4.8 Repeaters, Hubs, Bridges,
Switches and Routers
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Hubs and a Repeaters
• physical layer devices that do not understand
frames or packets. They understand the
symbols that encode bits as volts.
• A Repeater : They receives, amplifies (i.e.,
regenerates), and retransmits signals in both
directions.
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Hubs
• has multiple input lines that it joins
electrically. Frames arriving on any of the lines
are sent out on all the others.
• all stations connected to a hub are in the same
collision domain
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Bridges and Switches
• Data layer devices that extract the physical
addresses from the frame header and look it
up in a table to see where to send the frame.
• They only output incoming frames to the ports
for which those frames are destined.
• They can forward multiple frames at the same
time.
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Bridges and Switches (cont.)
• A Bridge : has relatively few ports to join
relatively few LANs.
• A Switch:
– Like a hub, has many ports (4 to 48 input lines ) to
plug directly individual computers into a switch
– Unlike in a hub, each station connected to a
switch is isolated to be in its own independent
collision domain
• if the port has a full-duplex point-to-point line, the
CSMA/CD algorithm is not needed.
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Hubs, Bridges and Switches
(a) A hub. (b) A bridge. (c) a switch.
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Routers
• a network layer device that extracts the logical
addresses (IP addresses) from the packet
header to choose an output line (the best
routing path) to forward packets between
computer networks.
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4.3 ETHERNET
IEEE STANDARDS
• the Computer Society of the IEEE started a
project, called Project 802, to set standards to
enable intercommunication among equipment
from a variety of manufacturers.
– Project 802 is a way of specifying functions of the
physical layer and the data link layer of major LAN
protocols.
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IEEE standard for LANs
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Ethernet evolution
• Classic Standard Ethernet.
• Standard Ethernet hubs.
• Switched Ethernet:
– Standard Ethernet:
– Fast Ethernet
– Gigabit Ethernet
– 10-Gigabit Ethernet
10 Mbps
100 Mbps
1 Gbps
10 Gbps
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4.3.1 Classic Standard Ethernet
• It is built as single long cable to which all the
computers were attached.
• The problems of classic Ethernet associated
with finding breaks or loose connections
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4.3.1 Classic Standard Ethernet
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4.3.4 Standard Ethernet with hubs
• Advantages:
– ease of maintenance :
• Adding or removing a station is simpler in this configuration
by plugging or unplugging a wire,
• cable breaks can be detected easily since a flaky cable or
port will usually affect just one station.
– being able to reuse existing wiring (i.e. the twisted
pairs wires of telephone company).
• Disadvantages:
– hubs do not increase capacity because they are
logically equivalent to the single long cable of classic
Ethernet. (it can not deal with increased load)
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4.3.4 Switched Ethernet
• A switch has the same advantages as a hub.
• However, A switch improves performance over a hub in
three ways.
– since there are no collisions, the capacity is used more
efficiently.
– with a switch multiple frames can be sent simultaneously
(by different stations).
– It has security benefits:
• With a hub, every computer that is attached can see the traffic
sent between all of the other computers.
• With a switch, traffic is forwarded only to the ports where it is
destined.
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Standard Ethernet cabling
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4.3.5 Fast Ethernet
• Upgrade the data rate from 10 Mbps to 100
Mbps), while maintaining compatibility with all
existing Ethernet standards (keep all the old
frame formats, interfaces, and procedural rules).
• fast Ethernet switches can handle a mix of 10Mbps and 100-Mbps stations (Autonegotiation)
– Autonegotiation: a mechanism that lets two stations
automatically negotiate the optimum speed (10 or
100 Mbps) and duplexity (half or full).
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4.3.5 Fast Ethernet
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4.3.6 Gigabit Ethernet
• increases performance tenfold (Upgrade the
data rate from 100 Mbps to 1 Gbps). while
maintaining compatibility with all existing
Ethernet standards(keep all the old frame
formats, interfaces, and procedural rules).
• Gigabit Ethernet switches can handle a mix of
10-Mbps ,100-Mbps and Gbps stations
(Autonegotiation)
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4.3.6 Gigabit Ethernet
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4.3.7 10-Gigabit Ethernet
• increases performance tenfold (Upgrade the
data rate from 1 Mbps to 10 Gbps). while
maintaining compatibility with all existing
Ethernet standards(keep all the old frame
formats, interfaces, and procedural rules).
• All versions of 10-gigabit Ethernet support
only full-duplex operation. Therfore, CSMA/CD
is no longer part of the design.
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4.3.7 10-Gigabit Ethernet (2)
• 10-Gigabit Ethernet is needed :
– inside data centers
– exchanges to connect high-end routers switches,
and servers.
– in long-distance.
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4.3.7 10-Gigabit Ethernet
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4.3.8 Ethernet Pros
• Ethernet is reliable ;
– Once the hub and switch architecture was adopted, failures
became extremely rare.
• Ethernet is cheap;
– Twisted-pair wiring is relatively inexpensive as are the hardware
components.
– It allowed the existing cabling (Twisted-pair) to be reused for a
time.
• Ethernet is easy to maintain;
– There is no software to install (other than the drivers)
– adding new hosts is as simple as just plugging them in.
– cable breaks can be detected easily
• Ethernet interworks easily with TCP/IP (The most widely
used communications protocol)
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