The CT3IP provides four T1 connections via DB-15 connectors and one DS3 connection via two BNC connectors. Each DS3 interface can provide up to 28 T1 channels (a single T3 group). Each channel is presented to the system as a serial interface (using a DB-15 connector) that can be configured individually. The CT3IP can transmit and receive data bidirectionally at the T1 rate of 1.536 Mbps. The individual T1 connections use 100-ohm twisted-pair serial cables to external channel service units (CSUs) or to a MultiChannel Interface Processor (MIP). External T1 channels do not provide CSU functionality and must connect to an external CSU. For wide-area networking, the CT3IP can function as a concentrator for a remote site. The T3 connection, provided by way of two female BNC connections for transmit (TX) and receive (RX), has an impedance of 75 ohms.