Synchronous digital hierarchy

The synchronous digital hierarchy —abbreviated as SDH, from English Synchronous Digital Hierarchy— is a set of data transmission protocols. It can be considered as the revolution in transmission systems, as a consequence of the use of fiber optics as a transmission medium, as well as the need for more flexible systems that support high bandwidths. The SDH hierarchy was developed in the US under the name of SONET or ANSI T1X1 and later the CCITT (Today ITU-T) in 1989 published a series of recommendations where it was defined under the name of SDH.

One of the objectives of this hierarchy was in the process of adapting the PDH (Plesiochronous Digital Hierarchy) system, since the new hierarchical system would be implemented gradually and had to coexist with the installed plesiochronous hierarchy. This is the reason why the ITU-T standardized the process of transporting the old frames in the new one. The basic SDH frame is STM-1 (Synchronous Transport Module level 1), with a speed of 155 Mbit/s.

Each frame is encapsulated in a special type of structure called a container. Once encapsulated, control headers are added that identify the content of the structure (the container) and the set, after a multiplexing process, is integrated into the STM-1 structure. The upper levels are formed by multiplexing several STM-1 structures at the byte level, giving rise to the STM-4, STM-16, STM-64 and STM-256 levels.

STM-1 frame structure

STM-1 frame.

Frames contain information on each of the network components: path, line and section, as well as user information. The data is encapsulated in specific containers for each type of tax signal.
Additional information called "path overhead" (Path overhead), which consists of a series of bytes used for network maintenance purposes, and which give rise to the formation of so-called virtual containers (VC). The result of the multiplexing is a frame made up of 9 rows of 270 octets each (270 columns of 9 octets). The transmission is carried out bit by bit in the direction from left to right and from top to bottom. The frame is transmitted at the rate of 8000 times per second (each frame is transmitted in 125 μs). Therefore, the binary regime (Rb) for each of the levels is:

STM-1 = 8000 * (270 columns * 9 rows * 8 bits)= 155 Mbit/s
STM-4 = 4 * 8000 * (270 columns * 9 rows * 8 bits)= 622 Mbit/s
STM-16 = 16 * 8000 * (270 columns * 9 rows * 8 bits)= 2.5 Gbit/s
STM-64 = 64 * 8000 * (270 columns * 9 rows * 8 bits)= 10 Gbit/s
STM-256 = 256 * 8000 * (270 columns * 9 rows * 8 bits)= 40 Gbit/s

Of the 270 columns that make up the STM-1 frame, the first 9 form the so-called "overhead or header" (overhead), independent of the overhead of the aforementioned virtual containers, while the remaining 261 constitute the payload (Payload).

SOH (Section Overhead)

The SOH (Section Overhead) is divided into two parts: The R-SOH and the M-SOH. The first of them (R-SOH) is used for applications between repeaters, which are comprised by the bytes of rows 1 to 3, while for use between multiplexing terminals (M-SOH) they correspond to the bytes of rows 5 to 9. The functions of each of the bytes that make up the SOH are detailed below.

