Download Repeaters and Bridges

Extending LANs
Based on Chapter 11 in Computer
Networks and Internets
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Size Limitations
• Recall that the combination of protocol and
wiring scheme places size restrictions on a
LAN
– Ethernet using 10Base5 (thick) allows segments
up to 500 m
– FDDI (Fiber Distributed Data Interface) allows
a single ring to be 100 kilometers (62 miles)
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Reason One
• Attenuation and interference:
– The farther a signal travels, the more it
attenuates (weakens). If the amplitude is
reduced, the signal-to-noise ratio may decrease.
And the information may be lost in the noise.
– The farther a signal travels, the more likely it is
to experience interference (noise) — other
unwanted waves adding to it.
– Increasing the amplitude would help but that
requires additional power and thus increasing
cost.
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Reason Two
• Sharing Speed:
– The longer a token ring (and the more
computers on it), the longer it takes before it is
a given computer’s turn to transmit.
– The longer a bus (and the more computers on
it), the more likely that two computers will
transmit simultaneously causing a collision
(CSMA/CD).
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Fiber-Optic Extensions
• The attenuation/interference limitation can be
overcome by changing the medium: wire  fiber
optic cable.
• Recall fiber optic cable has a much higher
bandwidth and is much less susceptible to
interference.
• Building a LAN entirely of fiber is expensive.
– One of the reasons FDDI did not become more
widespread is because of its cost.
• But fiber optic cable can be used to extend certain
connections in a LAN.
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Fiber Optic Extensions (Cont.)
• The connection between the computer and the bus
could be partially spanned by fiber optic cable.
• Electrical signal goes to a fiber modem.
• The fiber modem converts the signal into an
optical form for transmission over the optical
fiber.
• A second fiber modem receives the optical signal
and converts it back to an electronic signal.
• The electronic signal is placed on the bus in the
usual way.
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Fiber Extension (Fig. 11.1)
Note as shown above one is adding a single computer
that is a large distance from the rest of the network. But
this idea can be extended.
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Repeater
• Another solution to the attenuation/interference
problem is to use a repeater.
• A repeater receives a signal, amplifies it, and then
retransmits (forwards) it.
• Repeaters are used between segments of a local area
network (LAN).
– Segments have a fixed maximum length.
• Repeaters are also used in WANs, both wired and
wireless.
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Repeater (Cont.)
• In its simplest version an analog repeater
simply amplifies the signal it receives.
• Therefore, if any noise has corrupted the
signal, that noise is also amplified. This is
the best a repeater can do with an analog
signal.
• It is possible for a repeater to clean up a
digital signal provided it is not too noisy.
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It’s possible to clean up a digital signal
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A noise ridden digital signal.
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It’s possible to clean up a digital signal
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A cleaned-up digital signal.
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Repeater (Cont.)
• While noise can in some sense be removed
from a digital signal by a repeater, digital
signals tend to require more frequent
repeating.
• Whereas analog signal amplifiers are spaced
at 18,000 meter intervals, digital signal
repeaters are typically placed at 2,000 to
6,000 meter intervals.
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Repeater (Fig. 11.2)
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Repeaters
• Repeaters allow the LAN size to be extended.
• But the repeater solves only the attenuationinterference problem and not the sharing speed
problem.
• For this reason, some protocols place restrictions
on the number or arrangement of repeaters.
• Ethernet standards allow only a maximum of 4
repeaters between any two nodes.
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Multiple Repeaters (Fig. 11.3)
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MAC Bridges
• Like a repeater, a bridge (a.k.a. MAC bridge)
connects segments of a LAN, but a bridge is more
“intelligent.”
• Under “steady-state” conditions, a bridge only
passes a packet from segment A to segment B if the
packet’s destination is on segment B or beyond.
• In other words, while a repeater works at the
physical layer and sees the transmission only as a
wave, a bridge operates at the data-link layer and
understands the transmission as data, in particular its
destination and source addresses.
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Bridge
• A bridge is a “computer” with more than one NIC
card operating in promiscuous mode.
– It will probably not be used for anything but this
purpose as the processor will be quite occupied with
this task.
• Each NIC card is attached to a LAN segment.
• All transmissions on these segments are read by
the bridge.
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Bridge (Cont.)
• Most bridges are learning or adaptive bridges.
• When such a bridge is first connected, it does not
know which computers are on which segment.
• When a packet first arrives, the bridge “knows”
from which of its cards the message entered (i.e.
what segment the message came from) and it also
reads the source address.
• It has then learned the MAC address of a
computer (the source) and which of its ports that
computer is on.
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Bridges (Cont.)
• It does not yet know the side of the
destination computer, so it must transmit
(forward) the packet to all other ports.
