Types of sdh multiplexers. SDH technology. Synchronous digital hierarchy. "Transport and distribution networks"

Digital multiplexers are logical combined devices that are designed for controlled transmission of information from several data sources into a single output channel. In fact, such a device consists of several digital position toggle switches. Accordingly, we can come to the conclusion that a digital multiplexer is a switch of input signals into one output line. This article will consider a separate type of device - SDH optical multiplexers.

Such devices are designed to work with data streams using light beams that differ in amplitude or phase diffraction grating, as well as wavelength. SDH multiplexers transmit information over E1 channels and Ethernet lines in transport fiber optic networks. They operate over one or two optical fibers (single-mode or multimode) at speeds of 155, 520 Mbit/s at a wavelength of 1550/1310 nm. SDH multiplexers allow up to 126 pt communications.

The advantages of such devices include resistance to external influences, technical safety, and protection against hacking of transmitted information.

SDH multiplexers are simply scalable by including up to 3 additional modules for transmitting Ethernet channels, E1 streams, service communications, and a PM channel into the main module.

These devices are characterized by the highest “survivability” of the network. The implementation of E1 streams has a low jitter value, due to this, the standards for E1 are observed during synchronization drift, also when the synchronization of the STM-1 system is disrupted. The interface characteristics allow you to track an error in the communication channel and switch to a spare channel. The optical path and power supply are reserved according to the 1+1 scheme. In other words, when working over one fiber optic channel, in the event of cable damage, communication between subscribers is maintained.

SDH multiplexers are simply combined with other SDH type equipment. They can operate in both synchronous and asynchronous modes; the introduction of multimode and single-mode optical fiber is allowed. The SDH multiplexer supports remote configuration and management via TCP/IP, 10/100 BaseT.

Such switching devices are usually divided into two types: terminal and input/output. The difference between these types lies not in the composition of the ports, but in the placement of the device in the SDH network. The terminal multiplexer completes the aggregate channels, switching between them a huge number of output and input channels. The second type of device transmits aggregate lanes in transit, occupying a middle position on the highway. In this case, the information of tributary channels is derived from the aggregate flow or introduced into it.

Most manufacturers produce universal multiplexers of the SDH type, which are used as input/output, terminal, and cross-connectors, depending on the modules installed in them with tributary and aggregate ports.

In conclusion, we would like to add that fiber-optic multiplexers are gaining immense popularity due to the intense development of this type of communication. The future belongs to fiber optic technologies.

Good development of international standards describing the structure of SDH signals, functions and electrical parameters of equipment ensures compatibility of equipment from different manufacturers. This allows seamless interaction between operators of different networks.

Main characteristics of SDH

SDH technology is described in ITU-T recommendations (G.702, G.703, G.704, G.707, G.708, G.709, G.773, G.774, G.782, G.783, G .784, G.957, G.958, Q.811, Q.812), ETSI (ETS 300 147). The North American synchronous digital hierarchy is subject to the SONET standards system developed by ANSI (American National Standards Institute).
Let's consider the structure of SDH signals. This is a synchronous transport module STM-N, where N is determined by the SDH layer. Currently, the STM-1, STM-4, STM-16 and STM-64 systems are widely used. It is easy to see that the systems are built with a multiplicity of 4. Thus, the following hierarchy of speeds has been formed.

Synchronous digital hierarchy

The base layer of SDH is STM-1. It is characterized by its cycle with a repetition period of 125 μs. It is common to think of a loop as a rectangular table, although, of course, the data is transferred serially along the line. As can be seen from the figure, the STM-1 cycle contains 9 lines of 270 bytes (2430 bytes). The first 9 bytes in each line form the loop header.

The advantages of SDH include the modular structure of the signal, when the speed of the compressed signal is obtained by multiplying base speed by an integer. In this case, the structure of the cycle does not change and the formation of a new cycle is not required. This allows the desired channels to be extracted from a multiplexed signal without demultiplexing the entire signal.
The figure shows a diagram of multiplexing four STM-1 streams into one STM-4 stream. The figure shows that byte-by-byte multiplexing occurs in such a way that all sections of the section headers, the pointer and the useful signal are placed in the same way as before.

As a payload of a network built on the basis of SDH, PDH signals, ATM cells, and any unstructured digital streams with a speed of 1.5 to 140 Mbit/s and satisfying G.703 recommendations can be transmitted. This versatility is ensured by the use of containers that carry load signals over the SDH network.
Container principle well known and quite widely used in modern technology communications. This idea turned out to be very practical, because all operations on the network are performed on containers and do not affect their contents. Thus, complete network transparency for transmitted information is achieved.
The formation of containers for data transfer at different speeds is discussed below. All containers are placed in a part of the STM-1 cycle called Payload.
To avoid loss of synchronization, SDH equipment provides scrambling of transmitted signals. The point is that in useful information long strings of zeros or ones may be present. When transmitting electrical signals along lines (for example, in coaxial cable) this problem is eliminated by selecting the appropriate linear signal code.
According to the recommendation of ITU-T G.703, the CMI code (coded mark inversion code, two-level code with parcel inversion) should be used. In this code, the transmitted zero is always represented by a negative level in the first half of the message and a positive level in the second half. The transmitted 1 is represented by either a positive level or a negative level depending on the value of the previous bit.
In the vast majority of cases, optical communication lines are used to transmit STM signals. They use the linear NRZ (non return to zero) code.
It is to ensure timing differences in the transmitted STM signal over optical communication lines that the scrambling operation is used. The scrambler converts the original digital stream into a pseudo-random sequence. The pseudo-random sequence generator is built on the basis of a seven-bit shift register, modulo 2 adders (“exclusive OR”) and feedback according to the polynomial 1+X6+X7. The entire STM-N cycle except the first 9 bytes of the header is scrambled. The first header line carries a frame synchronization signal, which allows synchronization without prior descrambling.

The construction of an SDH network of any complexity is provided by a rather limited set of functional nodes. With their help, all operations for transmitting information and managing the network are performed.
The main functional unit of SDH is a multiplexer designed to organize the input/output of digital streams with a payload. There are two types of multiplexers: terminal and input/output. The main difference between them is their location on the network. Below, when considering typical SDH network designs, this difference will be indicated.
Cross-connectors usually do not directly service load input/output, but provide exchange between transport modules of the SDH network. Cross-connectors are used when combining networks or complex network topologies. In addition to specialized cross-connectors, local switching functions can be performed by a multiplexer.
A number of functional units, such as regenerators, equipment of linear paths and radio relay lines ensure the functioning of the actual transmission lines of the SDH network.
A mandatory functional unit of any serious SDH network is a management system, which ensures monitoring and control of all network elements and information paths.
SDH networks use two typical topological construction schemes: “ring” and “chain”. They are based on multiplexers. In the “ring” circuit, only input/output multiplexers (ADM -Add/Drop Multiplexer) are used, and in the “chain” circuit, terminal multiplexers (TM - terminal multiplexer) and input/output are used. As can be seen from the figure, each multiplexer has two pairs of main outputs, one is called “east”, and the other is called “west”. With their help, various redundancy or protection schemes are provided.
Protection schemes of the “1:1” type and the “1+1” type are formed by organizing two counter flows. In the first case, signals from each direction are analyzed during reception and the best one is selected for further processing. In the second scheme, there are two rings - the main and the backup. In case of failures in the main ring, a switch is made to the backup one; in the event of a break in the ring or failure of the multiplexer, a new ring is formed by organizing turns at the boundaries of the damaged section.

From the considered standard schemes or their variations, you can create an SDH network of any architecture and any complexity.