Fig. 1 Bytes of SOH

a) Framing signal A1, A2:
A1 and A2 are fixed frame synchronization patterns. A1 is set to 11110110 and A2 to 00101000.
b) J0 regenerator section trace:
The use of J0 is still under study.
c) Error monitoring B1, B2:'
Transmission errors are monitored in the regenerator and multiplexer sections. B1 is for the regenerator section and B2 for the multiplexer section.
d) Service channel for Engineering E1, E2:
E1 is accessible on regenerators and multiplexers, E2 only on multiplexers. Each circuit has a capacity of 64Kb/s.
e) User channel F1:
This is a 64 Kb/s data channel that can be used by any network operator for their purposes.
f) Data communication channel D1-3, D4-12:
These bytes are assigned as data communication channels to transmit information to multiplexers and regenerators and vice versa.
g) Automatic protection switching signaling K1, K2:
The information exchange between two ends in a multiplexer section is carried out through the K1 and K2 bytes. Part of K2 is also used to send MS-RDI (remote defect indication in multiplexer section) and MS-AIS (alarm indication signal in multiplexer section).
h) Synchronization state S1:
The S1 byte communicates to the next station the quality of the synchronization reference source used by the equipment.
Bits 1 through 4 of the S1 byte are reserved for quality used by individual operators. Bits 5 through 8 can take the following values:
0000 Unknown quality (existing synchronization network)
0001 Reserved
0010 Signal generated by a device that is synchronized to a clock according to Rec. ITU-T G.811
0011 Reserved
0100 Signal generated by a device that is synchronized to a SSU-A type clock
0101 Reserved
0110 Reserved
0111 Reserved
1000 Signal generated by a device that is synchronized to a SSU-B type clock
1001 Reserved
1010 Reserved
1011 Signal generated by equipment that is synchronized to a clock according to ITU-T Rec. G.813 Option I (SEC)
1100 Reserved
1101 Reserved
1110 Reserved
1111 Do not use the synchronization of this signal
i) Z1 and Z2 are spare bytes.
j) M1 Byte of Error indication in the Remote multiplexing Section.

POH (Path Overhead)

The POH (Path OverHead) has the mission of monitoring the quality and indicating the type of virtual container that you have. It is made up of the VC (Virtual Container) which is the payload entity that travels unchanged throughout the network, in addition to some bytes that are added and unpacked at the various termination points of the transport service. The bytes that are added will depend on the type of virtual container and are divided into two types Higher-order Path Layer and Lower-order Path Layer. The following table shows the bytes corresponding to the Higher-order Path Layer.

The second type of bytes that are added are those of the Lower-order Path Layer type that correspond to the VC-12. The following table shows the operation of each of them.

SDH Multiplexing

To be considered an international standard, the various existing PDH bit rate interfaces must be accommodated in the SDH framework. This is done by allowing different interfaces to be mapped in the SDH frame.

Fig. 2 Multiplexation SDH

SDH Multiplexing - 2Mbps (E1)

This multiplexing starts from the basic PDH unit, which is E1 (2 Mbit/s) to form an STM-1. 63 2 Mbit/s PDH signals can be carried. Below are the steps for mapping an STM-1 through an E1.

• The mapping of a 2 Mbit/s signal in the SDH plot is considered, the original PDH signal will be 2048 kbit/s, with a variation of 50 ppm. This is inserted into a container (C-12), where justification is performed using traditional stuffing techniques (bit fill). This is done to offset the frequency variations allowed in bit rates for PDH and SDH.
• The container is placed in a virtual container (VC-12) where the path overhead is added. This overhead is carried with the signal along the network, even when connected cross-shaped in different SDH plots. This allows the maintenance and monitoring of the signal through the network. Includes error detection, alarm indications, and a signal label.
• A pointer is added to the virtual container to form a tax unit (TU-12). This allows the SDH system to compensate for phase differences across the network or between networks.
• Three TU-12 are multiplexes in a tax unit group (TUG-2).
• Seven TUG-2 are multiplexed in a TUG-3. This is the unit of the same size that would be used for mapping, for example, an E3 signal in a SDH plot.
• Three TUG-3 are multiplexes through an administrative unit (AU-4) and in an administrative unit group (AUG) to form a STM-1 plot.

SDH Multiplexing - 34Mbps (E3)

To carry out this multiplexing, the previous steps are executed in a similar way. Up to 3 34Mbit/s signals can be transmitted.

• The frequency is adapted byte interleaving (C-3).
• 9 bytes overhead (VC-3).
• The pointer is added (TUG-3).
• Three (TUG-3) are multiplexed through (AU-4) and (AUG) to form a STM-1 plot.

SDH Multiplexing - 140Mbps (E4)

To multiplex PDH signals it is first necessary to adapt them to the SDH speed. The steps to carry out said multiplexing are given in a similar way to those developed in the previous points.

• The frequency of 140 Mbit/s to 149.76 Mbit/s should be increased by bit justification (C-4).
• Add a column of 9 overhead bytes (VC-4).
• Add the pointer (AU-4).

