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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
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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 • Effects of switching, bridging, and routing on network throughput Fast Ethernet technology and its benefits
CSMA/CD prevents multiple devices from transmitting at the same time.
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.
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.
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.
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)
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.
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.
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.
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 .
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).
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).
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.
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
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.
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.
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?
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.
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.
Routers? • Segment broadcast domains • Forward packets based on destination network layer addresses, i.e. IP • Segment collision domains
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%.
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.
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
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.
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
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
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.
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.
