One of the most common types of WAN connections is the point-to-point connection. As shown in Figure 1, point-to-point connections are used to connect LANs to service provider WANs, and to connect LAN segments within an enterprise network.
A LAN-to-WAN point-to-point connection is also referred to as a serial connection or leased-line connection. This is because the lines are leased from a carrier (usually a telephone company) and are dedicated for use by the company leasing the lines. Companies pay for a continuous connection between two remote sites, and the line is continuously active and available. Leased lines are a frequently used type of WAN access, and they are generally priced based on the bandwidth required and the distance between the two connected points.
Understanding how point-to-point serial communication across a leased line works is important to an overall understanding of how WANs function.
Communications across a serial connection is a method of data transmissions in which the bits are transmitted sequentially over a single channel. This is equivalent to a pipe only wide enough to fit one ball at a time. Multiple balls can only go into the pipe, but only one at a time, and they only have one exit point, the other end of the pipe. A serial port is bidirectional, and often referred to as a bidirectional port or a communications port.
This is in contrast to parallel communications in which bits can be transmitted simultaneously over multiple wires. As shown in Figure 2, a parallel connection theoretically transfers data eight times faster than a serial connection. Based on this theory, a parallel connection sends a byte (eight bits) in the time that a serial connection sends a single bit. However, parallel communications do have issues with crosstalk across wires, especially as the wire length increases. Clock skew is also an issue with parallel communications. Clock skew occurs when data across the various wires does not arrive at the same time, creating synchronization issues. Finally, most parallel communications supports only one-direction, outbound-only communication from the hard drive.
At one time, most PCs included both serial and parallel ports. Parallel ports were used to connect printers, computers, and other devices that required relatively high bandwidth. Parallel ports were also used between interior components. For external communications, a serial bus was primarily used for signal conversion. Because of their bidirectional ability, serial communications are considerably less expensive to implement. Serial communications use fewer wires, cheaper cables, and fewer connector pins.
On most PCs, parallel ports and RS-232 serial ports have been replaced by the higher speed serial Universal Serial Bus (USB) interfaces. However, for long-distance communication, many WANs use still serial transmission.