Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/02—Details
  • H04J3/14—Monitoring arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J2203/00—Aspects of optical multiplex systems other than those covered by H04J14/05 and H04J14/07
  • H04J2203/0001—Provisions for broadband connections in integrated services digital network using frames of the Optical Transport Network [OTN] or using synchronous transfer mode [STM], e.g. SONET, SDH
  • H04J2203/0057—Operations, administration and maintenance [OAM]
  • H04J2203/006—Fault tolerance and recovery

Definitions

• the invention relates to a method according to the preambles of attached claims 1 and 2 for receiving a signal used in a synchronous digital telecommunica ⁇ tions system.
• the current digital transmission network is plesiochronous, that is, each 2-Mbit/s basic multiplex system has a dedicated clock independent of any other system. It is therefore impossible to locate a single 2-Mbit/s signal in the bit stream of a higher-order system, but the higher-level signal has to be demulti ⁇ plexed through each intermediate level down to the 2 Mbit/s level to extract the 2-Mbit/s signal. For this reason, especially the construction of branch con ⁇ nections requiring several multiplexers and demulti ⁇ plexers has been expensive.
• Another disadvantage of the plesiochronous transmission network is that equipments from two different manufacturers are not usually compatible.
• STM-N transfer frames Synchronous Transport Modules
• the bit rates are multiples of the bit rate of the lowest level.
• all nodes of the synchronous transmission network are synchronized with a single clock. However, if some of the nodes would lose connection to the common clock, this would result in problems in con ⁇ nections between the nodes. It is also necessary that the phase of the frame can be detected easily at the reception.
• the SDH telecommunication utilizes a pointer, i.e. a number that indicates the phase of payload within a frame. The pointer thus points that byte in the STM frame from which the payload begins.
• the STM-N frame comprises a matrix with 9 rows and N x 270 columns so that there is one byte at the junction point between each row and the column.
• Rows 1-3 and rows 5-9 of the N x 9 first columns comprise a section overhead SOH, and the row 4 comprises an AU pointer.
• the rest of the frame structure is formed of a section having the length of N x 261 columns and containing the payload section of the STM-N frame.
• the row of the STM-1 frame is thus 270 bytes in length, as described above.
• the payload section comprises one or more administration units AU.
• Each byte in the AU-4 unit has its own location number.
• the above-mentioned AU pointer contains the location of the first byte of the VC-4 container in the AU-4 unit.
• the pointers allow positive or negative pointer justifications to be performed at different points in the SDH network. If a virtual container having a certain clock frequency is applied to a net ⁇ work node operating at a clock frequency lower than the above-mentioned clock frequency of the virtual container, the data buffer will be filled up. This requires negative justification: one byte is trans ⁇ ferred from the received virtual container into the overhead section of the frame to be transmitted while the pointer value is decreased by one. If the rate of the received virtual container is lower than the clock rate of the node, the data buffer tends to be emptied, which calls for positive justification: a stuff byte is added to the received virtual container and the pointer value is incremented by one.
• the SDH system comprises pointers on two levels: AU pointers and TU pointers, which indicate the first byte of a corre ⁇ sponding virtual container VC within the AU or TU unit, respectively.
• AU pointers and TU pointers, which indicate the first byte of a corre ⁇ sponding virtual container VC within the AU or TU unit, respectively.
• TU pointers which indicate the first byte of a corre ⁇ sponding virtual container VC within the AU or TU unit, respectively.
• the CCITT specifications relating to the pointer are set forth in Reference [1], which is referred to for a more detailed description.
• the AU-4 pointer for example, consists of nine successive bytes HI, Y, Y...H3, of which bytes HI and H2 are shown separately in Figure lb.
• the actual pointer value PTR consists of the ten last bits (bits 7 to 16) of the word formed by bytes HI and H2.
• the values of the TU-11, TU-12 and TU-2 pointers consist of the ten last bits of the word formed by bytes VI and V2.
• the TU-3 pointer consists of bytes HI and H2 in the same way as the AU-4 pointer ( Figure lb).
