The correct answer is "It is required for B2 class systems to protect against covert timing channels". Orange Book B2 class systems do not need to be protected from covert timing channels. Covert channel analysis must be performed for B2-level class systems to protect against covert storage channels only. B3 class systems need to be protected against both covert storage channels and covert timing channels.

From the glossary of Computer Security Basics: The Bell-LaPadula model is the security policy model on which the Orange Book requirements are based. From the Orange Book definition, "A formal state transition model of computer security policy that describes a set of access control rules. In this formal model, the entities in a computer system are divided into abstract sets of subjects and objects. The notion of secure state is defined and it is proven that each state transition preserves security by moving from secure state to secure state; thus, inductively proving the system is secure. A system state is defined to be 'secure' if the only permitted access modes of subjects to objects are in accordance with a specific security policy. In order to determine whether or not a specific access mode is allowed, the clearance of a subject is compared to the classification of the object and a determination is made as to whether the subject is authorized for the specific access mode." The Biba Model is an integrity model of computer security policy that describes a set of rules. In this model, a subject may not depend on any object or other subject that is less trusted than itself. The Clark Wilson Model is an integrity model for computer security policy designed for a commercial environment. It addresses such concepts as nondiscretionary access control, privilege separation, and least privilege. TEMPEST is a government program that prevents the compromising electrical and electromagnetic signals that emanate from computers and related equipment from being intercepted and deciphered. Source: RUSSEL, Deborah & GANGEMI, G.T. Sr., Computer Security Basics, O'Reilly, 1991. Also: U.S. Department of Defense, Trusted Computer System Evaluation Criteria (Orange Book), DOD 5200.28-STD. December 1985 (also available here).

Isochronous data is synchronous data transmitting without a clocking source, with the bits sent continuously and no start or stop bits. All bits are of equal importance and are anticipated to occur at regular time intervals.
*asynchronous, is a data transmission method using a start bit at the beginning of the data value, and a stop bit at the end of the value.
*synchronous, is a messageframed transmission method that uses clocking pulses to match the speed of the data transmission.
*pleisiochronous, is a transmission method that uses more than one timing source, sometimes running at different speeds. This method may require master and slave clock devices. Source: Communications Systems and Networks by Ray Horak (M&T Books, 2000).