The channelized T1/ ISDN PRI modules are available with and without a built-in CSU and with one or two ports. The channelized T1/ISDN PRI modules connect to an external CSU. The channelized T1/ISDN PRI CSU modules connect directly to the network. Each of the T1 modules provides up to 24 virtual channels per T1 port.The channelized E1/ISDN PRI modules are available with one or two E1 ports with balanced or unbalanced interfaces and are compatible with the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.703/G.704 specifications. The channelized E1/ISDN PRI modules receive and transmit data bidirectionally at the E1 rate of 2.048 Mbps and provide up to 30 virtual channels per E1 port.