Quasi-synchronous digital hierarchy in Europe and China:
Note: Take the primary group and the secondary group as examples, the speed of the secondary group here is not four times that of the primary group, because PDH adopts asynchronous multiplexing. The weakness of PDH:
(1)PCM multiplexing
PCM multiplexing is to encode and multiplex multiple signals directly. That is to say, the multi-channel analog voice signals are sampled separately with a period of 125μs, and then coded together to form multi-channel digital signals. (The formation of a primary group belongs to PCM multiplexing)
(2) Digital multiplexing
Digital multiplexing is the superposition of several low-order groups in the time gap to form a high-order group. That is, the formation of higher order groups adopts the method of digital multiplexing. For example, four primary groups form a quadratic group, and four primary groups can be superimposed on the time gap to form a quadratic group.
(1) Bit-by-bit multiplexing
Bit-by-bit multiplexing, that is, one bit of each low-order group (also called branch) is multiplexed each time to form a high-order group. Take a quadratic group formed by four primary groups as an example. First, take a bit code of the first primary group, then take a bit code of the second primary group, and then take the second bit code of each primary group in turn after taking a bit code of four primary groups.
(2) word-by-word multiplexing
word-by-word multiplexing, that is, one codeword of each low-order group (branch) is multiplexed each time to form a high-order group. Word-by-word multiplexing can ensure the completion of a codeword, but modern signal processing and switching are all in bytes, so SDH only uses word-by-word (8 bits) multiplexing, while traditional PDH mostly uses bit-by-bit multiplexing.
the primary problems to be solved in digital multiplexing are synchronization and multiplexing. Synchronization, that is, the digital rates of several low-order groups that are multiplexed are the same. If they are not synchronized, the multiplexed numbers will have overlapping and dislocation between symbols.
digital multiplexing methods are divided into synchronous multiplexing and asynchronous multiplexing.
synchronous multiplexing means that several low-order groups are controlled by the same high-stability master clock, so that the code rate (code speed for short) of these low-order groups is unified at the frequency of the master clock (so that several low-order group systems can be synchronized), and can be directly multiplexed. Synchronous multiplexing generates digital signal sequence under the control of clock, and the rate value of digital signal Su Lie is the same as the clock frequency. Although synchronous multiplexing does not need code speed adjustment, it needs code speed conversion.
(1) code speed transformation
Take four primary groups as an example. The code speed transformation is to leave a space for inserting additional codes in each primary group (insert additional codes when multiplexing) and increase the code speed from 248kbit/s to 2112 kbit/s. The reason of code rate conversion is that although the clocks of the multiplexed branches are all supplied by the same clock source, the code rate can be guaranteed to be equal, but in order to meet the needs of tapping at the receiving end, a certain number of frame synchronization codes need to be inserted; Moreover, in order to make the multiplexer and demultiplexer work normally, it is necessary to add business codes such as peer-to-peer alarm code, neighbor station detection and service practice (the above inserted symbols are collectively referred to as additional codes). Therefore, synchronous multiplexing needs code rate conversion.
(2) the process of code rate conversion
before the code rate conversion, the rate of four primary groups was 248 kbit/s, that is, there was a bit in 125 μs, and after the code rate conversion, it was hoped to increase to 2112 kbit/s. At this time, there was a bit in 125 μs, so each primary group needed to insert 8 bits in 125 μs, that is, it needed to insert 1 bit on average.
asynchronous multiplexing means that each low-order group uses its own clock. Because the clock frequency of each low-order group is not necessarily equal, the code rate of each low-order group is not exactly the same (this is asynchronous). Therefore, the code speed should be adjusted first to synchronize each low-order group before multiplexing. Therefore, although asynchronous multiplexing does not need code rate conversion, it needs code rate adjustment. Most PDH uses asynchronous multiplexing, because once the master clock fails, all related communication systems will be interrupted, so the synchronous multiplexing method is limited to local areas.
(1) Code speed adjustment method
Assuming that the code speeds of four primary groups are around, the code speeds of all four primary groups are synchronously adjusted to 2112kbit/s by inserting certain symbols, and then the secondary groups are multiplexed bit by bit. The code speed adjustment can be divided into:
(3) the difference between the code speed adjustment of asynchronous multiplexing and the code speed transformation of synchronous re-inoculation
(1) the code rate of the secondary group
The number of symbols in the secondary group within 125 μs is:
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there are 256bit in the primary group of 125 μs, of which 32 is the number of inserted codes in the asynchronous multiplexing process.
the digital rate is:
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(2) code speed adjustment
If there are four primary groups, the code rate will be, after inserting symbols in the code speed adjustment process, it will be multiplexed into secondary groups after code speed adjustment, and the code rate will be.
