Network Architecture

In practice, local area networks are not really thought of in terms of protocol layers but by what is called LAN architecture or network architecture. (Sometimes a network architecture is referred to as a LAN type or a LAN topology.) A network architecture, such as ethernet, provides a bundle of specifications governing media access, physical addressing, and the interaction of the computers with the transmission medium. When you decide on a network architecture, you are in effect deciding on a design for the Network Access layer.

A network architecture is a design for the physical network and a collection of specifications defining communications on that physical network. The communication details are dependent on the physical details, so the specifications usually come together as a complete package. These specifications include considerations such as the following:

• Access method— An access method is a set of rules defining how the computers will share the transmission medium. To avoid data collisions, computers must follow these rules when they transmit data.

• Data frame format— The IP-level datagram from the Internet layer is encapsulated in a data frame with a predefined format. The data enclosed in the header must supply the information necessary to deliver data on the physical network. You'll learn more about data frames later in this hour.

• Cabling type— The type of cable used for a network has an effect on certain other design parameters, such as the electrical properties of the bitstream transmitted by the adapter.

• Cabling rules— The protocols, cable type, and electrical properties of the transmission have an effect on the maximum and minimum lengths for the cable and for the cable connector specifications.

Details such as cable type and connector type are not the direct responsibility of the Network Access layer, but to design the software components of the Network Access layer, developers must assume a specific set of characteristics for the physical network. Thus, the network access software must come with a specific hardware design.