Brierley 1 Module 4 Module 4 Introduction to LAN Switching

Brierley 2 Objectives LAN congestion and its effect on network performance LAN congestion and its effect on network performance Advantages of LAN segmentation in a network Advantages of LAN segmentation in a network Advantages and disadvantages of using bridges, switches, and routers for LAN segmentation 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 Effects of switching, bridging, and routing on network throughput Fast Ethernet technology and its benefits

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

Brierley 6 The Ethernet/802.3 Interface Ethernet is known as a shared-medium technology – all the devices are connected to the same delivery media. 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. Ethernet media uses a data frame broadcast method of transmitting and receiving data to all nodes on the shared media.

Brierley 7 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). 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. 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.

Brierley 8  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. Performance of a shared media Ethernet/802.3 LAN can be negatively effected by several factors.

Brierley 9 Half-Duplex Design Loopback Tx Tx Rx Rx Collision Detection Ethernet Controller Tx Tx Rx Rx Collision Detection Ethernet Controller Transmit Receive Loopback Ethernet NIC Ethernet physical connector provides several circuits Most important are TX (transmit), RX (receive), and CD (collision detection)

Brierley 10 Half-Duplex Ethernet Design (Standard Ethernet) The most important of these circuits are the receive (RX), transmit (TX), and CD (collision detection0. 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 transmit (TX) circuit is active at the transmitting station. The receive (RX) circuit is active at the receiving station. The receive (RX) circuit is active at the receiving station.

Brierley 11 To the network this appears as a single one way bridge. To the network this appears as a single one way bridge. Both devices are contending for the right to use the single shared medium. 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. 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. 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. Then the host will determine if the network is clear before attempting to retransmit.

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Brierley 13 Congestion and Bandwidth To relieve network congestion more bandwidth is needed or the available bandwidth must be used more efficiently. 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. “Throwing bandwidth at the problem”. This could be attacking the symptom and not the problem.

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Brierley 15 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. 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 The greater the number of devices the greater the latency or propagation delay adding hosts simply increases collisions, increases jam signals decreasing throughput. adding hosts simply increases collisions, increases jam signals decreasing throughput.

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Brierley 17 Ethernet Transmission Times Each Ethernet bit uses a 100ns window for transmission. Each Ethernet bit uses a 100ns window for transmission. A byte is equal to eight bits. A byte is equal to eight bits. Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns). Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns).

Brierley 18 A 64 byte frame requires 51,200ns or 51.2 microseconds to transmit A 64 byte frame requires 51,200ns or 51.2 microseconds to transmit 1 Ethernet bit uses a 100ns window for transmission. 1 Ethernet bit uses a 100ns window for transmission. A byte is equal to eight bits. A byte is equal to eight bits. Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns). Therefore, one byte takes a minimum of 800ns to transmit (8 bits at 100ns = 800ns). 1 microsecond = 1000 nanoseconds 1 microsecond = 1000 nanoseconds 1 byte = 8 bits 1 byte = 8 bits (64 bytes at 800ns) = 51,200ns (64 bytes at 800ns) = 51,200ns (51,200ns/1000) = 51.2 microseconds). (51,200ns/1000) = 51.2 microseconds).

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Brierley 20 Extending Shared Media LANs using Repeaters Signal attenuation –Signal weakens as they travels through the network due to resistance in the medium. 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. A repeater is used to extend the geography of a LAN allowing more users to share that same network.

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Brierley 22 Improving LAN Performance The performance of a network can be improved in a shared media LAN by: 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

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Brierley 24 Segment LANs? Each segment uses the (CSMA/CD) protocol to manages traffic on the segment. Each segment uses the (CSMA/CD) protocol to manages traffic on the segment. By segmenting a network - less devices are sharing the same bandwidth By segmenting a network - less devices are sharing the same bandwidth Each segment is its own collision domain. Each segment is its own collision domain.

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Brierley 26 Segmented LANs? In a segmented Ethernet LAN messages passed between segments is transmitted on a network backbone using a bridge, switch, or router. 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. The backbone network is its own collision domain and uses CSMA/CD to manage between segments.

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Brierley 28 Segmentation with Bridges Bridges are Layer 2 devices, independent of Layer 3 protocols used by routers 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 transmit data frames regardless of which Layer 3 protocol is being used They are transparent to the other devices on the network. They are transparent to the other devices on the network. Bridges increase latency (delay) in a network by 10-30%. Bridges increase latency (delay) in a network by 10-30%.Why?

Brierley 29 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 determine through which interface the frame will be forward. 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 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. 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. Collision domains are created, not broadcast domains.

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Brierley 32 Segmentation using Routers Routers operate at network layer and base routing decisions on the Layer 3 IP protocol address. 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 perform higher level functions than do bridges consequently they operate at a higher latency.

