3 Switches and Bridges Make decisions when frames are received Select a path or circuit to send a frame to its destinationLayer 2 devicesincreases the number of collision domainsall hosts connected to the switch are still part of the same broadcast domainUsed toincrease available bandwidthreduce network congestionSwitch segments a LAN into microsegmentssegments with only a single hostCreates multiple collision-free domains
5 Hub Layer 1 device (physical layer) Ethernet concentrator or a multi-port repeaterNo decision made at this level (no addressing)Takes data signal in one portRegenerates, retimes and amplifies the data signalsSends (Broadcasts) data signal out all other portsAll users connected to the hub compete for the same bandwidth (share bandwidth)50% – 60% bandwidth availableIncrease collision domains (extends)Increase broadcast domains (extends)Only 1 device can transmit at a time
6 Bridge Layer 2 device (data link layer) Creates 2 network segments 2 collision domains – creates smaller collision domains2 bandwidth domainsDo not restrict broadcast traffic – (forwards broadcasts)Learns MAC address of all devices on each segmentUse this to build bridging tableForwards/blocks traffic based on tableMakes decisions based on MACIncrease latency by 10 to 30 percentSwitching occurs using softwareStore and forward deviceAdds 10% to 30% latency
7 Switch Layer 2 device (data link layer) Multiport bridge or switching hubsProvides microsegmentation (point-to-point link)It isolates traffic among segmentscreates a collision free environment between the source and destinationEach segment uses CSMA/CD (allows multiple communications on different segments)Each port has dedicated bandwidth (100% bandwidth available)Makes decisions based on MAC addressesHeld in Content Addressable MemorySwitching occurs using hardwareDecreases collision domain1 collision domain per segment (increases number of collision domains)Increases broadcast domain (Extends)Broadcasts sent out very port
8 Router Layer 3 device (network layer) Makes decisions based on network addressesIP AddressRouting tablesList of Layer 3 network address and the port to go out onRouter PurposeExamine incoming packets of Layer 3 dataChoose the best path for them through the networkSwitch them to the proper outgoing portReducesBroadcast domainCollision domain
9 Network Performance LANs are increasingly congested and overburdened Growing population of network usersMultitasking environmentincreased demand for network resourcesThe use of network intensive applicationse.g. WWW, multi media,Client/server applicationsThis has resulted ina need for more bandwidthslower response timeslonger file transfersnetwork users becoming less productive
10 Elements of Ethernet 802.3Used to transport data between devices on a network (computers, printers, and file servers)Multi-access broadcast technologyShared mediaUses CSMA/CD to allows one station transmit at a timeLatency as frames travel across mediaRepeaters extend distances (increase latency)Layer 2 devices improve performance
11 Network LatencyLatency, or delay, is the time a frame or a packet takes to travel from the source to the final destinationLatency sources:NIC DelayThe time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses1 microsecond for a 10BASE-T NICPropagation delaySignal takes time to travel along the cableAbout microseconds per 100 m for Cat 5 UTPNetworking devicesLayer 1 no decisions less latencyLayer 2 devices make layer 2 decisions increased latencyLayer 3 devices make layer 3 decisions most latency
12 Ethernet ?-BaseT Transmission The time it takes a frame to be transmittedNumber of bits being sent * Technology Bit time10 Mbps Ethernet bit has a 100 ns transmission window (bit time of 100 ns)A byte equals 8 bits1 byte is 8bits * 100ns = 800 ns to transmit100Mbps – 10ns1000Mbps – 1ns
13 Attenuation means that the signal weakens as it travels through the network The resistance in the cable causes loss of signal strength
14 Half Duplex EthernetA host could transmit or receive at one time, but not bothBefore transmittingHost checks media for signalIf no signal message is transmittedIf signal exists the transmission is delayedIf two or more hosts transmit at the same timeA collision occursJam signal setHosts stop sendingRun a back-off algorithm to generate a random delayWait for the random delay before attempting to retransmitOnly 1 host can transmit at a time50% - 60% bandwidth available
15 Full Duplex Transmission Transmission and receipt of packets at the same timeUse of two pairs of wires in the cable and a switched connection between each nodePoint-to-point connectiondedicated connection to switch portCollision free – 2 hosts can send simultaneouslyNo negotiation for bandwidthFull-duplex connections can use10BASE-T, 100BASE-TX, or 100BASE-FXNIC must have half duplex capabilitiesFull-duplex Ethernet offers100% of the bandwidth in both directionspotential 20 Mbps throughput -10 Mbps TX and 10 Mbps RX
16 LAN SegmentationData passed between segments is transmitted over the backbone of the network using a bridge, router, or switch.Each segment uses the CSMA/CD and maintains traffic between users on the segment. Each segment is its own collision domain.
