size of both the collision domains and the broadcast domains in a network. Routers are the most important traffic regulating devices on large networks. They enable virtually any type of computer to communicate with any other computer anywhere in the world. LANs typically employ a combination of Layer 1, Layer 2, and Layer 3 devices. Implementation of these devices depends on factors that are specific to the particular needs of the organization. Interactive Media Activity Drag and Drop: Devices Function at Layers After completing this activity, students will be able to identify the different OSI layers where networking devices function.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.2 Factors that impact network performance Today's LANs are becoming increasingly congested and overburdened. In addition to an ever-growing population of network users, several other factors have combined to test the limits of the capabilities of traditional LANs:
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.3 Elements of Ethernet/802.3 networks The most common LAN architecture is Ethernet. Ethernet is used to transport data between devices on a network. These devices include computers, printers, and file servers. All nodes on a shared Ethernet media transmit and receive data using a data frame broadcast method. The performance of a shared medium Ethernet/802.3 LAN can be negatively affected by several factors: Ethernet using CSMA/CD and a shared medium can support data transmission rates of up to 100 Mbps. CSMA/CD is an access method that allows only one station to transmit at a time. The goal of Ethernet is to provide a best-effort delivery service and allow all devices on the shared medium to transmit on an equal basis. A certain number of collisions are expected in the design of Ethernet and CSMA/CD. Therefore, collisions can become a major problem in a CSMA/CD network.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.4 Half-duplex networks Originally Ethernet was a half-duplex technology. Using half-duplex, a host could either transmit or receive at one time, but not both. Each Ethernet host checks the network to see whether data is being transmitted before it transmits additional data. If the network is already in use, the transmission is delayed. Despite transmission deferral, two or more Ethernet hosts could transmit at the same time. This results in a collision. When a collision occurs, the host that first detects the collision will send out a jam signal to the other hosts. Upon receiving the jam signal, each host will stop sending data, then wait for a random period of time before attempting to retransmit. The back-off algorithm generates this random delay. As more hosts are added to the network and begin transmitting, collisions are more likely to occur. Ethernet LANs become saturated because users run network intensive software, such as client/server applications, which cause hosts to transmit more often and for longer periods of time. The network interface card (NIC), used by LAN devices, provides several circuits so that communication among devices can occur.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.5 Network congestion Advances in technology are producing faster and more intelligent desktop computers and workstations. The combination of more powerful workstations and network intensive applications has created a need for greater network capacity, or bandwidth. The requirements have exceeded the 10 Mbps available on shared Ethernet/802.3 LANs. Today's networks are experiencing an increase in the transmission of many forms of media: There is also an increase in the number of users on a network. All these factors place an even greater strain on the 10-Mbps of available bandwidth. As more people utilize a network to share larger files, access file servers, and connect to the Internet, network congestion occurs. This can result in slower response times, longer file transfers, and network users becoming less productive. To relieve network congestion, more bandwidth is needed or the available bandwidth must be used more efficiently. Interactive Media Activity Drag and Drop: Bandwidth Requirements When the student has completed this activity, the student will be able to identify the bandwidth requirements for different multimedia applications on a network.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.6 Network latency Latency, or delay, is the time a frame or a packet takes to travel from the source station to the final destination. It is important to quantify the total latency of the path between the source and the destination for LANs and WANs. In the specific case of an Ethernet LAN, understanding latency and its effect on network timing is crucial to determining whether CSMA/CD for detecting collisions and negotiating transmissions will work properly. Latency has at least three sources: Latency does not depend solely on distance and number of devices. For example, if three properly configured switches separate two workstations, the workstations may experience less latency than if two properly configured routers separated them. This is because routers conduct more complex and time-consuming functions. A router must analyze Layer 3 data.
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.7 Ethernet 10 BASE-T transmission time All networks have what is called bit time or slot time. Many LAN technologies, such as Ethernet, define bit time as the basic unit of time in which ONE bit can be sent. In order for the electronic or optical devices to recognize a binary one or zero, there must be some minimum duration during which the bit is on or off. Transmission time equals the number of bits being sent times the bit time for a given technology.