1 Fall 2005 Extending LANs Qutaibah Malluhi CSE Department Qatar University Repeaters, Hubs,...
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Transcript of 1 Fall 2005 Extending LANs Qutaibah Malluhi CSE Department Qatar University Repeaters, Hubs,...
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Fall 2005
Extending LANs
Qutaibah MalluhiCSE DepartmentQatar University
Repeaters, Hubs, Bridges, Fiber Modems, and Switches
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LAN Distance Limitation
LAN technologies are designed with constraints of speed, distance and costs
Typical LAN technology can span, at most, a few hundred meters
Why limit the distance?– Length of medium affects fair, shared access to
medium » CSMA/CD - delay between frames, minimum frame length » Token passing - circulation time for token
– Length of medium affects strength of electrical signals and noise immunity
How can a network be extended to cover longer distances; e.g., the QU campus?
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LAN Extension
Most techniques use additional hardware to relay LAN signals between LAN segments
Resulting mixed technology– Must not violate design assumptions– Often part of original design
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Repeaters
May want to extend LAN medium – Ethernet - timing constraints allow longer medium – Signal strength constraints limit length
Repeater - bidirectional, analog amplifier that retransmits analog signals
Simply copy signals between segments – Do not understand frame formats – Do not have hardware addresses
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Ethernet Repeater
One repeater can effectively double the length of an LAN segment
Any Ethernet segment is limited to 500 meters Repeater can double to 1,000 meters
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Repeaters Limitations
Can't extend Ethernet with repeaters indefinitely CSMA/CD requires low delay; if medium is too long,
CSMA/CD won't work Ethernet standard includes limit of 4 repeaters between
any two Ethernet stations – With four repeaters, can extend Ethernet through a building
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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
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Hubs Physically
– Small electronic device– Has connections for several computers (e.g., 4 or 20)
Logically– Operates on signals– Propagates each incoming signal to all connections– Similar to connecting segments with repeaters– Does not understand frames
Extremely low cost Considering the Ethernet 4 repeater limitation, a
hub counts as a repeater
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Bridges
Connect two LAN 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 Relatively easy to use - just plug in Does not forward noise or collisions
– Isolate collisions, noise
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Frame Filtering
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 multicast packets
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
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Setting up Bridge Forwarding Table
Bridge examines source address in each frame Adds entry to list for LAN segment from which
frame was received Must forward any frame whose destination is not
in the list on every interface Startup bridge behavior
– Initially, the forwarding tables in all bridges are empty – 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 – May result in burst of traffic after, e.g., power failure
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Filtering Example
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Design with Filtering Bridges
Filtering bridge allows concurrent use of different LAN segments if traffic is local
U and V can exchange frames at the same time X and Y exchange frames
Can improve performance by careful design– Designers identify patterns of local communication and
isolate groups of communicating computers with bridges
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Bridging Between Buildings
Similar to extending AUI with fiber modems Can put bridge in one building with long
connection to LAN segment in different building
Avoids the costly extended AUI connection for each computer in remote building
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Cycles Of Bridges
Complex bridge connections may not be apparent– Adding one more bridge may inadvertently introduce a cycle– A circular path through bridged networks is called a cycle
Consider broadcasting when a cycle of bridges exists
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Breaking Bridge Cycles
Bridges must cooperate to broadcast frames exactly once on each segment
Use a Distributed Spanning Tree (DST) algorithm– Used to determine which bridges will forward broadcasts– Results in a unique path from each source to each
destination– Used by all bridges to
» Discover one another» Break cycle(s)
As each bridge joins the network, it communicates with other bridges on special hardware (typically multicast) address – Learns network topology – Performs spanning tree computation – Determines if bridge will form a cycle
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Spanning Tree Example
Initial logical broadcast topology
Broadcast topology after running the DST algorithm
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Fiber Optic Extensions
Can extend connection to a computer using fiber optic cable – Insert fiber modems and fiber optic cable into AUI cable
Optical fiber– Has low delay– Has high bandwidth
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 - different buildings (see next slide)
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Bridging Between Buildings
Similar to extending AUI with fiber modems Can put bridge in one building with long
connection to LAN segment in different building
Avoids the costly extended AUI connection for each computer in remote building
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Bridging Over Long Distances
Can use leased line, microwave, laser or satellite to connect two bridges and LAN segments
Using two bridges instead of one – Why? – WAN links are expensive and usually slow
» T1: 1.54 Mbps» Satellite 56 Kbps
– Filters at both ends, reducing traffic across slow link – Provides buffering at both ends, matching dissimilar
speeds
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Switches
Effectively a separate LAN segment for each port
Physically similar to a hub– hub shares single segment among all port
Logically similar to a bridge– Operates on packets– Understands addresses– Only forwards when necessary
With switching, multiple stations can transmit simultaneously
Provides much higher aggregate bandwidth
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Switches And Hubs
Switches are more expensive per port May make more sense economically to use hubs
for some stations and switches for others
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Summary LANs
– Have distance limitations– Can be extended
Optical fiber and modems can be used to extend AUI for single station
Repeater– Connects two LAN segments– Repeats and amplifies all signals– Forwards noise and collisions
Bridge– Understands and forwards entire frames– Uses addresses– Does not forward noise or collisions– Filtering bridges forward frames only as needed
» Allows simultaneous transmission on the segments – Therefore, careful design can improve the performance.
» Forwards all broadcast and multicast frames– DST used to break cycles
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Summary (cont’d)
Hub– Central facility in star-shaped network– Operates like a repeater
Switch– Central facility in star-shaped network– Operates like a set of bridged segments– More expensive that a hub but provide higher aggregate
bandwidth