The BLAS processes communicate with each other via calls to the BLACS.
For the sake of simplicity, we model a restricted subset of the possible
BLACS operations, namely point-to-point communication and
broadcast/combine operations along a row or column of the mesh.
It is customary to model the time for sending a message of 
items
between two processes by
where 
denotes the latency, and 
the inverse of
the bandwidth. The broadcast/combine operations are more complicated
since the BLACS allow the user to specify a topology argument
[10][8]. We estimate the cost of broadcasting
items using a split-ring topology to 
processes by
.
Similarly, the cost of a 1-tree combine operation of items
involving 
processes is estimated by
.
At this level of the model, it is not possible to determine the
values of 
because no assumption has been made so far on the
physical network to model. This justifies the introduction of
these functions 
.