Patent Grant
7593432
1. Field of the Invention
The invention relates to communication networks. More specifically, the invention relates to processing bit streams.
2. Description of the Related Art
A digital transmission line that uses wire-pair and coaxial cable is known as a T-carrier. T-carriers include T1 and T3 lines. A T1 line is a point to point digital communications circuit that carries 24 64 kbits/s channels (“Digital Hierarchy-Formats Specification”, American National Standards for Telecommunications, ANSI T1.107, 1995). The bits on the T1 circuit are sent as frames. Each frame consists of 24 8 bit channels resulting in 192 bits per frame (“Digital Hierarchy-Formats Specification”, American National Standards for Telecommunications, ANSI T1.107, 1995). The frames are sent at a rate of 8,000 frames per second (“Digital Hierarchy-Formats Specification”, American National Standards for Telecommunications, ANSI T1.107, 1995). This transfer rate provides an aggregate payload data rate of approximately 1.544 Mbits/s (“Digital Hierarchy-Formats Specification”, American National Standards for Telecommunications, ANSI T1.107, 1995). A framing bit for synchronization increases the size of each frame to 193 bits. The framing bit cycles through a framing bit pattern. A receiver searches for this framing bit pattern to achieve synchronization of the bit stream it is receiving. This bit format is referred to as digital signal level 1 (DS1).
A T-3 line is a digital transmission circuit that supports 28 T1 lines. The bit rate for a T1 line is approximately 44.736 Mbits/s. The bit format of the bit streams carried over T3 lines is referred to as digital signal level 3 (DS3). DS1 signals are multiplexed into DS3 signals. The multiplexing process is a 2 step process (“The Fundamentals of DS3”, 1992). Four DS1 signals are bit by bit interleaved to form a DS2 signal. Seven DS2 signals are multiplexed to form a DS3 signal.
Deframing more than one DS3 bit stream requires a network element with a 1:1 relationship of DSn deframers to DS3 bit streams. Alternatively, a DSn deframer with a 1:n relationship to DS3 bit streams becomes increasingly complicated and costly since the number of deframers and clocks increase linearly with the number of DS3 bit streams to be processed.
A method and apparatus for deframing signals is described. According to one embodiment of the invention, a method is provided for receiving a plurality of signals at a first clock rate, synchronizing the plurality of signals to a second clock rate, and deframing the plurality of signals.
These and other aspects of the invention will be better described with reference to the Detailed Description and the accompanying Figures.
The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known protocols, structures and techniques have not been shown in detail in order not to obscure the invention. Although the invention has been described with respect to DS3, DS2 and DS1 signals, the invention can also be applied to other signaling formats including E3, E2, E1, J1, etc.
The DSn deframing block 250 includes a DS3 deframing unit 203, a DS2 deframing unit 205, and a DS1 deframing unit 209. Some signals that flow into the DSn deframing block 250 enter the DS3 deframing unit 203. The DS3 deframing unit 203 is connected to the DS2 deframing unit 205. The DS2 deframing unit 205 connects to the DS1 deframing unit 209.
DS1 formatted bit streams are received at the DS1 receive and transfer unit 217 of the network element. In one embodiment of the invention, the DS1 bit streams are received from T1 lines (not shown) that are connected to the DS1 receive and transmit unit 217. DS1 signals can be carried in a number of ways including as SONET payload, microwave, etc. The DS1 receive and transmit unit 217 connects to a receiving DS1 buffer 207 and a transmitting DS1 buffer 237. The receiving DS1 buffer 207 is coupled to the DS1 deframing unit 209. DS1 signals received at the DS1 receive and transmit unit 217 follow a path to the DS1 deframing unit 209 via the receiving DS1 buffer 207.
The DS1 deframing unit 209 is coupled to an external memory unit 211. The DS1 deframing unit 209 is also coupled to a DS2 framing unit 239 and the DS3/DS1 data buffer 213. The DS3/DS1 data buffer 213 is coupled to a protocol receiving unit 215.
