Network switch

Surprisingly, there is very little written about interfacing a private line circuit to your network.  MPLS networks are not always the right solution for WAN connectivity.  For point-to-point networks, a private line or international private line could be the most cost effective solution.  But private lines typically do not come with routers and management.  This makes some people nervous about how they will make it work, but it shouldn’t be a real concern.

A Private Line is a TDM circuit, using Time Division Multiplexing to cost-effectively share your circuit with others on a carrier network.  To function, you need a CSU/DSU.

A CSU/DSU (Channel Service Unit/Data Service Unit, but no one calls it that!) is a digital-interface device used to connect a  router to a digital circuit (for example a T1, E1 p DS3 line).

A CSU/DSU operates at the physical layer (layer 1) of the OSI model.

So what does a CSU do? A CSU is acts as a buffer between a LAN and a public carrier’s WAN to ensure signals placed on the public lines are appropriately timed and formed for the network. The DSU manages timing errors and signal regeneration, providing a “modem-like” function , it converts digital data frames from the local area network (LAN) into frames appropriate to a wide-area network (WAN) and vice versa. The CSU receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit. The CSU can also echo loopback signals from the phone company for testing purposes.

The CSU/DSU can be a separate piece of hardware mounted on the wall.  More commonly, it is simply a card that is part of a router.  The WIC (WAN Interface Card) may contain an integrated CSU/DSU that can be inserted into a router slot.

Once your Private Line has timing set up between the far and near ends, you then simply configure your router as you would for any other wide area network.