Switching Concepts

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 thick Ethernet or thin Ethernet infrastructures • 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. Today’s LANs Router

9. Elements of Ethernet/802.3 networks • Broadcast data frame delivery of Ethernet/802.3 • The carrier sense multiple access/collision detect (CSMA/CD) method allows only one station to transmit at a time. • Multimedia applications with higher bandwidth demand such as video and the Internet, coupled with the broadcast nature of Ethernet, can create network congestion. • Normal latency as the frames travel across the layers • Extending the distances and increasing latency of the Ethernet/802.3 LANs by using Layer 1 repeaters.

10. 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

11. 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

12. Switching Concepts Introduction to LAN Switching Switch Operation

13. 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

14. 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

15. LAN Segmentation With Bridges • Segmentation provides fewer uses per segment • Bridges store, then forward frames based on Layer 2 addresses (CRC verified) • Layer 3 protocol-independent • Increase latency on the network

16. 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

17. LAN Segmentation With Routers • More manageable, greater functionality, multiple active paths • Smaller broadcast domains • Operates at Layer 3

18. 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

19. 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

20. 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

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

22. 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