This Recommendation covers digital line systems for the transmission of signals based on the 1544 kbit/s
hierarchy on symmetric pair cables and includes systems operating at the following bit rates:
1544 kbit/s,
3 152 kbit/s,
63 12 kbit/s.
ITU-T G.951-1988 history
1988ITU-T G.951-1988 DIGITAL LINE SYSTEMS BASED ON THE 1544 KBIT/S HIERARCHY ON SYMMETRIC PAIR CABLES