Layer 1 protocols (Physical layer)
ISDN = Integrated Services Digital Network
PDH = Plesiochronous Digital Hierarchy
T-carrier = (T1, T3 etc)
RS-232 = a serial line interface originally developed to connect modems and computer terminals
SDH = Synchronous Digital Hierarchy
SONET = Synchronous Optical NETworking

Layer 2 protocols (Data link layer)
ARP =Address Resolution Protocol
ARCnet
CDP = Cisco Discovery Protocol
DCAP = Data Link Switching Client Access Protocol
Econet = data unavailable
Ethernet = data unavailable
FDDI Fiber Distributed Data Interface
Frame Relay  = data unavailable
HDLC High Level Data Link Control
LocalTalk  = data unavailable
L2F Layer 2 Forwarding Protocol
L2TP Layer 2 Tunneling Protocol
PPP Point-to-Point Protocol
PPTP Point-to-Point Tunneling Protocol
SLIP Serial Line Internet Protocol (obsolete)
StarLan = data unavailable
Token ring = data unavailable

Layer 2+3 protocols
ATM Asynchronous Transfer Mode
Frame relay a simplified version of X.25
MPLS Multi-protocol label switching
Signalling System 7, also called SS7, C7 and CCIS7; a common PSTN control protocol.
X.25