(3) Composition of Digital Multiplexing System
Digital asynchronous multiplexing system mainly includes two parts:
Take four primary groups as an example. At the transmitter, there are four primary groups, and the code rate is about 248kbit/s, which are adjusted separately, adjusted to 2112kbit/s after inserting symbols, and then multiplexed to form a secondary group. At the receiving end, four primary groups with code rate of 2112kbit/s are obtained by demultiplexing, and then the code rate is recovered (corresponding to the code rate adjustment, the inserted symbols are removed, that is, "de-insertion"), and finally four primary groups with code rates of about 248kbit/s are obtained. Because the digital communication system is time-division multiplex communication, the processing and transmission of all signals are carried out in strict time, so the work of the digital multiplex system also needs to be controlled by timing pulses, so the transmitter and receiver in the digital multiplex block diagram have timing systems. Synchronization in the figure refers to the synchronization of timing at both ends of transmission and reception.
the frame period of quadratic group is 1.38μs and the frame length is 848 bit. Among them, there are at least 4× 25 = 82 bits of information codes (symbols before code speed adjustment of four primary groups) and at most 28bit of insertion codes.
there are 212 symbols within 1.38μs after the first group code speed adjustment:
* 25~26 information codes.
* There are 6~7 insertion codes, including ~1 insertion code for code speed adjustment (at most 1) and 3 insertion mark codes.
in frame 1 of secondary group, There are:
* 24-28 insertion codes (28 at most)
* -4 insertion codes (4 at most) for code speed adjustment
* 12 insertion mark codes
* There are at least 82 information codes
, in which the flag code is inserted to inform the receiver whether the 161st bit has been inserted or not, so that the receiver can "de-insert". And the insertion flag code adopts a three-bit insertion flag code, so as to prevent erroneous judgment at the receiving end caused by channel error code. ITU-T stipulates that the bit error rate of digital transmission system should be lower than minus 6 power of 1, that is, only one bit of 1 million bits is allowed to make an error. Select three bits to insert the flag code, and "choose two out of three":
When more than two "1" codes are received, it is considered to be inserted;
when more than two "" codes are received, it is considered that there is no insertion.
in this way, it is possible to prevent erroneous judgment at the receiving end due to channel error code with high probability.
four quadratic groups with nominal rate of 8.448 Mbit/s (instantaneous rate may be different) are respectively adjusted in code speed, and their rates are uniformly adjusted to 8.592 Mbit/s, and then they are bitwise multiplexed into cubic groups.
the structure of PCM tertiary group frame is as shown in the figure:
expansion:
SDH network is composed of some SDH network elements (nes), which transmit, multiplex, add/drop and cross-connect synchronous information on optical fiber. There is no switching equipment in SDH network, it is only the transmission receiving end between exchange offices. SDH network was first used for transmission between converters, and later used for ATM network (ATM is the abbreviation of Asynchronous Transfer Mode(ATM), which is a packet switching and multiplexing technology based on cells. It adopts connection-oriented transmission mode, which divides data into cells with fixed length and exchanges them through virtual connection. ATM integrates switching, multiplexing and transmission, and adopts asynchronous time division multiplexing in multiplexing, which distinguishes different channels by the header or header of information. ) and ATM switches, and now routers can use SDH network as data network to transmit ip datagrams. The concept of SDH network includes the following points:
Note that the high-order group here is four times that of the low-order group, because SDH adopts synchronous multiplexing.
the main task of the terminal multiplexer is to incorporate the low-speed branch signal into the STM-1 frame structure and convert it into STM-1 optical fiber signal through electrical/optical conversion, and the reverse process is just the opposite.
The function of the bifurcated multiplexer is to integrate synchronous multiplexing and digital cross-connection engineering, and it has the ability to add and drop any branch signal flexibly, which has great flexibility in network design. ADM also has optical/electrical and electrical/optical conversion functions, because ADM is divided and inserted on the basis of electrical signals, while optical signals are uploaded on the line.
the function of the regenerative repeater is to convert the optical pulse signal which has been greatly attenuated and distorted by dispersion after long-distance optical fiber transmission into an electrical signal, and then amplify, shape, retime and regenerate it into a planned electrical pulse signal, and then modulate the light source to convert it into an optical pulse signal and send it to the optical fiber for further transmission, so as to extend the transmission distance.
digital cross connect is used to realize cross connection between branches. Digital cross connect has applications in PDH network and SDH network. In PDH network, branches refer to PCM subgroups, etc. In SDH network, it refers to STM synchronous transfer module and so on.
the frame structure of SDH is the block state as shown in the following figure.
the frame structure of SDH always has 9 rows and 27×N columns, and each row and column of the frame structure of SDH corresponds to one byte, and N=1, 4, 16 and 64 SDH is transmitted bit by bit in the order from top to bottom and from left to right. The frame period of SDH is 125 μs, the frame length is 9×27×N×8 bit, and the rate is. Its main components are as follows:
For example, the capacity (rate) of AU-PTR in STM-16 is:
Among them, 125 μs is the frame period, and the number of bytes of AU-PTR in STM-16 is 1 row of 9×N bytes, that is, 1×9×16 bytes.