Types of telecommunications transport network services. Classification of transport networks

A modern transport network must ensure cost-effective aggregation of any client traffic and its reliable, high-quality transmission over communication channels. This can be achieved using a variety of transport technologies, many of which have been recently developed.

Widespread TDM technologies, based mainly on the principles of the synchronous SDH hierarchy (STM-N, VC-n, etc.), are currently being replaced by:

At the electrical level - Carrier Ethernet technologies (E/FE, GE, 10GE, 40GE and 100GE interfaces) and MPLS-Transport Profile. These technologies will provide ample opportunities to create connection-oriented carrier-class packet switched transport networks;

At the photonic level - technologies of the optical transport hierarchy OTH/OTN, similar to SDH, but unlike it, providing transparency of transmission and cross-connection of a set of TDM and packet traffic in any combination with their further transmission over the channels of wavelength-division systems optical radiation (systems with spectral multiplexing of channels) - WDM.

IP/MPLS service networks can provide services by interconnecting with the core network systems of fixed and mobile communications, with service provider points of presence, as well as with broadband access systems directly or on top of a carrier-class transport network. Packet switches with Carrier Ethernet/T-MPLS & MPLS-TP functionality are becoming an important element of the transport layer of the network, interoperating on top of existing NG SDH/MSPP networks and/or transparent and flexible OTN/WDM photonic layer. The flexible automated WDM photonic layer is equipped with software tunable and reconfigurable T&ROADM optical input/output nodes. These and other solutions, including the use of intelligent transport technologies ASON/GMPLS (Intelligent Optical Core), must be scalable in performance and open to modernization.

Convergence of transport solutions and Ethernet technologies: evolution to 40G and 100G

The IP transformation processes have stimulated research into increasing the capacity of transport networks for both traditional (TDM) and packet traffic.

For existing SDH synchronous transport hierarchy systems, transmission rates from STM-1 (155 Mbit/s) to STM-256 (40 Gbit/s), increasing from level to level with a factor of 4, are standardized. For optical transport hierarchy systems, transmission rates from OTU are standardized -1 (2.5/2.7 Gbit/s) to OTU-3 (40/43 Gbit/s), which also increase from level to level with a factor of 4. The Ethernet transmission speed (interfaces) grew with a factor of 10 and reached today 100 Gbit/s. The convergence of these technologies began with 10G transmission speeds. Research recent years have shown that this convergence is evolving towards 40G and 100G speeds. Current standardization supports this convergence and paves the way for the next generation of networks.

Initially proposed for data collection and processing centers, as well as for corporate computer networks, 40GE systems will, in all likelihood, be widely used at the level of transport networks with the introduction of a factor of 4, unusual for Ethernet technology (40GE relative to 10GE). At the backbone level of networks, transmission speeds of 100GE/OTN will be implemented with a coefficient of 2.5, unusual for transport networks, in relation to the 40GE/OTN level being implemented today.

Satisfying the requirements set by service providers is impossible without mastering data transfer rates in the range of up to 100 Gbit/s and higher.

Standards are currently being developed for new 40G and 100G protocols and interfaces. Back in July 2006 working group IEEE 802.3 WG created special group High Speed ​​Study Group (HSSG), which approved two MAC (Media Access Control) transmission rates a year later:

40GE for applications related to the interaction of servers (server-to-server), as well as servers and packet switches (server-to-switch);

100GE for switch-to-switch applications, including point-to-point connections between network clusters, etc.

The main efforts are aimed at selecting new technologies and solutions, including new line coding methods, which will allow the most efficient transmission of high-speed digital streams of 40 Gbit/s and 100 Gbit/s over the channels of WDM systems, which today operate mainly at speeds no higher than 10 Gbit/s (based on each optical channel).

To increase the transmission range of 40 Gbit/s and 100 Gbit/s streams over WDM system channels, multi-level linear codes(QAM, etc.), advanced error correction codes (SFEC), and coherent reception methods instead of differential signal detection. New methods are the future, but in the initial stages, 100-gigabit systems will be implemented with certain restrictions on transmission range on WDM systems already operating at the 10 Gbit/s level.

Transport solutions OTN/OTH

The Optical Transport Hierarchy (OTH), as defined in ITU Recommendations G.798 & G.709, provides methods for hosting, multiplexing and managing networks supporting various client signals in their native format, regardless of the types of protocols used. The standard describes a single Optical Data Unit (ODU)/Digital wrapper structure in which multiple existing data stream frames can be placed and then combined with other signals and further transmitted and controlled in uniform style with uniform functionality similar to that adopted in SDH systems.

