Signaling System No.7 Protocol Architecture And Sevices part 15

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Signaling System No.7 Protocol Architecture And Sevices part 15

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Signal Unit Delimitation A flag octet that is coded as 01111110 separates consecutive signal units on a signaling data link. The flag octet indicates the beginning or end of an SU.

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Nội dung Text: Signaling System No.7 Protocol Architecture And Sevices part 15

Signal Unit Delimitation A flag octet that is coded as 01111110 separates consecutive signal units on a signaling data link. The flag octet indicates the beginning or end of an SU. NOTE It is optional whether a single flag is used to mark both the beginning and end of an SU, or whether a common flag is used for both. The latter is the most common implementation. Because the 01111110 flag pattern can also occur in an SU, the SU is scanned before a flag is attached, and a 0 is inserted after every sequence of five consecutive 1s. This method is called bit stuffing (or 0 bit insertion). It solves the problem of false flags, because it prevents the pattern 01111110 from occurring inside an SU. The receiving MTP2 carries out the reverse process, which is called bit removal (or 0 bit deletion).After flag detection and removal, each 0 that directly follows a sequence of five consecutive 1s is deleted. Figure 6-5 shows how the sending node adds a 0 following five 1s while the receiving node removes a 0 following five 1s. Figure 6-5. Zero Bit Insertion and Deletion As another example, if the pattern 01111100LSB appears in the SU, the pattern is changed to 001111100LSB and then is changed back at the receiving end. This method continuously processes the stream of data on the link, inserting a 0 after five contiguous 1s without examining the value of the next bit. < Day Day Up > < Day Day Up >
Length Indicator MTP2 must be able to determine the SU type to process it. The length indicator (LI) provides an easy way for MTP2 to recognize the SU type. The LI indicates the number of octets between the LI and the CRC fields. Using telecommunications conventions, MTP2 measures the size of SUs in octets. An octet is simply another term for a byte; all SUs have an integral number of octets. The LI field implies the type of signal unit. LI = 0 for FISUs, LI = 1 or 2 for LSSUs, and LI >2 for MSUs. Because MSUs contain the actual signaling content, their size is relatively large compared to the two other types of SUs. NOTE Layers above the MTP can handle larger data streams than the MTP; however, these streams must be segmented into MSUs at MTP2 for transmission over the signaling link. The signaling payload is placed in the SIF, which is found in an MSU. The SIF can be up to 272 octets in size, rendering the maximum length for an MSU as 279 octets. If the MSU size is greater than 62 octets, the LI is set to the value of 63; therefore, an LI of 63 means that the SIF length is between 63 and 272 octets. This situation arises from backward-compatibility issues. The Red Book specified the maximum number of octets in the SIF as 62, and the Blue Book increased it to 272 (which was previously allowed only as a national option). (See the section "ITU-T (Formerly CCITT) International Standards" in Chapter 2, "Standards," for information about the meaning of the Red Book and Blue Book.) MTP2 uses the LI information to determine the type of SU with minimum processing overhead; therefore, the inaccuracy of the indicator above 62 octets is not an issue. MTP2 adds an overhead of six octets along with one additional octet for the MTP3 SIO when creating each MSU. This brings the total maximum size of a transmitted SU to 279 octets (272 maximum SIF size plus seven for MTP2 overhead and the SIO). NOTE In ANSI networks, when 1.536-Mbps links are used, a 9-bit length indicator is
used, and the actual SU length is checked against the LI value. < Day Day Up > < Day Day Up > Error Detection The error detection method is performed by a 16-bit CRC on each signal unit. These 16 bits are called check bits (CK bits). NOTE The process uses the Recommendation V.41 [ITU-T Recommendation V.41: CODE-INDEPENDENT ERROR-CONTROL SYSTEM, November 1988] generator polynomial X16 + X12 + X5 + 1. The transmitter's 16-bit remainder value is initialized to all 1s before a signal unit is transmitted. The transmission's binary value is multiplied by X16 and then divided by the generator polynomial. Integer quotient values are ignored, and the transmitter sends the complement of the resulting remainder value, high-order bit first, as the CRC field. At the receiver, the initial remainder is preset to all 1s, and the same process is applied to the serial incoming bits. In the absence of transmission errors, the final remainder is 1111000010111000 (X0 + X15). The polynomial that is used is optimized to detect error bursts. The check bits are calculated using all fields between the flags and ignoring any inserted 0s. The SP then appends the calculation to the SU before transmission as a two-octet field (CK field). The receiving SP performs the same calculation in an identical manner. Finally, the two results are compared; if an inconsistency exists, the SU is discarded, and the error is noted by adding to the Signal Unit Error Rate Monitor (SUERM). In this case, the error correction procedure is applied. < Day Day Up > < Day Day Up >
