9 extending lans fibre modems repeaters bridges and switches
TRANSCRIPT
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 1/30
11/11/2011 1
Data Communications & Networking: DIT 230
Extending LANs: Fibre Modems, Repeaters,Bridges and Switches
UNIT: 09
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 2/30
11/11/2011 2
Introduction
MAC protocols such as CSMA/CD require
time proportional to the length of the cable
Electrical signal weakens with distance
Network designers choose a combination of
capacity, delay and distance that can beachieved within a given cost hence:
LAN technologies are designed with
constraints of speed, distance and costs.
Typical LAN technology can span, at most, a
few hundred meters.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 3/30
11/11/2011 3
LAN Design for Distance
LANs use shared medium - Ethernet, token
ring
Length of medium affects fair, shared accessto medium
CSMA/CD - delay between frames, minimum
frame length
Token passing - circulation time for token
Length of medium affects strength of electricalsignals and noise immunity
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 4/30
11/11/2011 4
LAN Extensions
Several techniques extend diameter of LAN
medium
Most techniques use additional hardware LAN signals relayed between LAN segments
Resulting mixed technology stays withinoriginal engineering constraints while
spanning greater distance
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 5/30
11/11/2011 5
Fiber Optic Extensions
Fiber modems extend connection betweencomputer and transceiver
Can extend connection to a computer using
fiber optic cable
Fiber modems:
Convert AUI signals to digital signal Transmit digital signals via fiber optic cable
to other modem
Most often used to connect two LANs -
typically through a bridge on different
buildings
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 6/30
11/11/2011 6
Example of Fibre Modem Connections
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 7/30
11/11/2011 7
Repeaters
We may want to extend the LAN medium:
Ethernet - timing constraints allow longer
medium Signal strength constraints limit length
Repeater – a bidirectional, analog amplifier that retransmits analog signals.
One repeater can effectively double the
length of an LAN segment.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 8/30
11/11/2011 8
Figure below shows
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 9/30
11/11/2011 9
Ethernet Repeaters
Simply copy signals between segments
Do not understand frame formats
Do not have hardware addresses
Any Ethernet segment is limited to 500
meters Repeater can double to 1,000 meters
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 10/30
11/11/2011 10
Limits on Repeaters
• Can't extend Ethernet with repeaters
indefinitely.
• CSMA/CD requires low delay; if mediumis too long, CSMA/CD won't work.
• Ethernet standard includes limit of 4repeaters between any two Ethernet
stations.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 11/30
11/11/2011 11
Cont’d
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 12/30
11/11/2011 12
Characteristics of Repeaters
Very easy to use - just plug in
Repeaters simply re-transmit analog signals
Collisions affect entire network Transient problems - noise - propagates
throughout network
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 13/30
11/11/2011 13
Network Bridges
A network bridge is a device that connectsmultiple network segments (Also connect twoLAN segments)
Retransmits frames from one segment on other segment(s)
Handles complete frame
Uses NIC like any other station
Performs some processing on frame
Invisible to other attached computers
Bridging is most suitable to be used for LANnetwork traffic data load.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 14/30
11/11/2011 14
Advantages of Network Bridges
Bridges are simple to use and they are
relatively inexpensive.
Bridges have simple configuration modes
Bridges help to lower the data load over the
data link layer. Bridges are more reliable if one wants to
lower the bandwidth utilization.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 15/30
11/11/2011 15
Disadvantages of Network Bridges
All bridges are unable to read specific IP
address
Bridges cannot help to build a communicationnetwork between the networks of different
architectures
Unable to handle more complex and variable
data load such as occurring from WAN.
Bridges transfer all types of broadcastmessages
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 16/30
11/11/2011 16
Bridged LAN Segments
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 17/30
11/11/2011 17
Filtering Bridges
Bridges can do additional processing:
Don't forward collisions, noise
Only forward frames where necessary Bridge performs frame filtering and forwards
frames along LAN segments to destination
Learns location of stations by watching
frames
Forwards all broadcast and multicastpackets
This kind of bridge is also called a transparent
bridge, since it requires no special setup.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 18/30
11/11/2011 18
Frame Filtering
Bridge checks destination of each incoming
frame
Looks up destination in list of known stations Forwards frame to next interface on path to
destination
Doesn't forward frame if destination on
LAN segment from which frame was
received
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 19/30
11/11/2011 19
How Does A Bridge Set Up Its Forwarding Table?
