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Module 4Module 4

Introduction to LAN SwitchingIntroduction to LAN Switching

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ObjectivesObjectives

LAN congestion and its effect on network LAN congestion and its effect on network performance performance

Advantages of LAN segmentation in a network Advantages of LAN segmentation in a network Advantages and disadvantages of using Advantages and disadvantages of using

bridges, switches, and routers for LAN bridges, switches, and routers for LAN segmentation segmentation

Effects of switching, bridging, and routing on Effects of switching, bridging, and routing on network throughput Fast Ethernet technology network throughput Fast Ethernet technology and its benefits and its benefits

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

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The Ethernet/802.3 InterfaceThe Ethernet/802.3 Interface

Ethernet is known as a shared-medium Ethernet is known as a shared-medium technology – all the devices are connected technology – all the devices are connected to the same delivery media. to the same delivery media.

Ethernet media uses a data frame Ethernet media uses a data frame broadcast method of transmitting and broadcast method of transmitting and receiving data to all nodes on the shared receiving data to all nodes on the shared media. media.

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Standard EthernetStandard Ethernet using Carrier Sense using Carrier Sense Multiple Access/ Collision Detection Multiple Access/ Collision Detection (CSMA/CD) and a shared medium can (CSMA/CD) and a shared medium can support support data transmission ratesdata transmission rates of up to of up to 10 Gbps10 Gbps (gigabits per second (gigabits per second). ).

Goal of Standard EthernetGoal of Standard Ethernet is to provide is to provide a best effort delivery service and allow a best effort delivery service and allow all devices on the shared medium to all devices on the shared medium to transmit on an equal basis.transmit on an equal basis.

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The data frame broadcastThe data frame broadcast delivery nature delivery nature of of Ethernet/802.3 LANs Ethernet/802.3 LANs

CSMA/CD access methods CSMA/CD access methods allow allow only one station only one station to transmit at a time. to transmit at a time.

Network congestion due to increased bandwidth Network congestion due to increased bandwidth demands from multimedia applications such as demands from multimedia applications such as video and the Internet. video and the Internet.

Normal latency Normal latency (propagation delay) of frames as (propagation delay) of frames as they travel across the LAN layer 1 media and they travel across the LAN layer 1 media and pass through pass through layer 1, 2 and 3 networking layer 1, 2 and 3 networking devicesdevices. .

Extending the distancesExtending the distances of the Ethernet/802.3 of the Ethernet/802.3 LANs using LANs using Layer 1 Layer 1 repeaters. repeaters.

Performance of a shared media Performance of a shared media Ethernet/802.3 LAN can be negatively Ethernet/802.3 LAN can be negatively effected by several factors.effected by several factors.

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Half-Duplex Design

Loopback

TxTx

RxRx

CollisionDetection

EthernetController

TxTx

RxRx

CollisionDetection

EthernetController

Transmit

Receive

Loopback

Ethernet NIC Ethernet NIC

• Ethernet physical connector provides several circuits• Most important are TX (transmit), RX (receive), and CD

(collision detection)

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Half-Duplex Ethernet Design Half-Duplex Ethernet Design (Standard Ethernet)(Standard Ethernet)

The The most important of these circuitsmost important of these circuits are are the the receive (RX), transmit (TX), and CD receive (RX), transmit (TX), and CD (collision detection0(collision detection0. .

The The transmit (TX) circuittransmit (TX) circuit is active at the is active at the transmitting station. transmitting station.

The The receive (RX)receive (RX) circuitcircuit is active at the is active at the receiving station. receiving station.

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To the network this appears as a To the network this appears as a single single one way bridgeone way bridge..

Both devices are contending for the Both devices are contending for the right to use the single shared medium.right to use the single shared medium.

The CD (The CD (collision detection) collision detection) circuit on circuit on each node contends for the use of the each node contends for the use of the network when the two nodes attempt to network when the two nodes attempt to transmit at the same time. transmit at the same time.

