thus have no interleavers. For speech channels, vocoder bits are sorted into importance classes with different degrees of encoding protection applied to each class (GSM 05.03). Both 260-bit vocoder frames and 184-bit L2 control frames are coded into 456 bit L1 frames. On channels with 4-burst interleaving (BCCH, CCCH, SDCCH, SACCH), these 456 bits are interleaved into 4 radio bursts with 114 payload bits per burst. On channels with 8-burst interleaving (TCH, FACCH), these 456 bits are interleaved over 8 radio bursts so that each radio burst carries 57 bits from the current L1 frame and 57 bits from the previous L1 frame. Interleaving algorithms for the most common traffic and control channels are described in GSM 05.03 Sections 3.1.3, 3.2.3 and 4.1.4. Data Link Layer (L2) The Um data link layer, LAPDm, is defined in GSM 04.05 and 04.06. LAPDm is the mobile analog to ISDN's LAPD. Network Layer (L3) The Um network layer is defined in GSM 04.07 and 04.08 and has three sublayers. A subscriber terminal must establish a connection in each sublayer before accessing the next higher sublayer. Radio Resource (RR). This sublayer manages the assignment and release of logical channels on the radio link. It is normally terminated in the base station controller (BSC). Mobility Management (MM). This sublayer authenticates users and tracks their movements from cell to cell. It is normally terminated in the Visitor Location Register (VLR) or Home Location Register (HLR). Call Control (CC). This sublayer connects telephone calls and is taken directly from ITU-T Q.931. GSM 04.08 Annex E provides a table of corresponding paragraphs in GSM 04.08 and ITU-T Q.931 along with a summary of differences between the two. The CC sublayer is terminated in the MSC. The access order is RR, MM, CC. The release order is the reverse of that. Note that none of these sublayers terminate in the BTS itself. The standard GSM BTS operates only in layers 1 and 2. Um logical channels Um logical channel types are outlined in GSM 04.03. Broadly speaking, non-GPRS Um logical channels fall into three categories: traffic channels, dedicated control channels and non-dedicated control channels. Traffic channels (TCH) These point-to-point channels correspond to the ISDN B channel and are referred to as Bm channels. Traffic channels use 8-burst(Break) diagonal interleaving with a new block starting on every fourth burst and any given burst containing bits from two different traffic frames.