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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

Module 4

Introduction to LAN Switching



  • 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

Module 4

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

The ethernet 802 3 interface

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.

Module 4

  • 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.

Module 4

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.

Module 4













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

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.

Module 4

  • 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

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

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

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).

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  • 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

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

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

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

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

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%.


Module 4

  • 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

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.



  • Segment broadcast domains

  • Forward packets based on destination network layer addresses, i.e. IP

  • Segment collision domains

Module 4

More collision domains, but more bandwidth for each user

Segmentation with lan switches

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

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 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

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  • 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.

Module 4

Full DuplexEthernetController


Full DuplexEthernetController










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

Module 4


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

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.

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  • 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.

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Source MAC address is used to build this table

How a lan switch learns addresses

How a LAN Switch Learns Addresses

  • When messages are received by the switch their:

    • addresses are read and stored in the CAM (Content Address Memory).

  • Each time an address is stored it is time stamped.

    • This allows addresses to be stored for a set period of time.

Module 4

But more domains

Benefits of switching

Benefits of Switching

  • A LAN switch allows many users to communicate in parallel :

    • through the use of virtual circuits

    • dedicated network segments

    • in a collision free environment.

  • Very cost effective.

Symmetric switching

Symmetric Switching

  • A symmetric switch is optimized through even distribution of network traffic across the entire network .

  • All networks using the same bandwidth.

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before forwarding

Asymmetric switching

Asymmetric Switching

  • Asymmetric switching is best exemplified in client-server network traffic flows where multiple clients are simultaneously communicating with a server.

    • Each usually at a lower bandwidth than the server

Memory buffering

Memory Buffering

  • The area of memory where the switch stores the destination and transmission data is called the memory buffer.

  • This memory buffer can make use of two methods for forwarding packets:

    • port based memory buffering

    • shared memory buffering.

Module 4

  • Port based memory buffering packets are stored in queues that are linked to specific incoming ports.

    • Problem: One port may fill while another is empty.

  • Shared memory buffering deposits all packets into a common memory buffer that is shared by all the ports on the switch. (Better!)

3 frame transmission modes in a switch one variation

3 frame transmission modes in a switch(+one variation)

Three switching methods

Three Switching Methods

  • Store and Forward - the entire frame is received before any forwarding takes place.

    • Latency occurs while the frame is being received; the latency is greater with larger frames because the entire frame takes longer to read.

    • Error detection is high because of the time available to the switch to check for errors while waiting for the entire frame to be received.

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  • Cut-through the switch reads the destination address before receiving the entire frame.

    • The frame is then forwarded before the entire frame arrives.

    • This mode decreases the latency of the transmission and has poor error detection.

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Fragment-Free Switching

The switch reads only the 1st 64 bytes of the incoming frame and then forwards the frame to its destination port

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Means the switch is in cut through mode

Adaptive cut through

Adaptive Cut Through

  • Combines cut through with store and forward

    • The switch uses cut-through until there are a given number of errors

    • Then the switch will change to store and forward method

Module 4

Shared to Switched










The New Backbone

Emerging Trends: The Network Evolution

Module 4

Benefits of Switches

  • Number of collisions reduced

  • Simultaneous, multiple communications

  • High-speed uplinks

  • Improved network response

  • Increased user productivity

Module 41

Module 4

Switching Concepts


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