With the advance and development of sophisticated communication techniques, especially those involving considerably high data rates as may be encountered in high speed digital communication networks, particularly high speed optical digital communication systems, spare capacity or backup facilities are usually made a part of the network, in order to provide the network with the capability of maximizing the integrity of communication paths through it in the event of an outage or failure of an active communication link. A common solution to the problem of channel faults or data degradation is the provision of one or more protection channels that are normally maintained in a quiescent or standby condition; in response to instructions from channel monitoring equipment a protection channel may be substituted in place of an active channel that has been observed to have developed a fault or an adverse thruput condition that has led or could lead to a loss of data communication over the system.
Concomitant with the provision of such protection channel equipment is the need to ensure that the substitution of one channel for another is effected rapidly and without adversely affecting the data being transmitted over the channel. For low data rates the time window within which the channel substitution may occur permits a considerably wider error margin and does not necessarily require an observation of the clock signals themselves during the switching process. However, with the very high data rates of present day communication systems (e.g. 200-300 Mb/s) the degree of freedom allowed in switching between channels is significantly restricted. Still, as the demand for information transmission continuity is of paramount importance, some scheme must be provided to accurately control, within an extremely fine error window, the time at which the substitution of one channel for another is effected.
One such communication system in which the need arises is described in copending patent application Ser. No. 149,291, filed May 12, 1980, entitled Multi-channel, Repeatered, Fiber Optic Communication Network Having Fault Isolation System by P. Casper et al, and assigned to the assignee of the present application. The communication network described in that application employs a plurality of normally active optical communication channels each of which carries high speed (301 Mb/s) digital data, such as digital telephone traffic. One or more protection channels are provided to carry the digital traffic in the event of a detected problem with the normally active data channels. Advantageously, as described in that application, a channel switching system of the type to be explained in detail below is incorporated in the network in order that what is termed "hitless" switching between the active and protection channels may be effected. By "hitless" switching is meant the substitution of one data channel for another without the addition of or deletion of data bits from the link. Of course, this "hitless" or synchronized switching technique has applicability to data communications other than that described in the above cited application; and it is to be understood, therefore, that reference to such a network is not to be considered limitative of the invention but merely to illustrate an exemplary environment in which the present invention may be incorporated.