R&S M3SR 4100 Series, Operating Manual

Page: 342 / 706

M3SR Series 4100 Basics of ALE-2G LP
4.6 Operating Manual 6175.4760.02 – 01
4.3.2 Basics of ALE-2G LP
The ALE-2G LP (Automatic Link Establishment 2
nd
Generation with Linking Protection) pro-
vides additional FED-STD-1049 capability. Compliance with this standard ensures that all in-
formation in the link protocol, such as e.g. addresses and network relations, is protected
against tapping. The protective function only covers the linking procedure. To safeguard the
subsequent data or voice transmission, additional cryptographic measures must be taken at
the transmitting and receiving stations.
The link protection function is performed in compliance with FED-STD-1049, the
Rohde & Schwarz implementation supporting three of the five protection levels defined by
FED-STD-1049, namely AL-0, AL-1 and AL-2. Protection level AL-2 guarantees maximum
security (protection interval 2 s) at high demands on network synchronization, whereas pro-
tection level AL-1 provides slightly less security (protection interval 60 s) at lower demands
on network synchronization. The protection interval defines the time during which the input
quantities of the encryption algorithm are constant.
The 24 bit ALE words for linking are encrypted by means of the lattice algorithm. Input quan-
tities for this algorithm are the user-defined key, the frequency, date and time of day. For
maintaining synchronism, a time acquisition protocol is started as soon as a certain time un-
certainty threshold has been exceeded. The key can be 56 bits long, allowing as many as
2
56
different keys in the network.
This procedure assumes a time synchronous network. For initial synchronization and for
maintaining synchronism various procedures are used. One station within the network is the
time master station (= time server) providing the other stations (net stations) with the exact
time.
Each station determines its own time uncertainty, also taking into account the precision of
the internal clock. When time information from the time server is being received, delay and
internal processing times will also be allowed for. The radio uses a clock with a precision of
±0.5 ppm (parts per million), so that even with a 2 s protection interval (AL-2) re-synchroni-
zation will only be necessary after 23 days or more (see Table 4.2). With the transceiver be-
ing switched off, however, the clock precision is not better than ±7.5 ppm.
The FED-STD-1049 defines eight time quality levels.
Table 4.2 Time Uncertainty Definition
Time quality code Time uncertainty
0 none
1 20 ms
2 100 ms
3 500 ms
4 2 s
5 10 s
6 60 s
7 unbound