Brierley 33 Routers? Segment broadcast domains Segment broadcast domains Forward packets based on destination network layer addresses, i.e. IP Forward packets based on destination network layer addresses, i.e. IP Segment collision domains Segment collision domains

Brierley 34 More collision domains, but more bandwidth for each user

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Brierley 36 Segmentation with LAN Switches A switch segments a LAN into microsegments creating collision free domains from one larger collision domain, not broadcast domains. 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%. Switched Ethernet available bandwidth can reach close to 100%.

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Brierley 38 LAN Switch Latency Each switch on an Ethernet LAN adds latency to the network. 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. The type of switching used can help overcome the built in latency of some switches.

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Brierley 40 Full-Duplex Ethernet Overview Full duplex Ethernet allows the transmission of a packet and the reception of a packet at the same time. 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 Requires two pairs of conductors and a switched connection between each node

Brierley 41 Simultaneous transmission and reception of frames is called bidirectional traffic (two-way) and on a 10Mbps circuit yields 20Mbps of throughput. 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. The network interface cards (NICs) on both ends of the circuit require full duplex capabilities.

Brierley 42 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 Ethernet Controller Loopback Tx Tx Rx Collision Detection RX Full Duplex Ethernet Controller Loopback Tx Tx Rx Collision Detection

Brierley 43 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

Brierley 44 Full-Duplex Ethernet Design Standard Ethernet normally can only use % of the 100Mbps available bandwidth. This is due to collisions and latency. This is due to collisions and latency. Full duplex Ethernet offers 100% of the bandwidth in both directions. Full duplex Ethernet offers 100% of the bandwidth in both directions. This produces a potential 200Mbps throughput – 100Mbps TX and 100Mbps RX. This produces a potential 200Mbps throughput – 100Mbps TX and 100Mbps RX.

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Brierley 47 This virtual network circuit exists only when two nodes need to communicate this circuit is established within the switch. 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. It Allows multiple users to communicate in parallel via these virtual circuits.

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Brierley 49 Source MAC address is used to build this table

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Brierley 51 How a LAN Switch Learns Addresses When messages are received by the switch their: When messages are received by the switch their: addresses are read and stored in the CAM (Content Address Memory). addresses are read and stored in the CAM (Content Address Memory). Each time an address is stored it is time stamped. Each time an address is stored it is time stamped. This allows addresses to be stored for a set period of time. This allows addresses to be stored for a set period of time.

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Brierley 53 Benefits of Switching A LAN switch allows many users to communicate in parallel : A LAN switch allows many users to communicate in parallel : through the use of virtual circuits through 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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Brierley 55 Symmetric Switching A symmetric switch is optimized through even distribution of network traffic across the entire network. A symmetric switch is optimized through even distribution of network traffic across the entire network. All networks using the same bandwidth. All networks using the same bandwidth.

Brierley 56 before forwarding

Brierley 57 Asymmetric Switching Asymmetric switching is best exemplified in client-server network traffic flows where multiple clients are simultaneously communicating with a server. 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 Each usually at a lower bandwidth than the server

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Brierley 59 Memory Buffering The area of memory where the switch stores the destination and transmission data is called the memory buffer. 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: This memory buffer can make use of two methods for forwarding packets: port based memory buffering port based memory buffering shared memory buffering. shared memory buffering.

Brierley 60 Port based memory buffering packets are stored in queues that are linked to specific incoming ports. 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. 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!) Shared memory buffering deposits all packets into a common memory buffer that is shared by all the ports on the switch. (Better!)

Brierley 61 3 frame transmission modes in a switch (+one variation)

Brierley 62 Three Switching Methods Store and Forward - the entire frame is received before any forwarding takes place. 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. 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. 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.

Brierley 63 Cut-through the switch reads the destination address before receiving the entire frame. Cut-through the switch reads the destination address before receiving the entire frame. The frame is then forwarded before the entire frame arrives. The frame is then forwarded before the entire frame arrives. This mode decreases the latency of the transmission and has poor error detection. This mode decreases the latency of the transmission and has poor error detection.

Brierley 64 Fragment-Free Switching The switch reads only the 1st 64 bytes of the incoming frame and then forwards the frame to its destination port The switch reads only the 1st 64 bytes of the incoming frame and then forwards the frame to its destination port

Brierley 65 Means the switch is in cut through mode

Brierley 66 Adaptive Cut Through Combines cut through with store and forward Combines cut through with store and forward The switch uses cut-through until there are a given number of errors The switch uses cut-through until there are a given number of errors Then the switch will change to store and forward method Then the switch will change to store and forward method

Brierley 67 Emerging Trends: The Network Evolution Shared to Switched The New Wiring Closet VLAN System LAN Campus Switch The New Backbone The Old Wiring Closet HUB

Brierley 68 Benefits of Switches Number of collisions reduced Simultaneous, multiple communications High-speed uplinks Improved network response Increased user productivity

Brierley 69 Module 4 Module 4 Switching Concepts END