SS7 Basic Signal Unit format

The following excerpt from the "China domestic telephone network No.7 signal Mode technical Specifications (Interim provisions) gf001-9001".

There are three basic signal cell formats, namely the message Signal Unit (msu:message Signal unit), the link State Signal unit (Lssu:link status Signal unit), and the Fill signal Unit (fisu:fill-in Signal unit). They are distinguished by the length representation contained in the cell.
The Message Signal Unit (MSU) format is as follows:

The Link State Signal Unit (LSSU) format is as follows:

The Fill Signal Unit (FISU) format is as follows:

My understanding:
It is observed that, regardless of the message, it begins with F (sign code), BSN (back ordinal), BIB (back-to-front), FSN (Forward ordinal), FIB (forward-expression bit), and the length of each field is fixed. There is a difference from Li (length representation), so you can tell by analyzing Li to differentiate the message type.
Li=0 is the filling signal Unit (FISU);
Li=1 or 2 o'clock, is the link state Signal Unit (LSSU);
When Li>2, it is the message signal Unit (MSU).
Of the three basic signal units, the message Signal Unit (MSU) is the longest, because it carries a variety of call information, more complex, but also our focus on research objects.
In the message Signal Unit (MSU), the SIO (business information eight-bit group) structure is as follows:

The meaning of its business expression:

Of these, tup and IsUp are important two types of messages.

In the Child Business field, A and B bits are standby, which represents the priority of the message. C and D are also called network representations, meaning the following: