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.

Source MAC address is used to build this table

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.

But more domains

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

More collision domains, but more bandwidth for each user