SDH Pointers

A synchronous system is based on the fact that each clock is in phase and frequency of synchronization with the next. In practice this is impossible to achieve, therefore phase and frequency deviations will occur. Within a network the clock frequency is extracted from the line signal, however, phase variations can occur from jitter buildup on the network. Frequency interface variations in the network can occur. The way that SDH overcomes this problem is by using pointers to point to the address of the beginning of the virtual container within the frame. The value of the initial pointer corresponds to the phase difference between the arrival of the tributary unit and the empty tributary unit within the frame at the moment the tributary is mapped in the virtual container. If the phase varies between the read and write clocks in such a way that the digital termination stream input buffers show a tendency to overflow or run empty, a trim pointer will be produced. The following table provides a brief description of the pointers used for STM-N frame mapping.

Positive pointer justification

A positive pointer justification occurs when the input frequency is less than the output frequency, thus padding bytes are inserted that do not affect the data. Justification bytes are always inserted at the same location within the frame.

Negative pointer justification

A negative pointer justification occurs when the input frequency is greater than the output frequency, the H# bytes can carry actual VC4 information without affecting the payload data. Too much adjustment of pointers can cause jitter.

Mapping of SDH tributaries

ATM cell mapping

ATM cells are allocated to containers at different bit rates. These ATM cells are mapped by aligning each cell with the structure of the virtual or concatenated containers. Since the capability cannot be an integrating multiple of the length of ATM cells (53 bytes), one cell is allowed to cross the frame container boundary. The ATM cell information field (48 bytes) is encoded before being mapped, to guarantee delineation. A flow of ATM cells with a data rate that can be mapped is equal to the payload capacity of the VC. Unfortunately ATM was not accepted by the market as the solution to carry data over the SDH / SONET protocols. Its inherent bandwidth inefficiency, high cost, and complexity pushed ATM into specific niche markets, such as Frame Relay transport, xDSL access, and some military and scientific applications.

Signal mapping in HDLC frames

Signals in HDLC frames are mapped by aligning the byte structure of each frame with the byte structure of the VC. The range is from 1.5 Mbit/s to several Gbit/s using concatenation techniques. The 7EX HDLC flags are used between frames to fill the buffer, due to the discontinuous arrival of HDLC frame signals. HDLC frames are of variable length, one frame can cross the container boundary.

Sync on SDH

For synchronization in SDH, standards G.803 (Architecture of transport networks based on synchronous digital hierarchy) and G.811 (Timing characteristics of primary reference clocks) are taken into account, among others such as G.822, G.812, etc. Synchronizing refers to two or more items, events, or operations being scheduled to occur at a predefined moment of time or place. In electronic engineering, digital logic, and data transfer, synchronization implies that the device uses a clock signal.

Sync Networks

The synchronization network is the network that is responsible for distributing the information of synchronization to network elements that have to operate synchronously to satisfy the octet slip performance requirements of ITU-T Recommendation G.822.
The synchronous operation of the types of network elements is usually ordered in a certain geographical area, in which all these elements are synchronized with a "master clock". The zone in which all relevant network elements (in normal operation) are synchronized to a master clock is called a "synchronization zone”.
The master clock of a synchronization zone must meet the requirements described in ITU-T Recommendation G.811.

Nodal clocks

ITU-T Recommendation G.810 identifies two fundamental methods of synchronizing nodal clocks, namely master-slave synchronization and mutual synchronization. The master-slave synchronization: It is a suitable method for the synchronization of SDH networks; where a clock hierarchy is used in which each hierarchical level is synchronized with reference to a higher level. The highest level of the hierarchy is the PRC. The clock reference signals are distributed among the levels of the hierarchy by means of a distribution network that may use the infrastructure of the transport network. The hierarchical levels are the following:

Fig. 3 Structure of the watch hierarchy in direct synchronization

• PRC G.811
• Private watch (transit node) Rec. G.812.
• Private watch (local node) Rec. G.812.
• SDH Rec network element watch. G.813.