The first version of OTH was focused primarily on SDH client signals. Therefore, initially only 3 fixed types of ODU containers were defined in the G.709 recommendation:

ODU 1 for CBR 2G 5 (STM -16);

ODU 2 for CBR 10G (STM -64);

ODU3 for CBR40G (STM-256).

OTH structures are currently being considered to include client signaling such as

Ethernet 1GE, 10GE WAN/LAN, 40GE, 100GE;

OTH 2.5G, 10G, 40G, 100G;

SDH 2.5G, 10G, 40G;

FC 1G, 2G, 4G, 8G (10G).

OTN technology is the ideal remedy to create converged transport platforms that provide transparency when transmitting traffic related to any services over the optical channels of WDM systems, since it has its own separate header, similar to the header in SDH and making it possible to control and manage the network. Therefore, transparent joint transmission of a set of asynchronous (packet) and synchronous (TDM) traffic in any combination is supported.

In addition, OTN systems:

Very effective when supporting asynchronous packet-oriented services such as GE, 10GE, various levels Fiber Channel (FC), ESCON & FICON, without own funds monitoring at the physical level;

Allows you to detect and localize failures in the WDM network, significantly increasing the quality of the services provided;

They are the only technology that can carry the widespread 10GE LAN PHY client signals over IP/Ethernet;

They provide joint transmission of synchronous and asynchronous signals over one optical lambda channel of the WDM system.

It should, however, be noted that the standardization of OTN is not complete, in particular the algorithm for placing GE, FC and Video has not yet been fully developed, transparent placement of 10GE is specified in parallel in several different standards, for grouping and switching signals with transmission rates below 2.5 Gbit /c SDH systems are still used in practice. However, standardization continues, including the ODU4/100GE layer and the ODUflex layer for signals with rates lower than ODU-1 (sub-lambda channels).

OTN technology has every chance of becoming in the future a universal transparent electrical layer of optical backbone communication networks, expanding the well-established OAM methods in TDM/SDH to packet interfaces Ethernet type(including 10GE LAN PHY), FC, ESCON, Digital Video etc.

The role of ROADM at the photonic layer of the transport network

Reconfigurable optical add/drop multiplexers (ROADMs) simplify the planning and maintenance of DWDM networks by enabling the automation, with minimal human intervention, of adding, removing or rerouting optical channels. In existing networks, these processes are still carried out manually, requiring significant effort to adapt equipment and switch traffic, and require highly qualified personnel. The basis of ROADM was optical devices a new class, namely Wavelength Selective Switches (WSS) with one input (group signal) and many outputs for groups and/or individual channels, or with many inputs for groups and/or individual channels and one output.

It should be noted that if input, output, or channel rerouting/switching to another transmission direction is performed at a node, then all connections between network nodes, including transit connections through the node at the photonic level, must maintain a delicate balance between the parameters of individual optical channels (wavelengths) to achieve optimal parameters in the transmission system as a whole. Therefore, ROADM has the function of dynamically balancing the optical power levels of different optical channels.

As soon as transponders became available in WDM systems with the ability to adjust the wavelength of radiation throughout the C-band in accordance with a frequency grid with a step of 100 GHz and 50 GHz (up to 80-96 wavelengths of optical radiation in the C-band), a new one was discovered in ROADM limiting factor. Optical channels were output to fixed ROADM ports corresponding to a specific optical wavelength. Therefore, despite the flexibility of transponders, it was not possible to avoid manual operations to switch the channel to new directions.

As a result of the research, to prevent blocking of the optical channel, a colorless ROADM device was proposed, in which any user port can be used to organize a channel with any wavelength of optical radiation. At the next stage, directionless ROADMs were applied, in which an optical signal at any wavelength can be addressed to any port in any transmission direction. Input/output of the corresponding channel in any direction is carried out automatically, without disturbing the balance in the remaining optical channels transmitted through the node. This concept in Alcatel-Lucent network solutions is called Zero Touch Photonic (ZTP) - a network rebuilt through a control system, i.e. without “manual” intervention of personnel at the nodes (Fig. 1).

The presence of colorless & directionless T&ROADM systems in WDM network nodes is prerequisite implementation of ASON/GMPLS functionality at the photonic level of the network.

Intelligent transport solutions ASON/GMPLS

Next-generation networks must be more dynamic, ensure efficient use of resources and provide high levels of reliability and quality of on-demand services. In other words, it is necessary to ensure dynamic provision of network resources (the necessary bandwidth) to deliver any services at any time to any user. This is why the IETF extended the MPLS signaling and routing protocols beyond the IP network, and on this basis the General MultiProtocol Label Switching (GMPLS) protocol was developed.