After a collision occurs, the hosts After a collision occurs, the hosts will resume transmitting based on will resume transmitting based on the hold time calculated by a back-the hold time calculated by a back-off algorithm.off algorithm.

Then the host will determine if the Then the host will determine if the network is clear before attempting network is clear before attempting to retransmit. to retransmit.

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Congestion and Congestion and BandwidthBandwidth

To relieve network congestion To relieve network congestion more bandwidth is needed or the more bandwidth is needed or the available bandwidth must be used available bandwidth must be used more efficiently.more efficiently.

““Throwing bandwidth at the Throwing bandwidth at the problem”. This could be attacking problem”. This could be attacking the the symptomsymptom and not the problem. and not the problem.

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Propagation DelayPropagation Delay

Latency is also known as Propagation Latency is also known as Propagation delay is the time a frame or packet delay is the time a frame or packet requires to travel from the source to requires to travel from the source to destination on the network. destination on the network.

The greater the number of devices the The greater the number of devices the greater the latency or propagation greater the latency or propagation delaydelay

adding hosts simply increases adding hosts simply increases collisions, increases jam signals collisions, increases jam signals decreasing throughput .decreasing throughput .

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Ethernet Transmission TimesEthernet Transmission Times

Each Each Ethernet bitEthernet bit uses a 100ns window uses a 100ns window for transmission. for transmission.

A A bytebyte is equal to eight bits. is equal to eight bits. Therefore, Therefore, one byteone byte takes a minimum of takes a minimum of

800ns to transmit (8 bits at 100ns = 800ns to transmit (8 bits at 100ns = 800ns). 800ns).

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A A 64 byte frame64 byte frame requires 51,200ns or requires 51,200ns or 51.2 microseconds to transmit51.2 microseconds to transmit 1 1 Ethernet bitEthernet bit uses a 100ns window for uses a 100ns window for

transmission. transmission. A A bytebyte is equal to eight bits. is equal to eight bits. Therefore, Therefore, one byteone byte takes a minimum of takes a minimum of

800ns to transmit (8 bits at 100ns = 800ns). 800ns to transmit (8 bits at 100ns = 800ns). 1 microsecond = 1000 nanoseconds1 microsecond = 1000 nanoseconds 1 byte = 8 bits1 byte = 8 bits (64 bytes at 800ns) = 51,200ns(64 bytes at 800ns) = 51,200ns

(51,200ns/1000) = (51,200ns/1000) = 51.251.2 microseconds). microseconds).

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Extending Shared Media LANs Extending Shared Media LANs using Repeatersusing Repeaters

Signal attenuationSignal attenuation –Signal weakens as –Signal weakens as they travels through the network due to they travels through the network due to resistance in the medium. resistance in the medium.

AA repeater repeater is used to is used to extend the extend the geography of a LANgeography of a LAN allowing more users to allowing more users to share that same network. share that same network.

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Improving LAN PerformanceImproving LAN Performance

The performance of a network can The performance of a network can be improved in a shared media be improved in a shared media LAN by: LAN by: SegmentingSegmenting the network using the network using

bridges, routers, or switchesbridges, routers, or switches Using Using full duplexfull duplex transmitting transmitting Upgrade to a Upgrade to a faster Ethernetfaster Ethernet standard standard

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Segment LANs?Segment LANs?

Each segment uses the (CSMA/CD) Each segment uses the (CSMA/CD) protocol to manages traffic on the protocol to manages traffic on the segment. segment.

By segmenting a network - less devices By segmenting a network - less devices are sharing the same bandwidth are sharing the same bandwidth

Each segment is its own collision Each segment is its own collision domain. domain.

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Segmented LANs?Segmented LANs?

In a segmented EthernetIn a segmented Ethernet LAN messages LAN messages passed between segments is transmitted passed between segments is transmitted on a network backbone using a bridge, on a network backbone using a bridge, switch, or router. switch, or router.

The The backbonebackbone network is its own collision network is its own collision domain and uses CSMA/CD to manage domain and uses CSMA/CD to manage between segments. between segments.