• the AU and TU pointers have quite similar coding even in other respects; there are, however, some differences, which will be described in the following.
• the pointer value must be within a certain range in order to be acceptable.
• the acceptable decimal value of the AU-4 pointer is from 0 to 782, and the acceptable decimal value of the TU-12 pointer, for example, is from 0 to 139.
• These values are called offset values, as they (within the frame structure) indicate the offset between the pointer and the first byte of the corresponding virtual container.
• the new data flag NDF determined by the N bits allows arbitrary changes in the pointer value if they are the result of a change taking place in the payload.
• Normal operation (NDF_disabled) is indicated by the N-bit values "0110", and the new pointer value (NDF_enable) by the N-bit values "1001" (i.e. by inverting the bits of the normal state).
• NDF_disabled is indicated by the N-bit values "0110”
• NDF_enable by the N-bit values "1001” (i.e. by inverting the bits of the normal state).
• the new data flag together with the new pointer value, indicates a change in the alignment of the virtual container within the frame, if the change is caused by some other reason than positive or negative justification (the transmitter may force a new alignment on the virtual container within the frame structure).
• Concatena ⁇ tion means that e.g. AU-4 units are concatenated into one larger unit (so-called AU-4-Xc), which may trans ⁇ fer payloads which require a higher capacity than the C-4 container.
• AU-4-Xc a larger unit capable of trans ⁇ ferring greater payloads than the capacity of the C-2 container.
• bits 1 to 16 are ones, it signifies alarm (AIS, Alarm Indication Signal).
• the S-bits (bits 5 and 6) indicate on which level of hierarchy (e.g. TU-12) the operation is cur ⁇ rently taking place.
• the I- and D-bits of the 10-bit pointer word are used to indicate the positive and negative justifica- tion described above. If at least three out of five increment bits, or I-bits (bits 7, 9, 11, 13 and 15), are inverted, it signifies positive justification (if certain additional conditions are met).
• the interpretor of the receiver has three possible main states: the normal state NORM, the alarm state AIS (Alarm Indication Signal), and the loss of pointer state LOP, which will be described more close- ly below.
• the LOP state transition to the normal state NORM can take place if the same pointer is received three times in succession.
• this transition thus requires a time period corresponding to at least twelve STM-1 frames, and with TU-3 frames a time period corresponding to at least three STM-1 frames.
• this thus means that the loss of pointer state lasts at least 1,500 ⁇ s, whereas the time period after which the normal state can be resumed may actually be much longer.
• the object of the present invention is to eliminate the above-described drawback and provide a method which speeds up the transition of data trans ⁇ mission back to normal state. This is achieved by means of a method according to the invention, which is characterized by what is described in the character ⁇ izing portions of attached claims 1 and 2.
• the idea of the invention is that instead of starting the counting of the new pointer from the beginning, as defined in the CCITT recommendations, on transition from the normal state NORM or alarm state AIS to the loss of pointer state LOP, the counter is allowed to step normally in spite of the state trans ⁇ ition, whereby transition back to the normal state NORM will take place clearly more rapidly.
• Figure la shows an AU-4 pointer
• Figure lb shows bytes HI and H2 in the AU-4 pointer shown in Figure la in more detail;
• Figure 2 is a state diagram illustrating the main states of the receiver and transitions in the different states.
• the pointer interpretation effected in the receiver can be illustrated as a state diagram in which the interpretation machine of the receiver has the above-mentioned main states: the normal state NORM, the alarm state AIS (Alarm Indica- tion Signal), and the loss of pointer state LOP.
• NORM normal state
• AIS Alarm Indica- tion Signal
• NORM state event Definition active_point The new data flag (NDF) has the (active pointer) value "normal” (NDF_disabled), the SS-bits are correct, and the offset value is within the acceptable range and equal to the valid offset value.
• new point new The new data flag has the value pointer) "normal” (NDF_disabled), the SS-bits are correct, and the offset value is within the acceptable range and unequal to the valid offset value.
