Explain plan with high Network Cost

 Hi,
We have the next SQL:

select count (distinct(ef.fli_id)) AS num
FROM pp_fls_inf ef
JOIN pp_perss per ON ef.registro_per_id = per.per_id
JOIN pp_proff pro ON per.per_id=pro.per_id
LEFT OUTER JOIN pp_fli_tpr flt ON ef.fli_id = flt.fli_id
LEFT OUTER JOIN pp_tip_proff tpr ON flt.tpr_id=tpr.tpr_id
JOIN stk_niveles nvs on ((ef.nv1_id = nvs.nv1_id
        AND ef.nv2_id IS NULL
        AND ef.nv3_id IS NULL
        AND ef.nv4_id IS NULL)
    OR (ef.nv1_id = nvs.nv1_id