A bridge examines the source address in
each arriving frame.
It adds an entry to the list for the LANsegment from which the frame was received
(essentially recording from which “direction”
the frame arrived).
The bridge forwards each frame to each
interface that does not have the destinationaddress in its list.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 20/30
11/11/2011 20
Filtering Example
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 21/30
11/11/2011 21
Startup Behavior of Filtering Bridges
Initially, the forwarding tables in all bridgesare empty.
The first frame from each station on LAN is
forwarded to all LAN segments.
After all stations have been identified, frames
are only forwarded as needed. This behavior may result in a burst of traffic
after some events like power failures.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 22/30
11/11/2011 22
Designing with Filtering Bridges
• Filtering bridge allows concurrent use of different LAN segments if traffic is local.
• U and V can exchange frames at the sametime X and Y exchange frames.
• Designers identify patterns of local
communication and isolate groups of communicating computers with bridges.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 23/30
11/11/2011 23
Bridging Between Buildings
• This is similar to extending AUI with fiber modems.
• We can put a bridge in one building with a long
connection to a LAN segment in a differentbuilding.
• This avoids an extended AUI connection for each computer in the remote building.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 24/30
11/11/2011 24
Bridging Across Longer Distances
• Can use leased line, microwave, laser or
satellite to connect two bridges and LAN
segments• Using two bridges instead of one:
– Filters at both ends, reducing traffic across slow link
– Provides buffering at both ends, matching dissimilar
transmission speeds
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 25/30
11/11/2011 25
Bridges and Cycles• We can use multiple bridges to interconnect
many LAN segments.
• Stations on segment c sends frames tostations on segment g through B2, B1, B3and B6
• Broadcasts are forwarded through all bridges.
• Suppose another bridge connects g and f ?
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 26/30
11/11/2011 26
Cycles
• A circular path through bridged networks iscalled a cycle
• Adding B4 creates a cycle
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 27/30
11/11/2011 27
Eliminating Broadcast Cycles
Bridges must cooperate to broadcast framesexactly once on each segment.
The solution is from graph theory - spanning
trees - used to determine which bridges willforward broadcasts.
As each bridge joins the network, itcommunicates with the other bridges usingspecial hardware (typically multicast)addresses
The set of bridges learn the network topology;
performs the spanning tree computation; and
determines if the new bridge will result in a cycle.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 28/30
11/11/2011 28
Switching
A network switch is a small hardware device that joins multiple computers together within one localarea network (LAN).
• Effectively a separate LAN segment for each port.
• Similar to hub - hub shares single segment amongall ports.
• With switching, multiple stations can transmit
simultaneously.• Switching provides much higher aggregate
bandwidth.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 29/30
11/11/2011 29
Switches and Hubs
Network switches inspect data packets asthey are received, determining the source and
destination device of that packet, and
forwarding it appropriately.
A hub sends all packets out through all ports
A switch only sends the packet a packet outthrough the port that actually needs it. Cuts
down on broadcast traffic.
A switch is effectively a higher-performancealternative to a hub.
8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches
http://slidepdf.com/reader/full/9-extending-lans-fibre-modems-repeaters-bridges-and-switches 30/30
11/11/2011 30
Adv & Disad of Switches
Advantages : Reduces the number of Broadcast domains
Supports VLAN's
Intelligent device Compared to Bridges ,Switches are more H/w oriented
therefore operations are less CPU intense [Basicoperations]
Disadvantages: Not as good as a router in limiting Broadcasts
Handling Multicast packets needs quite a bit of
configuration & proper designing. At times switches when in Promiscuous mode is a
opening for Security attacks [Spoofing Ip address or capturing Ethernet Frames using ethereal]