Building Ethernet LANs with Switches

Building Ethernet LANs with Switches

1. HUB

   • When receive an electrical signal in one port,repeats the signal out all other ports
   • When two or more devices send at the same time, an electrical collision occurs, making both signals corrupt
   • devices must take turns by using carrier sense multiple access with collision detection (CSMA/CD) logic

2. Bridge

   • Separated devices into groups called collision domains.
   • Reduced the number of collisions that occurred in the network, because frames inside one collision domain did not collide with frames in another collision domain.
   • Increased bandwidth by giving each collision domain its own separate bandwidth, with one sender at a time per collision domain.
   • Buffer or queue the frame until the outgoing interface can send the frame

3. Switches

   • Like bridges, switches segment a LAN into separate collision domains, each with its own capacity
     • Switching Logic
       • Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address
       • Learning MAC addresses by examining the source MAC address of each frame received by the switch.
       • Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)
   • How Switches Learn MAC Addresses
     • Listening to incoming frames and examining the source MAC addres in the frame.
       • Flooding Frames
         • When there is no matching entry in the table, switches forward the frame out all interfaces (except the incoming interface)
     • Inactivity timer
   • Avoiding Loops Using Spanning Tree Protocol
     • Without STP, any flooded frames would loop for an indefinite period of time in Ethernet networks with physically redundant links
       • Blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments.
   • Switch Internal Processing
     • Switching Method
       • Fragment-free
         • The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.
       • Cut-through
         • The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.
       • Store-and-forward
         • The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.