  Molecular communication is set to play an important role in the design of
complex biological and chemical systems. An important class of molecular
communication systems is based on the timing channel, where information is
encoded in the delay of the transmitted molecule---a synchronous approach. At
present, a widely used modeling assumption is the perfect synchronization
between the transmitter and the receiver. Unfortunately, this assumption is
unlikely to hold in most practical molecular systems. To remedy this, we
introduce a clock into the model---leading to the molecular timing channel with
synchronization error. To quantify the behavior of this new system, we derive
upper and lower bounds on the variance-constrained capacity, which we view as
the step between the mean-delay and the peak-delay constrained capacity. By
numerically evaluating our bounds, we obtain a key practical insight: the drift
velocity of the clock links does not need to be significantly larger than the
drift velocity of the information link, in order to achieve the
variance-constrained capacity with perfect synchronization.
