re in the voice path of the call. Tones are sent at the beginning of the call (sometimes you can hear them) from switch to switch (when CAS signaling is used) to provide signals for the switch to make decisions about how to route your call. There are also signals within the channel that are not audible. These signals are bits embedded in the voice data; they are called the ABCD bits and are used to report on the status of phones (e.g., off-hook/on-hook). Out-of-band signaling, in contrast, works similarly to the FTP protocol: a channel is used to set up the call, and then a separate channel is chosen and used to deliver the payload (in this case, a voice call). Encoding Encoding refers to the method by which electrical signals are generated and decoded. There are two types of encoding in use on T1 links today: Alternate Mark Inversion (AMI), and Binary Eight Zero Substitution (B8ZS). Generally, AMI is used for voice circuits, and B8ZS is used for data. B8ZS can be used for voice, but AMI should not be used for data, for reasons noted next. AMI is a method of encoding that inverts alternate marks. In T1 signaling, there are two possible states: mark and space. Simply put, a mark is a one and a space is a zero. On a T1, a space is OV, and a mark is either +5V or -5V. AMI encodes the signal such that the polarity of each mark is the opposite of the one preceding it. Encoding This allows for some interesting error-detection techniques. For example, Fig- ure 21-2 shows two ones occurring in a row with the same polarity. This is considered an error (a bipolar violation, or BPV). If all ones were positive voltage, a voltage spike could be misconstrued as a valid one. As an added benefit, when the alternating marks are flipped, the average voltage of the physical line will always be OV, making the phys- ical T1 wires safe to handle. Space (0) Mark (1) Bipolar violation Figure 21-2. T1 AMI signaling T1s are asynchronous links, meaning that only one side of the link provides clocking. The far side of the link must rely on 