• NDF_enable new The new data flag has the value data flag) "new" (NDF_enable), the SS-bits are correct, and the offset value is within the acceptable range.
• AIS_ind (AIS The 16-bit pointer word has the alarm) value "1111 1111 1111 1111".
• inc ind The new data flag has the value (increment data) "normal” (NDF_disabled), the SS-bits are correct, the majority of I-bits are invert ⁇ ed, and the majority of D-bits are not inverted, and the value "new" for the new data flag (NDF_enable), increment data (inc_ind) or decrement data (dec_ind) has not been received in the three preceding frames.
• dec_ind The new data flag has the value (decrement data) "normal” (NDF_disabled), the SS-bits are correct, and the majority of D-bits are invert ⁇ ed, and the majority of I-bits are not inverted, and the value "new" for the new data flag (NDF_enable), increment data (inc_in ) or decrement data (dec_ind) has not been received in the three preceding frames.
• NORM_inv_point If AIS alarm, the value "new" (invalid pointer for the new data flag (NDF_ena- of the NORM ble) or the active pointer state) (active_poin ) is not received.
• new point new The new data flag (NDF) has the pointer) value "normal” (NDF_disabled), the SS-bits are correct, and the offset value is within the acceptable range.
• NDF new Data flag
• NDF enable new The new data flag has the value data flag
• NDF_enable new
• SS-bits are correct
• the offset value is within the acceptable range.
• AIS_ind AIS The 16-bit pointer word has the alarm
• new point new The new data flag has the value pointer) "normal” (NDF_disabled) , the SS-bits are correct, and the offset value is within the acceptable range.
• NDF_enable new The new data flag has the value data flag) "new" (NDF_enabled) , the SS- bits are correct, and the offset value is within the acceptable range.
• AIS_ind (AIS The 16-bit pointer word has the alarm) value "1111 1111 1111 1111”. inc_ind not defined
• invalid pointer pointer is not updated at all, of the LOP or (II) all pointer values that state) are unequal to the AIS alarm, or (III) all pointer values that are unequal to the AIS alarm and the value "new" for the new data flag.
• the above- mentioned bytes HI and H2 bits 1 to 16
• the transition of the receiver between the different main states and to another internal state within a main state is controlled by means of counters as defined in the following table.
• the index (N or 3) at the beginning of the name of the counter indicates the maximum value of the counter.
• the counters are modulo counters, i.e. when the counter value reaches its upper limit, the counter starts over again.
• N x NDF_enable NDF Counts successive NDF_enable counter
• N x inv_point (counter Counts successive inv point of an invalid pointer) events and controls transitio from NORM and AIS states to LOP state.
• AIS_ind (alarm Counts successive AIS ind counter) events and controls transitio from NORM and LOP states to AIS state.
• the counter of the new pointer controls the process for finding a correct frame synchronization if the pointer value has been changed erroneously.
• the offset values of three successive new pointers must be equal, otherwise the counter of the new pointer is reset. For this reason, it is likely to require a longer time period than three STM-1 frames (or twelve STM-1 frames if the STM-1 signal contains TU-12 or TU- 2 frames).
• N has a value of from 8 to 10. If an invalid pointer is received, the counter of the invalid pointer is incremented by one, other ⁇ wise the error counter is reset. If the counter of the invalid pointer reaches its maximum value, the LOP state is entered from the normal and alarm state.
• Figure 2 the state transitions described above are indicated by broken lines.
• Figure 2 also illustrates reception of a single new data flag (together with an acceptable pointer value) in the alarm state, which leads to transition to the normal state NORM. This is the only single event which causes transition from one main state to another.
• Figure 2 illustrates other changes occurring in the normal state. Such changes are (1) reception of increment or decrement data inc_ind/ dec_ind, and (2) reception of a single new data flag, both of these causing a change in a manner known per se in the offset value in the normal state, and recep ⁇ tion of the active pointer (active_point).
• the figure also illustrates internal changes occurring in the AIS and LOP states. As pointer interpretation as such does not fall within the scope of the present invention, Finnish Patent Applications 923 061 and 923 062 are referred to, where the interpretation process is illustrated by means of a flow diagram.
