MTP defines the lower three layers of the SS7 protocol stack. Layer 1 (MTP1) describes the Physical layer, defining how higher layer messages are represented on the transmission medium, including message delimiting. Layer 2 (MTP2) defines the Data Link layer, ensuring the reliable delivery of higher layer MSU (Message Signalling Units). Layer 2 also features FISU (Fill In Signalling Units) for link keep-alives, as well as LSSU (Link Status Signalling Units) in order to support signalling link establishment and maintenance. Layer 3 (MTP3) is comprised of two elements – SMH (Signalling Message Handling) and SNM (Signalling Network Management). SMH ensures higher layer signalling can be routed through the SS7 network (with the use of Point Codes). SNM is a route management facility for handling failures etc.