DE10354712A1 - Bit fault-free replacement circuit for ethernet and fibre channel does not use any extra frame overhead etc. to known gigabit ethernet, or fibre channel - Google Patents

Abstract

Bit fault-free replacement circuit for ethernet and fibre channel recognises transmission errors in real time and screens them out. Replacement circuit can be applied both to point-to-point connections and in optical rings.Travelling time differences, appearing in rings are compensated. No further transmission frame, overhead etc., in addition to gigabit ethernet, or fibre channel frame, is necessary. Transmitter and receiver processes are specified.

Description

equivalent circuit takes place today in (Wide Area Networks, WAN) essentially on SDH / Sonet Level. SDH / Sonet offers the possibility Hitless Protection Switching in rings.

hitless Protection switching means that the ring will fail if one fails Segmentes or network element closed again within 50 ms will and the transfer then trouble-free continues.

At the SDH BLSR will permanently transmit SDH signals in both directions. So z. Eg in SDH BLSR in picture 1 the SDH signal of station 1 transferred directly (clockwise) to station 2. Furthermore but it is also over Transfer station 4 and station 3 to station 2 (counterclockwise). The recipient (Station 2) uses the signal with the better quality. Falls so z. B. the direct transfer of Station 1 at station 2 off, the receiver uses the signal that against clockwise. Because the receiver can directly decide which of the two signals he uses this replacement switching technology is interruption free. Uninterrupted here means that the Switching to the spare channel within 50 ms of occurrence the error must be completed. Depending on the switchover criterion however, it can be switched much faster. In simple Bit errors caused by B1 and B2 bytes in the Regenerator (RSOH) and Multiplex Section Overhead (MSOH) can be monitored, an error already in the next transmission frame (frame) are detected. From the occurrence of the error to discovery so passes a maximum of a frame period of 125 μs. In case of loss of the transmission frame, detected by loss of frame (LOF) or out of frame (OOF) at least 4 transmission frames (500 μs) pass. During this Time all received errors are passed on.

The SDH / Sonet protocol was for transmission developed by language. Uncompressed speech comes at 64 kbps transmitted per channel. 30 voice channels are used in the PDH technique with signaling in E1 signals at 2.048 Mbps (32x64 kbit / s). The base rate for SDH (STM-1) is 155 Mbps and can transmit 63 E1 channels. Higher transfer rates have 4 times the transmission speed of the previous transmission rate.

Tabelle 1: SDH Signale

Table 1: SDH signals

meanwhile is the transmission of data has become more important than the transmission of language. In the area Local Area Network (LAN) has become Ethernet enforced. In the field of storage networks (Storage Area Network, SAN), Fiber Channel is widely used. If now different LAN or SAN need to be interconnected, so the data on the WAN transferred become. The following table shows common LAN and SAN standards.

Tabelle 2: LAN/SAN Standards

Table 2: LAN / SAN standards

The transmission rate is 25% higher as the capacity because of the 8B / 10B coding. This encoding converts 8 bit data in a 10 bit code, which is transmitted becomes. When comparing the transfer rates the LAN / SAN protocols with the available bandwidths of the WAN Logs falls on that the available Bandwidth except 2 Gigabit Fiber Channel just very inefficient is being used.

It There is a mechanism in SDH / Sonet, Virtual Concatenation, the various C-4 (C-3) containers bundles. A C-4 container can Transport 149.76 Mbit / s. Virtual Concatenation offers the like the possibility, z. B. Gigabit Ethernet without 8B / 10B encoding, the so-called MAC Frame (MAC, Media Access Controller) in 7 virtual bundled C-4 containers to transport a C-4-7vc with a capacity of 1048.32 Mbit / s. Virtual Concatenation is technically quite complicated and only brings then benefit if the remaining capacity of the WAN signal (eg a STM-16) actually is being used.

BLSR with Ethernet and Fiber Channel

One BLSR lets itself also directly with Ethernet or Fiber Channel realize. basis is the bit-error-free equivalent circuit as described in "Transmission Error Reduction by bit error-free equivalent circuit "(file number: 103 09 089.4-31 at German Patent and Trademark Office) is described.