Error Correction Two methods of error correction are available: basic error correction (BEC) and preventive cyclic retransmission (PCR) method. The basic method is used for signaling links using nonintercontinental terrestrial transmission and for intercontinental links that have a one-way propagation delay of less than 30 ms. The PCR method is used for all signaling links that have a propagation delay greater than or equal to 125 ms and on all satellite signaling links [115]. Where the one-way propagation delay is between 30 and 125 ms, other criteria must be considered that are outside the scope of this book (see [115]). Depending on other additional criteria, PCR can also be employed on links that have a one-way propagation delay between 30 ms and 125 ms [52]. For cases in which only one link in a linkset uses PCR, the other links should use PCR, regardless of their propagation delays. For example, if a single link in an international linkset is established by satellite, the PCR method should also be used for all other links in that linkset—even if the other links are terrestrial. (For information on linksets, see Chapter 4, "SS7 Network Architecture and Protocols Introduction.") This approach reduces the chances of different methods of error correction being provisioned at either side of the same link. Neither method tries to repair a corrupt MSU; rather, they both seek correction by MSU retransmission. For this reason, a signaling point has a retransmission buffer (RTB).The RTB stores copies of all the MSUs it has transmitted until the receiving SP positively acknowledges them. Basic Error Correction The basic method is a noncompelled, positive/negative acknowledgment, retransmission error correction system [51]. Noncompelled means that messages are sent only once—that is, unless they are corrupted during transfer. Positive/negative acknowledgment means that each message is acknowledged as being received, along with an indicator that the message was received error-free. Retransmission error correction system simply means that no attempt is made to repair the corrupt message; instead, correction is achieved through retransmission. In normal operation, this method ensures the correct transfer of MSUs—in the correct sequence and without loss or duplication—over a signaling link. Therefore, no resequencing is required at MTP2.
Basic error correction is accomplished using a backwards retransmission mechanism, in which the sender retransmits the corrupt (or missing) MSU and all subsequent MSUs. This method uses both negative and positive acknowledgments. Positive acknowledgments (ACKs) indicate the correct reception of an MSU, and negative acknowledgments (NACKs) are used as explicit requests for retransmission. Only MSUs are acknowledged and resent, if corrupt, to minimize retransmissions. FISUs and LSSUs are neither acknowledged nor resent if corrupt; however, the error occurrences are noted for error rate monitoring purposes. The basic error correction fields occupy a total of two octets in each SU and consist of an FSN, BSN, FIB, and BIB. The Forward Sequence Number (FSN) and Backward Sequence Number (BSN) are cyclic binary counts in the range 0 to 127. The Forward Indicator Bit (FIB) and Backward Indicator Bit (BIB) are binary flags that are used in conjunction with the FSN and BSN for the basic error correction method only. NOTE In ANSI networks, the sequence numbers extend up to 4095 for high-speed links. Links using bit rates of 64 kbps or lower are still limited to a maximum value of 127. Sequence Numbering Each SU carries two sequence numbers for the purpose of SU acknowledgment and sequence control. Whereas the FSN is used for the function of SU sequence control, the BSN is used for the function of SU acknowledgment. Before it is transmitted, each MSU is assigned an FSN. The FSN is increased linearly as MSUs are transmitted. The FSN value uniquely identifies the MSU until the receiving SP accepts its delivery without errors and in the correct sequence. FISUs and LSSUs are not assigned new FSNs; instead, they are sent with an FSN value of the last MSU that was sent. Because the FSN has a range of 127, it has to start from 0 again after it reaches a count of 127. This dictates that the RTB cannot store more than 128 MSUs. Positive Acknowledgment When the BIB in the received SU has the same value as the FIB that was sent
