Blocking occurs when an insufficient number of B-channels are available to make a call. Narrowband calls require only one channel, so blocking is less likely than with wideband calls that require multiple B-channels. Blocking also occurs for wideband calls when bandwidth is unavailable in the appropriate format, such as fixed, floating, or flexible.

To reduce blocking, Communication Manager selects trunks for both wideband calls and narrowband calls to maximize the availability of idle fixed channels for H0, H11, and H12 calls, and idle floating channels for N x DS0 calls that require a contiguous bandwidth. The strategy for preserving idle channels depends on the channel type. The chances for blocking are reduced if you use a flexible algorithm, assuming that the algorithm is supported on the other end.

The following table describes the blocking strategy for the different channel types.