GMPLS functionality with a distributed Control Plane separated from the Data Plaine was the next step in the evolution of MPLS technologies for use in transport networks. ITU-T has taken a deeper look at the networking aspects of this functionality in a series of recommendations for Automatically Switched Optical Network (ASON). OIF has completed the standardization of network interfaces. UNI user interfaces are used to access the ASON network to request services, monitor connections, ensure QoS in accordance with SLAs, collect failure messages, etc. NNI network interfaces are designed for communication between network elements, network domains and different networks. At this level, within the Control Plane, requests for connections are processed, organized and controlled, a certain amount of information is exchanged about available resources in network elements and connections, services are routed between network domains, etc.

One of the main advantages of an intelligent transport network with ASON functionality is the ability, at the request of users or a request from a centralized network management system, to automatically install:

Connections within a network built on equipment from one supplier;

End-to-end connections on a network built not only on equipment from different vendors, but also using different functional and connection-oriented technology layers, such as SONET/SDH (VC-N), WDM/OTN (OCH, ODU), T-MPLS /MPLS-TP (LSP, PW3), etc.

To implement ASON/GMPLS at the photonic level, T&ROADM systems are installed in WDM network nodes, providing switching of optical channels without additional O-E-O conversion. If T&ROADM systems have a connectivity coefficient N of up to 6-10 (the number of directions to which an optical channel can be switched from one network node at the photonic level), then in this case there is no need to keep up to 50% of the network capacity free to implement protective mechanisms with full redundancy channels like O-SNCP, OCP, etc. It is enough to have 10-25% of distributed free capacity on connections in the network to provide the ability to bypass affected areas based on ASON/GMPLS.

The same nodes can host automatic path switching systems that operate in accordance with the OTH/OTN standard at the electrical level and provide transparent data switching at the ODU and/or sub-lambda channel level (ODUflex), including GE, 10/100 Ethernet, Fiber Channel, FICON/ESCON, SONET/SDH, etc. ASON/GMPLS technology can also be implemented at the OTH/OTN network level (Fig. 2).

ASON/GMPLS functionality at the SDH layer has already been implemented on many networks. Similar functionality at the photonic level, providing network failures automatic recovery(without intervention of the control system operator in this process) optical lambda channels, implemented in 1626LM equipment and will begin to be implemented on operator networks in 2010 x

Category: .

The transport telecommunications network is the main part of the network infrastructure of any operator, be it a traditional telephony operator, a cellular operator, a wireline or wireless access to the Internet.

Modern transport telecommunication networks must be universal, i.e. are able to effectively support both 2G and 2.5G systems in use today, focused on transmitting traffic in TDM mode, and next generation networks - 3G and even 4G. The quality of the services provided completely depends on the quality of the transport telecommunications network. That is why, when choosing technology and education for building this section of infrastructure, operators are especially careful, attentive and picky. For example, if UMTS Release 99 systems are focused on transport based on ATM technology, then subsequent developments of UMTS Revision 5/6 are focused on IP solutions using Ethernet networks and MPLS technology. Therefore, the equipment of transport telecommunication networks must provide efficient transfer all types of traffic - TDM, ATM, IP.

The main methods of organizing transport telecommunication networks are fiber optic, satellite and wireless systems communications. The latter include radio relay systems, which are widely used in transport telecommunications networks of operators cellular communication and broadband access.

The transport telecommunications network of the mobile operator consists of two main segments (Fig. 1):

Distribution network (backhaul), connecting base stations with controllers and mobile switching centers (Mobile Switching Center (MSC));
backbone network providing high-speed transport between mobile switching centers.

Traditionally, the distribution network was built according to a “star” topology: in the center there was an MSC, and radio access systems (controller and base stations) were connected to it via a dedicated channel (usually E1 or NE1). If base stations are located in hard-to-reach areas, then they are often used to connect radio relay lines communications or satellite channels.

Cellular operators do not always have own channels between base stations, controllers and MSCs, they are often rented. Therefore, their desire to maximally load the rented capacities is understandable. However, it is necessary to take into account possible peak loads. The task arises of finding a compromise between the cost of renting channels and the quality of subscriber service during periods peak loads. It is difficult to solve using traditional circuit-switched (TDM) technologies.

Some mobile communication technologies inherently provide efficient use of channel resources, while others do not. For example, when transmitting regular GSM traffic additional procedures compression can be beneficial, but the traffic of CDMA systems on the E1 Frame Relay interfaces between the base station controllers and the MSC is already quite tightly packed.

Transport networks under construction must be universal, i.e. capable of effectively supporting both 2G and 2.5G systems in use today, focused on transmitting traffic in TDM mode, and next-generation networks.

