The PA-4E1G provide up to four E1 synchronous serial interfaces, which are compatible with and specified by G.703/G.704. The PA-4E1G network interfaces provide a connection between standard serial interfaces such as V.35 to telephone lines or Post, Telephone, and Telegraph (PTT) networks. Each PA-4E1G interface operates in full-duplex mode at E1 (2.046-Mbps) speed.The PA-4E1G interfaces do not operate in the data terminal equipment (DTE) and data communications equipment (DCE) modes that are typical of data communications interfaces. The PA-4E1G interfaces operate with either a line-recovered or an internal clock signal. The default is for a line clock signal that the interface recovers from the received data stream. The interface can also operate with an internal clock signal. The PA-4E1G generates the internal clock signal; the interface does not use the motherboard or system clock.Each PA-4E1G interface is available in either balanced (120-ohm) or unbalanced (75-ohm) mode; a unique port adapter supports each type. Neither the balanced and unbalanced modes nor the balanced and unbalanced cables are interchangeable; you cannot configure a balanced port to support an unbalanced line, nor can you attach an interface cable intended for a balanced line to an unbalanced port. Balanced interfaces typically use three conductors and three signal states: high, low, and ground. The high and low signals mirror each other. Unbalanced interfaces use only two signals: signal and ground. You can discover the mode of each interface by examining the agency approval label on each port adapter, or by using the show controller cbus command.