The feasibility of mutual synchronization is for further study.
Timing distribution between hierarchical node clocks should be done using a method that avoids intermediate pointer processing.
All elements in the SDH network operate under the same frequency clock, supplied by a signal source called the Primary Reference Clock (PRC). The ITU-T G.811 recommendation contains the performance specifications of the PRC, whose stability and frequency accuracy are in the order of ±10-11, possible thanks to a cesium oscillator.

Characteristics of clocks in SDH

In the ITU-T G.803 standard, emphasis is placed on the need for SDH clocks to conform to the Primary Reference Clock (PRC) and have a good short-term stability characteristic, in order to conform to the generic slip rate objectives of ITU-T Recommendation G.822.
It is further noted that, as long as the SDH clock meets the short-term stability template, there are no practical limitations to the number of pointer handlers that can be cascaded in an SDH network, to meet jitter requirements. output payload at an SDH/PDH border.
“Primary reference clocks need very high reliability and are likely to include repeat equipment, in order to ensure output continuity. However, any phase discontinuity due to internal clock operations shall not cause more than a lengthening or shortening of the interval width of the timing signal and shall not cause, at the clock output, a phase discontinuity greater than 1/8 of UI at clock output”. this is noted in ITU-T G.811.
The performance of the PRC is therefore not specified at internal reference points but rather at the external interface of the equipment. The output interfaces specified for the equipment in which the PRC may be contained are:

• Interfaces to 2048 kHz according to clause 10/G.703 with additional phase fluctuation and slow phase fluctuation requirements.
• Interfaces to 1544 kbit/s according to clause 2/G.703 with additional phase fluctuation and slow phase fluctuation requirements.
• Interfaces to 2048 kbit/s according to clause 6/G.703 with additional phase fluctuation and slow phase fluctuation requirements.
• Other interfaces (such as 8 kHz to 5 MHz of sinusoidal waves) are studied.

The distribution of the clock signal is manifested through ordinary transmission lines such as, in this case, an SDH transmission system. The "intermediate" network elements, such as regenerators, add and drop multiplexers, etc., are operated by means of a "slave mode", which uses a clock signal component extracted from the received STM-N signal.
Clock signal deterioration, such as accumulated jitter during transmission through a chain of network elements and transport lines, is reduced with high-performance slave clock equipment as specified in the G.812 recommendation for node transmission. transit and for local node.
An SDH network element has the ability to send an external clock signal directed towards the BITS (Integrated Construction Timing Source) to reduce the deterioration in the clock signal. The intermediate network element directly uses the extracted clock signal itself.
The clock signals necessary for the operation of the NE (Network Element) are produced by a clock circuit that runs mainly in slave mode. The reference sources available are:
- External input
An external clock signal from a primary reference clock (G.811), or BITS (G.812 transit or local), or the clock of a switching system is normally connected to this port.
- STM-N line signal
The clock component extracted from a line signal can be used as a reference source, whether it is connected to the east, to the west, or to a tributary direction. Then, the S1 byte of the SOH shows the quality level of the clock component. It instead shows the clock signal that originally generated the STM-N line signal, as long as the STM-N signal can be found from G.811 or G.812 T, L, or other.
- 2 Mb/s PDH signal in the tributary
Two of the 2 Mbit/s tributary signals can be selected as reference sources. This would be the case if, for example, the SDH system were installed in an isolated area with the synchronous clock communicated via a 2 Mbit/s signal generated by a PRC, or when the SDH system is synchronized to an ESS clock (switching system) instead of PRC.

Apart from being used in slave mode of operation, the NE clock circuit can also function as an independent clock source, for which there are two modes of operation:

- Hold mode
While the clock circuit operates in slave mode, all parameters such as frequency, phase etc they are memorized. When the circuit loses contact with the reference source, due to a line failure, for example, this stored information facilitates the flow of continuous operation without interruption. In this way, transmission disturbances caused by abrupt frequency and phase changes can be avoided.

