2.2.1 The concept of building a modern transmission systems
The development of science and the acceleration of technological progress is not possible without improving communications, data collection, transmission and processing of information.
Intensive development of new information technologies in recent years has led to the rapid development of microprocessor technology, which stimulated the development of digital transmission techniques.
Ultimately, this led to the creation of new high-speed WAN technologies: PDH, SONET, SDH, ISDN, Frame Relay and ATM.
One of the most advanced technology currently used for the construction of communication networks is the technology SDH synchronous digital hierarchy.
Interest in the SDH due to the fact that this technology has replaced the PCM pulse code modulation (PCM) and plesiochronous digital hierarchy of PDH (PDH).
Began actively implemented as a result of the mass installation of modern foreign digital PBX, allowing to operate with flows of 2 Mbit / s, and the creation of local regional SDH rings.
Synchronous Digital Hierarchy (SDH) has significant advantages over previous generations of systems, it allows you to fully realize the potential of fiber-optic transmission lines and create flexible, easy to operate and control the network, ensuring high-quality communications.
Thus, SDH concept allows optimally combine processes high-quality digital data transmission with automated management processes, control and network services in a single system.
SDH systems provide transmission speeds of 155 Mbit / s and above, and can be transported as digital signals of existing systems (eg, common on city network of PCM-30), as well as promising new services, including broadband.
SDH equipment is software-controlled and integrates the means of conversion, transmission, operational switching, control, control.
With the advent of modern fiber-optic cables (FOC) made possible a high transmission rate in linear tract (RT) digital transmission systems with a simultaneous lengthening of the regeneration sections up to 100 km or more.
Performance of LT exceeds the performance of digital paths in the cables with metal vapors 100 times or more that dramatically increases their cost-effectiveness.
Most repeaters it is possible to combine with the terminal or transit stations. From this it follows that the SDH - is not just a new transmission system, this fundamental change in network architecture management. The introduction of SDH is a qualitatively new stage in the development of digital communication network.
Therefore, the program of development of Kazakhstan in the long term the introduction of new information technologies belongs to one of the priority seats. Construction of fiber optic links in this area - the next logical step in the comprehensive modernization of communication of Kazakhstan, which will result in the creation of a powerful modern information network.
Advantages of fiber optic to copper lines are clear: high reliability and noise immunity, large data transmission speed, large bandwidth. FOTS raises means of telecommunications to a new, much higher level of development.
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This reliable telephone service, Internet access and other global projects, the implementation of which is currently virtually impossible at this site. Introduction FOTS raise to a higher level and a secondary communication network, with a significant extension of new services which require broadband connections (e.g., network communication technology - video, video conferencing, industrial television, computer networks, operating in real time).
The planned fiber-optic route is intended primarily to provide customers high-quality communications. Advantages of fiber optic to copper lines are clear: high reliability and noise immunity, large data transmission speed, large bandwidth. FOTS raises means of telecommunications to a new, much higher level of development.
This reliable telephone service, Internet access and other global projects, the implementation of which is currently virtually impossible at this site. Introduction FOTS raise to a higher level and a secondary communication network, with a significant extension of new services which require broadband connections (e.g., network communication technology - video, video conferencing, industrial television, computer networks, operating in real time).
The planned fiber-optic route is intended primarily to provide customers high-quality communications.
2.2.2 Transmission systems of PDH, characteristic features
In modern networks are operated as a plesiochronous system and synchronous digital hierarchy systems.
Standard PDH - plesiochronous digital hierarchy. Hierarchy recommended for digital transmission systems, something like a calendar hierarchy. For this purpose it was necessary to select a certain unit of measure "e" bit rate common to all countries and companies that produce equipment of transmission systems and allows you to measure the speed of the total digital streams. This "unit" rate worldwide is digital speech transmission rate of 64 kbit / c. Channel on which are transmitted at 64,000 bits / s, is called the primary digital channel. Possibilities any digital transmission system estimates the number of organized with the help of just such standard channels.
Combining flows leveling speed has been called plesiochronous (nearly synchronous), and the existing hierarchy of speeds transmission of digital streams, and hence the type of PCM transmission systems (PCM) - plesiochronous digital hierarchy (in the English writing Plesiohronous Digital Hierarhi, PDH).