The optimal transport telecommunications network of mobile operators must meet a number of criteria:
ensuring painless implementation of new mobile communication systems;
compliance with the requirements of next generation network architectures, in particular IMS;
preservation of investments;
availability effective means traffic management;
guarantees that the quality of communication services will not decrease, but rather increase;
providing convenient means maintenance and operation.

One of the ways to build an effective distribution network is to install multiservice edge devices at the radio network nodes (base stations and controllers) and at the MSC center, which package traffic into packets that optimize it for further transmission over the network. This approach will allow, on the basis of a single converged transport network, to support various equipment of radio segments: GSM (TDM), GPRS (TDM), CDMA 1 x EV-DO, UMTS (ATM), etc. Instead of many partially filled E1 streams, the operator will receive a relatively small number of channels , “densely” filled with bags, while QoS mechanisms guarantee high quality voice communication. Moreover, due to the efficient use of channel resources, operators will be able to connect new base stations using existing communication channels.

If there are fiber-optic lines in close proximity to the nodes where base stations, controllers and MSCs are located, then E1 streams can be multiplexed for transmission over the SDH network. The advantages of such networks are associated primarily with high reliability provided by ring protection circuits and developed means of operating support. Biggest savings is ensured when mobile network equipment is connected to an already existing network SDH, through which a variety of types of load can be transmitted: mobile telephony traffic, networks fixed line, video information, TV channels, etc.

The mobile backhaul network provides connections between a mobile base station (RBS) and a cellular switch at the edge of the backhaul network. Large operators mobile communications divide the transport channel architecture into two components (Fig. 2) - a grassroots radio access network (LRAN) and a radio access network high level(HRAN).

When installing radio equipment with Ethernet interfaces, it can also be connected to the SDH network. For this there are special technical solutions Ethernet over SDH, implemented, in particular, in SDH equipment Metropolis from Lucent-Alcatel. To improve the efficiency of transmitting Ethernet traffic over SDH networks, SDH has now been developed and standardized a whole series technologies: universal framing scheme (General Framing Concatenation, G.707), algorithms for adjusting the communication line capacity (Link Capactivy Adjustment Scheme, G.7024). Equipment supporting the mentioned technologies is optimized for building multiservice networks and is classified as next generation SDH systems (NG-SDH).

DISCIPLINE TUTORIAL

"TRANSPORT NETWORKS"

For students of specialty 210709

"Multichannel telecommunication systems"

Developed by KHIIK teacher Nekrasova EM

Khabarovsk 2014

References

1 Olifer V.G. Olifer N.A. Fundamentals of computer networks, textbook, St. Petersburg: Peter, 2009.

2 A.V. Roslyakov, M.Yu. Samsonov, I.V. Shibaeva. IP-Telephony - M.: Eco-trends, 2003.

3 S.V. Zapechnikov, N.G. Mioslavskaya, A.I. Tolstoy's basics of building virtual private networks, textbook. – Hotline– Telecom, 2003

4 Filimonov A.Yu. Construction of multiservice Ethernet networks - St. Petersburg: BHV - St. Petersburg, 2007

5 Baklanov I.G. “NGN: principles of construction and organization”, - M.: ECO-TRENDS, 2008

6 B.S. Goldstein A.B. Goldstein. "SOFTSWITCH" "BHV - St. Petersburg" 2006

7 Olifer V.G. Olifer N.A. Computer networks. Principles, technologies, protocols.

4th edition – St. Petersburg: Peter, 2010, 944 pp.

7 Goldshtein B.S., Pinchuk A.V., Sukhovitsky A.L. IP Telephony. - M.: Radio and communication, 2001c

Classification of transport networks. Review of technologies for the transport network (TN)

“The mind is not only about knowledge,

but also in the ability to apply knowledge in practice"

Aristotle.

First there was the word. The word contained some information intended to be transmitted from person to person. And then, gradually, people began to realize that for normal information exchange communications are necessary - from pigeon mail and camel caravans to telephones, computers and fiber optic highways. What has happened in the world of telecommunications today can be classified more as a revolution than an evolution, such is the difference between what the telephone was yesterday and the increased distribution of information and influence of the Internet today. Existing today telephone network public use(PSTN) and, along with it, circuit switching technology itself is at the stage of extinction. In its place is a packet-switched network that will serve the transmission of voice, video and data. The process of informatization is gaining momentum all over the world. In modern global world The level of informatization ensures the competitiveness and security of the country.