- Free operation mode
The clock circuit, which is basically a VCXO (Voltage Controlled Oscillator), operates freely without reference source. This is an excellent option for an area where a clock reference source is not available, and where SDH is used in a similar way to PDH.

Optical interfaces for SDH-related equipment and systems

This Recommendation specifies the parameters of optical interfaces for equipment and systems based on synchronous digital hierarchy to allow cross-compatibility (multi-vendor) on elementary cable sections.
These specifications are also intended to conform to ITU-T Rec. G.955 longitudinal compatibility of equipment of comparable hierarchical level and application. The Recommendation is based on the use of one optical fiber for each direction.

Classification of optical interfaces

Through the appropriate combination of transmitters and receivers, power balances can be obtained for optimized fiber optic line systems, in terms of attenuation/dispersion and costs with respect to the various applications. However, to simplify the development of cross-compatible systems, it is desirable to limit the number of application categories and corresponding sets of optical interface specifications for standardization. Three broad categories of application are contemplated:

Fig. 4 Optical interfaces classification

Intra-centres: Corresponding to interconnection distances less than approximately 2 km.
Intercentres: A short distance, corresponding to interconnection distances of approximately 15 km.
Interexchanges: Long distance, corresponding to interconnection distances of approximately 40 km in the 1310 nm window and approximately 80 km in the 1550 nm window.

Operating Wavelengths

To provide flexibility in the implementation of cross-compatible systems and to allow for future use of Wavelength-Division Multiplexing (WDM), it is desirable to support as wide a range of wavelengths as possible. system operation. The specification of operating wavelength ranges is affected by the following general considerations: fiber type, source characteristics, system attenuation range, and optical path dispersion.
The operating wavelength range is the maximum allowable range of source wavelengths. In this range, the source wavelengths can be selected for different fiber related impairments. The receiver must have the minimum range of operating wavelengths that corresponds to the maximum allowable range of wavelengths of the source. For SDH networks using fiber optic amplifiers, it might be necessary to limit the range of operating wavelengths.
The wavelength regions that allow the operation of the system are partially determined by the cutoff wavelength values of the fiber or fiber cable. For G.652 and G.653 fibers these values have been chosen in such a way as to allow single mode operation of the fiber cable for wavelengths of 1270 nm and above, although some administrations allow values as low as 1260 nm. For G.654 fiber cables, cutoff wavelength values for single-mode operation at 1530 nm and above have been accepted.
The allowable wavelength regions are further defined by the attenuation of the fiber.
While intrinsic scattering attenuation typically decreases with increasing wavelength, OH-ionic absorption can appear around 1385 nm and, to a lesser extent, around 1245 nm. Consequently, these absorption peaks and the cut-off wavelength define a region of wavelengths centered around 1310 nm.
Non-dispersion-shifted fibers conforming to ITU-T Rec. G.652 are optimized for use in this wavelength region. For longer wavelengths, bending attenuation appears for values of 1600 nm or higher and infrared absorption appears beyond 1600 m.
Consequently, these attenuations and the water vapor absorption peak appearing at 1385 nm define a second region of operating wavelengths around 1550 nm. ITU-T Rec. G.654 for cut-shifted fibers is limited to this region only. However, G.652 fibers and G.653 dispersion-shifted fibers can be used in this region.

Transmitters

Transmitting devices are:

• Photo transmitters (LED, light emitting diode)
• Multi-length lasers (MLM, multi-lengthinal mode)
• Monolongitudinal lasers (SLM, single-longitudinal mode).

Spectral Characteristics

• For LEDs and MLM lasers, the spectral width is specified by the medium quadratic value (RMS, root-mean-square) maximum of the width in standard operating conditions.
• For SLM lasers, the maximum spectral width is specified by the maximum total width of the crest of the central wavelength, measure 20 dB below the maximum width of the central wavelength in standard operating conditions. In the case of SDH networks using optical amplifiers, it is necessary to have a transmitter with spectral characteristics appropriate to achieve objective distances that exceed those defined for long-distance applications.