A protocol transmitting unit 231 is connected to a DS3/DS1 data buffer 233. The protocol transmitting unit 231 performs various functions such as protocol encapsulation. The data buffer 233 is connected to a DS3 framing unit 241 and a DS1 framing unit 235. The DS1 framing unit 235 connects to the transmitting DS1 buffer 237. Bit streams framed by the DS1 framing unit 235 follow a path to the DS1 receive and transmit unit 217 via the transmitting DS1 buffer 237. The DS1 framing unit 235 also connects to the DS2 framing unit 239. The DS2 framing unit 239 connects to the DS3 framing unit 241. The DS3 framing unit 241 is coupled to the transmit buffering unit 242 and the optical transmitting unit 225.
An optical receiving unit 229 connects to the transmit buffering unit 242. The optical receiving unit 229 performs various functions such as demapping STS formatted signals into DS3 signals. The optical receiving unit 229 also connects to the receive buffering unit 202.
In one embodiment of the invention, the set of buffers 306-309 are asynchronous First In First Out buffers (FIFOs). The inputs are written into the buffers 306-309 at the DS3 rate and read at the rate of the domain clock. In one embodiment of the invention, the domain clock runs at 100 Mhz in order to process 2 DS3 bit streams (each DS3 running at approximately 45 Mhz) per deframing slice. However, embodiments of the present invention are not so limited, as the domain clock can run at other clock rates that run faster than the sum of the clock rates of the incoming signals. Since each DS3 bit stream may originate from sources running at slightly different clock rates, valid bits accompany DS3 data read out of the asynchronous FIFOs in the clock domain.
The multiplexer 313 multiplexes the input selected by the selecting units 311 and 312 before sending the multiplexed input into a DS3 deframer 320. Each deframer slice includes the DS3 deframer 320, a DS2 deframer 322, and a DS1 deframer 324.
Each individual deframer processes successively lower bandwidth channels. Since each deframer handles two DS3 bit streams worth of data, though, each deframer actually processes approximately the same total number of bits. The DS3 deframer 320 handles two DS3 channels. The DS2 deframer 322 processes fourteen DS2 channels. The DS1 deframer 324 processes fifty-six DS1 channels. Input flows from the DS3 deframer 320 to the DS2 deframer 322, and then to the DS1 deframer 324. From the DS1 deframer 324 of each of the deframing slices 301, 303, and 305, bits flow into the DS1 data buffer 213 of
In addition to the DS3 inputs 302, 304 and inputs 306, 308, the deframing slice 301 receives DS1 bit streams from the receiving T1 buffer 207 of
In another embodiment of the invention, every deframing slice 301, 303, 305 only receives one DS3 bit stream input. In another embodiment of the invention, each deframer slice receives one DS3 bit stream input and a set of DS1 bit streams. In another embodiment of the invention, each deframing slice receives inputs from two sets of DS1 bit streams. In another embodiment of the invention, a deframing slice can have N inputs, each of the N inputs independently configurable for either a DS3 input or a set of DS1 inputs.
In one embodiment, each deframer 320, 322, 324 processes its set of channels in a time division multiplex fashion. For example, the DS3 deframer 320 works on the pair of DS3 channels in alternating cycles. The DS2 deframer 322 works on 14 DS2 channels in a circulatory fashion. The order the DS2 deframer 322 circulates through the DS2 subchannels depends on the order in which they are deframed by the DS3 deframer 320. In other words, the DS3 deframer 320 pushes DS2 subchannels into the DS2 deframer 322. Likewise, the order the DS1 deframer 324 circulates through its 56 DS1 subchannels is dictated by the DS2 deframer 322.
To accommodate 2 DS3 signals (transmitted at approximately 44.736 Mhz each) feeding into a deframing slice, the deframers run at approximately 100 Mhz. Each of the DS3 bit streams appears to flow through 50 Mhz deframers. Having the deframers outrun the bit streams insures that the deframers will be fast enough to deframe all incoming bits. In addition, although each bit stream needs a set of state for deframing (specifically, sync hunting which is a necessary aspect of deframing), the faster rate enables 2 DS3 bit streams to be deframed with one core. A single core logic for 2 DS3 bit streams provides a savings of space. In another embodiment of the invention, a faster clock speed for the deframers, such as 200 Mhz, enables a single core logic to process 4 DS3 bit streams. In another embodiment of the invention, a deframing slice receives N channels or inputs processed at M bits at a time. In such an embodiment, the core clock exceeds the following: sum(n=1 . . . N, clockrate[n]/M).
