are most network interface cards (NICs). In full duplex mode, there is no contention for the media. Thus, a collision domain no longer exists. Theoretically, the bandwidth is doubled when using full duplex. In addition to faster microprocessors and memory, two other technological advances made switches possible. Content-addressable memory (CAM) is memory that essentially works backwards compared to conventional memory. Entering data into the memory will return the associated address. Using CAM allows a switch to directly find the port that is associated with a MAC address without using search algorithms. An application-specific integrated circuit (ASIC) is a device consisting of undedicated logic gates that can be programmed to perform functions at logic speeds. Operations that might have been done in software can now be done in hardware using an ASIC. The use of these technologies greatly reduced the delays caused by software processing and enabled a switch to keep pace with the data demands of many microsegments and high bit rates. Web Links Switch Operation http://msridhar.freeshell.org/switching.htm
Content 8.1 Ethernet Switching 8.1.4 Latency Latency is the delay between the time a frame first starts to leave the source device and the time the first part of the frame reaches its destination. A wide variety of conditions can cause delays as a frame travels from source to destination: Web Links Latency http://whatis.techtarget.com/definition/ 0,,sid9_gci212456,00.html
Content 8.1 Ethernet Switching 8.1.5 Switch modes How a frame is switched to the destination port is a trade off between latency and reliability. A switch can start to transfer the frame as soon as the destination MAC address is received. Switching at this point is called cut-through switching and results in the lowest latency through the switch. However, no error checking is available. At the other extreme, the switch can receive the entire frame before sending it out the destination port. This gives the switch software an opportunity to verify the Frame Check Sum (FCS) to ensure that the frame was reliably received before sending it to the destination. If the frame is found to be invalid, it is discarded at this switch rather than at the ultimate destination. Since the entire frame is stored before being forwarded, this mode is called store-and-forward. A compromise between the cut-through and store-and-forward modes is the fragment-free mode. Fragment-free reads the first 64 bytes, which includes the frame header, and switching begins before the entire data field and checksum are read. This mode verifies the reliability of the addressing and Logical Link Control (LLC) protocol information to ensure the destination and handling of the data will be correct. When using cut-through methods of switching, both the source port and destination port must be operating at the same bit rate in order to keep the frame intact. This is called synchronous switching. If the bit rates are not the same, the frame must be stored at one bit rate before it is sent out at the other bit rate. This is known as asynchronous switching. Store-and-forward mode must be used for asynchronous switching. Asymmetric switching provides switched connections between ports of unlike bandwidths, such as a combination of 100 Mbps and 1000 Mbps. Asymmetric switching is optimized for client/server traffic flows in which multiple clients simultaneously communicate with a server, requiring more bandwidth dedicated to the server port to prevent a bottleneck at that port. Interactive Media Activity Drag and Drop: Switch Modes After completing this activity, the student will be able to identify the three types of switch modes. Web Links LAN Switching http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_ doc/lanswtch.ht
Content 8.1 Ethernet Switching 8.1.6 Spanning-Tree Protocol When multiple switches are arranged in a simple hierarchical tree, switching loops are unlikely to occur. However, switched networks are often designed with redundant paths to provide for reliability and fault tolerance. While redundant paths are desirable, they can have undesirable side effects. Switching loops are one such side effect. Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network. To counteract the possibility of loops, switches are provided with a standards-based protocol called the Spanning-Tree Protocol (STP). Each switch in a LAN using STP sends special messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence and to elect a root bridge for the network. The switches then use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths. Each port on a switch using Spanning-Tree Protocol exists in one of the following five states: A port moves through these five states as follows: The result of resolving and eliminating loops using STP is to create a logical hierarchical tree with no loops. However, the alternate paths are still available should they be needed. Interactive Media Activity Crossword Puzzle: Spanning Tree States After completing this activity, the student will be able to identify the function of spanning tree states. Web Links Understanding Spanning-Tree Protocol http://www.cisco.com/univercd/cc/td/doc/ product/rtrmgmt/sw_ntman/cwsimain/ cwsi2/cwsiug2/vlan2/stpapp.htm
Content 8.2 Collision Domains and Broadcast Domains 8.2.1 Shared media environments Understanding collision domains requires understanding what collisions are and how they are caused. To help explain collisions, Layer 1 media and topologies are reviewed here. Some networks are directly connected and all hosts share Layer 1. Examples are listed in the following: