5.26.3.1. Synchronization Interfaces 
All digital connections can be used for synchronization. The following
interface types exist:
ATM: STM1 or OC3 (bit rate 155,52 Mbps);
PMA: E1 (S2M, S2M FV, PCM30) (bit rate 2,048 Mbps), T1, T1.5 Accunet
(bit rate 1,544 Mbps);
BA: T0, S0, U-2B1Q (bit rate 160 Kbps) and data interfaces in accordance
with ITU X.21 with bit rates between 1,200 bps and 64,000 bps.
Depending on the network topology not all interfaces on the different
levels are allowed as tie trunk connections because the elastic buffers
have different sizes:
|
Level
|
Admissible tie trunk connections
|
|
1
|
S1, T1, S2, PCM30, STM1, S0, X.21
|
|
2
|
S1, T1 *, S2, PCM30, STM1, S0
|
|
3
|
S1, T1 *, S2, PCM30, STM1
|
|
4
|
S1, T1 *, S2, PCM30, STM1
|
-
* : In the case of network connections routed exclusively via AT&T's T 1.5 Accunet service only one level with T1 tie trunk connections is allowed because the network jitter/drift may be up to 90 µsec high, but the T1 interface components have only +/- 125 µsec buffers.In the OpenScape 4000 system, all available digital interfaces for synchronization can be set if the reference clock table size is matching. This makes it possible to deal with not only simple failures but also double and quadruple errors in a predefined manner. Always make sure that automatic changeover of reference clock routes in a network do not cause clock loops. Experience with many networks shows that leased lines used for tie trunks often cause problems because it is necessary to determine whether or not they are clocked. Also clock quality and availability of local exchange trunks is normally better than the quality and availability of leased lines.