The DS3 deframer 320 processes the streams 405, 407 and 409 and generates a data bit stream 406, a validity bit stream 408, and a subchannel bit stream 410 which flow into the DS2 deframer 322. The subchannel bit stream 410 also flows into a context memory 411. The context memory 411 includes a per-channel state memory and a sync hunt per-alignment memory for each pair of subchannels. The per-channel state memory and the sync hunt per-alignment memory for each deframer will be described later herein with references to FIG 6-12. Information 404 from the context memory 411 flows into the DS2 deframer 322.
The DS2 deframer 322 processes the streams 406, 408, 410 and the information 404 from the context memory 411 to generate a data bit stream 413, a validity bit stream 415, and a subchannel bit stream 417. The streams 413, 415, and 417 flow into the multiplexer 315. Updates 444 are written back to the context memory 411 from the DS2 deframer 322. Data bit streams also flow into the multiplexer 315 from the receiving T1 buffer 207. A data bit stream 427, a validity bit stream 425, and a subchannel bit stream 424 flow into the multiplexer 315 from the receiving T1 buffer 207. The data bit stream 427 and the data bit stream 413 are multiplexed to generate a data bit stream 412. The validity bit streams 415 and 425 are multiplexed to generate a validity bit stream 414. The subchannel bit streams 417 and 424 are multiplexed to generate the bit stream 416. The streams 412, 414, 416 flow into the DS1 deframer 324. The subchannel bit stream 416 also flows into a context memory 419. Information 430 from the context memory flows into the DS1 deframer 324. The context memory 419 and the information 430 stored in the context memory 419 are described later.
The DS1 deframer 324 processes the bit streams 412, 414, 416 and the information 430 from the context memory 419. After processing, the DS1 deframer 324 generates a data bit stream 418, a validity bit stream 420, and a subchannel bit stream 422. Updates 432 are written back to the context memory 419 from the DS1 deframer 324.
In one embodiment of the invention, each pair of DS1 subchannels (e.g. the pair of DS1 subchannels DS1 subchannel 0 of DS3 side 0 and DS1 subchannel 0 of DS3 side 1) has a read and write FIFO that is two 7 byte entries deep. This size provides space for 14 sync hunt states. The entries in each FIFO provide enough latency tolerance to keep the sync hunt logic working while the memory controller 321 serves other channels. Each one of the deframing slice's FIFOs are independently writable and readable. The memory controller 321 writes to the read FIFOs. The sync hunt core reads the read FIFOs. The sync hunt core writes to the write FIFOs and the memory controller reads from the write FIFOs. In one embodiment of the invention, the FIFOs are asynchronous because each DS3 bit stream may run at a different bit rate. In one embodiment of the invention, each DS1 sync hunt begins by flushing the read and write FIFOs of any possible stale sync hunt data. The sync hunt logic then allows the read FIFO to become full. After the read FIFO is full, sync hunting begins. The memory controller and asynchronous FIFOs ensure sufficient provision of bandwidth to the DS1 sync hunt logic. In another embodiment of the invention, the read/write FIFOs are larger to accommodate higher density deframing slices.
In one embodiment of the invention, the addressing pointers for the external memory unit 323 (shown in
As shown by
In one embodiment of the invention, each deframer sync hunts half of the total number of channels because the sync hunt memory is shared. Limiting the sync hunting reduces the space necessary to implement the deframer. In such an embodiment, each deframing slice can process channels from two DS3 bit streams, but sync hunts one of those bit streams. For example, the DS3 deframer sync hunts either the first DS3 bit stream or the second DS3 bit stream, but not both at the same time. Deframing (i.e., identification of payload and overhead bits) is conducted for both channels simultaneously as it is not costly to implement. The DS2 deframer sync hunts either a DS2 channel from the first DS3 bit stream or a DS2 channel from the second DS3 bit stream, but not DS2 channels from both DS3 bit streams. Similarly, the DS1 deframer sync hunts DS1 channels from either the first DS2 bit stream or the second DS3 bit stream.
