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Loss of Signal Detection Capability on CTP Bundles and SAToP Bundles


A loss of signal (LOS) alarm indicates that there is a physical link problem with the connection to the router receive port from the neighboring SONET equipment transmit port. An LOS alarm occurs when the port on the card is in service but no signal is being received. The cabling is not correctly connected to the ports, or no signal exists on the line. Possible causes for a loss of signal include upstream equipment failure or a fiber cut.

The CTP devices support a both-ended redundancy mechanism, in which two identical CTP circuit bundles are combined using Y cables at each end, enabling one bundle to act as a backup for the other. One of the bundles is in use (online), while the other is in the standby state (offline). Only the online bundle is allowed to drive the Y cable towards the user equipment, while the offline bundle is tristate. A communications channel (such as redundancy by using a hardware link that uses a special Y cable or redundancy based on a software link that does not depend on a signaling hardware like the Y cable) between ports at each end determines which of the two ports on the Y cable is currently online. When one bundle fails, the failed bundle transitions to the offline and places the other bundle in the online state.

Consider a sample configuration scenario in which two CTP bundles (four CTP ports) are used in a Y-cable redundancy format. Software-based redundancy is enabled. In this type of configuration, is the left primary link and is the left secondary link. is the right primary link and is the right secondary link. In this redundant configuration, the circuit is very robust, protecting against many types of failures, such network failures, power failures, and equipment failures. However, one type of failure is not detected, which is when a cable is pulled out.

Starting with CTPOS Release 7.2R1, CTP devices support the detection of a loss of signal, which denotes a physical link problem. The following conditions are supported:

  • In a serial both-ended Y-cable redundancy configuration (software-based Y cable link protocol), removal of Y cable leg from the CTP port of the online bundle must be able to force a switch to the standby bundle.

  • In a T1/E1 both-ended Y cable configuration (software-based Y cable link protocol), removal of Y cable leg from the CTP port of the online bundle must be able to force a switch to the standby bundle.

The T1/E1, CTP, and SAToP bundles support LOS detection and based on this signal, the run state of the bundles switches to TfFail, which initiates a software-based Y cable switchover to a redundant port. Also, for T1/E1 both-ended Y-cable redundancy configuration, only software-based Y cable link protocol is supported and hardware-based redundancy is not supported.

The way in which CTP redundancy works is by using the bundle state to make decisions. When a bundle is in the RUNNING state, the following processes occur:

  1. The remote CTP is operational and is able to generate and send packets into the IP network (towards us).

  2. The network is able to transport bundle OAM and payload packets from the remote CTP to the local CTP.

  3. A sufficient percentage of the bundle payload packets fills packet delay variation (PDV buffers) and maintain circuit data transport towards the locally connected user equipment.

Therefore, when a bundle is in the RUNNING state, it is “usable” and can be online in a redundant configuration.

Consider a network topology in which a failure occurs in the circuit path that does not cause the circuit to exit the RUNNING state. This phenomenon can be the case when the cable is pulled from the CTP port of a redundant online bundle. Although this condition might not typically be considered an actual failure, and instead more of a configuration error, this symptom can nevertheless be classified in the failure category. Therefore, a mechanism to be able to detect this condition in redundant setups and provide an online circuit switch to offline when the cable is removed is beneficial. CTP devices support the evaluation of LOS conditions on serial interfaces and T1E1 interfaces in CTP bundles and SAToP bundles.

Detection of LOS on Serial Interfaces

For serial interfaces, the determination of LOS condition is already performed in CTPOS releases earlier than Release 7.2R1. When a serial circuit is configured to use the TT input (on a data communication equipment [DCE] interface) for at least one of its five configured port clocks (for example, “Cfg Rate - Ext Clk), the external clock frequency is examined by the CTP device before the local bundle can go to the running state. If there is no external clock present or it is not the correct frequency, then the bundle transitions to the TtFAIL state and never go to RUNNING. Also, if the bundle is already in the RUNNING state, the external clock is verified every second to ensure that its frequency is still present and within range. If not, the bundle transitions from RUNNING to TtFAIL.

In the TtFAIL state, the bundle periodically transitions back to the EVAL state, where the external clock is checked again. If the clock fails or a bad frequency occurs, the bundle returns to the TtFAIL state. If the clock is properly functional, then the bundle transitions to the various states that eventually end in the RUNNING state. Such a method of change of states enable a graceful (if not instantaneous) recovery of a circuit where a cable is disconnected, but subsequently reconnected. Because removal of the cable on a serial port that is using an external clock can cause the bundle to exit the RUNNING state, that bundle switches offline, if currently online in a Y-cable redundancy setup.

Detection of LOS on T1/E1 Interfaces

The clock and data signals are embedded together on a T1/E1 interface in a single AMI (alternate mark inversion) electrical signal. The hardware line interface unit (LIU) that recovers the composite AMI signal into its component clock and data signals recovers a clock from the incoming AMI signal, even when none is present because it is based on a free running phase-locked loop (PLL) that generates a clock, even when it is not locked to an incoming signal. As a result, the CTP port interface receives an incoming external clock from the LIU, whether a valid T1/E1 signal is connected to the CTP or not. The LIU, however, cannot determine when it has a valid incoming T1/E1 signal, and in such a condition, the LIU indicates as a LOS status bit. This indication serves as the basis for detecting a cable disconnect in a Y-cable redundant configuration.

To use LOS as a way to take down a RUNNING bundle, the effective method implemented is to treat a T1/E1 LOS condition exactly the same as a serial port with a bad or missing external clock. When the CTP device performs its “check external clock” function, instead of returning an automatic success on T1/E1 ports, the LOS status bit is analyzed to determine whether it is a T1/E1 port. If the LIU LOS status indicates that there is no incoming signal, then the function returns a failure, which causes the bundle to move to the TtFAIL state. This state is the same as a missing external clock processing for a serial port. In this manner, the T1/E1 ports behave exactly the same way as serial ports.