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Improving LAN Performance
• The performance of a network can be improved in a shared media LAN such as Ethernet by using one or more of the following solutions: Segmenting the network using
Bridges, Routers, or LAN Switches Move to full duplex transmitting Upgrade to the Fast Ethernet
Standard
Why Segment LANs?A Cisco Segment• A network can be divided in smaller
units called segments. Each segment uses the (CSMA/CD) protocol and maintains traffic between users on the segment. By using segments in a network less users/devices are sharing the same 10Mbps when communicating to one another within the segment. Each segment is considered its own collision domain.
Why Segment LANs?
• In a segmented Ethernet LAN data passed between segments is transmitted on the backbone of the network using a bridge, switch, or router.
• The backbone network is its own collision domain and uses CSMA/CD to provide a best effort delivery service between segments.
Segmentation with Bridges
• Bridges are different than routers because they are Layer 2 devices, independent of Layer 3 protocols – they pass on data frames regardless of which Layer 3 protocol is being used and are transparent to the other devices on the network.
• Bridges increase the latency (delay)in a network by 10-30%.
• Why?
• A bridge is considered a store and forward device because it must examine the destination address (MAC) field in the frame and determine which interface to forward the frame.
• If there is no match in the table, the frame is flooded out all other interfaces
• Bridges "learn a network’s" segmentation by building address tables that contain the (MAC) address of each network device and which segment to use to reach that device.
• Smaller collision domains are created, not broadcast domains.
Segmentation with LAN Switches
• A switch segments a LAN into microsegments creating collision free domains from one larger collision domain, not broadcast domains.
• With switched ethernet implementation the available bandwidth can reach closer to 100%.
Using Full Duplex
• Node must– Be directly attached to a dedicated
switched port– Have installed network interface card
that supports full duplex
Full Duplex
Half Duplex
HUB
Full-Duplex Ethernet Design
• Standard Ethernet normally can only use 50-60% of the 10Mbps available bandwidth.
• This is due to collisions and latency.
• Full duplex Ethernet offers 100% of the bandwidth in both directions.
• This produces a potential 20Mbps throughput – 10Mbps TX and 10Mbps RX.
• This virtual network circuit exists only when two nodes need to communicate.
• This is why it is called a virtual circuit – it exists only when needed and is established within the switch.
• Allows multiple users to communicate in parallel via these virtual circuits.
How a LAN Switch Learns Addresses
• This means that as new addresses are read they are learned and stored in Content Address Memory (CAM).
• Each time an address is stored it is time stamped.
• This allows addresses to be stored for a set period of time.
Segmentation with Routers
• Routers operate at the network layer and base all of their forwarding decisions between segments on the Layer 3 protocol address.
• Because routers perform more functions than bridges they operate with a higher rate of latency. (Higher than other internetworking devices.)
Routers:
• Segment broadcast domains• Forward packets based on
destination network layer addresses
• Segment collision domains