Synchronizing a bit stream (sync hunting) comprises searching for a bit pattern formed by an alignment signal. For example, a DS3 frame includes seven subframes. Each subframe comprises eight 85 bit blocks. The first bit of each block is an overhead bit which includes bits of the alignment signal. For a DS3 signal, the alignment signal includes F-bits and M-bits. The F-bits or framing bits form a bit pattern “1001” in each subframe at blocks two, four, six, and eight. Each F-bit is separated by 170 bits. The M-bits or multiframing bits form a bit pattern “010”. The M-bits occur in the first block of the fifth, sixth, and seventh subframe. It should be understood that the invention is not limited to these bit patterns. In another embodiment of the invention, the logic searches for different bit patterns to synchronize a bit stream or signal. The sync hunting logic 527, 627 maintains multiple per-alignment state machines to be described. The sync hunt logic performs sync hunting concurrently for multiple per-alignment state machines using a single bit. The logic determines if the bit matches the F-bit pattern for one per-alignment state machine and the M-bit pattern for a different per-alignment state machine. The sync hunting is described in more detail with reference to
At block 801 of
At block 805, it is determined if X=N−1(N being the total number of per-alignment state machines). This check determines if the logic has iterated through all of the per-alignment state machines. If it is determined that X is not equal to N−1, then at block 807, X is incremented. From block 807, control flows back to block 803. If, at block 805, it is determined that X=N−1, then at block 808 X is reset. At block 809, another signal bit is received from the register 519 and saved as the second F-bit in the subframe alignment shift register for a per-alignment state machine X.
After this second F-bit is stored at block 809, a sync hunt state machine for the per-alignment state machine X is set to indicate state as “HUNTING—010” at block 811. The per-alignment state machine is described in Table 1.
At block 813 it is determined if X=N−1. If X does not equal N−1, then at block 815 X is incremented. From block 815 control flows back to block 809. If it is determined at block 813 that X equals N−1, then at block 817 X is reset. At block 819 a signal bit is received from the register 519. From block 819, control flows to both blocks 821 and 823. At block 821, framing bit pattern verification for a per-alignment state machine (X+85) MOD 170 is performed concurrently with verification of F-bits for a per-alignment state machine X at block 823.
At block 823, it is determined if the bit received at block 819 is the next expected F-bit for a per-alignment state machine X. If the received bit is the next expected F-bit for the per-alignment state machine X, then at block 831 it is determined if X=N−1. If at block 823 it is determined that the bit is not the next expected F-bit for the per-alignment state machine X, then at block 827 the sync hunt state machine for the per-alignment state machine X is set to indicate failure. From block 827 control flows to block 831. If X does not equal N−1, then at block 829 X is incremented. From block 829 control flows back to the block 819. If it is determined at block 831 that X equals N−1, then at block 833 it is determined if all per alignment state machines have failed or a time out has occurred. If all of the per-alignment state machines have failed or a timeout has occurred, then at block 835 the DS3 sync hunting restarts. In an alternative embodiment of the invention, a timeout forces the sync hunt logic to select one of the per-alignment state machines which have not failed. If it is determined at block 833 that all of the per-alignment state machines have not failed and a timeout had not occurred, then at block 837 it is determined if only one per-alignment state machine remains valid. If it is determined at block 837 that more than one per-alignment state machine still remains valid, then control flows to block 817. If only one per-alignment state machine remains valid, then it is determined if the per-alignment state machine indicates a state of “MAINTAIN—010” at block 838. If it is determined that the per-alignment state machine indicates “MAINTAIN—010”, then the DS3 framing pattern has been detected and at block 839 DS2 deframing begins. If it is determined at block 838 that the per-alignment state machines does not indicate “MAINTAIN—010”, then control flows to block 817.
Each state represents a transition state. The state S0 means the sync hunt logic is looking for the bit 0. The state S01 means the sync hunt state machine has stored a previous bit 0 and is looking for a bit 1. The state S010 means the sync hunt logic saw a 1 a the previous bit and is looking for a 0. The state SX1 represents a transition in the sync hunt to looking for the first X framing bit. The state SX2 means the sync hunt logic is hunting for the second X framing bit that should be the same as the P bit shown in table 1 (the first X framing bit). The state SP1 represents a transition in the sync hunt to looking for the first P framing bit. The state SP2 means the sync hunt logic is hunting for the second P framing bit that should be the same as the P bit shown in table 1 (first P framing bit).
