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A key characteristic of asynchronous transfer mode transmission protocol is the fixed length of ATM cells. The cells are 53 bytes in size and have 48 bytes of usable data along with five bytes of cell header. The cells are transferred via asynchronous time multiplexing. ### The evolution of ATM Initial specifications of asynchronous transfer mode were developed by ITU (International Telecommunication Union) back in 1980. The goal was to develop a communications protocol based on plesiochronous digital hierarchy (PDH) and [synchronous digital hierarchy (SDH)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdh), which allow the integration of various services. ATM not only supports line-switched data transfer, but also packet-based protocols such as frame relay or internet protocol. The masterminds in the development of ATM were initially telecommunications companies and the U.S. Department of Defence. In 1991 the ATM manufacturers set up the ATM forum, which is still the developer and submits specifications to ITU-T for standardisation. ### Cells and virtual connections of asynchronous transfer mode Characteristic features of asynchronous transfer mode protocol are its virtual connections and the cell structure in data transfer. One reason for the relatively small cells of 53 bytes is the minimisation of jitter when transmitting multiplex data streams. Large packets in a data transfer can therefore no longer block voice packets on a low-speed line. This minimises jitter of voice transmission. ATM uses virtual connections which may be either temporary or permanent. The standard specifies virtual paths (VPs) and virtual channels (VCs) for this purpose. Packets with identical VPIs (Virtual Path Identifiers) and VCIs (Virtual Channel Identifiers) follow the same route through the network. Different virtual connections are also used to multiplex various services within the network. ### Traffic Management in ATM Traffic management and traffic policing are important components of ATM. As soon as an ATM connection has been established, the nodes along the route receive information on the traffic class of the connection. This mechanism allows [bandwidths](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/bandwidth) on the network to be reserved for virtual connections and a defined quality of service. A network traffic contract for [telephone systems](t3://page?uid=16), for example, can be fulfilled by classifying packets, queuing and policing. 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