Juniper E Series Hardware Manual page 71

There are two input ports on the SRP I/O module for external clock sources.
These ports provide a way to ensure that the E-series router system clock
remains synchronized with the network's system clock. The primary clock is
labeled A; the secondary, redundant clock is labeled B.
Use the connectors appropriate for your locale (European or
North American standard):
European—Two 75-ohm E1 2.048-Mbps inputs with BNC connectors
North American—Two 100-ohm T1 inputs with three pin wire-wrap
connectors. Pins are labeled T (tip), G (ground), and R (ring). We recommend
using 26-AWG wire minimum.
NOTE: Use shielded cables to connect the external clock sources to the clock
source input ports.
Cabling E-series Routers
Cabling the SRP I/O Module
55