Abstract:
A telephony system having automatic timing adjustment for diverse routing of HFC systems has a signal stream routing through a signal line loop between a head-end and at least one communication unit. If a fault occurs within the signal line loop, recognized by the head-end when a short uplink burst from the communication unit does not align within a predetermined timing window, the head-end will cycle through predetermined delay parameters corresponding to various alternate signal line loops made up of redundant signal lines. Once the predetermined delay parameter corresponding to the current alternate signal line loop is determined, normal operation of the system resumes.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to wireline communication links, and in particular, to timing adjustment of communication signals along a wireline. 
     BACKGROUND OF THE INVENTION 
     In a wireline communication system, a single trunk line will service many different individual users. For instance, a telephony system will often have various trunk lines fanning out from a main control/switching station, and each of these trunk lines then fan out to individual users. Often a trunk line will run from a head-end (control station and switching network) to a service area node. Many different users will be fed to the node and then networked onto the trunk line. 
     These trunk lines are often fiber optic cables which are capable of carrying a tremendous number of calls in comparison to cables made of metals. These optic cables or lines can carry signals at speeds much greater than conventional metal lines. 
     Telephony cable lines, whether optic or metal, operate in pairs such that a trunk line will consist of a downstream cable and an upstream cable forming signal line loops for the telephony signal streams to follow. Generally the up and downstream cables are installed along the same route and have the same length, although not necessarily installed in the same trench or on the same utility line. However, these main up and downstream lines generally parallel each other. 
     A major goal of telephony systems is to supply dependable use to users so that the system may be accessed even during emergencies. To be reliable, the system must have backup in case a line goes down. When a line goes down, a fault is registered within the system indicating that the signal stream cannot be routed through the main signal line loop. A line can go down for many reasons including natural forces causing a break in the line, construction digging into the area where a line is laid and breaking the line, maintenance on the line by the operator, or any other number of occurrences. Accordingly, operators of telephony systems install redundant trunk lines so that the telephony signal streams may be routed through the redundant trunk lines to form an alternate signal line loop. Generally there will be a designated downstream redundant line and a designated upstream redundant line. If the main, or signal line loop downstream line is broken or disfunctional, the head-end will route the signal stream through the redundant downstream line while using the upstream line of the main, or signal line loop. Similarly, the redundant upstream line may be used in a similar manner. In fact, any combination of the four lines may be used by the head-end. 
     Although any combination of the four lines may be used, it should be noted that the two main lines (down and upstream) of the signal line loop usually follow the shortest path from the head-end to the service node. For reliability reasons, the redundant lines must follow a different path, often making broad detours resulting in much longer lines than the signal loop lines. Therefore, if a line of the signal line loop is near a construction site, for instance, and capable of being damaged by digging at the site, the redundant line will not be affected since its routing is away from the same area. 
     The longer length of the redundant line naturally delays the time that the signal stream will take to go from the head-end to the service node and back. This presents a problem with time based signaling protocols such as time division multiple access (TDMA) protocols. Delaying the time a signal stream takes to go to the service node and back beyond the delay expected as the signal follows the main signal line loop alters the anticipated position of the signal stream and control information within the stream once the signal stream returns to the head-end. Communication links are lost and an adjustment must take place to align the signal stream from the service node to the head-end to a position in the protocol that the head-end will be expecting the signal stream to be in. 
