The framing is completely different (24 channels per frame vs. 30/32 channels per frame), however the HDB3 encoding is for similar reasons for our B8ZS. It all has to do with ones density and ensuring the signal can be properly recovered by the repeaters. The framing picture is different from our perspective. You have the option of getting a framed or unframed E1, the difference being whether or not two channels are reserved for telco/signalling (there is no concept of RBS in the E-carrier system).