Another way to think about transmission time is the time it takes a frame to be transmitted. Small frames take a shorter amount of time. Large frames take a longer amount of time. Each 10 Mbps Ethernet bit has a 100 ns transmission window. This is the bit time. A byte equals 8 bits. Therefore, 1 byte takes a minimum of 800 ns to transmit. A 64-byte frame, the smallest 10BASE-T frame allowing CSMA/CD to function properly, takes 51,200 ns ( 51.2 microseconds). Transmission of an entire 1000-byte frame from the source station requires 800 microseconds just to complete the frame. The time at which the frame actually arrives at the destination station depends on the additional latency introduced by the network. This latency can be due to a variety of delays including all of the following: Interactive Media Activity Drag and Drop: 10BASE-T Transmission Times After completing this activity, students will be able to identify the transmission times of 10BASE-T.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.8 The benefits of using repeaters The distance that a LAN can cover is limited due to attenuation. Attenuation means that the signal weakens as it travels through the network. The resistance in the cable or medium through which the signal travels causes the loss of signal strength. An Ethernet repeater is a physical layer device on the network that boosts or regenerates the signal on an Ethernet LAN. When a repeater is used to extend the distance of a LAN, a single network can cover a greater distance and more users can share that same network. However, the use of repeaters and hubs compounds problems associated with broadcasts and collisions. It also has a negative effect on the overall performance of the shared media LAN.
Interactive Media Activity PhotoZoom: Cisco 1503 Micro Hub In this PhotoZoom, the student will view the Cisco 1503 Micro Hub.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.9 Full-duplex transmitting Full-duplex Ethernet allows the transmission of a packet and the reception of a different packet at the same time. This simultaneous transmission and reception requires the use of two pairs of wires in the cable and a switched connection between each node. This connection is considered point-to-point and is collision free. Because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth. Full-duplex Ethernet can use an existing cable infrastructure as long as the medium meets the minimum Ethernet standards. To transmit and receive simultaneously, a dedicated switch port is required for each node. Full-duplex connections can use 10BASE-T, 100BASE-TX, or 100BASE-FX media to create point-to-point connections. The NICs on all connected devices must have full-duplex capabilities. The full-duplex Ethernet switch takes advantage of the two pairs of wires in the cable by creating a direct connection between the transmit (TX) at one end of the circuit and the receive (RX) at the other end. With the two stations connected in this manner a collision free environment is created as the transmission and receipt of data occurs on separate non-competitive circuits. Ethernet usually can only use 50%-60% of the available 10 Mbps of bandwidth because of collisions and latency. Full-duplex Ethernet offers 100% of the bandwidth in both directions. This produces a potential 20 Mbps throughput, which results from 10 Mbps TX and 10 Mbps RX. Interactive Media Activity Drag and Drop: Full Duplex Ethernet After completing this activity, students will be able to identify the requirements for full duplex Ethernet.
Content 4.2 Introduction to LAN Switching 4.2.1 LAN segmentation A network can be divided into smaller units called segments. Figure shows an example of a segmented Ethernet network. The entire network has fifteen computers. Of these fifteen computers, six are servers and nine are workstations. Each segment uses the CSMA/CD access method and maintains traffic between users on the segment. Each segment is its own collision domain. Segmentation allows network congestion to be significantly reduced within each segment. When transmitting data within a segment, the devices within that segment share the total available bandwidth. Data passed between segments is transmitted over the backbone of the network using a bridge, router, or switch.
Content 4.2 Introduction to LAN Switching 4.2.2 LAN segmentation with bridges Bridges are Layer 2 devices that forward data frames according to the MAC address. Bridges read the sender's MAC address of the data packets that are received on the incoming ports to discover which devices are on each segment. The MAC addresses are then used to build a bridging table. This will allow the bridge to block packets that do not need to be forwarded from the local segment. Although the operation of a bridge is transparent to other network devices, the latency on a network is increased by ten to thirty percent when a bridge is used. This latency is a result of the decision making process prior to the forwarding of a packet. A bridge is considered a store-and-forward device. The bridge must examine the destination address field and calculate the cyclic redundancy check (CRC) in the frame check sequence field before forwarding the frame. If the destination port is busy, the bridge can temporarily store the frame until that port is available.
Content 4.2 Introduction to LAN Switching 4.2.3 LAN segmentation with routers Routers provide segmentation of networks, adding a latency factor of 20% to 30% over a switched network. This increased latency is because a router operates at the network layer and uses the IP address to determine the best path to the destination node. Figure shows a Cisco router. Bridges and switches provide segmentation within a single network or subnetwork. Routers provide connectivity between networks and subnetworks. Routers also do not forward broadcasts while switches and bridges must forward broadcast frames. Interactive Media Activity PhotoZoom: Cisco 2621 Router In this PhotoZoom, the student will view a Cisco 2621 router. Interactive Media Activity PhotoZoom: Cisco 3640 Router In this PhotoZoom, the student will view a Cisco 3640 router.
Content 4.2 Introduction to LAN Switching 4.2.4 LAN segmentation with switches LAN switching decreases bandwidth shortages and network bottlenecks, such as those between several workstations and a remote file server. Figure shows a Cisco switch. A switch will segment a LAN into microsegments which decreases the size of collision domains. However all hosts connected to the switch are still in the same broadcast domain. In a pure switched Ethernet LAN, the sending and receiving nodes function as if they are the only nodes on the network. When these two nodes establish a link, or virtual circuit, they have access to the maximum available bandwidth. These links provide significantly more throughput than Ethernet LANs connected by bridges or hubs. This virtual network circuit is established within the switch and exists only when the nodes need to communicate.
Content 4.2 Introduction to LAN Switching 4.2.5 Basic operations of a switch Switching is a technology that decreases congestion in Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) LANs. Switching accomplishes this by reducing traffic and increasing bandwidth. LAN switches are often used to replace shared hubs and are designed to work with existing cable infrastructures. Switching equipment performs the following two basic operations: Figures - show the basic operations of a switch.
Content 4.2 Introduction to LAN Switching 4.2.6 Ethernet switch latency Latency is the period of time from when the beginning of a frame enters to when the end of the frame exits