When we talk about communication between the User Equipment (UE) and the network in LTE, we usually think about signaling messages such as RRC (Radio Resource Control) or NAS (Non-Access Stratum) messages. However, LTE introduces additional communication paths at the MAC (Medium Access Control) layer. This involves the exchange of control commands between the UE and the network, specifically for managing various aspects of the communication, not for transmitting data traffic.

In contrast to UMTS, where only RRC and NAS messages serve as the communication between UE and network, LTE introduces special MAC structures known as ‘MAC CE’ (MAC Control Element) that carry specific control information. These MAC CEs are implemented as special bit strings within the LCID (Logical Channel Identifier) field of the MAC Header.

table from 3GPP specification 36.213 illustrates the various LCID types in the MAC header

MAC CE (Control Element):

Here are some key details about LTE MAC CE:

• A MAC CE is a form of in-band control signalling in LTE.
• MAC CEs are identified by reserved values in the LCID (Logical Channel ID) field.
• The LCID value indicates the type of control information carried by the MAC CE.
• Both fixed-length and variable-length MAC CEs are supported, depending on the use case.
• For downlink transmissions, MAC CEs is located at the beginning of the MAC PDU.
• For uplink transmissions, MAC CEs is located at the end, immediately before the padding.

MAC CEs specified for different purposes:

SCell activation/deactivation:

• Consists of one or four octets, identified by MAC PDU subheaders with LCID indices 58 or 57, respectively.
• Contains C-fields (mapped to component carriers) and an R-field.

Activation/Deactivation MAC control element of one octet

Activation/Deactivation MAC control element of four octets

DRX (Discontinuous Reception):

• DRX and long DRX command MAC CEs are identified by MAC PDU subheaders with LCID indices 60 and 59, respectively.
• Fixed size of zero bits.

Timing advance command:

• Identified by a MAC PDU subheader with LCID index 61.

Timing Advance Command MAC control element

UE contention resolution identity:

• Identified by a MAC PDU subheader with LCID index 62.
• Fixed size of 48 bits, containing UE contention resolution identities.

UE Contention Resolution Identity MAC control element

C-RNTI (Cell Radio Network Temporary Identifier):

• Identified by a MAC PDU subheader with LCID index 58.
• Fixed size with a single field containing the C-RNTI, 16 bits in length.

C-RNTI MAC control element

BSR (Buffer Status Report) MAC CEs:

• Short BSR format (fixed size), long BSR format (variable size), short truncated BSR format (fixed size), and long truncated BSR format (variable size).
• Identified by MAC PDU subheaders with LCID indices 59, 60, 61, and 62, respectively.

Short BSR and Truncated BSR MAC control element

Long BSR MAC control element