The figure shows an abstract SDH network that includes a long backbone and subnets at the ends of this backbone.
In city B, there are two ring architecture networks connected by a cross-connector. Through it, information flows can enter the backbone network, made according to the “chain” scheme. City A has one network of ring architecture. Data exchange with the backbone network is carried out using an input/output multiplexer (ADM). Due to the large length of the backbone network, in the absence of the need for intermediate data input/output points, regenerators are used to restore the signal shape. This type of organization is very rarely required. It is preferable to use input/output multiplexers instead of regenerators, which also provide regeneration digital signal.
The section of the network between two terminal multiplexers is called a route. Between two adjacent multiplexers (cross-connectors) is a multiplexer section, and between two adjacent regenerators or between a regenerator and a multiplexer (cross-connector) is a regeneration section.

Placing data in the STM-1 cycle (mapping)

As noted above, all payload is transmitted using containers. Let's consider possible types of containers, their internal structure and principles of formation.
The following container correspondence to useful information transmission speeds (in kbit/s) is determined:

This range of containers complies with international recommendations (ITU-T G.709) and integrates the European and North American SDH (SONET) system schemes. The European standard does not include the C2 container.
The picture shows general scheme placement of signals in a synchronous digital hierarchy.

The 140 Mbps (139,264 kbps) PDH signal when transmitted over the SDH network is housed in C-4 containers. S-4 containers follow with a period of 125 μs. The size of the C-4 container is precisely defined and is 2340 bytes (9 lines of 260 bytes) or 18720 bits. However, to accommodate all the bits of a 140 Mbps PDH signal, a container capacity of only 17408 bits (139,264 kbps: 8 kHz) is required. A value of 8 kHz corresponds to a repetition period of 125 µs. Thus, there is still space in the C-4 container that has not been filled with the PDH signal. This space contains:

coarse alignment bits and bytes (constant stuffing) to match the speed of the plesiochronic signal to the higher speed of the container;
fine alignment bits, positive stuffing is used (adding bits);
bits with information about the presence of fine alignment;
“ballast” bits that have no functional purpose.

To transmit container C-4 in the STM-1 stream, a path or path header PON (Path OverHead) of 9 bytes is added to it. As a result of this operation, a so-called VC-4 virtual container is formed, having a size of 2349 bytes (9 lines of 261 bytes).
Since STM-1 cycles are formed continuously and synchronously with respect to the entire network, flexible placement of VC-4 virtual containers in the STM-1 stream is used to ensure the transmission of plesiochronous signals. As will be shown below, the beginning of VC-4 is placed in one STM-1 cycle, the remainder in the next cycle.

Information about the beginning of the VC-4 virtual container and the location of its first byte is contained in the PTR (Pointer) pointer. The pointers are discussed in more detail below.
In the STM-1 cycle, the PTR and Payload pointer are collectively called the AU-4 administrative block.

The pointer is called AU-4 pointer (AU-4 PTR). Section headers (SOH) are added to the AU-4 block to obtain the complete structure of the STM-1 loop. The figure shows the relationship between the components of the STM-1 cycle when placing the C-4 container.

In an STM-1 cycle, 3 containers of PDH signals can be transmitted at a speed of 34 Mbit/s (34,368 kbit/s). These containers are called C-3. From a speed standpoint, the STM-1 loop can carry 4 signals at 34 Mbps, but only 3 C-3 containers are used for compatibility with the North American SONET system.
The C-3 container has a size of 756 bytes (9 lines of 84 bytes) or 6048 bits. The tracking period of the S-3 container is 125 μs. To transmit a PDH signal at 34 Mbps, a container capacity of only 4296 bits (34,368 kbps: 8 kHz) is required. Container C-3 is also designed to accommodate the DS-3 signal of the North American hierarchy (44 Mbit/s). To do this, only 5593 bits (44,736 kbit/s: 8 kHz) are used in the C-3 container.
The free bits remaining after the payload is placed are used in the same way as in the C-4 container. Only for precise alignment, two-way stuffing (adding and subtracting bits) is used.
A PON header is added to each C-3 container, resulting in a virtual VC-3 container with a size of 765 bytes (9 lines of 85 bytes).
There are two ways to place a VC-3 container in an STM-1 loop. With the first method, each virtual container VC-3 in the STM-1 cycle, more precisely in its PTR pointer, corresponds to a separate 3-byte pointer. The combination of the VC-3 container and the 3-byte pointer forms the AU-3 administrative block. The pointer is called the AU-3 pointer (AU-3 PTR) and indicates the beginning of the corresponding VC-3 in the STM-1 cycle. The ETSI standards describing SDH do not recommend this method for use.
The second method is based on converting three VC-3 blocks into one VC-4 block. To do this, a 3-byte pointer is added to the virtual container VC-3, resulting in a tributary block TU-3. Adding 6 fixed alignment bytes to it results in a TUG-3 tributary block group.

For transmission over the SDH network, the three received TUG-3 blocks are multiplexed byte-by-byte into a virtual container VC-4. The figure shows this process.

Note that in order to match container sizes (and therefore to match speeds), two columns of fixed alignment bytes are placed in the VC-4 container after the RON. The figure shows the relationship between the components of the STM-1 cycle when placing C-3 containers, according to ETSI recommendations.

In an STM-1 cycle, 63 containers of PDH signals can be transmitted at a speed of 2 Mbit/s (2,048 kbit/s). The container for transmitting this signal is called S-12. The tracking period of this container is 125 µs.
The container capacity is 34 bytes (8 lines of 4 bytes plus 1 line of 2 bytes) or 272 bits. A 2 Mbit/s PDH signal requires 256 bits (2048 kbit/s: 8 kHz).
Free bits left after payload placement are used in the same way as in C-4 and C-3 containers, double-sided stuffing is used for precise alignment.
A virtual container VC-12 is formed by adding a 1-byte PON to the beginning of the container. In this case, the 9th line of the container becomes 3 bytes, i.e. all information is shifted back by 1 byte.
VC-12 virtual containers are transmitted as part of a multiframe (or multiframe) having a period of 500 μs. Note that a multiframe is transmitted over several STM-1 cycles. The ROH bytes of each VC-12 container of one multiframe constitute the total RON header. The figure shows the components of a supercycle. The meaning of the POH bytes (V5, J2, Z6 and Z7) will be explained in the description of the header.

A TU-12 tributary block is formed by adding a pointer byte to the VC-12 container. The size of TU-12 is 36 bytes (9 lines of 4 bytes). From the VC-12 container multiframe, a TU-12 multiframe is formed by adding four pointer bytes (TU-12 PTR). Only the first three bytes of the pointer are meaningful; the fourth byte currently has no defined functionality. These pointers will be described in more detail below.
Three TU-12 blocks, by byte-byte multiplexing, form a TUG-2 group of 108 bytes in size (9 lines of 12 bytes). The seven TUG-2s are combined into a TUG-3 in the same way (Figure 5.13), with one column of fixed alignment bytes added.

In the resulting TUG-3 group, three bytes corresponding to the TU-3 PTR pointer are called NPI (Null Pointer Indicator) - an indicator of a “empty” (no value) pointer.
The STM-1 cycle is formed from TUG-3 blocks in the manner discussed above.

Container pointers

The pointer mechanism in SDH serves to synchronize between the various tributary signals and the STM frame. Thanks to pointers, there is no need for mutual coordination between the start of the SDH cycle and the tributary signal cycle packaged in a virtual container.
The pointers are always placed at precisely defined locations in the SDH signal structure, making it possible to access information without demultiplexing the entire signal. To equalize deviations in phase and transmission speed, double-sided stuffing of pointers is used.
There are three types of pointers:

administrative block indicators AU, AU-4 PTR and AU-3 PTR. The latter pointer is used in the North American version of SDH and will not be discussed in detail. The AU-4 pointer specifies the placement of the VC-4 virtual container in the STM-1 loop;

TU-3 tributary block indicator, TU-3 PTR. This type The pointer is used to place three virtual containers VC-3 in a virtual container VC-4;

indicators of tributary blocks TU-11, TU-12 and TU-2. These pointers serve to locate the corresponding virtual containers VC-11, VC-12 and VC-2. Each of these pointers is transmitted one byte in the first three cycles of 125 μs in one multicycle of 500 μs. The byte at the pointer position in the fourth multiframe frame has no meaning and is reserved for future use.

