xclusively, AMI signaling is a safe choice. Later in this chapter, we will further explore network signaling, when we introduce out-of- band signaling ESF/B8ZS is not to be confused with out-of-band signaling, which uses an entirely separate packet-switched network overlay to pass network information. Circuit-switched Networks Channel Service Units/Data Service Units Leased line connections rely on special terminating equipment. When the leased line is a T-1, this equipment is of two types, a channel service unit (CSU) and a data service unit (DSU). In most cases, a CSU includes a DSU. Customer premises equipment (CPE)-particularly equipment with a data rate lower than 56 Kbps-produces asynchronous bit streams. Asynchronous transmission relies on start and stop bits to distinguish between bytes of data. Furthermore, in asynchronous traffic, the interval between bytes is usually arbitrary. The digital signaling hierarchy, on the other hand, relies on synchronous signaling, in which senders and receivers exchange clocking information in order to identify the boundaries between units of data. CSUs and DSUs are used to condition asynchronous bit streams, SO that they can be passed over the synchronous network. DSUs connect to CPE via RS-232 or V.35 interfaces. They adjust between asynchronous and synchronous timing systems by putting bytes of data onto the network at precise intervals. This activity is known as rate adaptation. Second, a DSU converts digital pulses from the format used by video communication (or other computerized equipment) into the format used by the network. In computers and codecs, for example, ones are represented by a +5 volt charge and zeros are represented by the absence of voltage. This is called unipolar nonreturn to zero signaling. Carriers use bipolar signaling, in which a zero voltage level represents zeros, and ones alternate between +3 and -3 volts. In networks, bipolar signaling is preferable to unipolar signaling, which tends to build up a DC charge on the line. The CSU ensures that 