Organization of access to the networks of satellite operators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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The territory of Russia is characterized by an underdeveloped infrastructure of digital communication channels, especially land lines. To date, the most common, and sometimes the only means of organizing communications with high quality is satellite communications. More than 200 earth stations operating with geostationary satellites "Gorizon" have already been deployed on the territory of Russia, which makes it possible to connect users located almost anywhere in Russia. The article discusses how to more efficiently use the bandwidth of intercity and international communication lines to organize access to satellite systems and networks with fixed (dedicated) channels or multiple access with time division channels (Time Division Multiple Access - TDMA). The range of services provided is voice and data transmission.

The Russian market of telecommunication services is growing every year. The number of companies is increasing, the range of offers and types of services is expanding, and prices are dropping. This is especially noticeable in large cities of Russia, where already now the digital networks of one or several companies cover almost the entire territory of the city and anyone can receive the entire range of telecommunications services, from installing a simple telephone to accessing the Internet or other information and financial networks. If you need to connect offices located in different cities or even countries via a dedicated digital channel, or get access to a telecommunications network that is not represented in your city, then you have to contact not only local operators, but also long-distance and international communication operators (MMS ).

Due to the relative development of earth stations on the territory of Russia, the technical possibility of organizing long-distance and international digital channels using satellites is already being created. But the cost of a DS0 channel (64 kbps) in this case will be 4...7 times higher than a channel with the same speed, but within the city. Few potential users can afford such costs. How to reduce costs on a long-distance or international route and thus expand the range of users?

A possible solution is to combine several low-speed user channels in urban terrestrial wired networks into one DS0 channel (64 kbps) with its subsequent transmission to a satellite system. The foregoing will significantly reduce the cost of organizing a channel per user. There are two ways to implement such a scheme:

How is this solved by Golden Line?

The network of Golden Line, which has been operating on the Russian telecommunications market for 5 years, is one of the largest and most extensive transport networks in Moscow. The main task of the network is to provide access to any telecommunications networks and services, in particular, to operators of long-distance and international communications (MMS) from anywhere in Moscow. For this purpose, digital communication channels are organized with speeds from 1,2 kbps to 2,048 Mbps, for connections with circuit switching networks, X.25 and Frame Relay, as well as up to 155 Mbps using ATM technology.

Packet-switched X.25 networks became widespread in the 80s and are still used in many departmental structures. The X.25 standard was developed by the ITU back in 1976 and defines the interface between user terminal equipment and data transmission equipment of a packet switched network. Due to the support of error detection and correction technology, it is ideal for data transmission over poor quality communication lines. The high-speed distributed network of Frame Relay and ATM provides connection of users to MMS operators directly on these technologies. That is why the efficient use of these channels is achieved through dynamic bandwidth allocation. This article does not consider this option for organizing MMC lines, since it is related to the design of packet and frame switching networks and requires a separate presentation. It should be noted that X.25 networks do not support voice transmission due to high latency.

The rapidly developing technology Frame Relay (frame relay) is a modern circuit switching protocol similar to X.25, but using a simplified procedure for establishing and checking the quality of connections. It is designed to operate at higher speeds (up to 34 Mbps) with low latency (see the article by V. Neumann "What is frame relay?" in the journal "Communication: means and methods" No. 3, 4/1998) . For Frame Relay networks, the quality of connections, especially when transmitting voice, depends on many factors. This:

ATM (Asynchronous Digital Hierarchy) is a high-speed technology for transmitting information in cells of a constant size of 53 bytes, which makes it possible to provide services with various types of traffic. ATM allows you to transmit both speech and data on the water channel, that is, to support both constant and variable data rates. ATM is an ideal universal transmission medium that combines all the advantages of leased circuit networks and data transmission networks with dynamic bandwidth allocation.

But global ATM networks in the world are still underdeveloped and too expensive for users. Therefore, this technology is not widely used.

For the category of users who need a constant high-quality connection to transmit voice and data at low speeds, it is proposed to use speech compression and combine several data/speech channels into one channel at a speed of 64 kbps. Here, the channel switching/multiplexing technology is the most reliable and easy to implement, since most MMS networks use the same method of information transmission. The quality of data transmission does not change, but the quality of speech deteriorates slightly.

Let us consider in detail the technology of digital speech compression and the formation of a 64 kbps channel from several low-speed speech and/or data channels.

