STM-N--SynchronousTransportModulelevelN, synchronous transmission module n level

STM-N signal frame structure should be arranged as much as possible It is possible to make tributary low-speed signals evenly and regularly distributed within a frame. why? This is because it is convenient to realize the synchronous multiplexing, cross-connection (DXC), add/drop and exchange of branches. In the final analysis, it is convenient to directly add/drop low-speed tributary signals from high-speed signals. In view of this, ITU-T stipulates that the STM-N frame is a rectangular block frame structure with bytes (8bit) as the unit.

STM-N frame structure, it is composed of 9 rows × 270 columns (bytes), 8 bits per byte, a frame period of 125μs, and a frame rate of 8kHz (8000 frames per second) .

STM-1(N=1) is the most basic structure of SDH. Each frame period is 125μs, 19440 bits are transmitted (9×270×8), and the transmission rate is 19440×8000bit=155520kbit/s;


STM-N is formed by synchronous multiplexing of N STM-1s by inter-byte insertion, so its rate is N times that of STM-1.

SDH frame is composed of three parts: payload (payload), management unit pointer (AUP: Administrationunitpointer) and section overhead (SOH: Sectionoverhead).

1) Information payload

Information payload (payload) is a place where various information code blocks to be transmitted by STM-N are stored. The information payload area is equivalent to the carriage of the STM-N truck, and the cargo in the carriage is the packaged low-speed signal-the cargo to be transported. In order to monitor whether the goods (packed low-speed signals) are damaged in the transmission process in real time, the monitoring overhead byte-the channel overhead (POH) byte is added in the process of packing the low-speed signal. As part of the payload, POH is loaded on the truck STM-N along with the information code block and transmitted in the SDH network. It is responsible for monitoring, managing and controlling the channel performance of the packaged goods (low-speed signals).

For example, the STM-1 signal can be multiplexed into a 63×2Mbit/s signal. In other words, the STM-1 signal can be regarded as a transmission road. It is divided into 63 small roads, and each small road passes low-speed signals of the corresponding rate, then each small road is equivalent to a low-speed signal channel, and the function of the channel overhead can be seen as monitoring the transmission status of these small roads. These 63 2M channels are combined to form the big road of STM-1 signal-here can be called "segment". The so-called channel refers to the corresponding low-speed branch signals. The function of POH is to monitor the performance of these low-speed branch signals when they are carried by the STM-N truck and transported on the SDH network. The information net load is not equal to the effective load, because the information net load stores the packaged low-speed signal, that is, the corresponding POH is added to the low-speed signal.

2) Section overhead (SOH)

Section overhead (SOH) is necessary for network operation, management and maintenance (OAM) to ensure the normal and flexible transmission of information payload Of bytes.

For example: section overhead can be used to monitor whether all the goods in the STM-N truck are damaged during transportation, and the function of POH is to pass it when there is damage to the goods on the truck To determine which item of cargo is damaged. In other words, SOH completes the overall monitoring of the goods, and POH completes the monitoring of a specific piece of goods. Of course, SOH and POH also have some management functions.

Section overhead is divided into regeneration section overhead (RSOH) and multiplex section overhead (MSOH), and the corresponding section layer is monitored respectively. We said that the segment is actually equivalent to a large transmission channel, and the role of RSOH and MSOH is to monitor this large transmission channel.

RSOH, MSOH, and POH provide detailed monitoring functions for SDH signals. For example, in a 2.5G system, RSOH monitors the signal transmission status of the entire STM-16; MSOH monitors the transmission status of each STM-1 signal in STM-16; POH monitors each package in each STM-1 The transmission status of the low-speed tributary signal (for example, 2Mbit/s). In this way, through the layer-by-layer supervision function of the overhead, the transmission status of the signal can be easily monitored from the macro (whole) and micro (individual) perspectives, which is convenient for analysis and positioning.

3) Management unit pointer (AU-PTR)

The management unit pointer is located in the 9×N column of the 4th row in the STM-N frame, a total of 9×N bytes, What role does AU-PTR play? We said that SDH can directly sub/insert low-speed tributary signals (such as 2Mbit/s) from high-speed signals. Why is this happening? This is because the position of the low-speed tributary signal in the high-speed SDH signal frame is predictable, that is, it is regular. The foreseeable realization lies in the pointer overhead byte function in the SDH frame structure. AU-PTR is an indicator used to indicate the exact position of the first byte of the information payload in the STM-N frame, so that the receiving end can correctly separate the information payload according to the value of this position indicator (pointer value).

How do you understand this sentence? If a lot of goods are stored in a stack in the warehouse, and the placement of each piece of goods (low-speed branch signal) in each stack is regular (byte-interleaved multiplexing), then if you want to locate a certain warehouse The location of a piece of cargo only needs to know the specific location of the pile of goods, that is to say, as long as you know where the first piece of the pile of goods is placed, and then through the regularity of the position of the pile of goods, you can directly locate it. The accurate position of any piece of goods in the pile of goods, so that a specific piece of goods can be directly transported from the warehouse (direct distribution/insertion) (low-speed branch signal). The function of AU-PTR is to indicate the position of the first cargo in this pile of cargo. In fact, there are high-level and low-level pointers. The high-level pointer is AU-PTR, and the low-level pointer is TU-PTR (Tributary Unit Pointer). The function of TU-PTR is similar to that of AU-PTR, except that the cargo stack is indicated. It's smaller.


STM-N: STM stands for synchronous transmission mode, N stands for multiplexing level, N=1,4,16,64...corresponding to the line rate of 155M, 622M, 2.5G, 10G... it has nothing to do with the transmission medium itself. The transmission medium (that is, the physical layer of SDH) can be optical fiber, wired cable, or microwave. Using optical fiber as the transmission medium can maximize the advantages of SDH technology, so it is the most widely used.

Other transmission methods include: 51M is generally microwave transmission, which is widely used in Japan and North America, and there are few domestic applications. 155M wired cable transmission is used in the office. 155M and 2×155M and 622M microwave are used in different applications. Adapt to the area where the optical cable is laid or as the backup circuit of the optical fiber SDH.

IP, ATM, TDM can all be used as client signals for transmission via SDH. Specifically: TDM and signals are mapped into STM frames through VC-X, ATM cells VC-4-Xc are mapped into STM frames, and IP datagrams (including IPv4, IPX, etc.) are directly mapped to STM frames through point-to-point protocols. (IPoverSDH).

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