previously, this indicates a positive acknowledgment. The receiving SP acknowledges positive acceptance of one or more MSUs by copying the FSN value of the last accepted MSU into the SU's BSN, which it transmits. All subsequent SUs in that direction retain the same BSN value until a further incoming MSU requires acknowledgment. The BIB is set to the same value as the received FIB to indicate positive acknowledgment. Negative Acknowledgment When the BIB in the received SU is not the same value as the FIB that was sent previously, this indicates a negative acknowledgment. The receiving SP generates a negative acknowledgment for one or more MSUs by toggling the BIB's value. It then copies the FSN value of the last accepted MSU into the SU's BSN, which it transmits in the opposite direction. Response to a Positive Acknowledgment The transmitting SP examines the BSN of the received SU. Because they have been positively acknowledged, the MSUs in the RTB that have an FSN equal to or less than the BSN are removed. If an SU is received with a BSN that does not equal the previously sent BSN or one of the FSNs in the RTB, it is discarded. If an incorrect BSN is received three consecutive times, MTP2 informs MTP3 that the link is faulty, therefore resulting in an order for MTP2 to remove the link from service. The excessive delay of acknowledgment (T7) timer ensures that acknowledgments are received in an appropriate amount of time. Because the FSN values cannot be used again until they have been acknowledged, excessive delay would quickly exhaust the available FSNs. For example, if at least one outstanding MSU is in the RTB, and no acknowledgment is received within expiration of T7, a link failure indication is passed to MTP3. A list of MTP2 timers appears in Appendix G, "MTP Timers in ITU-T/ETSI/ANSI Applications." Response to a Negative Acknowledgment When the MSU receives a negative acknowledgment, retransmission occurs beginning with the MSU in the RTB having a value of 1 greater than the NACKed MSU. All MSUs that follow in the RTB are retransmitted in the correct sequence.
During this period, transmission of new MSUs is halted. At the start of retransmission, the FIB is inverted so that it equals the BIB again. The new FIB is maintained in subsequently transmitted SUs until a new retransmission is required. If an SU is received with a toggled FIB (indicating the start of retransmission) when no negative acknowledgment has been sent, the SU is discarded. If this occurs three consecutive times, MTP2 informs MTP3 that the link is faulty, resulting in an order for MTP2 to remove the link from service. Examples of Error Correction Figure 6-6 shows the fundamental principles of basic error correction by examining the error correction procedure for one direction of transmission between SP A and SP B. A similar relationship exists between the FSN/FIB from SP B and the BSN/BIB from SP A. Figure 6-6. Principles of Basic Error Correction Although the bit rate on the signaling link in either direction is the same, note that the number of SUs transmitted by the two SPs in a time interval is likely to differ because of the MSUs' variable lengths. As a consequence, an SP might receive a number of SUs before it can acknowledge them. Figure 6-7 shows an example of basic error correction with a differing number of SUs sent between two SPs in a given amount of time. Figure 6-7. Basic Error Correction [View full size image] In Figure 6-7, the FIB and BIB are set to 1 for both SPs at the start of the transmissions. The SU (c) acknowledges two MSUs positively (ii and iii). Because the SU (x) is a FISU, it takes on the FSN value of the MSU that was sent last. SP B
receives the MSU (vii) sent by SP A in error. SP B in SU (g) sends a negative acknowledgment. BIB is inverted, and BSN contains the FSN of the last correctly received SU. SP A detects negative acknowledgment upon receiving message (g) and, beginning with MSU (xi), resends all MSUs after the last positive acknowledgment in sequence. The SU (I) is the first positive acknowledgment since retransmission began. The error correction procedure operates independently in both directions. Figure 6- 8 shows how the FSNs and FIBs carried by SUs in the direction SP A to SP B, and the BSNs and BIBs carried by SUs in the direction SP B to SP A, act as the error correction and sequencing fields for messages that are sent from SP A to SP B. Independently from the error correction and sequencing being performed in the SP A–to–SP B direction, error correction and sequencing take place in the SP B-to-SP A direction. Figure L-1 in Appendix L, "Tektronix Supporting Traffic," shows a trace file with the FSN/BSN/FIB/BIB fields exchanged between two SPs. Figure 6-8. Relevance of Fields Related to the Direction of Transmission Figure 6-9 shows basic error correction in both directions. Figure 6-9. Basic Error Correction in Both Directions [View full size image] In Figure 6-9, SP A receives in error the MSU (c) sent by SP B. SP A in MSU (v) sends a negative acknowledgment. BIB is inverted, and BSN contains the FSN of the last correctly received SU. SP B detects the negative acknowledgment upon receiving the message (v) and resends all MSUs, beginning with MSU (c), after the last positive acknowledgment in sequence. FISU (x) is the first positive acknowledgment since retransmission began. Preventive Cyclic Retransmission The preventive cyclic retransmission (PCR) method is a noncompelled, positive