Just 10 years ago, any communication technology could have existed for 20-30 years. Now many technologies “die” within 1-2 years, because communication equipment is very much subject to obsolescence (that is, the equipment can still function, but it will no longer meet modern trends and requirements). And new equipment installed at stations requires qualified workers, so specialists working with new technologies need to continuously improve their knowledge and improve their skills.

The upcoming transition of the Internet to the more efficient IPv6 protocol will help implement more complex algorithms for servicing subscribers and even build the “Internet of Things”, when toothbrushes, refrigerators, and cars will have access to the network, and many sensors will be combined into self-organizing networks. And the number of “users” along the “machine-tomachine” (or M2M) line will number tens of billions of devices.

Telecom operators need to move from the consumer and try to generate services that are really very important for him, even with his participation. And we will all be happy. After all, happiness is like a butterfly - the harder you catch it, the more successfully it escapes. But if you shift your attention to other things, it will come and sit quietly on your shoulder.

It was digression. Now let’s look at the cover of this tutorial, which contains a drawing illustrating the concepts of “transport network” and “access network”.

Transport network is a set of network elements that provide traffic transmission. Transport is that part of the communication network that performs the functions of transferring (transporting) message flows from their sources from one access network to message recipients of another access network.

Access network is a set of network elements that provide subscribers with access to transport network resources in order to receive services. The access network connects the source (receiver) of messages with the access node, which is the boundary between the access network and the transport network.

From the picture on the cover of the manual it is clear that main technologies of a modern transport network are: WDM, NGSDH (new generation SDH), MPLS and, of course, 10GE.

A modern access network currently uses a huge number of various technologies, For example: various types DSL (ADSL, HDSL, VDSL); various types of optical access (FTTH - optics to an apartment, FTTB - optics to a building, FTTC - optics to a street cabinet); various types of radio access (Wi-Fi, WiMAX, LTE), MetroEthernet, GPON, etc.

Based on the type of subscriber terminals connected, BSS networks are divided into:

fixed line networks, providing connection of fixed subscriber terminals;

mobile networks, providing the connection of mobile (transportable or portable) subscriber terminals.

In addition, according to the method of organizing channels, networks are traditionally divided into primary and secondary (Figure 1.1).

Primary network is a set of channels and transmission paths formed by the equipment of nodes and transmission lines (or physical circuits) connecting these nodes. The primary network provides transmission channels (physical circuits) for secondary networks to form communication links.

Secondary network is a set of communication channels formed on the basis of the primary network by routing and switching in switching nodes and organizing communication between subscriber devices users.

Figure 1.1 – Telecommunication system structure

The construction of a classical telecommunication system is based on a primary network, which includes a signal propagation medium and signal transmission equipment that ensures the creation of standard channels and paths of the primary network. The primary network can be built on the basis of analogue transmission systems (ATS) or on the basis digital systems transmissions (PDH, SDH).

Typical channels and paths of the primary network are used by various secondary networks: telephony networks, data networks, radio communications, television, cellular networks.

It is very important to understand the classification of communication networks by territorial division :

mainline is a network connecting the nodes of the centers of the constituent entities of the Russian Federation. The backbone network ensures the transit of message flows between zone networks;

zonal(or regional) are communication networks formed within the territory of one or several constituent entities of the Russian Federation (regions);

local– these are communication networks formed within an administrative or otherwise defined territory and not related to regional communication networks. Local networks are divided into urban and rural;

international is a public network connected to communication networks of foreign countries.

IP telephony

Abbreviation VoIP (Voice Over Internet Protocol stands for Voice over Internet Protocol. Origins VoIP technologies are located back in 1876, when the American Alexander Bell carried out the first phone call and patented what he invented “ talking telegraph“This device did not have a bell, and the subscriber was called through the handset using a whistle. The emergence of VoIP dates back to 1995, when a small Israeli company VocalTec released the first program for Internet telephony. The program was called Internet Phone and was intended for calls from a home computer.

In networks based on the IP protocol, all data - voice, text, video - is transmitted in the form of packets. Any computer and terminal on such a network has its own unique IP address, and transmitted packets are routed to the recipient in accordance with this address indicated in the header. Data can be transferred simultaneously between many users on the same line. When problems arise, IP networks can change the route to bypass the faulty sections. In this case, the IP protocol does not require a dedicated channel for signaling.

Figure 2.1 – Packet switched network connection

Analog signal comes from the subscriber to the IP telephony gateway .

The following happens in the gateway:: At the first stage, voice digitization is carried out. The digitized data is then analyzed and processed to reduce the physical volume of data transmitted to the recipient. As a rule, at this stage, unnecessary pauses and background noise are suppressed, as well as compression. At the next stage, the received data sequence is divided into packets and protocol information is added to it - the recipient's address, the sequence number of the packet in case they are not delivered sequentially, and additional data for error correction. In this case, the necessary amount of data is temporarily accumulated to form a packet before it is directly sent to the network.