If the received bit is not the correct bit in accordance with the indicated state, then the block 851 the sync hunt state machine for this per-alignment state machine is set to indicate a state of fail. From block 851, control flows back to block 831 of
A DS3 master state machine controls the DS3 per-alignment state machines. The DS3 master state machine initializes and maintains the per-alignment state machines. The states of the DS3 master state machine are described in Table 3 below.
The states of the master state machine can be represented with a variety of values. In one embodiment of the invention, a S113_WAIT_WINNER state is not maintained.
In the DS2 format, a DS2 frame is comprised of four subframes. Each subframe includes six 49 bit blocks. Each block includes an overhead bit followed by 48 bits. An M-bit is the overhead bit for the first block of each subframe. The M-bits form either the bit pattern “0111” or “0110” in a given DS2 frame. An F-bit is the overhead bit for blocks three and six of each subframe. The two F-bits of a subframe form the bit pattern “01” in each subframe. The DS2 alignment bit patterns are meant to aid in the understanding of the invention and not as limitations upon the invention.
At block 1107, it is determined if X is equal to N−1. If X is not equal to N−1, then at block 1109 X is incremented. From block 1109, control flows back to block 1103. If at block 1107 it is determined that X does equal N−1, then at block 1111 X is reset. At block 1113, another signal bit is received from the register 615. From block 1113 control flows to both blocks 1115 and 1117. At block 1115 it is determined if the received bit is the next expected F-bit for a per-alignment state machine X in concurrence with verification of M-bit patterns for a per-alignment state machine (X+98) MOD 147 at block 1117. In other words, the search for valid F-bit and M-bit patterns are performed concurrently. If the received bit is the next expected F-bit for the per-alignment state machine X, then at block 1123 it is determined if X equals N−1. If at block 1115 it is determined that the received bit is not the next expected F-bit for the per-alignment state machine X, then at block 1119 the sync hunt state machine for the per-alignment state machine X is set to indicate a state of fail. For example, if per-alignment state machine is expecting a 1 but receives a 0, then synchronization represented by that state machine cannot be correct. From blocks 1119 and 1117 control flows to block 1123. If it is determined at block 1123 that X does not equal N−1, then at block 1121 X is incremented. From block 1121 control flows to block 1113. If at block 1123 it is determined that X equals N−1, then it is determined if all the per-alignment state machines have failed or a time out has occurred at block 1125. If all of the per-alignment state machines have failed or a time out has occurred, then at block 1127 DS2 sync hunting is restarted. In an alternative embodiment, a timeout forces the sync hunt logic to select one of the remaining per-alignment state machines as the winner. If it is determined at block 1125 that all the state machines have not failed or a time out has not occurred, then at block 1129 it is determined if only one per-alignment state machine has not failed. If it is determined at block 1129 that more than one per-alignment state machine remains valid, then control flows to block 1111. If only one per-alignment state machine has not failed, then it is determined at block 1130 if the per-alignment state machine indicates a state “MAINTAIN—01”. If the valid per-alignment state machine does not indicate the state “MAINTAIN—01”, then control flows to block 1111. If the valid per-alignment state machine indicates the state “MAINTAIN—01”, then the DS2 stream has been synchronized and DS2 deframing begins at block 1131.
If the sync hunt state machine of the per-alignment state machine indicates “MAINTAIN—01”, then at block 1137 it is determined if the received bit is the correct bit in accordance with a state indicated by the sync hunt state machine as shown in table 5.
As with the DS3 sync hunt state machine, each state represents a transition state. The state S0 means the sync hunt logic is looking for a bit 0. The state S01 means the sync hunt state machine has seen a previous bit 0 and is looking for a bit 1. The state S011 means the sync hunt logic has seen a bit 1 as the previous bit and is looking for a 1. The state X represents acceptance of any bit since the fourth M framing bit can be either a 0 or 1.
If the received bit is not the correct bit, then at block 1139 the sync hunt state machine is set to indicate a state of fail. From block 1139 control flows back to block 1123 of
Similar to the DS3 deframer, a master state machine controls the DS2 sync hunt logic. The states of the DS2 master state machine are shown in Table 6 below.