     Accordingly, a method is needed in a telephony system having redundant signal lines to automatically detect when a fault in a signal line loop has occurred and compensate for any delays caused by routing the signal stream through the redundant signal line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a telephony system according to a preferred embodiment of the present invention. 
     FIG. 2 is a diagram of a system protocol according to a preferred embodiment of the present invention. 
     FIG. 3 is a diagram showing a portion of the system protocol of FIG. 3 according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates how a redundant signal line system in a telephony system  110  may be arranged. Under normal operating conditions, a signal stream will route from a head-end  120  to a service node  130  through a signal line loop  140 . Signal line loop  140  is defined as two signal lines through which a signal stream is routed, one line, or downstream signal line  142 , routing the signal stream from a transmitter  122  in head-end  120  to a receiver  132  of service node  130 . A second signal line or upstream signal line  144  is routed from a transmitter  134  of service node  130  to a receiver  124  of head-end  120 . 
     A number of communication units  152  are coupled to a cable  150  which is then coupled to combiner/splitter  136  of service node  130 . Service node  130  facilitates communication between the number of communication units  152  and the telephony network  110 . In a cable telephony system, the number of communication units  152  will be cable access units attached to a user&#39;s premise, be it a home or business, which allow the user two-way communication on the cable network. 
     As is well recognized, telephony cable lines, such as those making up signal line loop  140 , are often buried in the ground. This is usually a relatively safe place for these lines and they remain undisturbed under normal conditions. However, if there is construction anywhere along the length of the line, the line can be damaged or broken causing loss of telephony service to any user connected to that signal line loop. As mentioned earlier, damage to these lines can happen because someone digging in an area is not aware of or careful about the lines, or a natural disaster breaks or damages the line. Failures may also occur in the active fiber optic transmitters and receivers, resulting in loss of service. Since telephony service is considered essential, particularly in times of emergencies, telephony operators build in redundant signal lines. Generally there is a second downstream signal line  146  and a second upstream signal line  148 . These redundant signals lines will not follow the same course as the signal lines of signal line loop  140 . 
     If for some reason one or both of the signal lines of signal line loop  140  are damaged, head-end  120  will route the signal stream through one or both of the redundant signal lines. For instance, if downstream signal line  142  is damaged, head-end  120  will route the signal stream through second downstream signal line and back up through upstream signal line  144 . Similarly, if upstream signal line  144  is damaged, the upstream path of the signal stream will be through second upstream signal line  148 . If both lines of the signal line loop  140  are damaged, the signal stream routes through second downstream signal line  146  and second downstream  148 . In any of these cases, an alternate signal line loop is formed. 
     In the preferred embodiment, each of the signal lines  142 ,  144 ,  146 , and  148  are optic lines made from fiber optics. A protocol used for the optic lines is a time division multiplexing (TDM) protocol on the downstream and time division multiple access (TDMA) protocol on the upstream. Accordingly, the time through each of the signal lines, or delay time, is important. The time required for the signal stream to route through one of the lines is determined and dependent upon the length of the signal line. Generally, the time required for the signal stream to route through signal line  142  will be nearly the same as the time required for the signal stream to route through signal stream  144  since the two lines are installed along the same route and generally the same length, although not required to be so. However, the time through either of the redundant lines,  146  and  148 , will be much greater since the lines must follow a detoured route making the optic lines much longer. 
     The delay for each of the lines between head-end  120  and user service node  130  in the preferred embodiment is predetermined and stored within head-end  120 . The predetermined delay parameters are determined according to the equations: 
     
