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Module 4

Module 4. Introduction to LAN Switching. Objectives. LAN congestion and its effect on network performance Advantages of LAN segmentation in a network Advantages and disadvantages of using bridges, switches, and routers for LAN segmentation

benedict-frost
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Module 4

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  1. Module 4 Introduction to LAN Switching

  2. Objectives • LAN congestion and its effect on network performance • Advantages of LAN segmentation in a network • Advantages and disadvantages of using bridges, switches, and routers for LAN segmentation • Effects of switching, bridging, and routing on network throughput Fast Ethernet technology and its benefits

  3. CSMA/CD prevents multiple devices from transmitting at the same time.

  4. The Ethernet/802.3 Interface • Ethernet is known as a shared-medium technology – all the devices are connected to the same delivery media. • Ethernet media uses a data frame broadcast method of transmitting and receiving data to all nodes on the shared media.

  5. Standard Ethernet using Carrier Sense Multiple Access/ Collision Detection (CSMA/CD) and a shared medium can support data transmission rates of up to 10 Gbps (gigabits per second). • Goal of Standard Ethernet is to provide a best effort delivery service and allow all devices on the shared medium to transmit on an equal basis.

  6. Performance of a shared media Ethernet/802.3 LAN can be negatively effected by several factors. • The data frame broadcast delivery nature of Ethernet/802.3 LANs • CSMA/CD access methods allow only one station to transmit at a time. • Network congestion due to increased bandwidth demands from multimedia applications such as video and the Internet. • Normal latency (propagation delay) of frames as they travel across the LAN layer 1 media and pass through layer 1, 2 and 3 networking devices. • Extending the distances of the Ethernet/802.3 LANs using Layer 1 repeaters.

  7. EthernetController EthernetController Transmit Tx Tx Loopback CollisionDetection Loopback CollisionDetection Receive Rx Rx Ethernet NIC Ethernet NIC Half-Duplex Design • Ethernet physical connector provides several circuits • Most important are TX (transmit), RX (receive), and CD (collision detection)

  8. Half-Duplex Ethernet Design (Standard Ethernet) • The most important of these circuits are the receive (RX), transmit (TX), and CD (collision detection0. • The transmit (TX) circuit is active at the transmitting station. • The receive (RX)circuit is active at the receiving station.

  9. To the network this appears as a single one way bridge. • Both devices are contending for the right to use the single shared medium. • The CD (collision detection) circuit on each node contends for the use of the network when the two nodes attempt to transmit at the same time. • After a collision occurs, the hosts will resume transmitting based on the hold time calculated by a back-off algorithm. • Then the host will determine if the network is clear before attempting to retransmit.

  10. Congestion and Bandwidth • To relieve network congestion more bandwidth is needed or the available bandwidth must be used more efficiently. • “Throwing bandwidth at the problem”. This could be attacking the symptom and not the problem.

  11. Propagation Delay • Latency is also known as Propagation delay is the time a frame or packet requires to travel from the source to destination on the network. • The greater the number of devices the greater the latency or propagation delay • adding hosts simply increases collisions, increases jam signals decreasing throughput .

  12. Ethernet Transmission Times • Each Ethernet bit uses a 100ns window for transmission. • A byte is equal to eight bits. • Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns).

  13. A 64 byte frame requires 51,200ns or 51.2 microseconds to transmit • 1 Ethernet bit uses a 100ns window for transmission. • A byte is equal to eight bits. • Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns). • 1 microsecond = 1000 nanoseconds • 1 byte = 8 bits • (64 bytes at 800ns) = 51,200ns • (51,200ns/1000) = 51.2 microseconds).

  14. Extending Shared Media LANs using Repeaters • Signal attenuation –Signal weakens as they travels through the network due to resistance in the medium. • A repeater is used to extend the geography of a LAN allowing more users to share that same network.

  15. Improving LAN Performance • The performance of a network can be improved in a shared media LAN by: • Segmenting the network using bridges, routers, or switches • Using full duplex transmitting • Upgrade to a faster Ethernet standard

  16. Segment LANs? • Each segment uses the (CSMA/CD) protocol to manages traffic on the segment. • By segmenting a network - less devices are sharing the same bandwidth • Each segment is its own collision domain.

  17. Segmented LANs? • In a segmented Ethernet LAN messages passed between segments is transmitted on a network backbone using a bridge, switch, or router. • The backbone network is its own collision domain and uses CSMA/CD to manage between segments.

  18. Segmentation with Bridges • Bridges are Layer 2 devices, independent of Layer 3 protocols used by routers • they transmit data frames regardless of which Layer 3 protocol is being used • They are transparent to the other devices on the network. • Bridges increase latency (delay) in a network by 10-30%. Why?

  19. A bridge is by default a store and forward device • It examines the destination MAC address to determine through which interface the frame will be forward. • If there is no match in the CAM table, the frame is flooded out all other interfaces • Bridges “learn” network segments by building an address table, a CAM (Content Address Memory), containing the (MAC) address of each network device that accesses the bridge and pairs it with its network segment. • Collision domains are created, not broadcast domains.

  20. Segmentation using Routers • Routers operate at network layer and base routing decisions on the Layer 3 IP protocol address. • Routers perform higher level functions than do bridges consequently they operate at a higher latency.

  21. Routers? • Segment broadcast domains • Forward packets based on destination network layer addresses, i.e. IP • Segment collision domains

  22. More collision domains, but more bandwidth for each user

  23. Segmentation with LAN Switches • A switch segments a LAN into microsegments creating collision free domains from one larger collision domain, not broadcast domains. • Switched Ethernet available bandwidth can reach close to 100%.

  24. LAN Switch Latency • Each switch on an Ethernet LAN adds latency to the network. • The type of switching used can help overcome the built in latency of some switches.

  25. Full-Duplex Ethernet Overview • Full duplex Ethernet allows the transmission of a packet and the reception of a packet at the same time. • Requires two pairs of conductors and a switched connection between each node

  26. Simultaneous transmission and reception of frames is called bidirectional traffic (two-way) and on a 10Mbps circuit yields 20Mbps of throughput. • The network interface cards (NICs) on both ends of the circuit require full duplex capabilities.

  27. Full DuplexEthernetController TX Full DuplexEthernetController Tx Tx CollisionDetection CollisionDetection Loopback Loopback RX Rx Rx Full-DuplexEthernet Design • Transmit circuit connects directly to receive circuit • No collisions • Significant performance improvement • Eliminates contention on Ethernet point-to-point links • Uses a single port for each full-duplex connection

  28. HUB Using Full Duplex Half Duplex Full Duplex • Nodes must • Be directly attached to a dedicated switched port • Have installed network interface card that supports full duplex

  29. Full-Duplex Ethernet Design Standard Ethernet normally can only use 50-60% of the 100Mbps available bandwidth. • This is due to collisions and latency. • Full duplex Ethernet offers 100% of the bandwidth in both directions. • This produces a potential 200Mbps throughput – 100Mbps TX and 100Mbps RX.

  30. This virtual network circuit exists only when two nodes need to communicate this circuit is established within the switch. • It Allows multiple users to communicate in parallel via these virtual circuits.

  31. Source MAC address is used to build this table

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