The AU-4 PTR and TU-3 PTR indicator bytes contain the following information:

the start address of the corresponding virtual container;

new data flag;

fine alignment bits;

indicator type label (AU-4 PTR, AU-3 PTR or TU-3 PTR). This label is currently unused and should have a fixed value;

bytes used when negative alignment is used.

The TU-11 PTR, TU-12 PTR, and TU-2 PTR pointer bytes contain information about the start address of the corresponding virtual container and a field to allow negative alignment.

The AU-4 PRT pointer values allow only every third byte of the payload area of an STM-1 cycle to be addressed. The range of addresses in which a “floating” start of the VC-4 container is possible begins after the AU-4 PTR block at address 0 and ends at address 782 in the next STM-1 cycle. The figure shows the beginning of the virtual container MS-4 from address 88.

Below is the structure of the AU-4 PTR index.

Bytes H1 and H2 contain the following fields:

new data flag field, N bits. This field can contain two status values “1001” and “0110”. The active status (“1001”) serves to notify the receiver that the pointer value has been changed. In subsequent cycles and during the alignment procedure, the inactive status (“0110”) is used;

pointer type label field, S bits. Not currently used and should be fixed to “10”;

pointer value field, 10 bits I and D. These bits have a dual purpose. They can define a pointer value from 0 to 782 in decimal. After transmitting the active status in N bits, the value of the pointer must match for at least three cycles. To perform negative alignment, all D bits are inverted and in the next AU-4 PTR the pointer value is decreased by 1 (decrement operation). With positive alignment, all I bits are inverted and in the next cycle an increment operation is performed (the pointer value is increased by 1). Pointer adjustments are allowed only once every four cycles to ensure that the pointer is correct.

According to ETSI recommendations, the “Y” and “1” bytes are not used and must have a constant value. Byte “Y” contains 1001SS11, where SS coincides with the pointer type label field and has the same value. Thus byte “Y” = “10011011”. Byte “1” always contains “11111111”. In the North American version, these bytes can be used as additional pointers.
The H3 bytes are reserve bytes for transmitting information during negative alignment.

TU-3 PTR indicators are used when placing three VC-3 containers in one VC-4 container. In this case, a TUG-3 tributary block group is formed from the VC-3 virtual container by adding a 3-byte pointer (TU-3 PTR) and 6 fixed alignment bytes.

The figure shows an addressing scheme using TU-3 PTR pointers. In the VC-4 container, following the POH routing header bytes and the fixed alignment bytes, three TUG-3 groups are byte-multiplexed. The range of addresses for the start of a VC-3 container within a TUG-3 group extends from 0 to 764.
In the example in this figure, the first VC-3 container starts at address 0, the second container starts at address 85, and the third container starts at address 594.
The structure of the H1, H2 and H3 bytes of the TU-3 PTR indicator completely coincides with the structure of the AU-4 PTR and a similar mechanism for aligning phases and signal speeds is used.

As previously stated, the superframe VC-12 virtual containers form a TU-12 superframe when a TU-12 PTR is added. The role of this pointer is similar to the AU-4 PTR and TU-3 PTR pointers, namely to fix the start of the virtual container. In this case, the beginning of a supercycle of virtual containers VC-12. The figure shows the placement of a VC-12 supercycle in a TU-12 supercycle.
The purpose and structure of the V1, V2 and V3 bytes is the same as the H1, H2 and H3 bytes. The only difference is in the SS bits. For the class of pointers under consideration, the values of these bits carry semantic load and determine the specific type of pointer. For TU-11 PTR the value should be “11”, for TU-12 PTR it should be “10” and for TU-2 PTR it should be “00”.
The ten-bit TU-12 PTR value field can contain a value from 0 to 139. This implies that a VC-12 multiframe can be transmitted using 4 or 5 STM-1 frames. In the example in the figure, the pointer value is 0, i.e. The VC-12 multiframe begins immediately after the V2 byte of the pointer and will only require 4 STM-1 cycles to transmit it. The V3 byte is reserved and serves to transmit information at the time of negative alignment. The alignment mechanism is similar to those discussed above.
When transferring VC-12 virtual containers in the STM-1 cycle, another special pointer is used. This is the so-called NPI indicator, which appears in place of the TU-3 PTR indicator when VC-12 containers are combined into a TUG-3 group.
In the NPI pointer, the new data flag field contains the active status (“1001”), and the ten-bit pointer value field has a constant, insignificant value - “1111100000”. Byte H3 is naturally not used in this case, since all alignment procedures are carried out at the level of TU-12 PTR pointers.

Container and signal headers (overhead)

Headers play an important role in the process of transferring useful information using SDH loops. The header is always separated from the transmitted payload. Thanks to this, the header bytes can be read, changed or appended without affecting the information itself.
It is known that the STM-1 cycle header consists of three parts:

PTR is an administrative unit (AU) indicator that defines the position of individual multiplexed signals (VC-4 and VC-3 containers) in the STM-1 frame.
RSOH is the header of the regeneration section, containing control, monitoring and cyclic synchronization signals to ensure the operability of the regeneration sections.
MSOH - multiplexer section header, provides interaction between multiplexers. They pass through regenerators without changes.

Together, RSOH and MSOH make up the sectional header (SOH -Section Overhead). Due to this header in the STM signal, control and synchronization networks are formed, which provide the transmission of synchronization signals, network management, monitoring and maintenance, and support service communication channels.
The figure shows the distribution map of the RSOH and MSOH header bytes.

Let's look at the purpose of these bytes:

A1, A2 - alignment signals, frame synchronization. Byte A1 contains the value “11110110”, A2 - “00101000”.
B1 - error control of the regeneration section. This byte (parity) is created from all the bits of the previous frame after scrambling and is written in the current frame before scrambling.
B2 - error control of the multiplexer section. These bytes are generated based on the entire unscrambled frame, with the exception of the bytes included in the RSOH header. The result is written to the appropriate positions before scrambling.
C1 - STM-1 cycle identifier. Assigned to each STM-1 before compaction into STM-N.
D1 - D3 - form a data transmission channel with a speed of 192 kbit/s in regeneration sections (DCC-R). Used only in the first STM-1 of the STM-N cycle. The DCC-R channel is used to transmit control commands and control signals between the regenerators and the network control center.
D4 - D12 - form a data transmission channel with a speed of 576 kbit/s in multiplexer sections (DCC-M). Used only in the first STM-1 of the STM-N cycle. The DCC-M channel creates a communication link between the multiplexers and the control center according to ITU-T G.784 recommendation.
E1 - forms a local service channel, which is used for voice communication between regenerators.
E2 - similar to E1, only between multiplexers.
F1 - SDH network operator channel. Provided for your own needs, data or voice transmission is possible. Used only in the first STM-1 of the STM-N cycle.
K1, K2 - signaling bytes in the automatic transfer switching system (APS). Used only in the first STM-1 of the STM-N cycle. In addition to the function of ensuring automatic switching in the K2 byte, bits 6, 7 and 8 are set to “1” when transmitting an AIS (Alarm Indication Signal) signal. Let us explain the purpose of the AIS signal; it is generated if an error is detected, for example, loss of frame synchronization STM-1 - sectional AIS or an error in the virtual container - path AIS. The generated AIS is sent in the same transmission direction as undistorted signals. Its purpose is to prevent the generation of alarm signals in downstream equipment. If the multiplexer receiver does not receive a signal or an AIS signal has been received, then the combination “110” is transmitted through bits 6, 7, 8 of the K2 byte. In this way, reception errors are reported to the remote party.
S1 - serves to indicate the presence of a clock signal (for example, from a master oscillator) in the incoming STM-N stream. Used only in the first STM-1 of the STM-N cycle.
M1 - called FEBE (Far End Block Error) and contains the number of blocks with errors detected using the B2 bytes. For STM-1, values from 0 to 24 are meaningful, and for STM-4 - from 0 to 96. Other values should not be generated.
Z1, Z2 - reserved for yet undefined functions.
N - reserved for national use.
The remaining bytes are reserved for future use.

