d a frame. The physical layer transmits the bits given by the data link layer to the physical media, such as an LAN cable. It may, for example, convert bits into light pulses for optical fiber cable transmission. In the receiving computer the message moves layer by layer upward. A corresponding layer at the other end as shown in Figure 6.12 strips off the header of a layer. None of the headers for layers below a certain layer n are passed up to the layer N. Thus, each layer receives the message as it was transmitted by a corresponding layer in the source machine. They act as if they were connected directly, not through the lower layers. For example, when the data link layer of the destination machine has checked through the error detection field in the trailer (T2) to determine that the frame is error free, the error check bytes are removed before the data are given to the net- work layer. Introduction to Telecommunications Network Engineering If the reader still feels that the preceding illustration was too abstract and wants to understand thoroughly the principle of protocols and layers, she may study the operation of one protocol stack, for example, TCP/IP, layer by layer. This is the most efficient way get a concrete grip on protocols; and when one protocol is understood, new ones are easy to learn. We will describe TCP/IP in more detail later in Section 6.6 and a comprehensive description is also given, for example, in [4]. 6.4 Access Methods To use data services, a user's computer has to access the data network. Vari- ous access technologies are available for business and residential needs. We review the most important access systems in this section. 6.4.1 Voice-Band Modems The word modem comes from the combination of the two devices, modula- tor and demodulator. Modulation converts a digital signal into an analog sig- nal for transmission through a channel, and demodulation performs the conversion back to the original digital baseband data signal. Voice-band modems are needed when an analog voice channel o