  With the help of information and communication technologies, studies on the
overall social networks have been extensively reported recently. However,
investigations on the directed Ego Communication Networks (ECNs) remain
insufficient, where an ECN stands for a sub network composed of a centralized
individual and his/her direct contacts. In this paper, the directed ECNs are
built on the Call Detail Records (CDRs), which cover more than 7 million people
of a provincial capital city in China for half a year. Results show that there
is a critical size for ECN at about 150, above which the average emotional
closeness between ego and alters drops, the balanced relationship between ego
and network collapses, and the proportion of strong ties decreases. This paper
not only demonstrate the significance of ECN size in affecting its properties,
but also shows accordance with the "Dunbar's Number". These results can be
viewed as a cross-culture supportive evidence to the well-known Social Brain
Hypothesis (SBH).