acknowledgment, cyclic retransmission, forward error correction system. This means that no negative acknowledgments are used and that the system relies on the absence of a positive acknowledgment to indicate the corruption of SUs. As in basic error correction, the FSN identifies the position of an MSU in its original transmission sequence, and the BSN identifies the most recently accepted MSU. Because PCR uses only positive acknowledgments, indicator bits FIB and BIB are ignored (they are permanently set to 1). The receiving SP simply accepts or discards an error-free MSU based on the FSN's value, which must exceed the FSN of the most recently accepted MSU by 1 (modulo 128). A transmitted SU is retained in the RTB until a positive acknowledgment for that SU is received. When one of the SPs no longer has new LSSUs or MSUs to send, it starts a PRC in which the MSUs in its RTB are retransmitted in sequence, beginning with the oldest one (the lowest FSN). If all MSUs have been acknowledged, resulting in an empty RTB, FISUs are transmitted. Any retransmitted MSUs that have already been accepted by the receiving SP but have not yet been positively acknowledged arrive out-of-sequence and are discarded. This method ensures that, if any MSUs are not accepted, the receiving SP receives fresh copies periodically until it gives a positive acknowledgment. Figure 6-10 shows a unidirectional example of how PCR works. Figure 6-10. PCR [View full size image] As shown in Figure 6-10, SP A has no more new LSSUs or MSUs to send after it transmits MSU (ii), so it begins a retransmission cycle with MSU (iii). At this point, SP A's RTB has only two MSUs (MSU with FSN = 125 and MSU with FSN = 126). After MSU (iii) has been retransmitted, a new MSU becomes available for transmission.After the new MSU (iv) has been transmitted, SP A finds itself without a new MSU or LSSU to send; therefore, it begins a retransmission cycle with MSU (v). Again, the retransmission cycle stops after just one MSU is retransmitted, because SP A finds itself with five new MSUs to send (vi to x). After the new MSU (x) has been transmitted, again SP A finds itself without a new MSU or LSSU to send, so it begins a retransmission cycle with MSU (xi). The retransmission cycle stops after just two MSUs have been retransmitted, because
SP A finds new MSUs to send (xiii). At this point, SP A has only one MSU in its RTB (MSU with FSN = 6). This primitive forward error correction, which assumes loss in the absence of an acknowledgment, allows retransmissions to take place much sooner than in basic error correction. This is why PCR is used on signaling data links with propagation times that make basic error correction impractical. As mentioned previously, PCR is used on signaling links that have long propagation times and for all signaling links established via satellite [115], because the basic error correction method would result in MSU queuing delays that are too great for call control applications (such as ISUP). Forced Retransmission Cycles Approximately 20 to 30 percent of the traffic load using PCR is new traffic (such as MSUs and LSSUs) [115]. This low utilization gives more-than-adequate capacity to perform enough retransmission cycles. During periods of heavy traffic load (new MSUs), the rate at which retransmission cycles take place can be severely impaired, because new MSUs and LSSUs have priority. Under these conditions, the RTB might become full, because it has limited capacity to store 128 messages; this impairs the error correction method. To overcome this impairment, PCR includes forced retransmission cycles in which MTP2 constantly monitors the number of MSUs and the number of octets in the RTB. If either of these two values exceeds a predetermined value, a forced retransmission cycle occurs. Both values are implementation-dependent. Setting the thresholds too low results in frequent use of the forced transmission procedure, which results in excessive delays for new transmissions. Likewise, setting the thresholds too high causes forced retransmissions to take place too infrequently. Unlike normal retransmission cycles, forced retransmission cycles end only when all MSUs in the RTB have been retransmitted. Note that LSSUs are always transmitted ahead of MSUs. If a new LSSU is queued for transmission, it is sent—regardless of the RTB's contents. Comparison with the Basic Error Correction Method The basic error correction method is preferred on links that have one-way propagation times of less than 30 ms [115], because this allows higher MSU loads
than with PCR. PCR achieves lower MSU loads because it expends a relatively large amount of time needlessly retransmitting MSUs that have already been received correctly (even though they have not yet been acknowledged). PCR links are highly underutilized because spare capacity is required to ensure that retransmissions can take place.

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