Better to know some
... than all
The Physical Layer
This section briefly describes general networking equipment types and the types of connections that can be established between them.
1. Data Terminal Equipment (DTE) refers to user equipment that convert outgoing user data into a transmission signal, and convert back the incoming signal into user data. DTEs may take many different shapes and forms. Examples include: terminals, terminal adapters, personal computers and mainframes. DTEs commonly reside at user sites.
2. Data Circuit-terminating Equipment (DCE) refers to the network equipment that connect DTEs to the network communication lines. In general, a DTE and a network line may use different types of signals (e.g., electrical versus optical). The necessary signal conversion between these two is performed by the DCE. A DCE may be a part of a DTE or be an entirely separate device. Modems and multiplexers are all examples of DCEs.
3. Data Switching Equipment (DSE) refers to network equipment used to connect DCEs together, thus providing switching capability to the network. DSEs correspond to the nodes in a network, and are responsible for routing data across the network. A DSE is commonly referred to as a switch. Digital telephone switches used in digital networks are examples.
1. Simplex. This is a unidirectional connection, i.e., data can only travel in one direction. Simplex connections are useful in situations where a device only receives or only sends data (e.g., a printer).
2. Half-duplex. This is a bidirectional connection, with the restriction that data can travel in one direction at a time.
3. Full-duplex. This is a bidirectional connection in which data can travel in both directions at once. A full-duplex connection is equivalent to two simplex connections in opposite directions.
Transmission is the act of transporting information from one location to another via a signal. The signal may be analog or digital, and may travel in different media.
All signals are either analog or digital. An analog signal is one in which information appears as a continuous variation of some property. Human speech is an example: it produces a continuous variation of air pressure. A digital signal, on the other hand, is one in which information appears as a sequence of binary values 0 and 1. To represent these two values, a signal is used in which only two wave shapes are allowed, one representing the binary value 0 and the other representing the binary value 1. By definition, therefore, a digital signal is a restricted form of an analog signal. A human speaker who only utters the two words zero and one is a crude example of a digital signal.
In electrical terms, signals appear as variation of some electrical property (e.g., voltage). In the analog signal example, the voltage freely varies between 0 and 5 Volts. In the digital signal, the voltage may assume only two values: 0 Volts to represent digital value 0 and 5 Volts to represent digital value 1. Since digital computers play a central role in data communication, in nearly all cases, digital signals are used. Analog signals are used in cases of equipment which date back to before the advent of digital technology. Existing analog telephone networks are a good example of the latter.