Extending LANs

1
Extending LANs

• Based on Chapter 11 in Computer Networks and
  Internets

2
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)

3
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.

4
Reason Two

• Sharing Speed
• The longer a token ring (and the more computers
  on it), the longer it takes before it is a given
  computers 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).

5
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.

6
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.

7
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.
8
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.

9
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.

10
Its possible to clean up a digital signal
A noise ridden digital signal.
11
Its possible to clean up a digital signal
A cleaned-up digital signal.
12
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.

13
Repeater (Fig. 11.2)
14
Repeaters

• Repeaters allow the LAN size to be extended.
• But the repeater solves only the
  attenuation-interference 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.

15
Multiple Repeaters (Fig. 11.3)
16
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 packets 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.

17
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.

18
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 packets source address,
  the bridge knows whether the packets must be
  forwarded or not.

20
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.

21
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.

22
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.

23
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.

24
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)
26
Even Greater Distances (Fig. 11.7)
2 half bridges
27
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
B
B
28
Bridged Networks (Cont.)
29
Possible Problem
Segment X
Bridge 1
Bridge 2
Segment Y
30
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).

31
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.

32
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

33
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.

34
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 networks physical topology is a ring and
  it is broken, then it goes down.
• However, if the networks physical topology is a
  star but its logical topology is a ring, then a
  new logical ring can be formed should a
  connection or node go down.

35
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.

36
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.

37
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.

38
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
• Theres an intermediate device known as an IP
  switch

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Other references

• http//www.whatis.com
• http//www.webopedia.com