onnection. Software and data-Software creates data and moves it from computer to computer by packaging the data packets, also known as protocol (or packet) data units (PDUs) that contain proper addressing and control infor- mation. These are the instructions that implement the LAN functionality according to published specifications (if a standard is being used). User interface-The user interface provides a way for the user to operate the software. Operating system-The operating system services the users' needs for files and devices such as printers and manages computer resources such as the disk, memory, and display functions. It also provides network management functions. Chapter 6 Local Area Networking LAN Characteristics LANs are grouped according to four key characteristics: The types of transmission media over which they can operate The transport technique they use to transmit data over the network (i.e., broadband or baseband) The access method, which is involved in determining which device or net- work node gets to use the network and when it gets to use it The topology, or mapping, of the network (i.e., the physical and logical connections between the nodes on the network) The following sections describe each of these four characteristics in detail. LAN Transmission Media This section talks specifically about the considerations in selecting the appropriate media for a LAN. For information on the characteristics and the advantages and disadvantages of the media types, see Chapter 2, "Traditional Transmission Media." First and foremost, when selecting transmission media for a LAN, you need to think about bandwidth. You must evaluate the kind of capacity you need: How much do you need per client? How much do you need per server? Typically, servers require more capacity than clients because they service multiple simultaneous sessions, but some clients in an organization might use applications such as three-dimensional modeling and simulation and therefore require as much capacity as some of the shared servers