• Later when a packet arrives having as a
destination the previous packet’s source
address, the bridge knows whether the
packets must be forwarded or not.
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Bridge (Cont.)
• The bridge develops a table of MAC addresses,
and after a time reaches its “steady state” in which
it knows the addresses of most of the active
computers.
– Tables are refreshed periodically in case a computer is
moved.
• It only transmits packets
– That are broadcast
– That are multicast
– That are unicast and have source and destination on
different ports of the bridge.
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Repeater vs. Bridge
• Repeaters lead to identical traffic on the connected
segments.
• Bridges reduce the amount of traffic on the
segments, freeing up the transmission line for
increased traffic provided a reasonable amount of
the traffic is intra-segmental (within a segment).
– Increases throughput
• When designing a multi-segmented network, one
wants to maximize intra-segmental
communication.
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Bridge
• Recall that error-checking takes place at the
data-link layer.
• Consequently, a bridge transmits only
packets thought to be error free.
• Collisions, noise, interference are not
transmitted across a bridge.
– Repeaters do transmit error ridden packets.
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Bridge vs Router
• A restriction on a bridge is that the
connected segments utilize the same
protocol.
• A router serves a somewhat similar purpose
but acts at a higher level (the network layer)
and is “more intelligent.”
• Occasionally a bridge and router are
combined in a product called a brouter.
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Long-distance bridge
• One can combine the ideas of the fiber
extension and the bridge to achieve a bridge
that extends over a long distance.
• The computer connected to the first LAN
(segment) is given a second NIC card so
that it can serve as a bridge to a second
LAN (segment).
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Long-distance bridge (Fig. 11.6)
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Even Greater Distances (Fig. 11.7)
2 half bridges
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Bridged Networks
• A bridged network does not necessarily just
form a long line with the end of one
segment bridged to the end of another
segment and a given segment bridged to at
most two others.
B
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B
B
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Bridged Networks (Cont.)
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Possible Problem
Segment X
Bridge 1
Bridge 2
Segment Y
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Possible Problem (Cont.)
• If a node on Segment X unicasts a message to a
node on Segment Y, then (in steady state) Bridge 1
will forward it to Segment Y as will Bridge 2. It
will arrive twice.
• If a node on Segment X broadcasts a message,
then Bridge 1 will forward it to Segment Y, then
the message will reach Bridge 2 and be forwarded
to Segment X, where it will reach Bridge 1 and be
forwarded to Segment Y, where ….
• We have an infinite loop (actually two counter
rotating infinite loops).
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Logical vs. Physical again
• Physically, loops are good because they can
provide a backup route should one route
fail.
• Logically, loops are bad, they lead to an
infinite cycling of messages.
• So long as the network is logically loop-less
(that is, a tree), it is OK.
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STP
• Spanning Tree Protocol (part of the IEEE 802.1
standard) allows for a bridged network that has
physical loops but is logically a tree.
• STP puts the bridges that lead to a loop into a
standby or blocked state (forming a logical tree).
• However, it stores alternate logical trees “in the
event that one bridge is unable to perform its
duties”
– Path redundancy
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STP (Cont.)
• Of all the logical trees, one wants the best, that is
the cheapest.
• There will be some “cost function” which will
depend on the throughput of the various
connections, the typical traffic patterns on those
connections and so on.
• STP will select the “minimal” tree, but that could
change, which is another reason that a blocked
bridge may later be activated.
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May the circle be unbroken
• Similar to the way one can have more than one
logical tree in case a bridge goes out, one can have
various logical rings in case a connection within
the ring is broken.
• If the network’s physical topology is a ring and it
is broken, then it goes down.
• However, if the network’s physical topology is a
star but it’s logical topology is a ring, then a new
logical ring can be formed should a connection or
node go down.
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FDDI Hub
• FDDI Hub contains the electronic circuitry
necessary to detect a broken link and
reconfigure the network.
• The FDDI logical network topology is a
ring, but the physical topology is star.
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Switch
• A switch is an intelligent hub.
• The hub operates at the physical layer,
forwarding an incoming signal to all other ports.
• A switch operates at the data-link layer,
forwarding a (unicast) message only to the
designated port.
• A switch is like a many-ported bridge with only
one computer on each segment.
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Switch vs. Hub
• For a given message a hub should be faster.
• With increased traffic, switches should
improve throughput, like bridges different
signals can be simultaneously transmitted
on the various segments.
– It allows different messages to be transmitted
“in parallel” — a given message is still sent
serially.
• Hubs are cheaper.
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Switch vs. Router
• Routers operate at a higher layer (the
network layer), so they are:
– more “intelligent” improving throughput for
heavier traffic
– slower for handling a given packet
– More expensive
• There’s an intermediate device known as an
“IP switch”
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Other references
• http://www.whatis.com
• http://www.webopedia.com
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