Signaling Link Initial Alignment The purpose of the signaling link alignment procedure is to establish SU timing and alignment so that the SPs on either side of the link know where SUs begin and end In doing so, you must inherently test a link's quality before putting it into use Example L-1 in Appendix L shows a trace file of two aligned SPs The signaling link alignment procedure ensures that both ends have managed to correctly recognize flags in the data stream Initial alignment is performed for both initial activation of the link (power on) to bring it to service and to restore a link following a failure Alignment is based on the compelled exchange of status information and a proving period to ensure that SUs are framed correctly MTP3 requests initial alignment, which is performed by MTP2 Because MTP2 operates independently on each link, the initial alignment procedure is performed on a single link without involving other links There are two forms of alignment procedures: the emergency procedure and the normal alignment procedure The emergency procedure is used when the link being aligned is the only available link for any of the routes defined within the SSP Otherwise, the normal alignment procedure is used Status Indications LSSUs are exchanged as part of the alignment procedure There are six different status indications, as shown earlier in Table 6-1 Only the first four indications are employed during the initial alignment procedure The alignment procedure passes through a number of states during the initial alignment: • • • • • • Idle Not Aligned Aligned Proving Aligned/Ready In Service Idle When an SP is powered up, the links are initially put in the idle state The idle state is the first state entered in the alignment procedure; it indicates that the procedure is suspended If the procedure fails at any time, it returns to the idle state Timer T17 (MTP3) prevents the rapid oscillation from in service to out of service Timer T17 is started when the link begins the alignment procedure No further alignment attempts are accepted from a remote or local SP until T17 has expired LSSUs of SIOS (out of service) are sent during the idle state LSSUs of this type are sent continuously until the link is powered down or until an order to begin initial alignment is received from MTP3 The FIB and the BIB of the LSSUs are set to 1, and the FSN and BSN are set to 127 Not Aligned When MTP2 receives an order to begin initial alignment, the SP changes the status of the transmitted LSSUs to indication SIO (out of alignment) and starts the timer T2 If T2 expires, the status of the transmitted LSSUs reverts to SIOS Aligned During T2 SIO, if SIN (normal alignment) or SIE (emergency alignment) is received from the remote SP, T2 is stopped, and the transmission of SIO ceases The SP then transmits SIN or SIE, depending on whether normal or emergency alignment has been selected and timer T3 is started The link is now aligned, indicating that it can detect flags and signal units without error If T3 expires, the alignment process begins again, transmitting LSSUs with a status field of SIOS The aligned state indicates that the link is aligned and can detect flags and signal units without error Proving Timer T4 governs the proving period, and the Alignment Error Rate Monitor (AERM) is used during this period The proving period is used to test the signaling link's integrity FISUs are sent and errors (CRC and signaling unit acceptance) are counted during the proving period LSSUs are also sent, indicating whether this is a SIN or SIE alignment The proving period is shorter for emergency alignment and as a result is not as thorough As previously stated, emergency alignment is selected if only one in service (or none) exists between two SPs If the local SP detects an emergency alignment situation, emergency alignment is used regardless of whether an SIN or SIE is received from the distant SP Similarly, emergency alignment is used if an SIE is received from the distant SP, even when the local MTP3 indicates a normal alignment situation (more than one in-service link between the two adjacent nodes) If four errors are detected during the proving period, the link is returned to state 00 (idle), and the procedure begins again Aligned/Ready When T4 expires, the transmission of SIN/SIE ceases, timer T1 is started, and FISUs are transmitted If timer T1 expires, the transmission of FISUs ceases, and LSSUs of type SIOS are transmitted In Service Timer T1 stops upon receiving either FISUs or MSUs When it stops, the SUERM becomes active Figure 6-11 shows the initial alignment procedure Figure 6-11 Procedure for Signaling Link Alignment [View full size image] < Day Day Up > < Day Day Up > Signaling Link Error Monitoring Error rate monitoring is performed both for an in-service link and when the initial alignment procedure is performed Signal Unit Error Rate Monitor (SUERM) and the Alignment Error Rate Monitor (AERM) are the two link error rate monitors that are used [51] The SUERM performs monitoring when the link is in service, and the AERM performs monitoring when the link is undergoing initial alignment to bring it into service The following sections describe these two link error rate monitors SUERM The SUERM is active when a link is in service, and it ensures the removal of a link that has excessive errors It employs