Extracting transmitted voice information from received packets happens in receiving gateway also in several stages. First, their ordinal sequence is checked. Since IP networks do not guarantee delivery time, packets with higher serial numbers may arrive earlier, moreover, the receiving time interval may also fluctuate.

To restore the original sequence and synchronization, packets are temporarily accumulated. However, some packets may be completely lost during delivery, or the delay in their delivery exceeds the acceptable variation. Under normal conditions, the receiving terminal requests retransmission of erroneous or lost data. But voice transmission is too critical in terms of delivery time, so in this case either an approximation algorithm is turned on, which makes it possible to approximately restore the lost ones based on the received packets, or these losses are simply ignored, and the gaps are filled with data randomly.

The data sequence thus obtained is decompressed and converted directly into an audio signal that carries voice information to the recipient.

Thus, with a high degree of probability, the received information does not correspond to the original (distorted) and is delayed (processing on the transmitting and receiving sides requires intermediate accumulation). However, within certain limits, the redundancy of voice information makes it possible to tolerate such losses.

Currently in IP telephony there is two main ways transmission of voice packets over an IP network:

1) through global network Internet (Internet telephony);

Will the operator be able to launch new services in the existing transport network, will it cope with the transmission of high-speed multimedia data traffic?

Operator concerns

With the transition to UMTS technology, the direct and return channels traffic transmission increases significantly.

Changes in the structure of transmitted traffic are also obvious. Until now, voice traffic prevails in mobile networks, but with the transition to 3G, the role of data services will increase, and their contribution to the total volume of traffic will increase significantly. At some point, IP traffic will become dominant, especially given the overall migration of voice from circuit to packet switching.

Instant abandonment of traditional technologies and transition to IP is impossible, and therefore the transport environment of the mobile operator must ensure gradual migration. The ability to transmit traffic over traditional protocols (TDM, ATM and FR) over an IP network using PWE3 (Pseudo Wire Emulation End-to-End) technology makes the IP environment universal in terms of supporting second and third generation services.

In general, two main segments can be distinguished in the transport network of a mobile operator: the backbone transport network and the radio access network (RAN). The principles of building a mobile operator's backbone network have their own characteristics, but in general they coincide with the principles of building other backbone networks.

The situation with the development of RAN transport networks is different. In second generation networks, operators use mobile low-speed TDM channels to connect base stations and controllers. Initially, they were forced to lease most of the channels from fixed and long distance communication, but now the situation is improving. Many cellular companies have their own optical infrastructure SDH/PDH, radio relay equipment and reduce the number of leased channels. As a result, operating costs for maintaining the network are reduced. However, few operators think of IP technology as possible way solving problems associated with expanding the RAN transport network, but it is the construction of IP-RAN that allows solving many problems of modernizing the access level.

As already noted, new services require increased bandwidth. If previously the capacity of a dedicated channel of 2 Mbit/s (E1) was sufficient to transmit traffic from the base station to the controller, then 3G BSs require four E1 channels. In the near future, base stations will need a bandwidth of 14.4 Mbit/s, and this is not the limit. To connect one BS, you will need a whole “bundle” of E1 channels, which is inconvenient and has a number of limitations.

Using IP as a transport medium makes it possible to easily obtain a bandwidth of 100 or 1 thousand Mbit/s, which is many times greater than the capacity of E1 channels.

Typical IP-RAN construction scenarios

Depending on the types of equipment used and the characteristics of transport networks, IP-RAN design options vary. We will look at different scenarios one by one.

The first scenario is typical for all second-generation operators planning the transition to 3G: this is the transmission of 2G BS traffic over Ethernet channels. Traditionally, second-generation mobile operator base stations are connected to controllers via TDM channels, through which both voice packets and signaling traffic are transmitted, as well as an equally important synchronization signal for coordinating the operation of all base stations and controllers. The advantage of TDM over Ethernet in mobile networks was that the latter could not synchronize the operation of the equipment. However, with the development of IP technologies, the problem was solved. Several technologies are now available to solve the problem of transmitting a clock signal over an IP network, for example, adaptive clock recovery technologies, synchronous Ethernet, etc. Consequently, the considered scenario for creating an IP-RAN network can be completely implemented based on Ethernet.