Average injected power

The average injected power at reference point S is the average power of a pseudo-random data sequence coupled to the fiber by the transmitter. It is expressed as a range to allow for some cost optimization and to take into account operating margins under standard operating conditions, transmitter connector degradations, measurement tolerances and aging effects. These values allow to determine the sensitivity values and the overload point for the receiver at the R reference point.
The convention adopted for the optical logic level is as follows:

• The light emission is represented by a logical "1".
• The absence of emission is represented by a logical "0".

Receiver

Proper system operation requires specification of minimum receiver sensitivity and minimum overload power level.

Considerations for enhancing the system

There are two possibilities to enhance the system:

• It may be convenient to enhance the system of the existing plesochronous method to the SDH method (for example, moving from a system to 139 264 kbit/s compatible with the specifications of the Rec. ITU-T G.955 to a STM-1 system based on this Recommendation).
• It may be convenient to boost the system from one level of the SDH to another (e.g. from STM-1 to STM-4).

Transport Network Architecture based on SDH

The main functions of SDH networks can be integrated into two large groups:

• Transport of information between 2 points efficiently and safely.
• Total management of services. (configuration, maintenance, evaluation of performance, etc.).

A transport network based on SDH technology can be decomposed into independent transport layer networks with a client-server association.

• Circuit layers are the carriers of the service.
• The travel layers provide the connection between network nodes.
• The transmission layers provide physical support.

The architecture of the transport network was based on the concepts of stratification and subdivision within each layer.
The architecture of SDH networks is defined by Recommendation G.803, in this recommendation a three-dimensional model is defined.
The network layer is a set of similar access points that can be associated to transfer information.
The adaptation function is the process by which layer information is adapted to be transported by the server layer network. Interlayer adaptation has the following processes:

• Codification
• Speed modification
• Alignment
• Justification
• Multiplexation

Monitoring the connection is done through:
Intrinsic Supervision:
Path layer connections can be monitored indirectly by using data available intrinsically from the multiplex section or higher order path server layers, and calculating the approximate state of the client's path connection from the available data.

Non-intrusive monitoring:
The connection can be directly monitored by the relevant overhead information in the regenerator section, multiplex section, higher order path or lower order path, and the approximate state of the connection is then calculated from the difference between the states. monitored at each end of the connection.
Sublayer Monitoring:
Connections can be monitored directly by overwriting some part of the original trail overhead capacity at the beginning of the connection. In the case of SDH, the overhead has been defined for this purpose in the higher and lower order path layers. When SDH cascading is applied, this monitoring method is known as tandem connection monitoring.

Techniques to improve availability in the Transport Network

SDH multiplex section protection

Failure events are detected by the Multiplex Section Termination (MST) function and reconfiguration uses protection switching functions found in the Multiplex Section Protection sublayer. The resulting reconfiguration may contemplate protection switching in multiple elements of the SDH network. The coordination of this switching in multiple elements of the SDH network is carried out by means of an Automatic Protection Switching (APS) protocol.

SDH Protection Rings

Fig. 5.1 MS-SP Ring operation

Fig. 5.2 MS-DP Ring operation

MS-SP Ring (Multiplex Section-Shared Protection Ring):
Only half of the capacity in each multiplexing section is used to carry traffic. Maximum 16 nodes. Total maximum distance of the structure of 1200 km. Switching times less than 50ms.
On failure:
– Adjacent Nodes detect the failure and perform a Bridge&Switch operation.
– The rest of the nodes perform a Full Pass-Through operation.
– In a switching situation, the traffic always circulates passing through all the nodes of the MS-SPRING ring.

MS-DP Ring (Multiplex Section-Dedicated Protection Ring):
Each direction of a bidirectional connection uses a different path following a direction of the ring. The opposite direction would be the backup. One drawback is that each bidirectional connection consumes BW across the entire ring. Maximum 16 nodes (due to signaling limitations).

