An SA may include attributes such as: cryptographic algorithm and mode; traffic encryption key; and parameters for the network data to be passed over the connection.
[1] An SA is a simplex (one-way channel) and logical connection which endorses and provides a secure data connection between the network devices.
The fundamental requirement of an SA arrives when the two entities communicate over more than one channel.
So both the base station and mobile subscriber will share the SAID, and they will derive all the security parameters.
In other words, an SA is a logical group of security parameters that enable the sharing of information to another entity.