ATM - asynchronous transfer mode tutorial
- an overview or tutorial of the basics of asynchronous transfer mode, used
in many data networking applications large and small.
The Asynchronous Transfer Mode (ATM) was developed to enable
a single data networking standard to be used for both synchronous channel
networking and packet-based networking. Asynchrnonous transfer mode also
supports multiple levels of quality of service for packet traffic.
In this way, asynchronous transfer mode can be thought of as
supporting both circuit-switched networks and packet-switched networks by
mapping both bitstreams and packet-streams. It achieves this by sending data in
a series or stream of fixed length cells, each of which has its own identifier.
These data cells are typically sent on demand within a synchronous time-slot
pattern in a synchronous bit-stream. Although this may not appear to be
asynchronous, the asynchronous element of the "Asynchronous Transfer Mode",
comes from the fact that the sending of the cells themselves is asynchronous and
not from the synchronous low-level bitstream that carries them.
One of the original aims of Asynchronous Transfer Mode was
that it should provide a basis for Broadband Integrated Services Digital Network
(B-ISDN) to replace existing PSTN (Private � ). As a result of this the
standards for Asynchronous Transfer Mode standards include not only the
definitions for the Physical transmission techniques (Layer 1), but also layers
2 and 3.
In addition to this, the development of Asysnchronous
Transfer Mode was focussed heavily on the requirements for telecommunications
providers rather than local data networking requirements, and as a result it is
more suited to large area telecommunications applications rather than smaller
local area data network solutions, or general computer networking.
While Asynchronous Transfer Mode is widely used for many
applications, it is generally only used for transport of IP traffic. It has not
become the single standard for providing a single integrated technology for
LANs, public networks, and user services.
Basic asynchronous transfer mode system
There are two basic elements to an ATM system. Any system can
be made up a number of each of these elements:
- ATM switch This accepts the incoming cells or information
"packets" from another ATM entity which may be either another switch or an
end point. It reads and updates the cell header information and switches the
information cell towards its destination
- ATM end point This element contains the ATM network interface
adaptor to enable data entering or leaving the ATM network to interface to
the external world. Examples of these end points include workstations, LAN
switches, video codecs and many more items.
ATM networks can be configured in many ways. The overall
network will comprise a set of ATM switches interconnected by point-to-point ATM
links or interfaces. Within the network there are two types of interface and
these are both supported by the switches. The first is UNI and this is used to
connect ATM end systems (such as hosts and routers) to an ATM switch. The second
type of interface is known as NNI. This connects two ATM switches.
ATM operation
In ATM the information is formatted into fixed length cells
consisting of 48 bytes (each 8 bits long) of payload data. In addition to this
there is a cell header that consists of 5 bytes, giving a total cell length of
53 bytes. This format has been chosen because time critical data such as voice
packets is not affected by very long packets being sent. The data carried in the
header comprises payload information as well as what are termed virtual-circuit
identifiers and header error check data.
ATM is what is termed connection orientated. This has the
advantage that the user can define the requirements that are needed to support
the calls, and in turn this allows the network to allocated the required
resources. By adopting this approach, several calls can be multiplexed
efficiently and ensuring that the required resources can be allocated.
There are two types of connection that are specified for
asynchronous transfer mode:
ATM networks can be configured in many ways. The overall
network will comprise a set of ATM switches interconnected by point-to-point ATM
links or interfaces. Within the network there are two types of interface and
these are both supported by the switches. The first is UNI and this is used to
connect ATM end systems (such as hosts and routers) to an ATM switch. The second
type of interface is known as NNI. This connects two ATM switches.
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