Module 4
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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

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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

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.













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%.


  • 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.


  • Segment broadcast domains

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

  • Segment collision domains

More collision domains, but more bandwidth for each user

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


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


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.

Source MAC address is used to build this table

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.

But more domains

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

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

  • All networks using the same bandwidth.

before forwarding

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

  • 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.

  • 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)

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.

  • 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.

Fragment-Free Switching

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

Means the switch is in cut through mode

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

Shared to Switched










The New Backbone

Emerging Trends: The Network Evolution

Benefits of Switches

  • Number of collisions reduced

  • Simultaneous, multiple communications

  • High-speed uplinks

  • Improved network response

  • Increased user productivity

Module 4

Switching Concepts


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