A multiblock is a 32 block container which consists of a concatenation of 32 blocks. An extended multiblock is a concatenation of multiple multiblocks, where E defines the number of multiblocks in an extended multiblock. A frame can be split between blocks and multiblocks, but there must be an integer number of frames in an extended multiblock. An extended multiblock is only necessary when a multiblock does not have an integer number of frames. If an extended multiblock is not used, because a multiblock contains an integer number of frames, then the E parameter is equal to 1 to indicate that there is one multiblock in an extended multiblock.

An extended multiblock is analogous to a multiframe in the 8B or 10B transport layer. The local extended mutiblock clock (LEMC) keeps track of the start and end of a multiblock for deterministic latency and data synchronization purposes in the same way the LMFC tracks the start and end of a multiframe in 8B or 10B encoding. The LEMC is reset by the SYSREF signal to a deterministic phase in both the transmitter and receiver in order to act as a timing reference for deterministic latency. The LEMC clock frequency is defined by Equation 9 where f_BIT is the serialized bit rate (line rate) of the SerDes interface. The frequency of SYSREF must equal to or an integer division of f_LMFC when using 64B or 66B encoding modes if SYSREF is a continuous signal.