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Segmentation with BridgesSegmentation with Bridges

BridgesBridges are Layer 2 devices, independent are Layer 2 devices, independent of Layer 3 protocols used by routers of Layer 3 protocols used by routers they transmit data frames regardless of they transmit data frames regardless of

which Layer 3 protocol is being used which Layer 3 protocol is being used They are transparent to the other They are transparent to the other

devices on the network. devices on the network. Bridges increase latency (delay) in a Bridges increase latency (delay) in a

network by 10-30%. network by 10-30%.

Why?Why?

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A bridge is by default a store and forward device A bridge is by default a store and forward device

It examines the destination MAC address to It examines the destination MAC address to determine through which interface the frame will be determine through which interface the frame will be forward. forward. If there is no match in the CAM table, the frame is If there is no match in the CAM table, the frame is

flooded out all other interfacesflooded out all other interfaces

Bridges “learn” network segments by building an Bridges “learn” network segments by building an address table, a CAM (Content Address Memory), address table, a CAM (Content Address Memory), containing the (MAC) address of each network device containing the (MAC) address of each network device that accesses the bridge and pairs it with its network that accesses the bridge and pairs it with its network segment. segment. Collision domains are created, Collision domains are created, notnot broadcast broadcast

domains.domains.

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Segmentation using RoutersSegmentation using Routers

RoutersRouters operate at network layer and operate at network layer and base routing decisions on the Layer 3 base routing decisions on the Layer 3 IP protocol address. IP protocol address.

Routers perform higher level functions Routers perform higher level functions than do bridges consequently they than do bridges consequently they operate at a higher latency.operate at a higher latency.

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Routers?Routers?

Segment broadcast domainsSegment broadcast domains

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

Segment collision domainsSegment collision domains

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More collision domains, but more bandwidth for each user

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Segmentation with LAN Segmentation with LAN SwitchesSwitches

A switch segments a LAN into A switch segments a LAN into microsegments creating collision free microsegments creating collision free domains from one larger collision domain, domains from one larger collision domain, notnot broadcast domains broadcast domains. .

Switched EthernetSwitched Ethernet available bandwidth available bandwidth can reach close to 100%.can reach close to 100%.

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LAN Switch LatencyLAN Switch Latency

Each Each switchswitch on an Ethernet LAN adds on an Ethernet LAN adds latency to the network. latency to the network.

The type of switching used can help The type of switching used can help overcome the built in latency of some overcome the built in latency of some switches.switches.

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Full-Duplex Ethernet OverviewFull-Duplex Ethernet Overview

Full duplex Ethernet allows the Full duplex Ethernet allows the transmission of a packet and the reception transmission of a packet and the reception of a packet at the same time. of a packet at the same time.

Requires two pairs of conductors and a Requires two pairs of conductors and a switched connection between each nodeswitched connection between each node

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Simultaneous transmission and Simultaneous transmission and reception of frames is called reception of frames is called bidirectional traffic (two-way) and on a bidirectional traffic (two-way) and on a 10Mbps circuit yields 20Mbps of 10Mbps circuit yields 20Mbps of throughput. throughput.

The The network interface cardsnetwork interface cards (NICs) (NICs) on both ends of the circuit require full on both ends of the circuit require full duplex capabilities.duplex capabilities.

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Full-Duplex Ethernet 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

TX

Full Duplex

EthernetController

Loopback

TxTx

RxRx

CollisionDetectionRX

Full Duplex

EthernetController

Loopback

TxTx

RxRx

CollisionDetection

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Using Full Duplex

• Nodes must– Be directly attached to a dedicated

switched port– Have installed network interface card

that supports full duplex

Full Duplex

Half Duplex

HUB

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Full-Duplex Ethernet DesignFull-Duplex Ethernet Design

Standard EthernetStandard Ethernet normally can only use normally can only use 50-60% of the 100Mbps available 50-60% of the 100Mbps available bandwidth. bandwidth. This is due to collisions and latency. This is due to collisions and latency. Full duplex EthernetFull duplex Ethernet offers 100% of offers 100% of

the bandwidth in both directions. the bandwidth in both directions. This produces a This produces a potentialpotential 200Mbps 200Mbps

throughput – 100Mbps TX and throughput – 100Mbps TX and 100Mbps RX.100Mbps RX.