Plesiochronous digital hierarchy was developed in the early 80s. In the hierarchy of great hopes, but it was not very flexible in order to enter into the digital stream "rushing" at a high speed or low-speed output from it flows must be fully "embroider", and then "sew" high flow. This requires installation of a large number of multiplexers and de multiplexers. It is clear that to do this operation is often quite expensive.
The system uses the principle of PDH plesiochronous multiplexing, according to which for multiplexing, for example, the 4-E1 (2048 kbit / s) into one stream E2 (8448 kbit / s) alignment procedure is performed such frequency signals occurring by stuffing. As a result, when multiplexing is necessary to make a step by step process of restoration of the original channels. For example, in the secondary digital telephone networks the most common use of E1. When sending this stream for PDH networks tract E3 must first perform incremental multiplexing E1-E2-E3-step demultiplexing and then E2-E3-E1 at each point E1 channel allocation. This is a major drawback PDH equipment - due to the increase in the number of necessary equipment for the separation of one or two streams. E1 primary digital channel DSO combines 32 channels, of which one DSO used for frame synchronization, another - for signaling. Still this stream consists of 32 time slots of 8 bits each. Frame repetition frequency 8 kHz, which gives the flow rate of 32 * 8 * 8 = 2048 kbit / s.
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The essence of the main drawbacks RDN is that the addition of equalizing bit makes it impossible to identify and output, for example, stream 64 Mbit / sec or 2 Mbit / s "hardwired" into the stream of 140 Mbit / s without complete demultiplexing or "embroidered" of this stream and removing leveling bit. One thing to "drive" the flow of long-distance or international calls from one call center to another "mixing" and "bark" them quite rare.
Another thing - to connect several banks and / or their separation via PDH network. In the latter case often have to either output stream 64 kbit / s and 2 Mbit / s from the flow of 140 Mbit / s to make it, for example, a bank branch, or vice versa to output a stream of 64 kbit / s and 2 Mbit / s from the bank for putting it back into the flow of 140 Mbps, the implementation of such a step input \ conclusion have to spend quite a complex operation the three-level demultiplexing ("bark") PDH signal removing \ adding leveling bit (all three levels) and his subsequent three-level multiplexing ("stitching") adding new leveling bits. the alarm. However, these tools are too weak.
If you have many users that require input \ output source (of 2 Mbit \ s) for the instrumental implementation of network flows require an excessively large number of multiplexers, as a result of operation of the network becomes economically advantageous.
Another bottleneck technology PDH - weak capacity in the organization of channels for purposes of control and flow control in the network and the almost complete absence of routing funds grassroots multiplexed streams, which is essential for use in data networks. Typically, for the purposes of identification and subsequent signaling flow is divided into groups of time slots or frames, which are then assembled in groups of several frames or multiframe. The latter, allowing the identification of the receiving side of the individual frames are supplied with additional bits of cyclic error-correcting codes and used PDH frames and multiframes increases (with an increase in the number of multiplexing and switching streams for routing), the possibility of error in tracking the "history" of current switching, and hence increases and the possibility of "losing" information not only about the current switching, but also about his "history" in general, which leads to disruption of routing all traffic scheme. So it would seem essential advantage of the method - a small "congestion headings" (Recommendation G.704 does not provide for the necessary for normal routing headers) - in fact, turns into another serious shortcoming as soon as there is a need for advanced routing, network PDH caused by the use of data.
And so, a number of shortcomings PDH:
-hard input / output digital streams at intermediate points;
-no funds network of automatic monitoring and control;
-multistage restore the presence of three different hierarchies.
2.2.3 SDH Standard
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These shortcomings RDN and a number of other factors led to the development in the United States is still one of the hierarchy - the hierarchy of SONET synchronous optical network, and in Europe Analytical SDH synchronous digital hierarchy, proposed for use in the fiber optic link. But due to unfortunate selected baud rate for STS-1, the decision was made - to abandon the creation of the SONET, and to create an it-based SONET / SDH at a rate of 51.84 Mbit / s of the first level of the SDH OC1. As a result, the NEO SONET / SDH STM-1 corresponds to the SDH hierarchy. Transmission speed SDH hierarchy presented in Table 3.1.
