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Switching Concepts

Switching Concepts. Introduction to Ethernet/802.3 LANs Introduction to LAN Switching Switch Operation. Switching Concepts. Introduction to Ethernet/802.3 LANs. Ethernet Technology Overview. Ethernet multi-access broadcast technology Uses CSMA/CD Collisions impact on network performance

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Switching Concepts

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  1. Switching Concepts Introduction to Ethernet/802.3 LANs Introduction to LAN Switching Switch Operation

  2. Switching Concepts Introduction to Ethernet/802.3 LANs

  3. Ethernet Technology Overview • Ethernet multi-access broadcast technology • Uses CSMA/CD • Collisions impact on network performance • Layer 2 devices can improve performance • Media includes CAT5(e), fibre, wireless • Speeds from 10Mbps to 10,000 Mbps

  4. Network Growth • Bandwidth needs have increased • Internet/intranet/email • Multimedia • Increasing use of enterprise servers • Ethernet has developed to meet challenge • 10Mbps, 100Mbps, 1000Mbps, 10Gbit • Coaxial, Twisted Pair, Fibre Optic, Wireless • Repeaters, hubs, bridges, switches, routers • BUT you must understand the features of all this technology to gain best performance in your network design!

  5. Hubs • Layer 1 devices • Regenerate, retime, amplify signals • 1 collision/bandwidth domain • Broadcasts propagated out of every port • Only 1 device can transmit at a time • Only 50-60% bandwidth available

  6. Bridges • Layer 2 device • Splits network into 2 collision/bandwidth domains • Broadcasts are forwarded • Local traffic stays local • Checks Layer 2 MAC addresses in 802.3 frame

  7. Switches • Layer 2 device • Learns MAC addresses of devices attached to each port • Each switchport is a collision domain • More collision domains BUT smaller collision domains • Broadcasts still sent out of every port • Each switchport has dedicated bandwidth • 100% bandwidth available

  8. Transmission Time & Latency • Bit time – time taken to recognise 1 bit • Minimum frame size - 64 bytes – 512 bits • Maximum frame size – 1518 bytes – 12,144 bits • Transmission time is always 512 bit times • 10Mbps – 64 byte frame - 51,200 ns (100ns bit time) • 100Mbps – 64 byte frame 5,120 ns (10 ns bit time) • 1000Mbps – 64 byte frame – 512 ns (1ns bit time) Times above do not include • Time taken to propagate signal along medium • Delays introduced by hubs/switches/routers/NICs etc

  9. Types of Transmission Half-duplex • Host checks medium for signal – if clear host transmits • Only 1 host can transmit at a time • Collisions – jam signal generated, back-off algorithm before retransmission • 50-60% bandwidth available Full duplex • Host can transmit immediately • 2 hosts can transmit simultaneously • No collisions • 100% bandwidth available • Requires dedicated connection to a switchport

  10. Switching Concepts Introduction to LAN Switching Switch Operation

  11. Overview • Maximum availability for the least cost • Reduce the effects of collisions on available bandwidth • Reduce the effect of broadcasts on available bandwidth • Deploy network hardware (media/switches/routers) to overcome bottlenecks & meet bandwidth requirements

  12. LAN Segmentation - bridges • A bridge splits a LAN into 2 segments • It creates 2 collision domains • Adds 10-30% latency • Learns MAC addresses • Keeps local traffic local • Forwards broadcasts

  13. LAN Segmentation - Switches • Each switchport is a collision domain – “micro-segmentation” • 100% bandwidth available to each switchport • Every switchport can send/receive simultaneously • Host to switch/switch to switch connection creates full duplex link

  14. LAN Switch Operation - 1 • When a switch starts up it sends a broadcast out of all ports to learn host MAC addresses • When a frame is received for an unknown destination a broadcast is sent to discover • Addresses are added to a switching table mapping them to the port on which they were learned • When a frame is received for a known destination it is switched to the appropriate port

  15. LAN Switch Operation - 2 • Switches contain RAM – known as CAM “Content Addressable Memory” • Stores MAC address table • Used as frame buffer • Used to queue frames in asymmetric switching – switchports operating at different speeds e.g. 10 and 100 Mbps

  16. Switching Methods Cut-through Switching • Fast-forward – as soon as destination address is read switching starts • Fragment-free – after 64 bytes have been received (minimum valid frame size) frame is switched Store & Forward Switching • Entire frame is received before switching Increased Latency

  17. Terminology • Ignoring a frame – filtering • Copying a frame – forwarding • Microsegmentation – dividing a network into smaller segments (using a switch)

  18. Broadcasts • Bridges & switches cannot block layer 2 or layer 3 broadcasts • Adding bridges or switches to a network extends the broadcast domain but creates additional collision domains – a 24 port switch creates 24 collision domains • Routers can inspect layer 3 packets and create broadcast domains – a router with 3 ports creates 3 broadcast domains

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