The equipment used by Golden Line for this task was developed by Newbridge. Its important advantage is that both pulse-code modulation (PCM) speech compression and channel combining are performed by the same device - a module called a digital signal processor (Digital Signal Processor -DSP). Each processor has 6,10, 20, or XNUMX circuits, each circuit can be configured as a Voice Compressor (VC) or a Subrate Merger (SRM). Up to seven DSP modules can be installed in each node I/O multiplexer

The operating principles and standards supported by this equipment are described below.

Voice compression

Like most telecommunications equipment developers, Newbridge supports two methods of speech compression - its own development - HCV (8 and 16 kbps) and the standard one, according to the Recommendations of the Technology Sector - ITU-T. G.728 - LD-CELP 16 kbps and ITU-T G.729 - A-CELP 8 kbps. Algorithms developed by Newbridge allow the most flexible and efficient use of 64 kbit/s bandwidth. But at the same time, the compression / decompression procedure can only be carried out on Newbridge equipment. Using standard speech conversion algorithms, any equipment that supports these methods can be selected for decompression. The quality of compressed speech at 16 kbps is equivalent to that at 32 kbps when using adaptive differential pulse code modulation (ADCM) used in long distance telephony. And at 8 kbps, compression supports toll-quality speech.

Thus, when using compression at a rate of 8 kbps, a channel at a rate of 64 kbps can contain up to eight speech channels, and at 16 kbps - up to four speech channels. The compressed channels are packed into a 64 kbit/s channel using the "transparent" rate adaptation method, which does not require information about frame synchronization and signaling. The 64 kbit/s channel consists of eight elements at 8 kbit/s, labeled B7 to BO. Compressed speech at 8 and 16 kbps is placed in the appropriate number of elements. The telephone signaling is transmitted inside the user information. This method of signaling is called "in-band". On fig. 1 shows the scheme of combining channels and their placement in a 64 kbps aggregate channel.

Aggregation of low-speed data channels

Combining low-speed data channels with speeds from 1,2 to 19,2 kbps occurs according to the same principles and on the same equipment as the combination of voice channels. The data is fed directly to the SRM low-speed channel combiner, where information from several users is grouped into one 64 kbit / s channel.

Newbridge offers two methods of link aggregation:

X.50 is a European multiplexing standard developed in accordance with ITU-T Recommendations that describes a mechanism for combining several synchronous low-speed channels into one 64 kbps channel. The standard is adopted to define the interface between public data networks on the international site.

DDS is a North American standard similar to X.50 developed by AT&T and supports multiplexing of synchronous and asynchronous data channels. The table shows the number of low-rate channels that can be transmitted on a 64 kbit/s channel in one 2,048 Mbit/s frame cycle (interface described in ITU-T Rec. G.703).

When compared, it is obvious that the use of the HCM multiplexing method is much more efficient than the others.

The organization scheme of multiplexing is shown in fig. 2.

Organization of access to MMS operators

The above methods for efficient use of the bandwidth of the 64 kbps channel allow developing various schemes and projects for connecting users to satellite communication networks, while multiplexing methods in the 64 kbps channel can combine both speech and data.

On fig. 3 shows a diagram of the implementation of the connection of users to the ground node of satellite communications.

The interface between the I/O multiplexer and the satellite system can be any one of those described in ITU-T Recs V.24, X.21, V.35, or G.703. In the case of using the G.703 standard, it is possible to connect up to 30 aggregate channels at a speed of 64 kbps in one stream of 2,048 Mbps. These interfaces are widely used in private and public data transmission networks with time division channels and are available in almost any telecommunications equipment - switches, multiplexers and routers. The satellite system or modem must have modules for connecting to data terminal equipment with the above interfaces. Such systems can be, for example, a user satellite station VSAT-NEXTAR from NEC or low-speed satellite modems SDM-100 from EFData and DMD2401 from Radyne.

In conclusion, we note that the efficient use of the bandwidth of MMS channels could be useful for many satellite operators in order to reduce tariffs for services and thereby attract additional customers. A similar scheme has been implemented and is successfully operating in a joint project between British Telecom and Golden Line to provide voice channels with compression at speeds of 8 and 16 kbit/s to Moscow banks to enter the London Stock Exchange. The use of voice and data transmission technology in one or two 64 kbps channels seems to be the best solution for organizing corporate networks of companies with representative offices in different cities and countries.

Author: S. Laryushkin, Moscow

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