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

Introduction to Ethernet/802.3 LANs Switching Concepts Introduction to Ethernet/802.3 LANs

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

Network Growth Bandwidth needs have increased Internet/intranet/email Multimedia Increasing use of enterprise servers Ethernet was 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!

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

Bridge 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

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

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

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

Introduction to LAN Switching Switch Operation Switching Concepts Introduction to LAN Switching Switch Operation

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

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

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 and switch to switch connections create full duplex links

LAN Switch Operation - 1 When a switch starts up it sends a broadcast out of all ports to learn the 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

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

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

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

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 inspects layer 3 packets and creates broadcast domains – a router with 3 ports creates 3 broadcast domains