Channels are defined by the number and position of their corresponding burst period. The capacity associated with a single timeslot on a single ARFCN is called a physical channel (PCH) and referred to as "CnTm" where n is a carrier index and m is a timeslot index (0-7). Each timeslot is occupied by a radio burst with a guard interval, two payload fields, tail bits, and a midamble (or training sequence). The lengths of these fields vary with the burst type but the total burst length is 156.25 symbol periods. The most commonly used burst is the Normal Burst (NB). The fields of the NB are: Midamble A 26-bits training sequence that helps in multipath equalisation at the center of the burst "Stealing bits" each side of the midamble, used to distinguish control and traffic payloads Payload two 57-bit fields, symmetric about the burst Tail bits 3-bit field, at each end of the burst Guard period 8.25-symbols at the end of the burst There are several other burst formats, though. Bursts that require higher processing gain for signal acquisition have longer midambles. The random access burst (RACH) has an extended guard period to allow it to be transmitted with incomplete timing acquisition. Burst formats are described in GSM 05.02 Section 5.2. Multiplexing and timing Each physical channel is time-multiplexed into multiple logical channels according to the rules of GSM 05.02. One logical channel constitute of 8 burst periods (or physical channels) which is called a Frame. Traffic channel multiplexing follows a 26-frame (0.12 second) cycle called a "multiframe". Control channels follow a 51-frame multiframe cycle. The C0T0 physical channel carries the SCH, which encodes the timing state of the BTS to facilitate synchronization to the TDMA pattern. GSM timing is driven by the serving BTS through the SCH and FCCH. All clocks in the handset, including the symbol clock and local oscillator, are slaved to signals received from the BTS, as described in GSM 05.10. BTSs in the GSM network can be asynchronous and all timing requirements in the GSM standard can be derived from a stratum-3 OCXO. Coding The coding sublayer provides forward error correction. As a general rule, each GSM channel uses a block parity code (usually a Fire code), a rate-1/2, 4th-order convolutional code and a 4-burst or 8-burst interleaver. Notable exceptions are the synchronization channel (SCH) and random access channel (RACH) that use single-burst transmissions and