In addition to the SOH section header, the ETSI recommendations define three types of path headers (POH -Path Overhead), these are VC-4 POH, VC-3 POH and VC-12 POH.
The PON header is added to the corresponding C containers, forming virtual containers. The figure below shows the header data bytes.

Let's consider the purpose of the indicated bytes for VC-4 POH and VC-3 POH:

J1 - this byte is the first byte of the virtual container and is used to transmit 64-byte information about the route of such a container. This information is transmitted cyclically, one byte every 64 cycles.
B3 is a check byte for detecting errors in the virtual container. Before the procedure of scrambling a virtual container, this check byte is calculated from all its bytes, and the parity method is used. The generated byte is written to field B-3 of the subsequent container, again before the procedure for calculating the control byte and scrambling.
C2 - signal mark. Serves to indicate the contents of the virtual container. The following values for this label are defined:
- C2 = 00h - VC-3 and VC-4 container paths are not formed.
- C2 = 01h - VC-3 and VC-4 container paths are formed, but there is no useful information.
- C2 = 02h - the VC-4 path is formed to transmit 3 TUG-3 groups.
- C2 = 12h - the VC-4 path is formed to transmit a 140 Mbit/s signal.
- C2 = 13h - the VC-4 path is formed and serves for the transmission of ATM cells.
- All other values are reserved for future use.
G1 - this byte is used to signal errors in the reverse direction. Using this byte, a message about its status and quality indicators is transmitted towards the beginning of the path. The first four bits are called FEBE (Far End Block Error) and convey the number of bad blocks determined using the control byte B3. Values from 0 to 8 make sense; all others are interpreted as 0, i.e. like the absence of errors. The fifth bit is an indicator of a failure and is called FERF (Far End Receive Failure) and is set to “1” when receiving AIS, loss or error in the signal, or incorrectly formed end-to-end paths. The remaining bits of the G1 byte are unused.
F2, Z3 - reserved for the purposes of organizing service lines of the network operator. There is currently no precise specification for this capability.
H4 - indicator (counter) of the position of useful information distributed over several cycles (supercycle when transmitting a virtual container VC-12). Using this indicator, you can determine the presence of a supercycle and identify individual cycles of the supercycle.
Z4 - not used, reserved.
Z5 - reserved for operational purposes. Used by the network operator both to count incoming errors and to organize a communication channel.

The VC-12 virtual container path header is formed during multiframe transmission and consists of four bytes. The previous figure shows the distribution of these bytes within a multicycle.
V5 - this header byte is used to detect errors, transmit a signal label and indicate the status of the path. For each task, the corresponding bits of this byte are predefined. Bits 1 and 2 are used for parity error detection. Bit 1 provides parity control for the odd (byte count - 1, 3, 5 and 7) bits of all bytes of the previous VC-12 virtual container. Accordingly, bit 2 is used to check the parity of even (according to the count in a byte - 2, 4, 6 and 8) bits. There is no parity check on the V1, V2, V3 and V4 bytes that form the TU-12 pointer. The exception is the V3 byte if negative alignment is present. Bit 3 is the FEBE indicator, set by the receiving side and evaluated by the transmitting side. Is kind of feedback. When at least one error is detected using bits 1 and 2, it is set to the value “1” and this informs the path source about the presence of errors. If no errors were detected, then its status is “0”. Bit 4 is not used. Bits 5, 6 and 7 convey the signal label. The value “000” indicates that the VC-12 container path is not formed. The value is “001” - the path is formed, but not defined (a non-standard signal is transmitted). The value is “010” - an asynchronous signal is transmitted. Value “100” - a synchronous signal is transmitted. The remaining combinations of values (“101”, “110”, “111”) indicate that the path is formed and reserved for future use. Bit 8 is an alarm indicator, FERF signal. Set to “1” and informs the transmitting side about signal loss or AIS reception.
J2 - used to transmit a path label, which allows you to track the continuity of the connection along the path.
Z6, Z7 - reserved for future use.

The figure shows the areas of “responsibility” of each type of header.

Error Control and Management in SDH Networks

Using the corresponding bytes and bits of the STM frame headers and virtual containers, monitoring and control procedures are carried out on the SDH network.

Bit Interleaved Parity (BIP) is used to detect bit errors. This procedure is based on the method of adding “1” to an even number. If there is an odd number “1” in a certain bit sequence, then an additional “1” is set in the check bit. And vice versa, if the number “1” is even, then “0” is set in the control bit.
SDH uses codewords of varying lengths to provide parity. The principle of forming these words is the same. The entire controlled bit sequence is conditionally divided into blocks, equal to length specific code word. Then the resulting blocks are added bit by bit in accordance with the “exclusive OR” rule. The resulting result is the desired control codeword. In other words, the number “1” located at the corresponding bit positions is counted.
The resulting codeword is transmitted in the corresponding header of the next STM cycle or virtual container. At the receiving side, the code word is again calculated and compared with the received word from the subsequent information block. If these words coincide, then a conclusion is made about the reception without distortion. The code words used in SDH are shown in the figure:

In the regeneration section, the word BIP-8 is used, located in byte B1 of the RSOH header. This word is formed from all the bits of the frame after the scrambling operation and is placed in the B1 byte of the next frame before scrambling. Recall that the entire frame except the first 9 bytes of the RSOH header is subject to scrambling. The BIP-8 word is checked in each multiplexer and regenerator.
The multiplexer section uses the BIP24 codeword, which is located in the B2 bytes of the MSOH header. This is true for the STM-1 cycle. When using STM-N, the codeword will be BIP-Nx24. The BIP-24 codeword is generated before the scrambling operation from the entire STM-1 frame except the first 3 rows of SOH (this is RSOH). The resulting value is placed in the B2 bytes of the next frame before scrambling it. Thus, the value of BIP-24 does not change in regenerators.
For virtual containers VC-3 and VC-4, the BIP-8 codeword is used, located in byte B3 of the POH path header. This word is formed from all the bits of the virtual container and placed in the RON of the next container. When generating BIP-8, pointer bits are not taken into account.
The VC-12 virtual container uses the BIP2 codeword, which is located in bits 1 and 2 of byte V5 of the PON path pointer. The BIP-2 word is formed from the entire VC-12 multiframe and placed in the subsequent multiframe. The figure shows the actions of each type of BIP.

The receiving side generates several types of signals carrying emergency information. There are two types of signals - error indicators. These are FEBE (Far End Block Error) - block error at the far end and FERF (Far End Receive Failure) - failure to receive at the far end. There are path and section signals.
First, let's look at the conditions for generating the FEBE signal. This signal is sent to the sending side to notify the detected errors using BIP codewords.
To transmit path FEBE of virtual containers VC-3 and VC-4, bits 1 - 4 of the G1 byte of the PON header are used. For BIP-8, a maximum of 8 parity violations can be detected. The FEBE code contains the number of such violations and can range from 0 to 8. All other values are interpreted as 0.
Bit 3 of byte V5 of the PON path header is used to transmit the VC-12 virtual container FEBE. If this bit is “0”, then no parity violations were detected in the BIP-2 codeword.
To transmit the sectional FEBE of the STM-1 frame, the M1 byte of the MSOH header is used. For STM-1 the FEBE value can be from 0 to 24, and for STM-N it can be from 0 to Nx24.
The FERF signal notifies the transmitting end that an AIS signal has been detected at the receiving end or that it is unable to receive. Here we are talking about receiving signals from SDH multiplexers located further down the chain. Those. The FERF alarm signal moves in the same direction as the transmitted signal.