       
         Delay 0   =d   1 / u   1 =τ d1 +τ u1   
       
     
     
       
         Delay 1   =d   1 / u   2 =τ d1 +τ u2   
       
     
     
       
         Delay 2   =d   2 / u   1 =τ d2 +τ u1   
       
     
     
       
         Delay 3   =d   2 / u   2 =τ d2 +τ u2   
       
     
     where d 1  is a length of a first downstream line ( 142 ), u 1  is a length of a first upstream line ( 144 ), d 2  is a length of a second downstream line ( 146 ), u 2  is a length of a second upstream line ( 148 ), τ d1  is a propagation delay for d 1 , τ d2  is a propagation delay for d 2 , τ u1  is a propagation delay for u 1 , and τ u2  is a propagation delay for u 2 . Therefore, if one of the signal line loop  140  lines is unusable for signal stream routing, head-end  120  will cycle through the delay parameters until it finds the delay parameter that corresponds to the alternate signal line loop formed by routing the signal stream through one of the redundant lines. 
     Head-end  120  must be programmed to automatically detect when one of the lines is unusable, or in other words, when a fault in a line has occurred. If a line goes down, head-end must be able to immediately reroute the signal stream through a redundant line to maintain continuous service to the users. To do so, head-end must be able to detect the fault, preferably independent of the actual rerouting mechanism. 
     FIG. 2 graphically shows how head-end  120  detects a fault in the signal lines according to a preferred embodiment of the present invention. When a given route of the signal stream is established having an associated delay, head-end  120  will expect a response, or time alignment, from the communication units  152  within a predetermined timing window  202 . The response which head-end  120  will be looking for is, in the preferred embodiment, shortened uplink bursts (SUBs) (shown as elements  204  in FIG. 2 indicating that the SUBs may be anywhere within predetermined timing window  202 ) from the communication units  152 . In the preferred embodiment, predetermined timing window  202  is an access channel  202  of the system protocol  200  as shown in FIG.  2 . 
     In the event that a signal line is switched, the SUBs will shift either to the left or to the right of predetermined timing window, or access channel,  202  as shown in FIG.  3 . In the preferred embodiment, access channel  202  lies between a system broad-cast channel  302  and a traffic channel  304 . Head-end  120  detects that the SUBs have shifted into the adjacent channels, whether system broadcast channels (indicated by  306 ) or traffic channels (indicated by  308 ). 
     This method of detecting a fault assumes that the head-end receiver can reliably detect that a shift has occurred by detecting the presence of shortened bursts within the adjacent channels. It also assumes that the differential signal line fixed delay is limited to a single slot time. Certainly head-end receivers can be and are designed to recognize short bursts in adjacent channels. Limiting the total fiber round trip differential between any given combination of the signal lines to approximately 60 kilometers maintains any shift in the SUBs to one slot time. 
     When the downlink signal to the communication units  152  is interrupted requiring a switch to a different signal line during active calls, the active communication units  152  will loose synchronization with the traffic channel  304  at the head-end  120 . The active communication unit  152  will attempt to reconnect by sending the SUBs to the head-end  120 . If the SUBs are within the predetermined timing window  202 , head-end  120  will assign a new time alignment value to the active communication unit  152  and the call will continue. 
     If head-end  120  detects that the SUBs are out of bounds, it will begin cycling through the delay parameters, one at a time. For each delay parameter, head-end  120  adjusts receiver  124  of head-end  120  to the predetermined timing window to compensate for the new delay parameter. If the new delay parameter does not correlate with the current alternate signal line loop, the return SUBs from the communication units  152  will not be within predetermined timing window  202  and head-end  120  will cycle to the next delay parameter and adjust to the new delay parameter. Head-end  120  cycles through the delay parameters until the SUBs from communication units  152  are within predetermined timing window  202 . 
     If the fault occurred in the system broadcast channel  302  (left of the predetermined timing window  202 ), head-end  120  will cycle through the delay parameters backwards through the list (relative to the current delay parameter, where the delay parameters are arranged in order of increasing time delay). If the fault occurred in the traffic channel  304  (right of the predetermined timing window  202 ), head-end  120  will cycle forward through the list. If the fault occurred within the predetermined timing window  202  (partially within predetermined timing window  202  and partially outside), then the sign of the fault (positive or negative) will determine which direction head-end will cycle through the delay parameters where positive cycles forward and negative cycles backward. 
     Once head-end  120  has selected the delay parameter correlating with the alternate signal line loop, the fault condition will be cleared and the system  10  will stabilize. The timing adjustment is applied to the receiver  124  of head-end  120 . 
     In the preferred embodiment, head-end  120  cannot detect a fault unless at least one communication unit  152  is active. If none of the communication units  152  are active when a signal line is interrupted or an alternate signal line loop is established, head-end  120  will not detect a fault until at least one of the communication units  152  becomes active. At this time, the active communication unit  152  recognizes the loss of synchronization and transmit the SUBs. 
     A second embodiment of the present invention requires at least one communication unit  152  coupled near the service node  130  dedicated to transmitting SUBs at a constant rate. The method of detecting and correcting a fault is the same as the preferred embodiment. The dedicated communication unit  152  constantly transmits the SUBs. The advantage of the second embodiment of the present invention is that if there are no communication units  152  actively involved in calls, head-end  120  can still detect if a fault has occurred. For instance, in the middle of the night when there are no active calls, a signal line may be interrupted or broken and head-end  120  may correct for timing due to the continuous transmissions of SUBs from the dedicated communication unit  152 . The disadvantage of the second embodiment of the present invention is cost. Each service node  130  requires at least one dedicated communication unit  152 , and preferably two. Therefore, although the preferred embodiment of the present invention will not detect a fault during no-call periods, the disadvantage of the high cost of placing a dedicated communication unit at each service node  130  is outweighed by the inconvenience caused when an active call in initiated and must wait a second or so for the proper delay parameter to be chosen. 
     It should be recognized that the present invention may be used in many different sound recognition systems. All such varied uses are contemplated by the present invention.