• 1. Bessere Signalqualität durch Reduzierung der Übertragungsfehler
• 2. Höherwertiger Service durch bitfehlerfreie Ersatzschaltung (interessant z. B. für TV Live Übertragungen)
• 3. Niedrigere Kosten durch einfachere Systeme
• 4. Höhere Reichweite in optischen Ringen durch niedrigere Übertragungsrate und Ersatzschaltgewinn (so lassen sich die Kosten weiter reduzieren, wenn auf Verstärker bzw. Regeneratoren verzichtet werden kann)

The main advantages are:

• 1. Better signal quality by reducing transmission errors
• 2. High-quality service due to bit-error-free replacement circuit (interesting eg for TV live transmissions)
• 3. Lower costs through simpler systems
• 4. Higher range in optical rings due to lower transmission rate and replacement switching gain (thus the costs can be further reduced if amplifiers or regenerators can be dispensed with)

One BLSR with Ethernet / Fiber Channel transmits the same optical signal in both directions. Will now be in the receiver error detected in one of the two received signals, it will other signal used. Both Ethernet and Fiber Channel offer for error detection both 8B / 10B codes and cyclic redundancy exam (CRC, cyclic redundancy check) via the transmission frame. So that bit error free can be switched back and forth the be both signals in sync with each other. This will be buffered by the signal with the shorter one Runtime reached. To find the signal with the shorter duration, have to evaluated the overheads of the Fiber Channel or the Ethernet frames become.

synchronization

Fiber Channel provides information in the overhead, which is the confusion make the frame very unlikely.

Tabelle 3: Fibre Channel Overhead

Table 3: Fiber Channel overhead

It is not enough, only the sequence identification (SEQ_ID) and the sequence counter (SEQ_CNT) to consider to synchronize the incoming frames in the receiver. There depending on the class frames in any order from a fiber Channel fabric can be sent is not guaranteed that the sequence counter the received frame increases in consecutive order.

The synchronization mechanism works as follows:
A receiver stores the received 10 bit codes of a channel (channel A) and searches for the 10 bit coded start of frame (SOF, Start of Frame). If a Start of Frame (SOF) is found, the following 20 bytes up to and including SEQ_CNT are decoded and loaded into the comparator. Now, the second channel (channel B) is examined for an overhead.

If now the same overhead is found in the comparator, the two channels are pre-synchronized and the first channel (channel A) is now read from the memory. In the presynchronization states, channel A is read from memory and the read-out overheads are compared with the overheads received in channel B. If the overhead matches n consecutive times, the channels are synchronized. If the same overhead is not found before the memory overflows, it is obvious that the stored cherte channel has the greater duration.

Now the other channel (channel B) is stored and the overhead in loaded the comparator and compared with the overheads of channel A. If the overheads match, so are the channels pre-synchronized. In the presysynchronization states Channel B is read from memory and the read out overheads compared with the received in the channel A overheads. Is that true? Overhead n times, are the channels synchronized.

vote the error-free received 10 bit codes m times in a row do not match, so the synchronization machine does not go back to the state synchronous.

at Ethernet frame, the overhead is lower. This increases the probability confusing the frame. Confusing the frame means that the receiver is faulty synchronized.

Tabelle 4: Ethernet Rahmen

Table 4: Ethernet frame

To reduce the likelihood of a mis-synchronization, it is necessary to compare more than just an overhead. A synchronization mechanism that runs through several states until it is synchronized or desynchronized offers:
Because Ethernet often transfers large files in a series of Ethernet frames with the same sender and destination address and maximum frame size, the overhead for the series of Ethernet frames is the same. In order to keep the synchronization mechanism independent of the higher protocol layers, it makes sense to increase the number of bytes compared for synchronization (eg 20 bytes as in Fiber Channel) and to trigger the synchronization mechanism on overhead transitions. The synchronization mechanism is otherwise the same as the synchronization mechanism in Fiber Channel.