17 LAN Segmentation with Bridges Operation of a bridge is transparent to other network devicesBridge increases latency by 10% to 30%Due to decision making processBridge is a store-and-forward deviceExamine the destination address fieldCalculate the cyclic redundancy check (CRC)Forward the frameIf the destination port is busy, the bridge can temporarily store the frame until that port is availableForward broadcasts
18 LAN Segmentation using Routers Provide segmentation of networksLatency is increased by 20% to 30% over a switched networkrouter operates at the network layerUses IP address to determine the best path to destinationProvide connectivity between networks and subnetworksRouters also do not forward broadcasts
19 LAN Segmentation with Switches Segment LAN into microsegmentsDecreases collision domains sizeExtends broadcast domainVirtual network circuit is established within the switch and exists only when the nodes need to communicate
21 Switchingreceiving incoming frame on one interface and delivering that frame out another interfaceRouters use Layer 3 switching to route a packetBased on network address/ ip addressHierarchical addressing – more security and flow controlSwitches use Layer 2 switching to forward framesbased on destination MAC address informationIf it does not know where to send the frame, it broadcasts the frame out all ports to the networkWhen a reply is returned, the switch records the new address in the CAM.
22 How do Switches and Bridges Learn Addresses? Switch usingContent Addressable Memory
23 Bridges and switches learn in the following ways: Reading the source MAC address of each received frame/datagramRecording the port on which the MAC address was received.Bridge/Switch learns which devices are connected to each portThe device MAC addresses and port are stored in the addressing table (Content Addressable Memory)When a packet arrivesHost and destination address information is identifiedCAM stores host MAC addresses and associated port numbersAddresses are learned dynamicallyAt each store it is stamped and will be held for a set time periodIf it is not stamped within the time period it is removed from CAMDestination address is compared with a CAMIf there is a match, and the outgoing port is the same as the incoming port the packet is discardedIf there is a match, and the outgoing and incoming ports are different the packet is forwarded out the outgoing portIf there is no match, packet is forwarded out all ports except the port it was received on (flooding)
24 How do Switches and Bridges Filter Frames Bridges are capable of filtering frames based on any Layer 2 fieldsBridge can be programmed to reject/not forwardAll frames sourced from a particular networkBased on upper network layer protocolsfilters out unnecessary broadcast and multicast packetsIgnoring a frame is called filtering.Copying the frame is called forwarding.
25 Symmetric and Asymmetric Switching based on the way bandwidth is allocated to the switch portsSymmetric switchswitched connections between ports with the same bandwidth (all 10Mbps or all 100Mbps)Asymmetric switchswitched connections between ports of unlike bandwidthcombination of 10 and 100 Mbps portsEnables more bandwidth to be dedicated to the server switch port in order to prevent a bottleneckMemory buffering is required (keeps the frames contiguous between different data rate ports)
27 Memory BufferingEthernet switch use memory buffering to store and forward framesAlso be used when the destination port is busyMemory buffer is where the switch storesMethods for forwarding framesPort-based memory bufferingframes are stored in queues that are linked to specific incoming ports – 1 queue per incoming portFrame is transmitted to the outgoing port when all the frames ahead in the queue are successfully transmittedA busy destination port can delay the deliver of framesShared memory bufferingAll frames are placed in a common memory buffer – 1 queueBuffer memory space required by a port is dynamically allocatedThe number of frames stored in the buffer is restricted by the size of the memory buffer, and not limited to a single port bufferUsed in asynchronous switching, where frames are being exchanged between different rate ports.
28 Switching Methods Store and Forward Cut-Through Entire frame is received before any forwardingIncreases latencyFilters can be applied to destination and source addressesFrame can be checked for errors (discarded)Cut-ThroughAt least the frame destination address must be read before the frame can be forwardedDecreases latencyReduced error detection
29 Types of Cut-Through Switching Fast-forward Lowest level of latencyImmediately forwards packet after reading destination addressNo error checkingDestination network adapter will discard the faulty packet upon receiptFragment-free Filter out collision fragments before forwarding beginsReads first 64 bits to identify if a collision occurred
31 Hybrid Transmission Mode Combination of cut-through and store-and-forwardCalled adaptive cut-through or error sensingUses cut-through until it detects a given number of errorsOnce the error threshold is reached, the switch changes to store-and-forward mode
32 Switches and Broadcast Domains one transmitter tries toreach one receiverone transmittertries to reach only a subsetor a groupof the entire segment.one transmittertries to reach all receiversin the network
33 When a device wants to send out a Layer 2 broadcast Destination MAC address in the frame is set to all onesFF:FF:FF:FF:FF:FF in hexadecimalMAC broadcast domainWhen a switch receives a broadcastit forwards it to each port on the switch except the incoming portEach attached device must process the broadcast frameBroadcasts reduce available bandwidth