4.2. Introduction to Serial Devices

The Unix kernel provides devices for accessing serial hardware, typically called tty devices (pronounced as it is spelled: T-T-Y). This is an abbreviation for Teletype device, which used to be one of the major manufacturers of terminal devices in the early days of Unix. The term is used now for any character-based data terminal. Throughout this chapter, we use the term to refer exclusively to the Linux device files rather than the physical terminal.

Linux provides three classes of tty devices: serial devices, virtual terminals (all of which you can access in turn by pressing Alt-F1 through Alt-Fnn on the local console), and pseudo-terminals (similar to a two-way pipe, used by applications such as X11). The former were called tty devices because the original character-based terminals were connected to the Unix machine by a serial cable or telephone line and modem. The latter two were named after the tty device because they were created to behave in a similar fashion from the programmer's perspective.

SLIP and PPP are most commonly implemented in the kernel. The kernel doesn't really treat the tty device as a network device that you can manipulate like an Ethernet device, using commands such as ifconfig. However, it does treat tty devices as places where network devices can be bound. To do this, the kernel changes what is called the “line discipline” of the tty device. Both SLIP and PPP are line disciplines that may be enabled on tty devices. The general idea is that the serial driver handles data given to it differently, depending on the line discipline it is configured for. In its default line discipline, the driver simply transmits each character it is given in turn. When the SLIP or PPP line discipline is selected, the driver instead reads a block of data, wraps a special header around it that allows the remote end to identify that block of data in a stream, and transmits the new data block. It isn't too important to understand this yet; we'll cover both SLIP and PPP in later chapters, and it all happens automatically for you anyway.