a leaky bucket counter, which is initially set to The counter is increased by for each SU that is received in error The counter is decreased by for each block of D consecutive SUs received without error, if it is not at If the link reaches a threshold of T, MTP2 informs MTP3, which removes it from service For a 64-kbps link, the values of D and T are 256 and 64, respectively NOTE In ANSI networks, high-speed links (1.536 Mbps) use an errored interval monitor, which differs in its threshold and counting values from those used by the SUERM on low-speed links (see Figure 6-13) Refer to ANSI T1.111 for more information Figure 6-13 SUERM Counter The SUERM enters octet counting mode if an SU fails the acceptance procedure (seven or more consecutive 1s, length is not a multiple of bits, or SU length is not between and 279 octets) For every block of N octets counted during octet counting mode, the SUERM is increased by If the octet counting mode continues for a significant period of time (meaning that SUs cannot be identified from the received data), the link is removed from service The SUERM reverts to normal mode if a correctly checked SU is received AERM The AERM is active when the link is in the proving period of the initial alignment procedure The counter is initialized to at the start of the proving period and is increased for every LSSU that is received in error If octet counting mode is entered during the proving period, the counter is increased for every block of N octets that is counted The proving period is aborted if the counter reaches a threshold value of Ti; it is reentered upon receiving a correct LSSU, or upon the expiration of the aborted proving period Different threshold values Tin and Tie are used for the normal and emergency alignment procedures, respectively If the proving is aborted M times, the link is removed from service and enters the idle state The values of the four parameters for 64-kbps and lower bit rates (both for ITU and ANSI) are as follows: • • • • Tin = Tie = M=5 N = 16 < Day Day Up > < Day Day Up > Processor Outage The processor outage condition occurs when SUs cannot be transferred to MTP3 or above This could be the result of a central processing failure or communication loss between MTP2 and Levels 3/4 when a distributed processing architecture is used A processor outage condition won't necessarily affect all signaling links in an SP, nor does it exclude the possibility that MTP3 can control the operation of the signaling link When MTP2 recognizes a local processor outage condition, it transmits LSSUs with the status field set to status indication processor outage (SIPO) and discards any MSUs it has received When the distant SP receives the SIPO status LSSU, it notifies its MTP3 and begins to continuously transmit FISUs Note that the affected links remain in the aligned state < Day Day Up > < Day Day Up > Flow Control Flow control allows incoming traffic to be throttled when the MTP2 receive buffer becomes congested When an SP detects that the number of received MSUs in its input buffer exceeds a particular value—for example, because MTP3 has fallen behind in processing these MSUs—it begins sending out LSSUs with the status indicator set to busy (SIB) These LSSUs are transmitted at an interval set by timer T5, sending SIB (80 to 120 ms), until the congestion abates The congested SP continues sending outgoing MSUs and FISUs but discards incoming MSUs It also "freezes" the value of BSN and the BIB in the SUs it sends out to the values that were last transmitted in an SU before the congestion was recognized This acknowledgment delay would normally cause timer T7, excessive delay of acknowledgment, at the distant SP to time out; however, timer T7 restarts each time an SIB is received Therefore, timer T7 does not time out as long as the distant SP receives SIBs Timer T6, remote congestion, is started when the initial SIB is received If timer T6 expires, it is considered a fault, and the link is removed from service Timer T6 ensures that the link does not remain in the congested state for an excessive period of time When congestion abates, acknowledgments of all incoming MSUs are resumed, and periodic transmission of the SIB indication is discontinued When the distant SP receives an SU that contains a negative or positive acknowledgment whose backward sequence number acknowledges an MSU in the RTB, timer T6 is stopped, and normal operation at both ends ensues Figure 6-14 depicts flow control using LSSUs with status indication busy Figure 6-14 Flow Control Using Status Indication SIB NOTE The mechanism for detecting the onset of congestion is implementation-specific and should be chosen to minimize the oscillation between the onset and abatement of congestion   ... until an order to begin initial alignment is received from MTP3 The FIB and the BIB of the LSSUs are set to 1, and the FSN and BSN are set to 127 Not Aligned When MTP2 receives an order to begin... flags and signal units without error Proving Timer T4 governs the proving period, and the Alignment Error Rate Monitor (AERM) is used during this period The proving period is used to test the signaling. .. period The proving period is used to test the signaling link''s integrity FISUs are sent and errors (CRC and signaling unit acceptance) are counted during the proving period LSSUs are also sent,