1 Fall 2005 Extending LANs Qutaibah Malluhi CSE Department Qatar University Repeaters, Hubs, Bridges, Fiber Modems, and Switches

2 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?

3 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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5 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

6 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

7 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

8 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

9 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

10 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

11 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

12 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

13 Filtering Example

14 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

15 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

16 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

17 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

18 Spanning Tree Example Initial logical broadcast topology Broadcast topology after running the DST algorithm

19 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)

20 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

21 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

22 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

23 Switches And Hubs  Switches are more expensive per port  May make more sense economically to use hubs for some stations and switches for others

24 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

25 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