The second scenario is also typical for second generation networks, where most of traffic consists of voice information. When two people are talking, one of them, as a rule, speaks, and the second listens, therefore, when using TDM technologies, the channels are at least half loaded with uninformative traffic, that is, silence. All uninformative packets can be detected on devices accessing the IP network and discarded as unnecessary. Before being sent to the network, information packets can be optimized on the access device using a principle similar to file archiving. All this makes it possible to significantly reduce the volume of traffic transmitted from the base station and the need for bandwidth, reduce the volume of transmitted information and operating costs for maintaining the transport network.

The third scenario is typical when there are base stations supporting ATM technology. In this case, access devices must support the ATM IMA standard for connecting base stations and PWE3 technology for organizing virtual ATM channels over an IP network. In terms of the methods of organizing virtual channels and transmitting a clock signal, the third scenario is similar to the first.

The fourth scenario is typical for European mobile operators, which previously relied on well-developed transport ATM networks and could not immediately refuse their further use. In European 3G networks, there is a division of traffic across different transmission media. Thus, voice traffic and clock signals are traditionally transmitted through an ATM network, which guarantees high quality of service. A additional traffic services that are not critical to the quality of service are sent over the new IP transport infrastructure. This does not mean that European companies do not trust IP technologies to transmit key traffic, but only indicates that they are trying to relieve the network as much as possible with a minimum of additional investments. Ethernet channels and copper DSL lines can be used as IP access channels, which can significantly reduce the cost of building an IP-RAN.

The fifth scenario is used when deploying a new generation IP-based BS. Such base stations can use an aggregated multicast channel consisting of several E1 streams. In this case, when connecting several BSs via microwave or wired channels to one access device, a rational solution is to terminate Multilink PPP sessions on the access device and aggregate IP traffic into a single stream. Traffic from each base station is determined according to its IP address.

The last, sixth, scenario is dictated by the transition of operators to third generation networks. This process will not be immediate, and the dynamics of demand for new services are difficult to predict. Operators continue to receive high incomes from 2G networks and are not going to shut them down, so the operation of second and third generation BS on the same site is not excluded. IN in this case The access device must receive traffic of different types from base stations (IP, TDM, ATM) and ensure its transmission over virtual IP channels. The clock signal is also transmitted over an IP network.

Most of the difficulties in building an IP-based RAN are caused by the need to “tailor” the capabilities of packet technology to the requirements of mobile equipment that originally worked with the TDM and ATM protocols. However, new IP technologies, such as PWE3 or transmission of a clock signal over IP channels, allow operators to build universal multi-service transport networks to provide 2G and 3G services and develop additional services.

Note that Huawei was the first to offer the market base stations connected to an IP network, supporting Ethernet and TDM-over-IP technologies. At the same time, customers are provided not with individual network elements, but comprehensive solutions IP-RAN. Not limited to new base stations, Huawei has released a whole line of CX series equipment supporting TDM, ATM, IP over MPLS traffic technologies and implemented clock signal transmission over IP. The high density of E1, IMA E1, FE ports allows you to connect second and third generation base stations to one CX device. To improve the reliability of the IP-RAN solution, RPR and RRPP reliable ring structure technologies are implemented at the access layer. In cases where building access rings is not possible, CX devices provide network construction tree topology based on STP and RSTP protocols.

Alexey Gordienko ( [email protected]) - Data Communications Equipment Manager at Huawei

Transport communication network is a network that provides the transfer of different types of information using various transmission protocols.

Transport networks can be divided three levels. Networks first level - local or local. They are organized in urban or rural areas. Networks of the second level – regionalor intrazonal. The third level is the global (backbone) network. When building transport networks at different levels, uniformity is maintained in the methods of transporting information, methods of network management and synchronization. The differences in networks of different levels consist only in the hierarchy of speeds used, the architecture of networks (ring, star, linear, etc.), and the power of cross-connection nodes. Fiber-optic transmission lines, radio relay and satellite trunks, and coaxial cables are used as transmission lines in transport networks.

Figure 2.8 shows the structure of a local (city) transport network based on SDH technology.

Rice. 2.8 Structure of the city’s transport network based on technologySDH

For the construction of modern transport and corporate networks of any level, the most widely used are network technologies PDH/PDH, SDH/SDH and ATM. ATM technology, unlike PDH and SDH technologies, covers not only the primary or transport network level, but also combines the levels of secondary networks and access networks with the primary network. In recent years, such technologies such as DWDM, IP over ATM and IP over SDH. Currently, the greatest progress has been made in creating backbone networks based on the above technologies. New technologies for transmitting IP traffic have emerged with unified connections of IP routers using technologies such as WDM, DWDM, SDH and dark fiber as the channel medium. Transport networks use a hierarchy of transmission speeds in accordance with international ITU-T recommendations and the most widely used European standard, which is used on Russian communication networks. PDH technology supports the following levels of digital channel hierarchy: subscriber or main channel E0 (64 kbit/s) and user channels of levels E1 (2.048 Mbit/s), E2 (8.448 Mbit/s), E3 (34.368 Mbit/s), E4 ( 139.264 Mbps). The digital channel level E5 (564.992 Mbit/s) is defined in ITU-T recommendations, but in practice it is not usually used. PDH digital channels are the input (payload) for the user interfaces of SDH networks.