For virtual containers VC-3 and VC-4, the FERF path signal is transmitted in bit 5 of the G1 byte. To do this, it is set to “1”. For a VC-12 virtual container, the FERF signal is transmitted by bit 8 of byte V5. The FERF path signal is established if:

for BIP-8, the Bit Error Rate BER is greater than or equal to 10 -4;

there is an error in the J1 byte, distortion of information about the route of the virtual container;

There is no virtual container signal.

The FERF signal for STM-1 is transmitted in bits 6 - 8 of the K2 byte, the value is 110. Sectional FERF is set if:

for BIP-24 the BER value is greater than or equal to 10 -3;

AIS signal detected in section header;

loss of FAS frame synchronization signal;

loss of STM-1 signal.

AIS signal (Alarm Indication Signal) - an emergency indication signal is generated when a number of errors are detected in the received signal. The purpose of the AIS signal is to prevent the generation of error messages in downstream multiplexers or regenerators. Reception of an AIS signal causes response actions (such as channel blocking) only in certain terminal equipment.
The AIS signal is used in PDH and SDH. In SDH, when an AIS signal is detected, the STM-1 or STM-N frame is completely stored and forwarded. In PDH, this signal indicates the impossibility of FAS frame synchronization in further sections. This occurs because the frame synchronization bytes and the PDH complex word are filled with log. “1” to transmit AIS signal.
SDH distinguishes between tract AIS and sectional AIS. Path AIS corresponds to the virtual containers of the SDH hierarchy. For tributary blocks TU - 1, 2, 3, the indicator is set to “1” in the case of AIS TU. For administrative blocks AU - 3, 4, the indicator is set to “1” for AIS AU. These constant signals are transmitted in the STM-1 cycle as corrupted tributary blocks.

Control and monitoring signals on SDH networks are carried in the RSOH and MSOH headers using D bytes. In the STM-N cycle, only the D bytes of the first STM-1 are used to transmit these signals.
To organize technological communication between components The geographically distributed SDH network uses voice communication channels. These channels are formed by the E bytes of the RSOH and MSOH headers.

The main element of the SDH network is the multiplexer (see Figure 1). It is usually equipped with a number of PDH and SDH ports: for example, PDH ports at 2 and 34/45 Mbit/s and SDH STM-1 ports at 155 Mbit/s and STM-4 at 622 Mbit/s. SDH multiplexer ports are divided into aggregate and tributary. Tributary ports are often called I/O ports, and aggregate ports are called linear ports. This terminology reflects typical SDH network topologies, where there is a distinct backbone in the form of a chain or ring along which data streams coming from network users through I/O ports are transmitted (i.e., flowing into an aggregated stream: tributary literally means “tributary” ).

SDH multiplexers are usually divided into terminal (Terminal Multiplexor, TM) and input/output (Add-Drop Multiplexor, ADM). The difference between them is not the composition of the ports, but the position of the multiplexer in the SDH network. The terminal device completes the aggregate channels by multiplexing a large number of input/output channels (tributary) into them. An I/O multiplexer transits aggregate channels, occupying an intermediate position on the backbone (in a ring, chain, or mixed topology). In this case, data from tributary channels is entered into or output from the aggregate channel. The multiplexer's aggregate ports support the maximum STM-N speed level for this model, the value of which serves to characterize the multiplexer as a whole, for example, an STM-4 or STM-64 multiplexer.

Sometimes a distinction is made between so-called cross-connectors (Digital Cross-Connect, DXC) - unlike input/output multiplexers, they perform switching of arbitrary virtual containers, and not just a container from an aggregate stream with the corresponding container of a tributary stream. Most often, cross-connectors implement connections between tributary ports (more precisely, virtual containers formed from these tributary ports), but cross-connectors can also be used for aggregate ports, i.e., VC-4 containers and their groups. The latter type of multiplexer is still less common than the others, since its use is justified with a large number of aggregate ports and a mesh network topology, and this significantly increases the cost of both the multiplexer and the network as a whole.

Most manufacturers produce universal multiplexers that can be used as terminal, input/output and cross-connectors - depending on the set installed modules with aggregate and tributary ports. However, the ability to use such multiplexers as cross-connectors is very limited, since manufacturers often produce multiplexer models with the ability to install only one aggregate card with two ports. A configuration with two aggregate ports is the minimum configuration that provides operation in a network with a ring or chain topology. This multiplexer design is not very expensive, but can complicate network design if you want to implement a mesh topology at the maximum speed for the multiplexer.

In addition to multiplexers, the SDH network may include regenerators; they are necessary to overcome limitations on the distance between multiplexers, which depend on the power of optical transmitters, the sensitivity of receivers and the attenuation of the fiber-optic cable. The regenerator converts the optical signal into an electrical one and vice versa, while restoring the signal shape and its timing parameters. Currently, SDH regenerators are used quite rarely, since their cost is not much less than the cost of a multiplexer, and the functionality is incommensurable.

The SDH protocol stack consists of four layers of protocols.

The physical layer, called photonic in the standard, deals with encoding bits of information using light modulation.
The section layer maintains the physical integrity of the network. In SDH technology, a section refers to each continuous piece of fiber optic cable through which a pair of SONET/SDH devices are connected to each other, for example, a multiplexer and a regenerator, a regenerator and a regenerator. It is often called a regenerator section, meaning that the end devices are not required to perform the functions of this multiplexer layer. The regenerator section protocol deals with a specific part of the frame header, called the regenerator section header (RSOH), and based on overhead information can perform section testing and support management control operations.
The line layer is responsible for transmitting data between two network multiplexers. This layer protocol operates on STS-n layer frames to perform various multiplexing and demultiplexing operations, as well as user data insertion and deletion. It also carries out line reconfiguration operations in the event of failure of any of its elements - optical fiber, port or neighboring multiplexer. The line is often called a multiplex section.
The path layer controls the delivery of data between two end users on the network. A path (path) is a composite virtual connection between users. The path protocol must accept incoming data in a user format, such as E1 format, and convert it into synchronous STM-N frames.

Let us describe the main elements of an SDH-based data transmission system, or SDH functional modules. These modules can be interconnected in an SDH network. The logic of operation or interaction of modules in the network determines the necessary functional connections of the modules - the topology, or architecture of the SDH network.

An SDH network, like any network, is built from separate functional modules of a limited set: multiplexers, switches, concentrators, regenerators and terminal equipment. This set is determined by the main functional tasks solved by the network:

collection of input streams through access channels into an aggregate block suitable for transportation in the SDH network - a multiplexing problem solved by terminal multiplexers - TM access networks;

transportation of aggregate blocks over a network with the ability to input/output input/output streams is a transportation problem solved by input/output multiplexers - ADMs, which logically control the information flow in the network, and physically control the flow in the physical environment that forms a transport channel in this network;

overloading of virtual containers in accordance with the routing scheme from one network segment to another, carried out in dedicated network nodes, is a switching or cross-connection problem solved using digital switches or cross-switches - DXC;

combining several flows of the same type into a distribution node - a concentrator (or hub) - a concentration problem solved by concentrators;

restoration (regeneration) of the shape and amplitude of a signal transmitted over long distances to compensate for its attenuation - a regeneration problem solved with the help of regenerators;

pairing the user's network with the SDH network is a pairing problem solved using terminal equipment - various matching devices, for example, interface converters, speed converters, impedance converters, etc.

2. Functional modules of sdh networks

Multiplexer.

The main functional module of SDH networks is the multiplexer. SDH multiplexers perform both the functions of a multiplexer itself and the functions of terminal access devices, allowing you to connect low-speed PDH hierarchy channels directly to their input ports. they are universal and flexible devices that allow you to solve almost all of the problems listed above, i.e. in addition to the multiplexing task, perform the tasks of switching, concentration and regeneration. This is possible due to the modular design of the SDH multiplexer - SMUX, in which the functions performed are determined only by the capabilities of the control system and the composition of the modules included in the multiplexer specification. It is customary, however, to distinguish two main types of SDH multiplexer: terminal multiplexer and input/output multiplexer.