• the present invention may utilize either the same interpretation or the same interpretation so modified that each increment or decrement data item is accepted only if the value "new" for the new data flag (NDF_enable), the increment data (inc_ind) or the decrement data (dec_ind) has not been received in the three preceding frames and if the S-bits have been received correctly.
• NDF_enable new data flag
• dec_ind decrement data
• transition from the loss of pointer state LOP back to the normal state requires a time period corresponding to at least twelve STM-1 frames in the case of TU-12 and TU-2 frames and a time period corresponding to at least three STM-1 frames in the case of TU-3 frames.
• this transition is speeded up in a very simple way, i.e. by neglecting that resetting of the counter of the new pointer counting successive new pointers (new_point) which would be caused by transition from NORM state to LOP state.
• transition from LOP state back to NORM state can be accepted in the case of TU-12 and TU-2 frames after four STM-1 frames, and in the case of TU-3 frames after one STM-1 frame in cases shown in the following table.
• the columns on the left side of the table present the interpretation according to the present invention; columns on the right side present, for the sake of comparison, the interpretation according to the CCITT recommendations.
• Successive STM-1 frames have been numbered from 1 to 53.
• the example concerns a case where the STM-1 signal contains TU-12 frames (or TU-11 or TU-2 frames), the received pointer being known after each V2 byte.
• the values of the received pointers are 20, 39, 24, 120, 39, 39, 21, 39, 39, 39, 39, 39 and 39.
• the counter of the new pointer is not reset in the method according to the invention upon transition to the LOP state after frame 33, when the counter of the invalid pointer achieves its upper limit.
• NORM transition back to the normal state NORM takes place when the counter of the new pointer achieves the value three, this transition takes place in the method according to the invention eight STM-1 frames earlier than in the method complying with the CCITT recommendations.
• the improvement in the recovery rate to be achieved by means of the method according to the invention depends on the reading of the counter of the new pointer upon transition to the LOP state. If the counter of the new pointer steps to the value three at the same time as the upper limit of the counter of the invalid pointer is reached, no transition to the LOP state takes place but the pointer interpretation machine remains in normal state.
• the counter of the new pointer counts the number of successive identical new pointers, whereby it steps e.g. at frame 6 to the value 1, as the value of the obtained pointer deviates from the preceding value, but it will not step onwards from the value 1 e.g. at frames 10 and 14, as the obtained pointer values deviate from the preceding values.
• transition to the LOP state results from the counter of the invalid pointer reaching its upper limit.
• the transition may equally well result from the NDF counter reaching its upper limit (cf. Figure 2).
• Both of the above examples concern a case where the LOP state is entered from the NORM state.
• the LOP state may equally well be entered from the alarm state AIS, in which case the resetting of the counter of the new pointer caused by the state transition is neglect ⁇ ed when the transition takes place. Both transitions take place if N successive invalid pointers (cf. Figure 2) are received. In the case of the AIS state, this is the only event that causes transition to the LOP state.
• the invention is equally applicable e.g. in the corresponding American SONET system or any other similar system where the frame structure consists of a predetermined number of bytes of fixed length, and which frame structure comprises a pointer indicating the phase of the payload within the frame structure.
• the predetermined number of new pointers after which the normal state (NORM) is entered may vary, particularly if the recommendations are changed in this respect.

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• 1993
  • 1993-11-12 FI FI935026A patent/FI94813C/fi active IP Right Grant
• 1994
  • 1994-11-10 WO PCT/FI1994/000506 patent/WO1995013673A1/en active IP Right Grant
  • 1994-11-10 EP EP95900153A patent/EP0728389B1/en not_active Expired - Lifetime
  • 1994-11-10 US US08/640,936 patent/US5724342A/en not_active Expired - Lifetime
  • 1994-11-10 DE DE69432900T patent/DE69432900T2/de not_active Expired - Lifetime
  • 1994-11-10 AU AU81085/94A patent/AU8108594A/en not_active Abandoned

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