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2.2 Network Devices
Connectors, Repeaters, Hubs, Bridges,
Switches, Routers, NIC’s
Network Devices
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Network is interconnection of devices.
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For these connection we need to use the
connecting devices.
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Also called as Network Control Devices.
The purpose
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Allow a greater number of nodes to be
connected to the network.
Extend the distance over which a network can
extend.
Localize traffic on the network.
Can merge existing networks.
Isolate network problems so that they can be
diagnosed more easily.
Devices and the layers at
which they operate
Connectors
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To connect cable between two computers.
Connectors are of different type such as –
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Twisted Pair cable
Co-axial Cable
Fibre optic cable.
Connectors are type such as
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Jacks
Plugs
Sockets and ports
Connectors
Example:
 RS232 and V35 for serial interface
 RJ45 and BNC connectors for Ethernet.
 SC or ST connectors for fibre optic
Repeaters
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Signal attenuation or signal loss – signal
degrades over distance
Repeaters clean, amplify, and resend signals
that are weakened by long cable length.
Built-in to hubs or switches
A repeater operates only at the PHYSICAL layer.
It connects two segments of the same network.
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Single port, multi-port repeaters.
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Repeaters
Function of a repeater
HUB
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A hub is used as a central device.
Connects the computers in star topology.
Hubs are simple devices that direct data
packets to all devices connected to the hub.
Hubs regenerate and retime network signals
hubs work at the OSI physical layer
They cannot filter network traffic.
They cannot determine best path
They are really multi-port repeaters
Types of Hub
Passive hub
1.
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Active hubs or Multiport repeaters
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is just a connector - connects the wires coming from
different branches.
The signal pass through a passive hub without
regeneration or amplification. (distance 300 feet)
They regenerate or amplify the signal before they are
retransmitted (distance -2000 feet).
Intelligent Hub
 Regenerate the signals and perform network
management and intelligent path selection.
Bridges
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Operates in both the PHYSICAL and the data
link layer.
As a PHYSICAL layer device, it regenerates the
signal it receives.
As a data link layer device, the bridge can
check the PHYSICAL/MAC addresses (source
and destination) contained in the frame.
Bridge
Bridges
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A bridge has a table used in filtering decisions.
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It can check the destination address of a frame
and decide if the frame should be forwarded
or dropped.
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For frame to be forwarding, it specify the port.
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Limit or filter traffic - keeps local traffic local
yet allow connectivity to other parts (segments).
Function of Bridge
Characteristics of Bridges
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Routing Tables
– Contains one entry per station of network.
– Is used to determine the network of destination.
Filtering
– Packets are filtered with respect to their destination
and multicast addresses.
Forwarding
– the process of passing a packet from one network to
another.
Learning Algorithm
– the process by which the bridge learns how to reach
stations on the internetwork.
Types of Bridges
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Transparent Bridge
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Also called learning bridges
Build a table of MAC addresses as frames arrive.
Ethernet networks use transparent bridge
Duties are : Filtering frames, forwarding and
blocking
Source Routing Bridge
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Used in Token Ring networks
Frame contains not only the source and destination
address but also the bridge addresses.
Advantages And Disadvantages
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Advantages of using a bridge
– Extend physical network
– Reduce network traffic with minor segmentation
– Reduce collisions
– Connect different architecture
Disadvantages of using bridges
– Slower than repeaters due to filtering
– Do not filter broadcasts
– More expensive than repeaters
Differences Between Bridges
and Repeaters
Repeaters
Bridges
OSI layer
Physical layer
Data link layer
Data
regeneration
Regenerate data at
the signal level
Regenerate data
at the packet
level
Yes
Reduce network No
traffic
Switched networks
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Shared ethernet networks perform best when
kept to 30-40 percent full capacity
This is a result of CSMA/CD
A LAN switch is a high-speed multiport bridge
which segments each port into its own
collision domain and can access the full
bandwidth
Devices and the layers at
which they operate
Layer
Name of Layer
Device
3
Network
Routers, layer 3
switches
2
Data Link
Switches,
bridges, NIC’s
1
Physical
Hubs
Switches
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Each port is a simulated segment to itself
Store and Forward Switches
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Do error checking on each frame after the entire
frame has arrived into the switch
If the error checking algorithm determines there is
no error, the switch looks in its MAC address table
for the port to which to forward the destination
device
Highly reliable because doesn’t forward bad frames
Slower than other types of switches because it holds
on to each frame until it is completely received to
check for errors before forwarding
Cut Through Switch
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Faster than store and forward because
doesn’t perform error checking on frames
Reads address information for each frame as
the frames enter the switch
After looking up the port of the destination
device, frame is forwarded
Forwards bad frames
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Performance penalty because bad frames can’t
be used and replacement frames must be sent
which creates additional traffic
Fragment free cut through
switch
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Combines speed of cut through switch with
error checking functionality
Forwards all frames initially, but determines
that if a particular port is receiving too many
bad frames, it reconfigures the port to store
and forward mode
Preferred switching solution
Unmanaged/Intelligent
switches
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Unmanaged – provides LAN’s with all the
benefits of switching
Fine in small networks
Intelligent switches tracks and reports LAN
performance statistics
Have a database ASIC (application specific
integrated circuit) on board to collect and
store data which you view through a software
interface
Layer 3 switch
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By definition a switch filters or forwards frames
based on MAC addresses. This makes a switch a
layer 2 device.
Now we have layer 3 switches which have routing
capability. If a data frame can’t be switched it is
routed.
Each port is a separate LAN port, but the forwarding
engine actually calculates and stores routes based
on IP addresses, not MAC addresses
Usually support only IP or IP and IPX
VLAN Switches
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Virtual local area network
Each port on a switch defines a collision domain
The entire switch forms a single broadcast domain
VLANs can define multiple broadcast domains
Network traffic that is directed to all computers on
the network can be segmented to transmit only on a
specific VLAN.
Improves bandwidth on a the VLAN’s because each
VLAN filters the network-to-network broadcast traffic
as well as the collision traffic from other VLAN’s
Physical Layer Broadcast
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Physical layer broadcasts – implemented by
non-switched Ethernet networks through
shared cabling and hubs
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Each bit that is transmitted is physically received
by every station
Switches and VLAN’s don’t do physical layer
broadcasts
MAC-level broadcast
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MAC-level broadcast – deal with how to handle
MAC level broadcast frames; that is the data frames
that have a broadcast destination MAC address
MAC-level broadcast frames are addressed to all
MAC addresses on a given network (not a network
segment, but an actual network as defined by its
network address)
A regular switch forwards all broadcast frames out
all ports, but a VLAN switch forwards broadcast
frames only to ports that are part of the same VLAN
Multiple switches can be part of the same VLAN
VLAN Switches
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None of the VLAN’s can communicate unless
each VLAN is connected to a router or layer 3
switch
Each VLAN is separating collision traffic
associated with MAC Addresses (layer 2) and
each VLAN is separating the network-tonetwork broadcast traffic. In other words each
VLAN is acting as a separate network so a
layer 3 device is necessary for them to
communicate