The Terminal Multiplexer TM is a multiplexer and terminal device of an SDH network with access channels corresponding to the PDH and SDH access tribes of the hierarchy (Fig. 6). The terminal multiplexer can either introduce channels, i.e. switch them from the input of the trib interface to the linear output, or output channels, i.e. switch from the linear input to the output of the trib interface.

The ADM input/output multiplexer can have the same set of tribes at its input as the terminal multiplexer (Fig. 6). It allows you to input/output their corresponding channels. In addition to the switching capabilities provided by TM, ADM allows for end-to-end switching of output streams in both directions, as well as closing the receive channel to the transmit channel on both sides ("east" and "west") in the event of failure of one of the directions. Finally, it allows (in the event of an emergency failure of the multiplexer) to pass the main optical flow past it in bypass mode. All this makes it possible to use ADM in ring-type topologies.

Figure 5.1 - Synchronous multiplexer (SMUX): TM terminal multiplexer or ADM input/output multiplexer.

Regenerator is a degenerate case of a multiplexer that has one input channel - usually an STM-N optical tribe and one or two aggregate outputs (Fig. 7). It is used to increase the allowable distance between SDH network nodes by regenerating payload signals. Typically this distance is 15 - 40 km. for a wavelength of the order of 1300 nm or 40 - 80 km. - for 1500 nm.

Figure 5.2 - Multiplexer in regenerator mode

Hubs

Hub(hub) is used in star-type topological circuits; it is a multiplexer that combines several, usually of the same type (from the input ports) streams coming from remote network nodes into one distribution center SDH networks, not necessarily also remote, but connected to the main transport network.

This node may also have not two, but three, four or more linear ports of the STM-N or STM-N-1 type (Fig. 5.3) and allows you to organize branch from the main stream or ring (Fig. 5.3a), or, conversely, connecting two external branches to the main stream or ring (Fig. 5.3) or, finally, connecting several mesh network nodes to the SDH ring (Fig. 5.3c). In general, it allows you to reduce the total number of channels connected directly to the underlying SDH transport network. The multiplexer of the distribution node in the branch port allows local switching of the channels connected to it, allowing remote nodes to communicate through it with each other without loading the traffic of the main transport network.

Figure 5.3 – Synchronous multiplexer in hub mode

Switch.Physically, the capabilities of internal channel switching are embedded in the SDH multiplexer itself, which allows us to talk about the multiplexer as an internal or local switch. In Fig. 8, for example, the payload manager can dynamically change the logical mapping between the TU and the access channel, which amounts to internal circuit switching. In addition, the multiplexer, as a rule, has the ability to switch its own access channels (Fig. 9), which is equivalent to local channel switching. Multiplexers, for example, can be assigned local switching tasks at the level of access channels of the same type, i.e. tasks solved by concentrators (Fig. 9).

In general, you have to use specially designed synchronous switches - SDXC, which perform not only local, but also general or pass-through (end-to-end) switching of high-speed flows and synchronous transport modules STM-N (Fig. 3.5). An important feature of such switches is the absence of blocking of other channels during switching, when switching of some TU groups does not impose restrictions on the processing of other TU groups. such switching is called non-blocking.

Figure 8 - I/O multiplexer in internal switch mode.

Figure 9 - I/O multiplexer in local switch mode.

Figure 10 - General or pass-through high-speed channel switch

There are six different functions performed by a switch:

Routing of virtual containers VC, carried out based on the use of information in the ROH routing header of the corresponding container;

Consolidation or merging (consolidation/hubbing) of virtual containers VC, carried out in hub/hub mode;

Translation of a stream from a point to several points, or to a multipoint, carried out when using the point-to-multipoint communication mode;

Sorting or rearranging (drooming) of virtual containers VC, carried out in order to create several ordered VC flows from the total VC flow arriving at the switch;

Access to the virtual container VC, carried out when testing equipment;

Input/output (drop/insert) of virtual containers, carried out during operation of the input/output multiplexer;

SDH was originally created to transmit a large number of relatively low-speed digital channels (E1, E2, EZ). However, new generations of SDH implement methods (virtual container coupling) that make it possible to transmit high-speed streams of any traffic (ATM, IP) at speeds up to 10 Gbit/s. Due to this, TDM traffic telephone networks and data traffic is transmitted integrated and SDH equipment has acquired multi-service properties. High fault tolerance and short recovery time for SDH networks are of no small importance.

The technology has become widespread - to date, more than 150 thousand SDH networks have been built in the world and about 150 thousand SONET networks in the USA. Thus, SDH can be considered the dominant technology in backbone networks and city-scale networks (Metropolitan Access Network - MAN). Additional advantage SDH is a significant reduction in the cost of solutions, which occurred as a result of increasing production volumes of this equipment.

1. Digital primary network - principles of construction and development trends

Primary network is a set of standard physical circuits, standard transmission channels and network paths of a telecommunication system, formed on the basis of network nodes, network stations, terminal devices of the primary network and the transmission lines of the telecommunication system connecting them. The modern telecommunication system is based on the use of a digital primary network based on the use of digital transmission systems. As follows from the definition, the primary network includes the signal transmission medium and transmission system equipment. A modern primary network is built on the basis of digital transmission technology and uses electrical and optical cables and radio air as transmission media.

Let's consider that part of the primary one, which is associated with the transfer of information in digital form. As can be seen from Fig. 1.1, a modern digital primary network can be built on the basis of three technologies: PDH, SDH and ATM.

Rice. 1.1. Place of the digital primary network in the telecommunication system

The primary digital network based on PDH/SDH consists of multiplexing nodes (multiplexers) that act as converters between channels of different levels of the standard hierarchy bandwidth(below), regenerators that restore the digital flow on long paths, and digital cross-connections that perform switching at the level of channels and paths of the primary network. The structure of the primary network is shown schematically in Fig. 1.2. As can be seen from the figure, the primary network is built on the basis of standard channels formed by transmission systems. Modern systems transmissions use electrical and optical cable as a signal transmission medium, as well as radio frequency means (radio relay and satellite systems transfers). A digital signal of a typical channel has a certain logical structure, including the cyclic structure of the signal and the type of linear code. The cyclic structure of the signal is used for synchronization, multiplexing and demultiplexing processes between different levels hierarchy of channels of the primary network, as well as to control block errors. Linear code ensures noise immunity of digital signal transmission. The transmission equipment converts a digital signal with a cyclic structure into a modulated electrical signal, which is then transmitted through the transmission medium. The type of modulation depends on the equipment used and the transmission medium.

Rice. 1.2. Primary network structure.

Thus, within digital transmission systems, electrical signals of various structures are transmitted; at the output of digital transmission systems, digital primary network channels are formed that comply with standards for transmission speed, cyclic structure and linear code type.

Typically, the channels of the primary network arrive at communication nodes and end in the line hardware shop (LAS), from where they are crossed for use in secondary networks. We can say that the primary network is a bank of channels, which are then used by secondary networks (network telephone communication, data networks, special-purpose networks, etc.). It is important that for all secondary networks this bank of channels is the same, from which it follows mandatory requirement so that the primary network channels comply with the standards.

The modern digital primary network is built on the basis of three main technologies: plesiochronous hierarchy (PDH), synchronous hierarchy (SDH) and asynchronous transfer mode (ATM). Of the listed technologies, only the first two can currently be considered as the basis for building a digital primary network.

ATM technology as a technology for building a primary network is still young and not fully tested. This technology differs from PDH and SDH technologies in that it covers not only the primary network level, but also the technology of secondary networks (Fig. 1.1), in particular, data networks and broadband ISDN (B-ISDN). As a result, when considering ATM technology, it is difficult to separate the primary network technology portion from the secondary network portion.

