1.2. Why use a serial console?

Linux is an good operating system for deployment at unstaffed sites. Linux is also good at hosting critical network infrastructure such as DNS and DHCP servers. These servers are generally installed at every site of an organisation including sites which may be too small or too remote to have information technology staff.

System administration of these remote computers is usually done using SSH, but there are times when access to the console is the only way to diagnose and correct software failures. Major upgrades to the installed distribution may also require console access.

In these cases the serial console is attached to a modem. Access to the console is gained from a remote computer by dialing into the modem. This allows the console to be reached from any telephone socket.

Clusters of personal computers can outperform mainframe computers and form competitive supercomputers for some applications. See the Cluster-HOWTO for more information on clustering.

These clusters are typically assembled into 19 inch telecommunications equipment racks and the system unit of each computer is typically one rack unit (or 1.75 inches) tall. It is not desirable to put a keyboard and monitor on each computer, as a small cathode ray tube monitor would consume the space used by sixteen rack units.

A first glance it seems that a monitor and keyboard switch is the best solution. However the VGA signal to the monitor is small, so even with the switch the monitor cannot be placed very far away from the rack of computers.

It is desirable to allow the consoles to be monitored in the operators' room of the computer center, rather than in the very expensive space of the machine room. Although monitor switches with remote control and fiber optical extensions are available, this solution can be expensive.

A standard RS-232 cable can be 15 meters in length. Longer distances are easily possible. The cabling is cheap. Terminal servers can be used to allow one terminal to be access up to 90 serial consoles.

This is useful in two very different cases.

Kernel programmers are often faced with a kernel error message that is displayed a split second before the computer reboots. A serial console can be used to record that message. Another Linux machine can be used as the serial terminal.

Some secure installations require all security events to be unalterably logged. A way to meet this requirement is to print all console messages. Connecting the serial console to a serial printer can achieve this.[1]

Linux is increasingly being as the operating system in embedded applications. These computers do not have keyboards or screens.

A serial port is a cheap way to allow software developers to directly access the embedded computer. This is invaluable for debugging. Most chip sets designed for embedded computers have a serial port precisely for this purpose.

The shipping product need not present the RS-232 port on an external connector. Alternatively the RS-232 port is often used for downloading software updates.

Linux is increasingly being used as the operating system inside telecommunications equipment. The Carrier Grade Linux consortia hopes to accelerate and coordinate this trend.

Most telecommunications equipment is remotely managed from a distant computer. However, site technicans (called craft personnel in telco-speak) need to access the equipment to test installation changes, check the status of reported faults, and so on. The terminal used by the craft personnel is called the craft terminal. The craft terminal plugs into the craft interface on the equipment.[2]

It is common, but not universal, industry practice for craft terminals to be a DEC VT100 terminal (or an emulator). The craft interface is commonly assumed to be an RS-232 DCE interface, running at 9600bps with 8 data bits, no parity, one stop bit, no handshaking and no status lines. An RJ45 connector is sometimes used in place of RS-232's standard 25-pin D connector. The DCE interface allows the craft terminal to be connected without a special null modem cable.