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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

Switching Concepts

Introduction to Ethernet/802.3 LANs

Introduction to LAN Switching

Switch Operation

switching concepts1

Switching Concepts

Introduction to Ethernet/802.3 LANs

ethernet technology overview
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
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!
slide5
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
bridges
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
switches
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
elements of ethernet 802 3 networks
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.
transmission time latency
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
types of transmission
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
switching concepts2

Switching Concepts

Introduction to LAN Switching

Switch Operation

overview
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
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 with bridges
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
lan segmentation switches
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
lan segmentation with routers
LAN Segmentation With Routers
  • More manageable, greater functionality, multiple active paths
  • Smaller broadcast domains
  • Operates at Layer 3
lan switch operation 1
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
lan switch operation 2
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
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
Terminology
  • Ignoring a frame – filtering
  • Copying a frame – forwarding
  • Microsegmentation – dividing a network into smaller segments (using a switch)
broadcasts
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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