Let us consider in more detail the history of the construction and the differences between plesiochronous and synchronous digital hierarchies. PDS circuits were developed in the early 80s. There were three of them: 1) adopted in the USA and Canada as the primary signal speed digital channel The PCC (DS1) selected a speed of 1544 kbit/s and gave the sequence DS1 - DS2 - DS3 - DS4 or a sequence of the form: 1544 - 6312 - 44736 - 274176 kbit/s. This made it possible to transmit, respectively, 24, 96, 672 and 4032 DS0 channels (64 kbit/s BCC); 2) adopted in Japan, the same speed was used for DS1; gave the sequence DS1 - DS2 - DSJ3 - DSJ4 or the sequence 1544 - 6312 - 32064 - 97728 kbit/s, which made it possible to transmit 24, 96, 480 or 1440 DS0 channels; 3) adopted in Europe and South America, the speed of 2048 kbit/s was chosen as the primary speed and gave the sequence E1 - E2 - E3 - E4 - E5 or 2048 - 8448 - 34368 - 139264 - 564992 kbit/s. The specified hierarchy allowed the transmission of 30, 120, 480, 1920 or 7680 DS0 channels.

The ITU-T Standardization Committee developed a standard according to which: - firstly, the first three levels of the first hierarchy, four levels of the second and four levels of the third hierarchy were standardized as the main ones, as well as cross-multiplexing schemes of hierarchies; -- secondly, the last levels of the first and third hierarchies were not recommended as standard.

These hierarchies, known collectively as the plesiochronous digital hierarchy PDH, or PDH, are summarized in Table 1.1.

Table 1.1. Three PDS schemes: AC-American; YAS-Japanese; EU-European.

But PDH had a number of disadvantages, namely: - difficult input/output of digital streams at intermediate points; -- lack of network automatic monitoring and control tools; -- multi-stage restoration of synchronism requires quite a lot of time; The presence of three different hierarchies can also be considered a disadvantage.

The indicated disadvantages of PDH, as well as a number of other factors, led to the development in the USA of another hierarchy - the hierarchy of the synchronous optical network SONET, and in Europe a similar synchronous digital hierarchy SDH, proposed for use on fiber-optic communication lines (FOCL). But due to unsuccessfully chosen transmission speed for STS-1, it was decided to abandon the creation of SONET, and to create SONET/SDH on its basis with a transmission speed of 51.84 Mbit/s of the first level of OS1 of this SDH. As a result, OC3 SONET/SDH corresponded to the STM-1 SDH hierarchy. The transmission rates of the SDH hierarchy are presented in Table 1.2.

Table 1.2. Transmission rates of the SDH hierarchy.

The PDH and SDH hierarchies interact through procedures for multiplexing and demultiplexing PDH streams into SDH systems.

The main difference between the SDH system and the PDH system is the transition to a new multiplexing principle. The PDH system uses the principle of plesiochronous (or almost synchronous) multiplexing, according to which, to multiplex, for example, four E1 streams (2048 kbit/s) into one E2 stream (8448 kbit/s), a procedure is performed to equalize the clock frequencies of incoming signals using the stuffing method. As a result, when demultiplexing it is necessary to perform step by step process restoration of original channels. For example, in secondary digital telephony networks the most common use is the E1 stream. When transmitting this stream over the PDH network in the E3 path, it is necessary to first carry out step-by-step multiplexing E1-E2-E3, and then step-by-step demultiplexing E3-E2-E1 at each E1 channel allocation point.

The SDH system performs synchronous multiplexing/demultiplexing, which allows direct access to PDH channels that are transmitted in the SDH network. This rather important and simple innovation in technology has led to the fact that in general the multiplexing technology in the SDH network is much more complex than the technology in the PDH network, the requirements for synchronization and quality parameters of the transmission medium and transmission system have increased, and the number of parameters essential for network operation. As a result, the operating methods and measurement technology of SDH are much more complex than those for PDH.

The International Telecommunication Union ITU-T provides a number of recommendations standardizing transmission speeds and interfaces of PDH, SDH and ATM systems, multiplexing and demultiplexing procedures, the structure of digital communication lines and standards for jitter and wander parameters (Fig. 1.3).

Rice. 1.3. Standards for a primary digital network built on the basis of PDH, SDH and ATM technologies.

Let's consider the main trends in the development of a digital primary network. At the moment, an obvious trend in the development of multiplexing technology on a primary communication network is the transition from PDH to SDH. If in the field of communications this transition is not so obvious (in the case of low traffic, PDH systems are still used), then in the field of operation the trend towards orientation towards SDH technology is more obvious. Operators creating large networks, are already focused on using SDH technology. It should also be noted that SDH allows direct access to a 2048 kbit/s channel through the procedure of input/output of the E1 stream from paths of all levels of the SDH hierarchy. The E1 channel (2048 kbit/s) is the main channel used in digital telephony networks, ISDN and other secondary networks.

2. SDH technology

Features of SDH technology: provides synchronous transmission and multiplexing. Elements of the primary SDH network use one master oscillator for synchronization, as a result, issues of constructing synchronization systems become especially important;

Provides direct multiplexing and demultiplexing of PDH streams, so that a loaded PDH stream can be allocated at any level of the SDH hierarchy without the step-by-step demultiplexing procedure. The direct multiplexing procedure is also called the I/O procedure;

Relies on standard optical and electrical interfaces, which ensures better compatibility of equipment from different manufacturers;

Allows you to combine PDH systems of the European and American hierarchy, ensures full compatibility with existing systems PDH and, at the same time, enables the future development of transmission systems, since it provides high-capacity channels for transmission of ATM, MAN, etc.;

Provides better control and self-diagnosis of the primary network. A large number of fault signals transmitted over the SDH network makes it possible to build control systems based on the TMN platform. SDH technology provides the ability to manage an arbitrarily extensive primary network from a single center.

Let's highlight general features building a synchronous hierarchy:

Supports only tribes as input signals of access channels (note from trib, tributary - component signal, slave signal or load, load flow) PDH and SDH;

Tribes must be packaged in standard labeled containers, the sizes of which are determined by the tribe's level in the PDH hierarchy;

The position of the virtual container can be determined using pointers that eliminate the contradiction between the fact of synchronous processing and a possible change in the position of the container within the payload field;

Multiple containers of the same level can be chained together and treated as one continuous container used to accommodate custom payloads;

A separate header field of 81 bytes is provided.

The SDH hierarchy includes several STM levels. As an example of the use of layers in an SDH network, Fig. 2.1 shows a primary SDH network, including rings of a backbone network built on STM-16 streams, regional networks built on STM-4 streams, and local networks with STM-1 streams.

Fig.2.1. Example of a primary network built on SDH technology

In the process of introducing SDH technology, the emergence of combined SDH/PDH networks is likely at the first stage. SDH technology is usually implemented in the form of “islands”, united by channels of the existing primary network (Fig. 2.2). At the second stage, the "islands" are combined into a primary network based on SDH. As a result, at the present stage it is necessary not only to consider SDH technology, but also to focus on studying combined networks and interaction processes between SDH and PDH.

Fig.2.2.Example of a combined primary PDH/SDH network

3. SDH network composition. Topology and architecture

SDH network composition.

Collection of input streams through access channels into an aggregate block suitable for transportation in the SDH network - a multiplexing problem solved by terminal multiplexers - TM access networks;

Transporting aggregate blocks over a network with the ability to input/output input/output streams is a transportation problem solved by input/output multiplexers - ADMs, which logically control the information flow in the network, and physically control the flow in the physical environment that forms a transport channel in this network;

Overloading of virtual containers in accordance with the routing scheme from one network segment to another, carried out in dedicated network nodes, is a switching or cross-commutation problem, solved using digital switches or cross-switches - DXC;

Combining several flows of the same type into a distribution node - a concentrator (or hub) - a concentration problem solved by concentrators;

Restoration (regeneration) of the shape and amplitude of a signal transmitted over long distances to compensate for its attenuation is a regeneration problem solved with the help of regenerators - devices similar to repeaters in a LAN;

Interfacing the user's network with the SDH network is a pairing task solved using terminal equipment - various matching devices, for example, interface converters, speed converters, impedance converters, etc.

