Section: 6.2.2 MAC header for Random Access Response
Content: The MAC header is of variable size and consists of the following fields: - E: The Extension field is a flag indicating if more fields are present in the MAC header or not. The E field is set to "1" to indicate at least another set of E/T/RAPID fields follows. The E field is set to "0" to indicate that a MAC RAR or padding starts at the next byte; - T: The Type field is a flag indicating whether the MAC subheader contains a Random Access ID or a Backoff Indicator. The T field is set to "0" to indicate the presence of a Backoff Indicator field in the subheader (BI). The T field is set to "1" to indicate the presence of a Random Access Preamble ID field in the subheader (RAPID); - R: Reserved bit, set to "0"; - BI: The Backoff Indicator field identifies the overload condition in the cell. The size of the BI field is 4 bits; - RAPID: The Random Access Preamble IDentifier field identifies the transmitted Random Access Preamble (see clause 5.1.3). The size of the RAPID field is 6 bits. The MAC header and subheaders are octet aligned. NOTE: For NB-IoT, the Random Access Preamble IDentifier field corresponds to the start subcarrier index.