Abstract: As a formal notation for telecommunication systems, SDL (Specification and Description Language, CCITT Z.100) has been widely used for modeling, analyzing, and simulations of communication protocols and/or real time systems. This paper first introduces a Timed-SDL to incorporate timing and probabilistic specifications into the original SDL functional descriptions. Then this Timed-SDL is used to specify, the formal models of communication protocols. These formal models also support validations and performance evaluations of communication protocols. The principles of the "observer" concept are adopted in the paper. The "observer" concept associates each SDL process in a system model with an observer process. Each observer process allows: (i) to date all the time stamps upon the occurrences of the inbound and outbound signals of the process being observed (Performance prospect); (ii) to describe the temporal properties that are needed to be validated on the process being observed (Validation prospect). By using the "observer" concept, both prospects of formal specification of communication protocols are investigated based on the Timed-SDL.