Multiplexer. The main functional module of SDH networks is the multiplexer.

SDH multiplexers perform both the functions of a multiplexer itself and the functions of terminal access devices, allowing you to connect low-speed PDH hierarchy channels directly to their input ports. they are universal and flexible devices that allow you to solve almost all of the problems listed above, i.e. in addition to the multiplexing task, perform the tasks of switching, concentration and regeneration. This is possible due to the modular design of the SDH multiplexer - SMUX, in which the functions performed are determined only by the capabilities of the control system and the composition of the modules included in the multiplexer specification. It is customary, however, to distinguish two main types of SDH multiplexer: terminal multiplexer and input/output multiplexer. Terminal multiplexer TM is a multiplexer and terminal device of an SDH network with access channels corresponding to the PDH and SDH access tribes of the hierarchy (Fig. 3.1.). The terminal multiplexer can either introduce channels, i.e. switch them from the input of the trib interface to the linear output, or output channels, i.e. switch from the linear input to the output of the trib interface. The ADM input/output multiplexer can have the same set of tribes at the input as the terminal multiplexer (Fig. 3.1.). It allows you to input/output their corresponding channels. In addition to the switching capabilities provided by TM, ADM allows for end-to-end switching of output streams in both directions, as well as closing the receive channel to the transmit channel on both sides ("east" and "west") in the event of failure of one of the directions. Finally, it allows (in the event of an emergency failure of the multiplexer) to pass the main optical flow past it in bypass mode. All this makes it possible to use ADM in ring-type topologies.

Rice. 3.1.Synchronous multiplexer (SMUX):

terminal multiplexer TM or input/output multiplexer ADM.

Regenerator is a degenerate case of a multiplexer that has one input channel - usually an STM-N optical tribe and one or two aggregate outputs (Fig. 3.2.). It is used to increase the allowable distance between SDH network nodes by regenerating payload signals. Typically this distance is 15 - 40 km. for a wavelength of the order of 1300 nm or 40 - 80 km. - for 1500 nm.

Rice. 3.2.Multiplexer in regenerator mode.

Switch. Physically, the capabilities of internal channel switching are built into the SDH multiplexer itself, which allows us to talk about the multiplexer as an internal or local switch. In Figure 3.3, for example, the payload manager can dynamically change the logical mapping between the TU and the access channel, which is equivalent to internal circuit switching. In addition, the multiplexer, as a rule, has the ability to switch its own access channels (Fig. 3.4.), which is equivalent to local channel switching. Multiplexers, for example, can be assigned local switching tasks at the level of access channels of the same type, i.e. tasks solved by concentrators (Fig. 3.4.). In the general case, you have to use specially designed synchronous switches - SDXC, which perform not only local, but also general or pass-through (end-to-end) switching of high-speed flows and synchronous STM-N transport modules (Fig. 3.5). An important feature of such switches is the absence of blocking of other channels during switching, when switching of some TU groups does not impose restrictions on the processing of other TU groups. such switching is called non-blocking.

Rice. 3.3.Input/output multiplexer in internal switch mode.

Rice. 3.4.Input/output multiplexer in local switch mode.

Rice. 3.5.General or pass-through switch of high-speed channels.

There are six different functions performed by a switch:

Routing of virtual containers VC, carried out based on the use of information in the ROH routing header of the corresponding container;

Consolidation or merging (consolidation/hubbing) of virtual containers VC, carried out in hub/hub mode;

Translation of a stream from a point to several points, or to a multipoint, carried out when using the point-to-multipoint communication mode;

Sorting or rearranging (drooming) of virtual containers VC, carried out in order to create several ordered VC flows from the total VC flow arriving at the switch;

Access to the virtual container VC, carried out when testing equipment;

Input/output (drop/insert) of virtual containers, carried out during operation of the input/output multiplexer;

SDH network topology.

Point-to-point topology.

The network segment connecting two nodes A and B, or point-to-point topology, is the most simple example basic topology of the SDH network (Fig. 3.6.). It can be implemented using terminal multiplexers TM, both according to a scheme without redundancy of the receive/transmit channel, and according to a scheme with 100% redundancy of the 1+1 type, using the main and backup electrical or optical aggregate outputs (reception/transmission channels).

Rice. 3.6.Point-to-point topology implemented using TM.

Topology "series linear circuit".

This basic topology is used when the traffic intensity on the network is not so high and there is a need for branches at a number of points along the line where access channels can be introduced. It can be presented either as a simple sequential linear circuit without redundancy, as in Fig. 3.7, or as a more complex circuit with redundancy of the 1+1 type. The latter version of the topology is often called a "simplified ring".

Rice. 3.7. “Serial linear circuit” topology implemented on TM and TDM.

A star topology that implements the function of a hub.

In this topology, one of the remote network nodes, connected to the switching center or SDH network node on the central ring, plays the role of a hub, or hub, where part of the traffic can be output to user terminals, while the rest can be distributed to other remote nodes (Fig. 3.9.)

Rice. 3.9. Star topology with a multiplexer as a hub.

Ring topology.

This topology (Fig. 3.10.) is widely used to build SDH networks of the first two levels of the SDH hierarchy (155 and 622 Mbit/s). The main advantage of this topology is the ease of organizing 1+1 type protection, thanks to the presence in the SMUX synchronous multiplexers of two pairs of optical reception/transmission channels: east - west, making it possible to form a double ring with counter flows.

Rice. 3.10.Ring topology with 1+1 protection.

Linear architecture for long-distance networks.

For long-distance linear networks, the distance between terminal multiplexers is greater or much greater than the distance that can be recommended from the point of view of the maximum permissible attenuation of the fiber-optic cable. In this case, on the route between TMs (Fig. 3.14), in addition to multiplexers and a pass-through switch, regenerators must also be installed to restore the fading optical signal. This linear architecture can be represented as serial connection a number of sections specified in recommendations ITU-T G.957 and ITU-T G.958.

Rice. 3.14.Long-distance SDH network with point-to-point communications and its segmentation.

In the process of developing an SDH network, developers can use a number of solutions typical for global networks, such as forming their own “backbone” or backbone network in the form of a mesh (mush) structure, which allows organizing alternative (backup) routes used in the event of an emergency. problems when routing virtual containers along the main path. This, along with the inherent SDH networks internal redundancy allows you to increase the reliability of the entire network as a whole. Moreover, with such reservation, alternative signal propagation media can be used on alternative routes.

Methods for parity control and error detection in the SDH system

The SDH system uses a method for monitoring error parameters without disconnecting the channel, which is called the parity method (Bit Interleaved Parity - B1P). This method, like CRC, is an evaluation method, but it gives good results when analyzing SDH transmission systems. The parity control algorithm is quite simple (Fig. 5.1). Parity check is performed on a specific frame data block within data groups of 2, 8 and 24 bits (BIP-2, BIP-8 and BIP-24, respectively). These groups of data are organized into columns, then for each column its parity is calculated, i.e. an even or odd number of ones in a column. The counting result is transmitted as a code word to the receiving side. At the receiving side, a similar calculation is made, compared with the result, and a conclusion is drawn about the number of parity errors. The comparison result is transmitted in the opposite direction to the flow.

Fig. 5.1. Parity control algorithm.

The parity method is evaluative because multiple errors can cancel each other out in a parity sense, but it provides an acceptable level of assessment of the quality of a digital transmission system. Since SDH technology involves the creation of section headers and a path header, the parity method makes it possible to test the parameters of a digital transmission system from section to section and from the beginning to the end of the route. For this purpose, special bytes are used (see above) as part of the SOH and PON headers. For example, the number of errors detected in channel B3 is transmitted in byte G1 PON VC-4 of the next cycle. Figure 5.2 shows a diagram of section-by-section monitoring of the BIP error parameter. The bytes associated with them in the digital transmission system are shown in Table 5.1.