A modern digital primary or transport network, as a rule, is built on the basis of a combination of PDH and SDH equipment. Digital channels of the transport network with a capacity (transmission speed) from 64 kbit/s to 39813.12 Mbit/s are created based on PDH and SDH technologies (Table 8.4.1, Table 8.4.2). PDH and SDH technologies interact with each other through multiplexing and demultiplexing procedures for digital streams E1, E3 and E4 PDH in SDH equipment. Table 8.4.1 shows general characteristics main digital channel E0 and network paths E1, E2, E3 and E4 PDH.

SDH technology compared to PDH has the following features and advantages:

 provides for synchronous transmission and multiplexing, which leads to the need to build network synchronization systems;

 provides direct multiplexing and direct demultiplexing (input-output) of PDH digital streams;

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

 allows you to combine PDH systems of the European and American hierarchy;

 provides full compatibility with PDH, ATM and IP equipment;

 provides multi-level management and self-diagnosis of the transport network.

ATM technology, based on statistical multiplexing of different input signals, was first developed as part of the broadband B-ISDN technology. It is designed for high-speed transmission of heterogeneous traffic: voice, data, video and multimedia, and is focused on using the physical layer of high-speed network technologies such as SDH, FDDI, etc. In ATM technology, the basic values ​​of transmission speeds for access interfaces (user interfaces) correspond to digital channels E1 (2 Mbit/s), E3 (34 Mbit/s), E4 (140 Mbit/s) PDH , ATM (25 Mbit/s), Fast Ethernet, FDDI (100 Mbit/s) and some others. The basic speeds of linear transmission interfaces correspond to the transmission speeds of STM-N digital channels (N=1, 4, 16, 64 (Table 2)) of the SDH system.

ATM technology was the first technology on the basis of which, instead of standard and numerous networks (telephone, telegraph, fax and data networks), it was planned to build a single digital network based on the widespread use of fiber optic lines. However, due to the high cost of ATM equipment and the widespread penetration of the IP protocol into global networks, these plans were not fully implemented. IP technology is the basis of the Internet and is a set of protocols called the TCP/IP protocol stack, and the IP Transmission Control Protocol is called the Internet Protocol. It is he who implements the internetwork exchange. The main advantage is that the TCP/IP protocol stack provides reliable communication between network equipment from different manufacturers. The protocols of the TCP/IP stack describe the format of messages and indicate how errors should be handled, providing a mechanism for transmitting messages over the network, regardless of the type of equipment used. However, during the existence of the TCP/IP protocol stack, weaknesses and shortcomings of the TCP/IP protocol architecture have emerged. In many cases, IP technology cannot meet the demands of new applications. First of all, it must provide higher throughput. However, this is not enough. There is a need to complement IP technology with bandwidth management capabilities that provide applications with the QoS they require.

The development of information and telecommunication technologies is constantly stimulated by the search for opportunities and technologies that can most effectively combine networks, turning them into multi-service broadband and ultra-wideband. Currently, the greatest progress has been made in the creation of global backbone networks based on IP over ATM and IP over SDH technologies. New technologies for transmitting IP traffic have emerged, providing for unified router connections through systems and media, such as WDM, DWDM, and dark fiber. An example of such technology is the one proposed in 1999. Cisco Systems developed the SRP (Spatial Reuse Protocol) protocol, which later became known as DPT (Dynamic Packet Transport). DPT technology embodies the best qualities of such technologies as SDH, FDDI, etc. DPT technology allows you to avoid intermediate protocols of other network technologies, for example, SDH and ATM when transmitting IP traffic over fiber. The main advantages of DPT technology include the following. The use of the SDH format (STM-1 level) allows DPT traffic to be transmitted over SDH networks, thereby ensuring their compatibility. In this case, the trunk paths occupy bandwidth only between the points of transmission and reception of signals, which allows more efficient use of the bandwidth of the ring topology of the DPT network. DPT technologies have developed traffic backup capabilities through the implementation of recovery mechanisms in the ring network topology. The use of the IP protocol allows for end-to-end monitoring of the entire DPT network, from the backbone (transport) to access networks.