The alignment signal for the DS1 superframe format is formed by interleaving framing and superframing bits. The first bit of each DS1 frame is a bit for the alignment signal. The interleaved framing and superframing bits form the pattern “100011011100”. The DS1 superframe alignment bit patterns are meant to aid in the understanding of the invention and not as limitations upon the invention.
The bit sequences identified in table 7 do not occur in the framing bit stream for DS1 super frame formatting.
At block 1419 a signal bit is received from the register 713. At block 1421 it is determined if the received bit is the expected bit in accordance with the indicated state of the per-alignment state machine X as shown in Table 8 below.
These states are based on the super frame framing bit stream 100011011100.
If the received bit is not the expected bit, then at block 1423 the per-alignment state machine is updated to indicate a state of fail. If the received bit is the expected bit, then at block 1431 the per-alignment state machine is updated. Control flows from block 1423 and block 1431 to block 1437. At block 1437 it is determined if X=N−1. If X does not equal N−1, then at block 1439 X incremented. Control flows from block 1439 to block 1419. If at block 1437 it is determined that X equals N−1, then at block 1425 it is determined if all per-alignment state machines have failed or a timeout has occurred. If all state machines have not failed and a timeout has not occurred, then at block 1427 it is determined if only one state machine remains valid. If, at block 1425, it is determined that all of the per-alignment state machines have failed, then at block 1435 DS1 super frame sync hunting is restarted. If it is determined at block 1427 that only one state machine remains valid, then synchronization has been found for the DS1 signal and at block 1429 DS1 super frame deframing begins. If it is determined at block 1427 that more than one per-alignment state machine is valid, then at block 1441 X is reset and control flows back to 1419.
The alignment signal for the DS1 extended superframe format comprises framing bits (F-bits) positioned at the beginning of every block that is a multiple of four (i.e., the first bit of blocks 4, 8, 12, 16, etc). The F-bits form the pattern “001011” over 24 frames. The pattern is repeated every 24 frames. The DS1 extended superframe alignment bit patterns are meant to aid in the understanding of the invention and not as limitations upon the invention.
At block 1605, it is determined if X=N−1. If X is not equal to N−1, then at block 1607 X is incremented. From block 1607, control flows to block 1603. If it is determined at block 1605 that X=N−1, then at block 1609 a bit is received and stored as a second F-bit in the per-alignment state machine X[0]. An illustration of storing F-bits in the per-alignment state machines is described with reference to
At block 1611, the per-alignment state machine X is set to indicate “HUNTING—101”. At block 1613 it is determined if X=N−1. If X is not equal to N−1, then at block 1615, X is incremented. Control flows back to block 1609 from block 1615. If it is determined at block 1613 that X is equal to N−1, then at block 1617 X is reset. At block 1619 another signal bit is received from the register 713. At block 1621 frame bit verification is performed. At block 1625 it is determined if X=N−1. If X is not equal to N−1, then At block 1623 X is incremented. From block 1623 control flows to block 1619. If X is equal to N−1, then at block 1627 it is determined if all per-alignment state machines have failed or timeout has occurred. In an alternative embodiment of the invention, a timeout forces the sync hunt logic to select one of the valid per-alignment state machines as a winner. If all state machines have failed or timeout has occurred, then at block 1629 DS1 sync hunting is restarted. If it is determined at block 1627 that all per-alignment state machines have not failed or a timeout has not occurred, then at block 1631 it is determined if only one per-alignment state machine remains valid. If more than one per-alignment state machine remains valid, then control flows to block 1617. If only one per-alignment state machine remains valid, then it is determined at block 1632 if the valid per-alignment state machine indicates a state “MAINTAIN—101”. If the valid per-alignment state machines does not indicate this state, then control flows to block 1617. If it is determined at block 1632 that the valid per-alignment state machine indicates the state “MAINTAIN—101”, then synchronization has been found for the DS1 extended superframe signal and at block 1633 DS1 extended super frame deframing begins.
Each state represents a transition state for DS1 extended super frame sync hunting. The state S0 means the sync hunt logic is looking for the bit 0. The state S00 means the sync hunt state machine has stored a previous bit 0 and is looking for a bit 0. The state S001 means the sync hunt logic saw a 0 as the previous bit and is looking for a 1. The state S0010 indicates that the sync hunt state machine has stored a previous bit 1 and is looking for a bit 0. The state S00101 means the sync hunt state machine has stored a previous bit 0 and is looking for a bit 1. The state S001011 indicates that the sync hunt state machine has stored a previous bit 1 and is looking for a bit 1.
