escalator at high speed and jump straight onto the third step. Behind that child might be an injured athlete with a cane, who cautiously waits while several stairs pass, until confident of stepping on the center of the stair. So people get on the escalator at varying rates and in different places; no consistent timing determines their presence. The escalator scenario describes the modern broadband network. SDH/SONET is a synchronous network infrastructure. When bits get into an SDH/SONET frame, they all travel at OC-3 or OC-12 or one of the other line rates that SDH/ SONET supports. But access onto that network might be asynchronous, through an ATM switch, where a movie might be coming in like a fire hose of information Chapter 5 Data Communications Basics through one interface and next to it a dribble of text-based e-mail is slowly passing through. One stream of bits comes in quickly, and one comes in slowly, but when they get packaged together into a frame for transport over the fiber, they're trans- ported at the same rate. Error Control Error control, which is a process of detecting and/or correcting errors, takes a num- ber of forms, the two most common of which are parity checking and cyclic redun- dancy checking. In ASCII-based terminals, which use asynchronous transmission, most often the error control is parity checking. Parity checking is a simple process of adding up the bit values to come up with a common value, either even or odd. It doesn't matter which one, but once you've selected either even or odd, every terminal must be set to that value. Let's say we're using odd parity. If you add up the bits for char- acter #1 in Figure 5.6, you see that they equal 2, which is an even number. We need odd parity, SO the terminal inserts a 1 bit to make that a 3, which is an odd number. For character #2 the bits add up to 3, SO the terminal inserts a 0 as a parity bit to maintain the odd value. The terminal follows this pattern with each of the six characters, and then it sends all the bits across the network 