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

2. Introduction to Ethernet/802.3 LANs
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/
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!

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

7. 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

8. 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
(100ns bit time) yields 10Mbps
( 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

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. Introduction to LAN Switching Switch Operation
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 and switch to switch connections create full duplex links

14. 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

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