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Chapter 11 Extending LANs
1. Distance limitations of LANs2. Connecting multiple LANs together3. Repeaters4. Bridges 5. Filtering frame6. Bridged network7. Broadcast cycles8. Switching and Switched LANs
Note: Sections 11.3, 11.8, 11.9, 11.10 & 11.15 in the text will not be covered
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Introduction
• LAN technologies are designed with constraints of speed, distance and costs
• Typical LAN technology can span, at most, a few hundred meters
• How can a network be extended to cover longer distances?
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LAN design for distance• Many LANs use shared medium - Ethernet, token ring • Length of medium affects fair, shared access to
medium – set max length to ensure no significant delay – CSMA/CD (time to reach every part of Ether) & Token
passing time proportional to network size
• Length of medium affects strength of electrical signals and noise immunity – signal becomes weaker as it travels
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LAN extensions
• Several techniques extend connectivity of LAN medium
• Most techniques use additional hardware to relay LAN signals between LAN segments
• Resulting mixed technology stays within original engineering constraints while spanning greater distance
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Repeaters• A device that joins 2 cables to extend LAN medium
– continuously monitors signal on cables
– amplifies and transmit signal to other cable
• One repeater can effectively double the length of an LAN segment
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Limits on repeaters• A pair of repeaters can be used to connect 3 segments (see fig 11.3 in your text)• 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
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Characteristics of repeaters• Very easy to use - just plug in
• Simply copy signals between segments – Do not understand frame formats
– Do not understand hardware addresses
• Limitations: Repeaters may re-transmit unwanted signals – Collisions affect entire network
– Transient problems - noise - propagates throughout network
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Bridges• Also an electronic device that connects two LAN segments
• Retransmits frames from one segment on other segment(s)
• Handles complete frame
– Performs some processing on frame
– listen to frame traffics on each segment
– verify frame arrived intact; ensure no interference
– forward frame to other segment
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Bridged LAN segments• 2 connected segments behave like a single LAN• computers on segment 1 can send frames to computers in segment 2 and vice versa
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Characteristics of bridges
• Relatively easy to use - just plug in
• more popular than repeaters
• does more than just forwarding frames
• Isolate collisions, noise
• typical bridge may consist of a conventional
computer
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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 – don’t forward frame to next segment if
destination in the same segment
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Frame filtering
• Bridge checks destination of each incoming frame • Looks up destination in list of known stations
– Destination not on same LAN: Forwards frame to next LAN segment on path to destination
– Doesn't forward frame if destination on same LAN segment from which frame was received
• Bridge needs to know location of each computer attached to the LANs that it is connected to
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How does bridge set up table?
• Learns location of stations by listening to frames in promiscuous mode (examines all frames regardless of whether they are unicast, multicast or broadcast frames)
• Bridge examines source address in each frame – Adds entry to list for LAN segment from which frame was
received
• examines destination address in each frame – use destination address to determine whether to forward
frame to next LAN segment
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Filtering example
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Startup behavior of filtering bridges
• 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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Multiple Bridges• Can use multiple bridges to interconnect many LAN segments • Station of segment c sends frames to station on segment g through B2, B1, B3 and B6
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Bridges and cycles
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Broadcast cycles• Broadcasts are forwarded through all bridges
• Recall that bridge use destination address to determine whether to forward frame (see slide 14)
• Bridge always forward a frame sent to a broadcast address
• Problem: Bridged network with cycles may have broadcast cycles.
• Eg, computer on LAN segment a sends a broadcast frame– B1 & B2 forward frame to segments b and c respectively
– B4 forward frame to d; B3 forward another copy of the frame to d
– Segment d received 2 copies
– B4 forward B3’s copy to segment c; B3 forward B4’s copy to segment b
– … and the cycle continues
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Eliminating broadcast cycles• To prevent infinite loops:
– Bridges must cooperate to broadcast frames exactly once on each segment
– prevent bridged network from forming cycles
• Solution from graph theory - spanning tree used to determine which bridges will forward broadcasts
• As each bridge joins the network, it communicates with other bridges on special hardware (typically multicast) address
– Learns network topology
– Performs spanning tree computation (called distributed spanning tree algorithm)
– Determines if bridge will form a cycle
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Switching
• Device that connects to 1 or more computers & allow them to send/receive data• Effectively a separate LAN segment for each port • Similar to hub - hub consists of single box with multiple ports each attach to a computer• Switched LAN: consists of a single electronic device that transfers frames among many
computers
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Switching
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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 • hub simulates a single shared medium
– at most 2 computers can communicate at any time– only one computer can send at a time
• switch simulates a bridged LAN with one computer per segment– parallelism: With switching, multiple stations can transmit
simultaneously – Provides much higher aggregate bandwidth: up to one-half
of the computers can send data simultaneously
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Summary• Repeater acts as amplifier and retransmits analog signals
• Bridge accepts entire incoming frame and retransmits
– Doesn't forward collisions
– Avoids collisions on destination segments
• Filtering bridge forwards frames only as needed
– Allows simultaneous use of LAN segments for local transmission
– Forwards all broadcast and multicast packets
• Switches provide full LAN speed to each port by simulating separate LAN segments