that does not matter to
the other domain).
This sheet discusses segmenting using bridges and
routers (switching is covered in the next chapter). 53
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Ethernet Collisions
In a traditional LAN, several users would all share
the same port on a network device and would
compete for resources (bandwidth). The main limitation
of such a setup is that only one device can
transmit at a time. Segments that share resources
in this manner are called collision domains,
because if two or more devices transmit at the
same time, the information “collides,” and both
end points must resend their information (at different
times). Typically the devices both wait a random
amount of time before attempting to retransmit.
This method works well for a small number of
users on a segment, each having relatively low
bandwidth requirements. As the number of users
increases, the efficiency of collision domains
decreases sharply, to the point where overhead traffic
(management and control) clogs the network.
the same port on a network device and would
compete for resources (bandwidth). The main limitation
of such a setup is that only one device can
transmit at a time. Segments that share resources
in this manner are called collision domains,
because if two or more devices transmit at the
same time, the information “collides,” and both
end points must resend their information (at different
times). Typically the devices both wait a random
amount of time before attempting to retransmit.
This method works well for a small number of
users on a segment, each having relatively low
bandwidth requirements. As the number of users
increases, the efficiency of collision domains
decreases sharply, to the point where overhead traffic
(management and control) clogs the network.
What Problems Need to Be Solved?
Ethernet is a shared resource in which end stations
(computers, servers, and so on) all have access to the
transmission medium at the same time. The result is
that only one device can send information at a time.
Given this limitation, two viable solutions exist:
• Use a sharing mechanism: If all end stations are
forced to share a common wire, rules must exist
to ensure that each end station waits its turn
before transmitting. In the event of simultaneous
transmissions, rules must exist for retransmitting.
• Divide the shared segments, and insulate them:
Another solution to the limitations of shared
resources is to use devices that reduce the number
of end stations sharing a resource at any given time.
(computers, servers, and so on) all have access to the
transmission medium at the same time. The result is
that only one device can send information at a time.
Given this limitation, two viable solutions exist:
• Use a sharing mechanism: If all end stations are
forced to share a common wire, rules must exist
to ensure that each end station waits its turn
before transmitting. In the event of simultaneous
transmissions, rules must exist for retransmitting.
• Divide the shared segments, and insulate them:
Another solution to the limitations of shared
resources is to use devices that reduce the number
of end stations sharing a resource at any given time.
Why Should I Care About Ethernet?
Ethernet was developed in 1972 as a way to connect
newly invented computers to newly invented
laser printers. It was recognized even at that time
as a remarkable technology breakthrough.
However, very few people would have wagered
that the ability to connect computers and devices
would change human communication on the same
scale as the invention of the telephone and change
business on the scale of the Industrial Revolution.
Several competing protocols have emerged since
1972, but Ethernet remains the dominant standard
for connecting computers into local-area networks
(LAN). For many years Ethernet was dominant in
home networks as well. Ethernet has been mostly
replaced by wireless technologies in the home networking
market. Wireless or Wi-Fi is covered in
Part VIII, “Mobility.”
newly invented computers to newly invented
laser printers. It was recognized even at that time
as a remarkable technology breakthrough.
However, very few people would have wagered
that the ability to connect computers and devices
would change human communication on the same
scale as the invention of the telephone and change
business on the scale of the Industrial Revolution.
Several competing protocols have emerged since
1972, but Ethernet remains the dominant standard
for connecting computers into local-area networks
(LAN). For many years Ethernet was dominant in
home networks as well. Ethernet has been mostly
replaced by wireless technologies in the home networking
market. Wireless or Wi-Fi is covered in
Part VIII, “Mobility.”
Evolution of Ethernet
When Metcalfe originally developed Ethernet, computers were connected to a
single copper cable. The physical limitations of a piece of copper cable carrying
electrical signals restricted how far computers could be from each other on
an Ethernet. Repeaters helped alleviate the distance limitations. Repeaters are
small devices that regenerate an electrical signal at the original signal strength.
This process allows an Ethernet to extend across an office floor that might
exceed the Ethernet distance limitations.
The addition or removal of a device on the Ethernet cable disrupts the network
for all other connected devices. A device called an Ethernet hub solves
this problem. First, each port on a hub is actually a repeater. Second, hubs let
computers insert or remove themselves nondisruptively from the network.