Table 3.1 - SDH hierarchy bit rates
| SDH level | Transmission speed Mb / s |
| ST-1 | 155,520 |
| ST-4 | 622,080 |
| ST-16 | 2487,320 |
SDH - Synchronous Digital Hierarchy. SDH - is a set of digital structures, standardized for the purpose of transporting it necessary to adapt the load on the physical network. SDH - calculated as a transport signal existing PDH, with the speed specified in recommendation G.703 as the signals of new broadband services. At the same time significantly increase the reliability and survivability of networks, their flexibility, quality of communication, Linear SDH signals are arranged in a so-called synchronous transport modules STM. The first of these STM - 1 corresponds to the speed of 155 Mbit / s. Each subsequent rate is four times greater than the previous one and formed by a synchronous byte multiplexing. Already standardized STM - 4 (622 Mbit / s) and STM - 16 (2.5 Gbit / s).
In the SDH network using the principle of container traffic to be pre-arranged transportation of signals in standard containers C. All operations are performed with the containers, regardless of their content. This ensures the transparency of the SDH network, it is possible to transport a variety of SDH signals, streams of ATM cells, or any new signals. There are four levels of containers. All of them, together with SDH signals placed in them are listed in Table 3.
Table 3.2- SDH containers four levels
| Level | Container | SDH signal Mbit/s |
| C-11, C-12 C-2 C-3 C-4 | 1,5; 2 34 and 45 |
An important feature of SDH network is to divide it into three functional layers, which are subdivided into sublayers (Table 3.3). Each layer maintains the overlying layer and has a certain access point. Layers have their own control and management tools, simplifies operation on liquidation of consequences of failures and reduce their impact on the overlying layer. The independence of the layers enables you to implement, upgrade or replace them without affecting other layers.
Table 3.3 - SDH division into functional layers
| Layer | Sublayer |
| Channels Paths | Lower order |
| Higher-order | |
| Transfer funds | Multiplex Section Regeneration |
| Physical Environment: Fiber optic, radio relay link, copper cable |
The most important for the following discussion are network layers: channels, paths and sections, a network of canals - a layer that serves the user's own. These terminals are connected to the sets of SDH terminal equipment trunks. channel network connects different sets SDH terminal equipment through the switching station.
Channel groups are combined in different orders group paths, forming a network of paths. There are two paths of network layers (from top to bottom over SDH hierarchy) of the lower and higher order. The layer paths provide software and remote monitoring and control connections. All the paths end at operational switching equipment, part of the SDH multiplexers.
Group paths are organized in line, the construction of which depends on the transmission medium (OM, RRL). This network transmission layer. It is divided into two parts: a layer sections (upper) layer and a physical medium. The network layer is divided into two sections. The top layer is a multiplex section (MS). MS - provides transfer of information between the points where terminate or switch paths. The bottom layer of regeneration section (RS) - provides the transfer of information between the regenerators and the end points or switching paths. New digital hierarchy was conceived as a high-speed.
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Application of SDH-technology:
- Transportation of data streams in the ATM networks. This SDH equipment transmits the signal over long distances, carries ATM Cross Connect - streams and allows you to organize the ATM network with complex topology, even for a linear arrangement of ATM switches;
- The transfer of a large number of E1. Primary E1 digital circuit 32 combines the channel DCO (main digital channel), one of which is used to DCO frame synchronization, another - for signaling. This stream frame consists of 32 time slots of 8 bits each. Frame repetition frequency 8 kHz, which gives the flow rate of 32 x 8 x 8 = 2048 kbit / s.
- Creation of fail-safe transport networks with a fast recovery time performance (on this indicator SDH significantly superior to other technologies).
With the spread of SDH - technology, when combined networks of various operators sharply there is a problem of global synchronization of nodes, and this point should not be underestimated.
The trend of recent years - the replacement of currently existing PDH SDH networking systems, as well as the use of not only the operators of technology, but also for the construction of highways corporate information systems.
Working documents