Ring (Subnetwork Connection Protection Ring):
Employee in a ring. Each unidirectional connection uses both paths in the ring (it is a 1+1). It does not have the limitation of 16 nodes. Supports the failure of a node.

Management aspects of transport network elements in SDH

Current SDH networks are basically built from four different types of equipment or network elements (ITU-T G.782): Regenerators, Terminal Multiplexers, Insertion and Extraction Multiplexers, and Multiplexer Distributors. These devices can support a wide variety of network configurations, even the same device can work interchangeably in different modes, depending on the functionality required in the node where it is located. Figure 6 shows a block diagram of a generic SDH element, without considering optional amplifiers or boosters.

Intermediate Regenerators or IR (Intermediate Regenerators)

As their name indicates, they regenerate the clock signal and the amplitude ratio of the digital signals at their input, which have been attenuated and distorted by the dispersion of the optical fiber through which they travel. The regenerators obtain the clock signal from the incoming bit string.

Terminal Multiplexers or TM (Terminal Multiplexers)

It is an element that is used in a point-to-point link. It will implement only the line termination and the function of multiplexing or demultiplexing several tributaries in an STM-N line. In the generic element of Figure 8, the TM STM-4 would have a single STM-4 optical aggregate interface (with transmission and reception) and, depending on the configuration, several electrical tributary interfaces (1.5 Mbit/s, 2 Mbit/s, 34 Mbit/s, 45 Mbit/s, 140 Mbit/s, STM-1) or optical (STM-1).

Add and Drop Multiplexers (ADM)

They are in charge of extracting or inserting plesiochronous or synchronous tributary signals from any of the two STM-N aggregate signals that it receives (one in each direction of transmission), as well as making way for those that are desired. Brings flexibility to the SDH network

Distributors multiplexers or DXC (Digital Cross-Connect)

They allow the interconnection without signal blocking at an equal or lower level, between any of its input and output ports. DXCs support access signals, both plesiochronous and sychronous, at various levels.

SONET/SDH speeds

The higher level signals are formed by the multiplexing of various level 1 signals (STM-1), creating a family of STM-N signals, where N indicates the number of level 1 signals that compose it. Table 1 indicates the denominations of the electrical signals and optical carriers, as well as their speeds and the points of coincidence with those of SONET.

In the table above, the upload bandwidth is the line speed minus the line and section bandwidth.

Note that the data rate progression starts at 155 Mbit/s and increases in multiples of 4. The only exception is OC-24, which is standardized in ANSI T1.105, but is not a standard SDH rate of the ITU-T G.707. Other rates such as OC-9, OC-18, OC-36, and OC-96 and OC-1536 are sometimes described, but have probably never been deployed. They are certainly not common and are not supported by standards.

The next speed of 160 GB/s OC-3072/STM-1024 has not been standardized yet, due to the cost of high-speed transceivers, as multiplexes at 10 and 40 Gbit/s wavelengths are cheaper.

Advantages and Disadvantages of SDH

SDH has a number of advantages over the plesiochronous digital hierarchy (PDH).

Some of these advantages are:

• The multiplexing process is much more direct. The use of pointers allows a simple and rapid location of the information tax signals.
• Signal processing is performed at STM-1 level. High speed signals are synchronous with each other and are in phase because they are generated locally by each node of the network.
• Tax plots of line signals can be subdivided to accommodate plesochroman loads, ATM traffic or smaller units. This means mixing traffic of different types giving rise to flexible networks.
• Electrical and optical compatibility between the equipment of the different suppliers thanks to international standards on electrical and optical interfaces.
• A STM1 has the ability to group multiple E1 and T1 in a multiplexed way, that is, the speeds are universalized by occupying the corresponding VCs, the capacity of the STM1 is sufficient.

As for the disadvantages we have to:

• Some current PDH networks already have some flexibility and are not compatible with SDH.
• The need for synchronism between the nodes of the SDH network requires that all services work under the same time frame.
• The compatibility principle has been above the bandwidth optimization. The number of Bytes destined for the section header is too large, which leads to lose efficiency.