Finally, hubs simplify Ethernet troubleshooting and administration. As networks
grow larger, companies need to fit more and more computers onto an
Ethernet. As the number of computers increases, the number of collisions on
the network increases. As collisions increase, useful network traffic decreases
(administrative traffic actually increases because of all the error messages getting
passed around). Networks come to a grinding halt when too many collisions
occur.
Ethernet bridges resolve this problem by physically breaking an Ethernet into
two or more segments. This arrangement means that devices communicating
on one side of the bridge do not collide with devices communicating on the
other side of the bridge. Bridges also learn which devices are on each side and
only transfer traffic to the network containing the destination device. A twoport
bridge also doubles the bandwidth previously available, because each port
is a separate Ethernet.
Ethernet bridges evolved to solve the problem of connecting Ethernet networks
to Token Ring networks. This process of translating a packet from one LAN
technology to another is called translational bridging.
As Ethernet networks continue to grow in a corporation, they become more
complex, connecting hundreds and thousands of devices. Ethernet switches
allow network administrators to dynamically break their networks into multiple
Ethernet segments.
Initially, switches operated as multiport Ethernet bridges. But eventually, as the
cost per port decreased significantly, Ethernet switches replaced hubs, in which
each connected device receives its own dedicated Ethernet bandwidth. With
switches, collisions are no longer an issue, because connections between computer
and switch can be point-to-point, and the Ethernet can both send and
receive traffic at the same time. This ability to send and receive simultaneously
is called full duplex, as opposed to traditional Ethernet, which operated at half
duplex. Half duplex means that a device can receive or transmit traffic on the
network, but not at the same time. If both happen at the same time, a collision
occurs.
This is different from subnetting in a couple of distinct ways. First, Ethernet is
a Layer 2 protocol, and subnetting has to do with IP addressing (which is a
Layer 3 function). Second, IP addressing is a logical segmentation scheme, and
switching is a physical separation, because each end station has a dedicated
physical port on the switch.
single copper cable. The physical limitations of a piece of copper cable carrying
electrical signals restricted how far computers could be from each other on
an Ethernet. Repeaters helped alleviate the distance limitations. Repeaters are
small devices that regenerate an electrical signal at the original signal strength.
This process allows an Ethernet to extend across an office floor that might
exceed the Ethernet distance limitations.
The addition or removal of a device on the Ethernet cable disrupts the network
for all other connected devices. A device called an Ethernet hub solves
this problem. First, each port on a hub is actually a repeater. Second, hubs let
computers insert or remove themselves nondisruptively from the network.
Finally, hubs simplify Ethernet troubleshooting and administration. As networks
grow larger, companies need to fit more and more computers onto an
Ethernet. As the number of computers increases, the number of collisions on
the network increases. As collisions increase, useful network traffic decreases
(administrative traffic actually increases because of all the error messages getting
passed around). Networks come to a grinding halt when too many collisions
occur.
Ethernet bridges resolve this problem by physically breaking an Ethernet into
two or more segments. This arrangement means that devices communicating
on one side of the bridge do not collide with devices communicating on the
other side of the bridge. Bridges also learn which devices are on each side and
only transfer traffic to the network containing the destination device. A twoport
bridge also doubles the bandwidth previously available, because each port
is a separate Ethernet.
Ethernet bridges evolved to solve the problem of connecting Ethernet networks
to Token Ring networks. This process of translating a packet from one LAN
technology to another is called translational bridging.
As Ethernet networks continue to grow in a corporation, they become more
complex, connecting hundreds and thousands of devices. Ethernet switches
allow network administrators to dynamically break their networks into multiple
Ethernet segments.
Initially, switches operated as multiport Ethernet bridges. But eventually, as the
cost per port decreased significantly, Ethernet switches replaced hubs, in which
each connected device receives its own dedicated Ethernet bandwidth. With
switches, collisions are no longer an issue, because connections between computer
and switch can be point-to-point, and the Ethernet can both send and
receive traffic at the same time. This ability to send and receive simultaneously
is called full duplex, as opposed to traditional Ethernet, which operated at half
duplex. Half duplex means that a device can receive or transmit traffic on the
network, but not at the same time. If both happen at the same time, a collision
occurs.
This is different from subnetting in a couple of distinct ways. First, Ethernet is
a Layer 2 protocol, and subnetting has to do with IP addressing (which is a
Layer 3 function). Second, IP addressing is a logical segmentation scheme, and
switching is a physical separation, because each end station has a dedicated
physical port on the switch.
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