If the received bit is the correct bit, then the bit is shifted into the sync hunt state machine at block 1655. Control flows from block 1655 to block 1623 of
A master state machine regardless of extended super frame or super frame formatting controls DS1 sync hunting. The states of the master state machine are shown in table 11.
As described above, each successive deframer tags the multiplexed bit stream with successively more information. For example, the DS3 deframer tags the bit stream with DS2 subchannel numbers and an indication of the DS3 channel (i.e., even or odd DS3 input). The DS2 deframer adds indicator bits indicating DS1 subchannels. The DS1 deframer extracts the Facility Data Link Channel and tags it, creating a new data link channel for every data channel. The DS3 deframer tags bits to distinguish overhead bits from information bits. A stuffing bit identifying a bit as a DS3 information bit is replaced by the DS2 deframer with a stuffing bit identifying the bit as a DS2 overhead bit or DS2 information bit. The same is done by the DS1 deframer.
Change of frame alignment feed forwarding increases the efficiency of deframing. As soon as the DS3 deframer 320 finds the DS3 alignment and begins deframing, the DS2 deframer 322 will begin sync hunting for DS2 alignment. The DS2 deframer will not look at every bit from the DS3 deframer, though. The DS3 deframing logic is stuffing overhead bits, thus enabling the DS2 deframer to ignore bits which are not part of the DS2 alignment signal. Likewise, as soon as the DS2 deframer 322 finds the DS2 alignment and begins deframing, the DS1 deframer 324 will begin sync hunting for DS1 alignment. The accelerated sync hunting enabled by change of frame alignment forwarding provides the time for sharing sync hunting memory.
The deframing logic described herein enables the production of network elements with a high density of deframers. The validity bits used for synchronizing bit streams reduces cost and complexity of a network element to process a large number of bit streams. The density or number of bit streams that can be processed is not hindered by the generation of individual clocks for each channel or subchannel. Typically, the number of clocks increases linearly with the number of subchannels to be processed. Using the deframing logic described herein, the clock speed increases with the density of bit streams, but deframing is performed in one clock domain.
The described sync logic sync hunts by searching approximately half of the subframes of each frame. Such a design enables sharing of memory to maintain state machines for multiple subchannels. Sharing memory reduces the cost and complexity to implement the deframers. Furthermore, less space is used for memory to maintain state machines for deframing.
The techniques shown in the figures can be implemented using code and data stored and executed on computers. Such computers store and communicate (internally and with other computers over a network) code and data using machine-readable media, such as magnetic disks; optical disks; random access memory; read only memory; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. Of course, one or more parts of the invention may be implemented using any combination of software, firmware, and/or hardware.
While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. Alternative embodiments can implement the loop controls of the sync hunt logics and deframing logics in a variety of ways. In addition, as previously described, deframers running at a fast clock speed can process bit streams transmitted at a slower clock rate. The increased deframer density leads to alternative embodiments with the per-alignment state machines for the DS2 and DS3 deframers stored in external memory. In another embodiment, a single external memory unit stores the per-alignment state machines for all deframers. In another embodiment of the invention, each deframing slice of a network element processes a single DS3 input within a single clock domain. In another embodiment of the invention, each deframing slice of a network element processes a single DS3 input within a single clock domain and shares a single memory unit to store states. In another embodiment of the invention, data formats may vary across channels or subchannels. For example, a deframing slice may receive a DS3 input and an E3 input as long as the domain clock outruns the sum of the incoming signal rates. In another exemplary embodiment of the invention, a DS2 signal may include three E1 signals instead of four DS1 signals. The E1 and DS1 signals can be deframed in the same clock domain.
The method and apparatus of the invention can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting on the invention.
This application claims the benefit of U.S. Provisional Application No. 60/280,694, entitled “A Method and Apparatus for Processing Multiple Communications Signals in One Clock Domain”, filed Mar. 31, 2001.
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| Number | Date | Country | |
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| 20040073714 A1 | Apr 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60280694 | Mar 2001 | US |
