What is an ATM network?

[Monty1]

ATM (Asynchronous Transfer Mode). Layer 2 protocol of the OSI model. Developed by CNET and a few manufacturers of computer components. The ATM technique is based on segmentation data cell size. The header of the cell contains information allowing each cell to find its way: it shows between the identity of another (VCI Virtual Channel Identifier) which the cell belongs. The VCI value is local to the switch, its value changes the output of each switch, indicating a new value for the next node. The ATM switch has a table giving the VPI / VCI based on the output VPI / VCI entry. An entry in this table is created each time a connection is established, the route is then established for the duration of the connection. Currently, two types of switching technologies are available:

• Packet switching (and frame relay), well suited for data transmission.
• Circuit switching, which takes into account the isochronous nature of telephone service

[Monty1]

Cell switching combines the simplicity of circuit switching with the flexibility of packet switching. Simplicity through the length of the cell that enables the design of cell switches are relatively simple and efficient. On the other hand, if we agree to limit the size of the queues in the switches and do not use the access links to most of the opportunities, the small cell size allows isochronous circuit emulation. Flexibility because the cells are packets with headers, to establish virtual connections that can be multiplexed. The flow is adapted to the characteristics of the source, rather than being imposed by the access link.

ATM allows data transfers at speeds ranging from 25Mbps to over 622Mbps (it is expected to reach more than 2Gbps Fibre optics).

What is a cell?

The basic idea of cell-switched networks is to transmit all data in small packets of fixed size called cells. The cell size is of some importance because it affects the form of higher layer protocols. So that the receiver can correctly interpret the data in the cell, it must contain control information describing the relationship of this cell with another in the data stream. The cell size is 48 bytes of payload and 5 bytes header, 53 bytes.

Fields of application of ATM

International networks with transmission of data, voice and image (for industry or the media) Real-time applications (video, voice) Network-based business: a platform for the transmission of IP VPN Voice Application Hosting with QoS Rapid transmission of large volumes of data.

[Monty1]

Transit Connect and cells across the network

There are three distinct phases:

• The establishment of the connection
• The transfer of data through the virtual channel established
• The release of the connection

During the procedure for establishing the connection, a virtual circuit is reserved through the ATM network. Routing is established during this phase, which subsequently optimizes the transmission delays. The virtual circuit is created from two identifiers :

• VPI (Virtual Path Identifier), 8-bit field in the header of an ATM cell. Virtual path number. The physical medium consists of a set of virtual paths which are themselves composed of other virtual paths. ATM switches use the fields of virtual path identifiers and virtual channel to identify the next virtual channel link on which a cell must pass to reach its final destination.
• VCI (Virtual Circuit Identifier) field of 16 bits in the header of an ATM cell. The circuit identifier or virtual channel identifier is used with the virtual path to determine the next destination of a cell when it passes through a series of ATM switches to reach its destination. ATM switches use the fields of virtual channel identifiers and virtual path to identify the next virtual channel link of the network by which a cell must pass to reach its final destination.

Addressing a ATM virtual circuit is a couple VPI / VCI. This system allows a routing very easily: a virtual circuit is in fact a sequence of pairs VPI/VCI used to go from router to router, until the recipient. In fact, when establishing the connection, each router in the ATM network that makes up the virtual circuit creates a routing table that allows cells to transit to the virtual path arriving adequate.

[Monty1]

AAL (ATM Adaptation Layer)

These layers are loaded to segment and reassemble the cells from applications. ATM has been designed to carry different data streams, video, voice or data. But the transport of these different types of data streams that might require different types of services, for example: (the data constraints are not the same for voice transmission). To meet these diverse application requirements, various AAL layers have been defined:

• AAL1: Supports video applications at a constant rate, voice transport.
• AAL2: Supports audio and video applications with variable flow.
• AAL3 / 4: This type of AAL is adapted to secure data transfer.
• AAL5: Suitable for data transport.

Multiplexing

It is the ability to transmit on a single physical medium (lane high speed) data from multiple devices (lanes slow speed).

• The way high speed: it is the communication channel between the multiplexer and demultiplexer, it supports all traffic.
• Vaginal delivery rate: the communication channel between the user terminal to the multiplexer, it supports the user traffic.

Multiplexer: Equipment for combining the signals from the transmitters for transmission over high-speed track. The demultiplexer and the equipment on which the receivers are connected to high speed track.

Frequency division multiplexing: Also known as MRF (frequency division multiplexing or FDM Frequency Division Multiplexing) to share the frequency band towards high speed into a series of several narrower channels, which will circulate continuously on the road High speed data from different channels low speed. Multiplexing Statistical multiplexing Statistical multiplexing uses the characteristics of time, with the difference that it transmits on the high speed lane roads with low speed data. This type of multiplexing is based on statistics of throughput of each low line speed. Thus, the high line speed does not transmit the white (electronics has specific problems encountered at speeds over 100 MHz).

TDM: time division multiplexing, also called MRT (Division Multiplexing or TDM time, Time Division Multiplexing) is used to sample the low speed channel signals to transmit successively towards high speed by allocating the entire bandwidth, even if they do not have data to transmit (it is not possible to process signals continuously, for simplicity, the signals are sampled at a steady pace making it possible to have bandwidth without issue).