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Network Devices
Ethernet - Architecture
What are internetworking
devices?
•
Internetworking devices are
products used to connect networks
• As computer networks grow in size
and complexity, so do the
internetworking devices used to
connect them.
The purposes of having
devices
• First, they allow a greater number of nodes
to be connected to the network.
• Second, they extend the distance over which
a network can extend.
• Third, they localize traffic on the network.
• Fourth, they can merge existing networks.
• Fifth, they isolate network problems so that
they can be diagnosed more easily.
What internetworking devices operate at
the physical layer (layer 1) of the OSI
model?
• A repeater can provide a simple solution if either
of these two problems exists.
• When signals first leave a transmitting station,
they are clean and easily recognizable
• However, the longer the cable length, the weaker
and more deteriorated the signals become as they
pass along the networking media.
What is the disadvantage associated with
using a repeater?
• it can't filter network traffic. Data,
sometimes referred to as bits, arriving at
one port of a repeater gets sent out on all
other ports
• data gets passed along by a repeater to all
other LAN segments of a network
regardless of whether it needs to go there or
no
What problem could occur as a result of
too much traffic on a network?
• if segments of a network are only connected
by non-filtering devices such as repeaters,
this can result in more than one user trying
to send data on the network at the same time
• If more than one node attempts to transmit
at the same time, a collision will occur.
• When a collision occurs, the data from each
device impact and are damaged
Hub
• Multi-port repeaters are often called hubs
• Hubs are very common internetworking
devices
• Generally speaking, the term hub is used
instead of repeater when referring to the
device that serves as the center of a star
topology network.
Hubs
• Intelligent hubs have console ports, to allow
monitoring of the hubs status and port activity.
• Passive hubs just repeat any incoming signals to
every port available, therefore does not act as a line
repeater.
• Passive hubs just split signals to multiple ports but
do not regenerate the signals, which means that
they do not extend a cable’s length. They only allow
two or more hosts to connect to the same cable
segment.
• Active hubs regenerate signals.
• Hubs utilise star topology.
Advantages
• As an active hubs regenerate signals, it
increases the distance that can be spanned by
the LAN (up to 100 meters per segment).
• Hubs can also be connected locally to a
maximum of two other hubs, thereby
increasing the number of devices that can be
attached to the LAN.
• Active hubs are usually used against
attenuation, which is a decrease in the
strength of the signal over distance.
Disadvantages
• Bandwidth is shared by all hosts i.e. 10Mbs
shared by 25 ports/users.
• Can create bottlenecks when used with
switches.
• Have no layer 3 switching capability.
• Most Hubs are unable to utilise VLANS.
What internetworking device can be
used to filter traffic on the network?
• One way to solve the problems of too much
traffic on a network and too many collisions
is to use an internetworking device called a
bridge.
• A bridge eliminates unnecessary traffic and
minimizes the chances of collisions
occurring on a network by dividing it into
segments
Bridges
• A network bridge connects multiple network segments at
the data link layer (layer 2) of the OSI model.
• Bridges are similar to repeaters or network hubs, devices that
connect network segments at the physical layer
• However a bridge works by using bridging where traffic
from one network is managed rather than simply rebroadcast
to adjacent network segments
• In Ethernet networks, the term "bridge" formally means a
device that behaves according to the IEEE 802.1D standard this is most often referred to as a network switch in
marketing literature.
At what layer of the OSI model
do bridges operate?
• Because bridges operate at the data link layer, layer 2,
they are not required to examine upper-layer
information.
A bridge works at the data-link (physical network) level of a
network, copying a data frame from one network to the next
network along the communications path.
How are bridge dataforwarding decisions limited?
•
Although bridges use tables to
determine whether or not to
forward data to other segments
of the network, the types of
comparisons and decisions they
make are relatively low level,
simple ones
What types of network traffic
problems is a bridge incapable
of solving?
• Bridges work best where traffic from one
segment of a network to other segments is
not too great.
• However, when traffic between network
segments becomes too heavy, the bridge can
become a bottleneck and actually slow
down communication
Transparent and Source Route
Bridging
•
•
Transparent Bridging
This method uses a forwarding
database to send frames across
network segments. The
forwarding database is initially
empty and entries in the database
are built as the bridge receives
frames. If an address entry is not
found in the forwarding
database, the frame is
rebroadcast to all ports of the
bridge, forwarding the frame to
all segments except the source
address.
Also known as adaptive bridges.
•
Route Source Bridging
With source route bridging two
frame types are used in order to
find the route to the destination
network segment. Single-Route
(SR) frames comprise most of the
network traffic and have set
destinations, while AllRoute(AR) frames are used to
find routes. Bridges send AR
frames by broadcasting on all
network branches; each step of
the followed route is registered
by the bridge performing it.
ADVANTAGES
•
•
•
•
•
Self configuring
Primitive bridges are often inexpensive
Reduce size of collision domain
Transparent to protocols above the MAC layer
Allows the introduction of management performance information and access control
• LANs interconnected are separate and physical
constraints such as number of stations,
repeaters and segment length don't apply
DISADVANTAGES
• Does not limit the scope of broadcasts
• Does not scale to extremely large networks
• Buffering introduces store and forward delays on average traffic destined for bridge will be
related to the number of stations on the rest of
the LAN
• Bridging of different MAC protocols
introduces errors
• Because bridges do more than repeaters by
viewing MAC addresses, the extra processing
makes them slower than repeaters
• Bridges are more expensive than repeaters
How many addressing schemes
are there in networking?
• One of the addressing schemes is the
MAC address
• The second addressing scheme in
networking makes use of what is called
the IP address
How do IP addresses differ
from MAC addresses?
• Like MAC addresses, every IP address is
unique. No two IP addresses are ever alike.
• However, while MAC addresses are
physical addresses that are actually hardcoded into the NIC card and occur at the
data link layer
• IP addresses are implemented in software
and occur at the network layer of the OSI
model.
Switches
• A switch is a multi-port bridge.
• It operates at OSI data link layer 2.
• It stores MAC addresses in an internal lookup table.
• Temporary switched paths are created between the
frame’s source destination.
• Some Switches have limited layer 3 IP routing
capabilities.
• Switches can be configured to use VLANS.
• Switches support spanning tree protocol to create
resilient networks.
What are routers?
• Routers are another type of internetworking
device.
• These devices pass data packets between
networks based on network protocol or
layer 3 information.
• Routers have the ability to make intelligent
decisions as to the best path for delivery of
data on the network.
Routers
• Routers are OSI network layer 3 devices
• Using interface modules can connect different layer
2 technologies e.g. Ethernet, FDDI, token ring etc…
• Routers have the capability to interconnect network
segments or entire networks (WANS/MANS).
• These devices examine incoming packets to
determine the destination address of the data. It
then examines its internal routing table to choose
the best path for the packet through the network,
and switches them to the proper outgoing port.
What network problems can
routers help resolve?
• The problem of excessive broadcast traffic
can be solved by using a router
• Routers are able to do this, because they do
not forward broadcast frames unless
specifically told to do so
How do routers differ from
bridges?
• First, bridging occurs at the data link layer
or layer 2,while routing occurs at the
network layer or layer 3 of the OSI model.
• Second, bridges use physical or MAC
addresses to make data forwarding
decisions. Routers use a different
addressing scheme that occurs at layer three