Disharmony,
The MS out of memory means that the “RAM” of the phone is full and it cannot store incoming frames anymore RLC frames.
You might ask : “Why is the MS storing RLC frames ?”
Mainly because it is trying to recover lost data : when a frame is not decoded correctly by the mobile, it stores the bad frame and wait for a re-transmission of this same frame. Even if the second identical frame can’t be decoded, by comparing both frames, the MS might be able to recover the actual frame. (cf. puncturing schemes)
Therefore, when the MS memory is full, this feature can’t be used anymore. The MS has to understand each frame one by one. It means the BTS has to use a much “safer” (or “resistant”) coding scheme, without a need to match the previous RLC coding scheme’s family.
The charts were performed by 3GPP, in order to know which coding scheme to use under different conditions of BER, coding scheme, and MS memory’s state.