Substitute circuit at frame level

If both channels are synchronized, bit error-free switching between them can be performed. In order to reliably detect transmission errors, the CRC-32 of the Ethernet / Fiber Channel frames must be evaluated. The probability that a CRC-32 will not detect transmission errors in a frame is 1 to 2 ³² , or 1 to 4 294 967 296 in other words. Thus Fiber Channel frames will be all 1 bit errors, 2 bit errors and all odd number errors detected by bit errors.

Become now the frames of both channels evaluated with the help of the CRC-32 and in case that a channel one CRC-32 has errors, the other channel, which has no CRC-32 error has forwarded, so reduce the forwarded transmission errors considerably. Recognizable errors are only forwarded if they are in both Copies of the same frame occur. For evaluation of the CRC-32 must the entire frame to be cached.

Starting from known bit error rates BER _in (BER, bit error rate) in both independent channels results in a bit error rate of the forwarded bit error, which is dependent on the length of the frame. BER _outCRC = BER _inCanalA · BER _inKanalB · _Frame length (in bits) where BER _in · frame length (in bits) is the probability that a transmission error occurs in one frame, which means that errors in the other channel can not be corrected.

Tabelle 5: maximale Bitfehlerraten in Abhängigkeit von den Eingangsfehlerraten BER_in For Ethernet, the maximum frame length is 1624 bytes, for Fiber Channel 2212 bytes. If one now assumes the maximum frame length and the same bit error rates BER _in both channels, the following values result for BER _out :

Table 5: maximum bit error rates as a function of the input error rates BER _in

Replacement circuit on 8B / 10B Code level

A further error reduction leaves reach if you recognize errors by the 8B / 10B code are reduced. This error reduction is used when both received frames have CRC errors.

• • in beiden Rahmen im gleichen Kodewort ein Fehler auftritt
• • ein Fehler in einem 8B/10B Kode nicht erkannt wird

The equivalent circuit at 8B / 10B level occurs when a faulty detetified frame in one channel in an 8B / 10B code word fails, while the corresponding 8B / 10B code word of the other channel is error free. The equivalent circuit does not work when

• • An error occurs in both frames in the same codeword
• • an error in an 8B / 10B code is not detected

The simplest and thus most likely error pattern that of the 8B / 10B code is not detected is a complementary error, consisting of a 1 error and a 0 error in the same 10 bit code.

The probability that a complementary error will occur in a frame that has already been erroneously detected by CRC is approximately:
9 · BER _in · 0.5 = 4.5 · BER _in
1 is the probability that an error is in an 8B / 10B code,
9 · BER _in is the probability that an error will occur in the remaining 9 bits of the 8B / 10B code
0.5 is the probability that this error is complementary

The Probability that in both detected as faulty frame the error occurring in the same 8B / 10B code is approximately: 1 / frame length (in Byte)

This means that the error with the approximate probability:
1 - (4.59 · BER _in + 1 / frame length (in bytes))
can be corrected by additional equivalent circuit on 8B / 10B code level.

As the output error rate of the combined equivalent circuit of the system thus results
BER _outSYS = (4.5 * BER _in + 1 / frame length (in bytes)) · BER _outCRC

Tabelle 6: maximale Augangsfehlerraten in Abhängigkeit der Rahmenlänge und Eingangsfehlerraten und sich daraus ergebengender Ersatzschaltgewinn (NPG, Net Protection Gain) The following table shows the Net Protection Gain (NPG):

Table 6: maximum initial error rates as a function of the frame length and input error rates and resulting replacement switching gain (NPG, Net Protection Gain)

Claims (1)

Bit-error-free equivalent circuit for Ethernet and Fiber Channel, characterized in that: 1. transmission errors are detected in real time and hidden by switching to the equivalent switching channel. 2. the equivalent circuit can be used both in point to point connections, as well as in optical rings can be compensated for 3. Runtime differences occurring in rings. 4. In addition to the Gigabit Ethemet or Fiber Channel frame no further transmission frame, overhead, etc. is necessary.