Asynchronous Transfer Mode (AT) is a technology, with which the data traffic will transfer into small packages, called, with firm length (53 byte) coded cells and over asynchronous time multiplexing. The cell technology has the advantage compared with transmission techniques with variable package size (for example Ethernet) that the cells can be passed on by so-called cell Relay (similar Frame Relay) more efficiently.

Overview and developing history

ATMs was developed as network standard, which uses the synchronous (PDH, SDH) transportation technologies and them further useful capability characteristics adds. Not only line-obtained data communication is supported by AT, but also like IP, Frame Relay etc. was based contrary to the simple and durable Ethernet or TCP/IP technology, which can lead in load situations to incalculable results, offers AT warranties regarding effective bit rate, Delay and jitter, which is usually called (apart from other characteristics) quality OF service (QoS).

The problem to unite various data streams of different kind of being able to co-operate thus e.g. both with synchronous and package-based networks was solved, as both bit types of current (synchronous or package-been based) at the intersections are converted on a new bit stream with at cells. The cells PDH typically in the utilizable data of or SDH formatted data streams sent. Asynchronously with AT it means that transmitters and receivers with within wide limits clock rates from each other deviating can work: the receiver examines error check for each cell again by means of headers, whether a sufficient synchronisation exists, and accomplishes if necessary a new synchronisation.

In the original conception AT was the key technology for the "“Broadband Integrated Services Digital Network"” (broadband ISDN), which should form the Backbone net for the existing similar telephone network (Plain old of telephones system, POTS). The complete at standard consists therefore of definitions for the Layer 1 to 3 link layer and switching layer) of the OSI model. Responsible with the development of at standards telecommunications companies (Telcos) were predominant, in addition, the American department OF Defense (DoD). From this reason many of the existing telecommunications technologies became and - conventions in AT integrates.

Today AT is a technology with very large efficiency, with support for uses of the global Internet and Telefonie Backbones over the DSL technology up to the private LAN. At technology is standardized by AT the forum.

Use with broadcast and broadcasting stations

The use of at of technology finds a special use with sending and broadcasting corporations in Germany. Over the glass fiber net made available by T-system, larger production companies and transmitter dispatch such as N24, RTL and WDR to real time their pictorial material to the different broadcasting stations. By satellite technology (that so-called Uplink) also larger distances (intercontinental) can be bridged over at net. The consist the so-called "“Muxer"” (multiplexing) of an EN and a decoder. The technology offers also the basis for live-switching from Studio to Studio.

At standard

Nearly all operators of communications networks furnished, use at networks within the Backbone range however no at signaling, but firm interconnecting. In the wide-band entrance net as multiplex Layer almost exclusively AT is used (to DSLAM, RACE). AT could become generally accepted as technology for local of networks only within the high speed range. Its high complexity and the associated costs prevented the wide employment as integrated network solution within the office range. Some fundamental principles of at standards like e.g. the possibility of priorisieren certain types of the data traffic were transferred later to MPLS, general minutes for efficient Switching below Layer 3.

At layer model (levels):

Management functions (OAM) are defined for AT in substantially stronger extent than for IP. They cover configuration management, error management and power measurement.

Tasks of the layers:

physical connection to other systems, preferential medium SDH

Transport and switching of at cells

Task of the AAL is to adapt data of higher layers to the format of the utilizable data field of at cell to convey and to the opposite side control information. One differentiates between 5 service types, about which however so far only the simplest greater importance has. IP uses at of adaptation Layer 5 (AAL5). The adaptation of the AAL5 takes over mainly fragmenting and Reassemblierung for the IP packages, which do not fit into the short utilizable data field.

In the context of at layer model the cell synchronisation is accomplished. The receiver looks for the HEC in Zellenkopf.Der HEC for the synchronisation consists of a check total of the remaining cell head. Tune the check totals of the first 4 bytes with that 5. , One speaks byte of synchronisation.

At concepts

Why

The reason for the use of small data "“cells"” was the reduction of the jitter with the multiplex one of data streams.

At the time, when AT was developed, STM-1-Leitungen with 155 Mbit/s (135 Mbit/s pay load) was a fast optical network connection, whereby many PDH lines of the networks at that time were slower still another whole piece, between 1,544 Mbit/s and 45 Mbit/s in the USA, and 2-34 Mbit/s in Europe.

With these data rates a typical packet of full length (1500 Byte/12000 bit) needs about 89 for the serialization of the data. With slower connections, for example 1.544 Mbit/s T1, the same package 7.8 milliseconds.

Now we imagine a Sprachsignal divided on packages, which must divide the line with large volume data traffic. No matter how small the language packages could be made, they would always meet the packets of full size, and the waiting periods, until the line is times free, would be a repeated of the pure serialization time of 7,8 ms. For language traffic this was completely clear inakzeptabel. Even if one filtered the jitter, the delay was so large that one would have needed echo compensation even in the local network. At that time this was directly too expensive.

At solution consisted now of it, all packages, both data and language splitting in 48 byte large parts and adding a Routing header of 5 byte, so that they could be joined later again, and this 53-Byte-Zellen then in place of packages to multiplex. This procedure reduced the waiting loop time nearly by the factor 30, whereby one could save the echo compensation.

The rules for the allocation and RH assembly of packages and Streams in cells are called AT of adaptation Layers: The two most important are AAL 1, which for Streams (e.g. language) is responsible, and AAL 5 for nearly all kinds of packages. Which AAL is used in each case, is not encodiert in the cell. Instead it is configured between two terminator points or agreed upon on basis of a virtual connection.

Since then the glass fiber TZE became many faster. Today a Ethernet package of full length needs to keep only 1.2 short on an optical connection with 10 Gbit/s range, which makes it actually no longer necessary to use small packages in order the latencies. Some people drew from the fact the conclusion that AT in the network Backbone became redundant thereby.

For slower connections (2 Mbit/s and smaller) AT is still meaningful. For this reason many ADSL systems use AT between the physical layer and a Layer-2-Protokoll such as PPP or Ethernet.

For what virtual

AT been based on connections, which can be furnished both firmly, and by means of a ISDN similar signaling only for a certain time to be switched can. For this purpose Virtual Paths (VPs) became and Virtual Channels (VCs) defines. Each at cell has 8 and/or 12 bits is enough for Virtual Path Identifier (VPI) and a Virtual a Channel Identifer (VCI) of 16 bits in their header. While these cells pass at network, the Switching is reached by change of the VPI/VCI values. Although the VPI/VCI values remain on the other hand alike thus not necessarily by an end of the connection, this corresponds to the concept of a connection, since all packages with equivalent VPI/VCI values take the same way (contrary to IP, where a package could achieve its goal by means of another route than preceding and following packages).

Virtual connections have also the advantage that one can use them as Multiplexing Layer for different services (language, Frame Relay, IP, SNA etc.), which can divide then a common at connection, without disturbing itself mutually.

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