30B + D is the primary rate for E1 configurations. What ever happened to E1's second separate channel? What is the aggregate bit rate of 2B + D? First for CCITT, then for North American ISDN ? Distinguish TE1 and TE2 in the standard ISDN network interface. European ISDN would have an NT12 interface. The equipment at this interface would belong to whom? In North America, this interface is split: NT1 and NT2. Discuss the difference.