Why we use unaligned PER in case of RRC and aligned PER ASN format in case of S1/X2 ?

Question

As per standard we use ASN Encoding and Decoding format to make our code platform independent and language independent.

In case of LTE RRC- We use unaligned PER and for S1/X2 we use aligned PER ...

Is there any specific reason for different ASn Encoding/Decoding format we have used ???

Answer 1

There are two most common method of Encoding in telecom BER and PER i.e. basic encoding rules and packet encoding rules. If you see the structure of BER its is basically TLV (TLIV sometime) or something similar which means TAG + Length + Value. So for each IE there is a overhead of TAG+Length which is 4 bytes in general and is a huge overhead.

This overhead is ok if interface is wire-line but on air each bit/byte is costly specially because of the cost and scarcity spectrum. You want to minimize all overhead on Air and maximize the use of data that is the reason why packet encoding is used at RRC/ S1AP(contains NAS message) etc.

Comments

My bad read the question as PER and BER...

---

Yes Salil,That is right ...is the same reason is also valid for using aligned and unaligned version of PER as X2/S1 interfaqced is wire-line and UE-ENB RRC interface is wireless ...

---

Yes reason is similar.
With the unaligned encoding, the bits are packed with no regard for octet (byte) boundaries. With aligned encoding, certain types of data structures are aligned on octet boundaries, meaning there may be some number of wasted padding bits. So aligned is between BER and unaligned PER.
Unaligned encoding uses the least number of bits, but presumably at some cost in processing time which is a tradeoff and x1/s1 interfaces chosen aligned in place of unaligned.