Cisco 3 - Switching Perrine. J Page 16/4/2016 Chapter 4 Switches The performance of shared-medium Ethernet is affected by several factors: data frame broadcast delivery nature of Ethernet/802.3 LANs Carrier Sense Multiple Access/Collision Detect (CSMA/CD) access methods allows only one station to transmit at a time multimedia applications with higher bandwidth (video/internet) can create network congestion normal latency of frames traveling across layer 1 through layer 2,3,4 of the network. Also latency added by the extension of repeaters extending the distances by repeaters Ethernet is defined by IEEE802.3 using CSMA/CD access over half-duplex. When collision is detected by a host, it sends a jam signal. When hosts detect the jam signal, they will wait a random period of time before attempting to transmit. The NIC cards generate this random period of time through their algorithm.

Cisco 3 - Switching Perrine. J Page 26/4/2016 Chapter 4 Latency/delay is the time a frame takes to travel from the source station/node to its final destination on the network. Latency has 3 sources (says Cisco): time to take the source NIC to place voltage pulses on the media & time it takes the receiving NIC to interpret these pulses. actual propagation for the signal to travel down the cable number of layers that the signal must go through (layer 1,2,3,4) Latency does not depend on distance & number of devices solely. EX. A routers conduct more complex & time consuming decision-making functions.

Cisco 3 - Switching Perrine. J Page 36/4/2016 Chapter 4 Ethernet defines a ‘bit time’ as the basic unit of time in which one bit can be sent. Transmission time equals the number of bits being sent times the bit time for a given technology i.e. the time it takes a frame to actually be transmitted. For 10 Mbps Ethernet bit has a 100 ns window for transmitting. Hence a byte takes a minimum of 800 ns (100 X 8) A 64 byte frame takes ns ( 64 X 8 X 100) = 51.2 µs Transmission time, as an example, could be described as the time it takes to move a packet from the data link layer to the physical layer.

Cisco 3 - Switching Perrine. J Page 46/4/2016 Chapter 4 Because of attenuation, LAN is limited in distance, approximately 100 meters. One can increase this distance using a repeater (a layer 1 device). It regenerates the signal and passes it on allows longer distances increase the collision domain size increase the broadcast domain size One can get more bandwidth by having a full-duplex Ethernet. It has the properties: needs 4 wires consider to be point-to-point is collision free NIC must have a full-duplex capabilities offers 100% of bandwidth in both directions. For 10 Mbps Ethernet, get 10 Mbps throughput on TX and 10 Mbps throughput on RX.

Cisco 3 - Switching Perrine. J Page 56/4/2016 Chapter 4 By using segmentation, fewer devices are sharing the same bandwidth. By dividing the network into segments, one can decrease network congestion by creating smaller collision domains. Bridges can be used for segmentation. Bridges learn a network’s segmentation by building address tables that contain the address of each network device and which segment to use to reach that device. Bridges are layer 2 devices, and hence use the MAC address for its decisions. Routers can be used for segmentation. Router operates at layer 3 which creates the highest level of segmentation though their main purpose remains in best path selection & switching. LAN switches can be used for segmentation, called microsegmentation & increased the efficiency of bandwidth. reduces collision domain though still is in the same broadcast domain full bandwidth between the connection of the two PCs.i.e. point-to-point connections

Cisco 3 - Switching Perrine. J Page 66/4/2016 Chapter 4 One segment – many collisions

Cisco 3 - Switching Perrine. J Page 76/4/2016 Chapter 4 One segment – many collisions 2- segment – fewer collisions

Cisco 3 - Switching Perrine. J Page 86/4/2016 Chapter 4 2- segments – fewer collisions 3 segments – even fewer collisions Each segment is its own collision domain By increasing the number of segments, one reduces the number of collisions increases the number of collision domains

Cisco 3 - Switching Perrine. J Page 96/4/2016 Chapter 4 Ethernet/LAN Switch A LAN switch is a very high-speed multiport bridge with one port for each node/segment of the LAN. Therefore creates microsegments (between the nodes) creates collision free domains (between the nodes) though all nodes connected to LAN switch still belongs to the same broadcast domain

Cisco 3 - Switching Perrine. J Page 106/4/2016 Chapter 4 Switching is a technology that decreases congestion by reducing traffic & increasing the utilization of bandwidth. LAN switch often replace shared HUBs by a direct replacement. The switch maintains MAC address tables and are more functional than bridges because of their increase speed. Typically switch using switching via hardware. Types of switch modes: store & forward fragment free fast forward Switches uses layer 2 switching for forwarding frames out of its ports. If it does not know where to send the frame, it broadcasts the frame out all of its ports to the network to learn the correct destination.

Cisco 3 - Switching Perrine. J Page 116/4/2016 Chapter 4 Switches learn the addresses dynamically and stored in content- addressable memory (CAM). Each time an address is stored, it is given a time-stamp, and each time the address is read, it is given a new time-stamp. If an address is not read in a certain amount of time it is removed from its list.

Cisco 3 - Switching Perrine. J Page 126/4/2016 Chapter 4 Symmetric switching provides switched connections between ports with the same bandwidth. 10 Mbps switch

Cisco 3 - Switching Perrine. J Page 136/4/2016 Chapter 4 Asymmetric switching provides switched connections between ports with different bandwidth. 10 Mbps 100 Mbps 10 Mbps switch

Cisco 3 - Switching Perrine. J Page 146/4/2016 Chapter 4 Queues in switches: port based memory - queues for incoming ports A packet is transmitted to the outgoing port only when all the packets ahead of it in the queue have been successfully transmitted. shared memory - used a common memory buffer The packets in the buffer are linked dynamically to the transmit port

Cisco 3 - Switching Perrine. J Page 156/4/2016 Chapter 4 Store-and-forward: Entire frame is received before any forwarding takes place. validates CRC looks up the route. Cut-through (as soon as the destination address is read, the frame is forward) Fast-forward-switching might send the frame out with errors Fragment-free-switching filters out collision fragments before forwarding begins collision will occur in the first 64 bytes - no errors, then any packet greater than 64 bytes is assumed error free.

Cisco 3 - Switching Perrine. J Page 166/4/2016 Chapter 4 Latency hierarchy for switching methods: Fast Forward Fragment-free Store & forward Lowest highest

Cisco 3 - Switching Perrine. J Page 176/4/2016 Chapter 4 Virtual LAN (VLAN) is logical grouping of network/user devices into segments implemented by software still in one broadcast domain group can be based on function department application based on IEEE 802.1q

Cisco 3 - Switching Perrine. J Page 186/4/2016 Chapter 4 Spanning-Tree Algorithm (STP) is used to calculate a loop-free path. A switch uses Spanning-Tree Protocol on all Ethernet & Fast Ethernet based VLANs. The Spanning-Tree Protocol states: 1. blocking - no frames forward 2. listening - no frames forward, listening for frames 3. learning - no frames forwarded, learning address 4. forwarding frames - frames forwarded, learning addresses 5. disabled - no frames forwarded