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This virtual network circuit exists only This virtual network circuit exists only when two nodes need to communicate this when two nodes need to communicate this circuit is established within the switch.circuit is established within the switch.

It Allows multiple users to communicate in It Allows multiple users to communicate in parallel via these virtual circuits.parallel via these virtual circuits.

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Source MAC address is used to build this table

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How a LAN Switch Learns How a LAN Switch Learns AddressesAddresses

When messages are received by the When messages are received by the switch their:switch their: addresses are read and stored in the addresses are read and stored in the

CAM (Content Address Memory). CAM (Content Address Memory).

Each time an Each time an address is storedaddress is stored it is time it is time stamped. stamped. This allows addresses to be stored for a This allows addresses to be stored for a

set period of time. set period of time.

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But more domains

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Benefits of SwitchingBenefits of Switching

A A LAN switchLAN switch allows many users to allows many users to communicate in parallel :communicate in parallel : through the use of virtual circuitsthrough the use of virtual circuits dedicated network segments dedicated network segments in a collision free environment. in a collision free environment.

Very cost effective.Very cost effective.

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Symmetric SwitchingSymmetric Switching

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

All networks using the same bandwidth.All networks using the same bandwidth.

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before forwarding

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Asymmetric SwitchingAsymmetric Switching

Asymmetric switchingAsymmetric switching is best is best exemplified in client-server network traffic exemplified in client-server network traffic flows where multiple clients are flows where multiple clients are simultaneously communicating with a simultaneously communicating with a server.server. Each usually at a lower bandwidth than Each usually at a lower bandwidth than

the serverthe server

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Memory BufferingMemory Buffering

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

This This memory buffermemory buffer can make use of two can make use of two methods for forwarding packets:methods for forwarding packets: port basedport based memory buffering memory buffering shared memoryshared memory buffering. buffering.

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Port based memory bufferingPort based memory buffering packets are stored in queues that are packets are stored in queues that are linked to specific incoming ports. linked to specific incoming ports. Problem: One port may fill while Problem: One port may fill while

another is empty.another is empty.

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

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3 frame transmission modes 3 frame transmission modes

in a switchin a switch (+one variation)(+one variation)

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Three Switching MethodsThree Switching Methods

Store and Forward - the entire frame is Store and Forward - the entire frame is received before any forwarding takes received before any forwarding takes place. place. LatencyLatency occurs while the frame is occurs while the frame is

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

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

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Cut-throughCut-through the switch reads the the switch reads the destination address before receiving the destination address before receiving the entire frame. entire frame. The frame is then forwarded before The frame is then forwarded before

the entire frame arrives. the entire frame arrives. This mode decreases the latency of This mode decreases the latency of

the transmission and has poor error the transmission and has poor error detection. detection.

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Fragment-Free SwitchingFragment-Free Switching

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

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Means the switch is in cut through mode

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Adaptive Cut ThroughAdaptive Cut Through

Combines cut through with store and Combines cut through with store and forwardforward The switch uses cut-through until there The switch uses cut-through until there

are a given number of errorsare a given number of errors Then the switch will change to store and Then the switch will change to store and

forward methodforward method

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Emerging Trends: The Network Evolution

Shared to Switched

TheNew

WiringCloset

VLANSystem

LANCampus

Switch

The New Backbone

TheOld

WiringCloset

HUB

HUB

HUB

HUB

HUB

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Benefits of Switches

• Number of collisions reduced

• Simultaneous, multiple communications

• High-speed uplinks

• Improved network response

• Increased user productivity

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Module 4Module 4

Switching ConceptsSwitching Concepts

ENDEND