HP Certified: HP-UX System Administration

Hosts in a network are connected using a communication medium. In most of the cases, this communication medium is a cable. The physical layout of this cable shows network topology. Commonly used network topologies are bus, star, and ring. Networks of medium-to-large size use multiple topologies. The selection of a network topology depends on many factors, including cost and reliability of the network.

Network devices serve different purposes. Other than network cables and connectors, transceivers, repeaters, and hubs are used at the physical layer. A transceiver connects the AUI port on the network interface adapter to a network cable. The repeater is used to extend the length of a network cable by regenerating and reshaping electrical signals. The hub is a central part in a star-connected network. These also serve as a multiport repeater. On the data link layer level, bridges and switches are used. A bridge connects network segments and creates additional collision domains. It filters traffic and stops errors, including collisions, to pass through to other network segments. A switch provides multiple parallel paths for data transfer at the data link layer. Routers operate at the network layer of the OSI model and are used to connect multiple networks. A router can also connect networks with different protocols at the first three layers of the OSI model. Gateways can be used to connect networks using different protocols at all network layers .

A MAC address or station address is the physical address of a host. It is a permanent address assigned by the manufacturer of a network interface adapter. It is a 48-bit long number and must be unique for all hosts on a LAN. The lanscan command can be used to display the MAC addresses of all network interface adapters installed in a system.

An access method is used to access network cables and transfer data. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used in Ethernet networks, while the token passing method is used in token ring and token bus networks. CSMA/CD employs a technique whereby each network station checks the network cable for any existing network traffic before sending its own data. If the cable is free, the host starts sending its data. If two hosts start sending data at the same time, a collision occurs and is detected by all sending hosts. In case of collision, all hosts wait for a random time and retry data transfer. In token passing networks, a data token is passed in an orderly fashion from one host to another in the network. If a host wants to transfer some data, it waits for the token.

Multiple standards of Ethernet networks are in use. The most common are 10BASE-2, 10BASE-5, 10BASE-T, and 100BASE-T. Each of these standards employs a particular type of cable and spans a defined cable length. The data transfer speeds are also defined for each standard. FDDI networks operate at 100 Mbits per second and use a dual ring of fiber optic cable.