An ingress TDM network segment (101) and an egress TDM network segment (102) are connected by a packet-switched network (40). An ingress inter-working function (20) receives a TDM traffic flow (11) and an ingress data clock (12). A first mapping function (21) maps the TDM data (11) into first data containers using an ingress reference clock (22). The first mapping function conveys a difference between the ingress data clock (12) and the ingress reference clock (22) by a rate of justification operations to the mapped data within the first data containers. A second mapping function (23) maps the first data containers into second data containers using an ingress line clock (24). The mapping conveys a difference between the ingress reference clock (22) and the ingress line clock (24) by a rate of justification operations to the mapped data within the second data containers. Re-mapping is performed at boundaries between packet sub-networks (41, 42, 43).