Chapter 11 Extending LANs 1. Distance limitations of LANs 2. Connecting multiple LANs together 3. Repeaters 4. Bridges 5. Filtering frame 6. Bridged network 7. Broadcast cycles 8. Switching and Switched LANs Note: Sections 11.3, 11.8, 11.9, & in the text will not be covered

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?

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

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

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

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

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

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

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

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

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

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

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

Filtering example

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

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

Bridges and cycles

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 –B 1 & B 2 forward frame to segments b and c respectively –B 4 forward frame to d; B 3 forward another copy of the frame to d –Segment d received 2 copies –B 4 forward B 3 ’s copy to segment c; B 3 forward B 4 ’s copy to segment b –… and the cycle continues

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

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

Switching

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

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