Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and
adopted by the ITU-T. The combination of ATM and SONET will allow high-speed
interconnection of all the world's networks. In fact, ATM can be thought of as
the "highway" of the information superhighway.
1. Design Goals
challenges faced by the designers of ATM, six stand out.
is the need for a transmission system to optimize the use of high-data-rate
transmission media, in particular optical fiber. In addition to offering large
bandwidths, newer transmission media and equipment are dramatically less
susceptible to noise degradation. A technology is needed to take advantage of
both factors and thereby maximize data rates.
system must interface with existing systems and provide wide-area
interconnectivity between them without lowering their effectiveness or requiring
design must be implemented inexpensively so that cost would not be a barrier to
adoption. If ATM is to become the backbone of international communications, as
intended, it must be available at low cost to every user who wants it.
d) The new
system must be able to work with and support the existing telecommunications
hierarchies (local loops, local providers, long-distance carriers, and so on)
e) The new
system must be connection-oriented to ensure accurate and predictable delivery.
f) Last but
not least, one objective is to move as many of the functions to hardware as
possible (for speed) and eliminate as many software functions as possible
(again for speed).
2. Frame Networks
ATM, data communications at the data link layer had been based on frame
switching and frame networks. Different protocols use frames of varying size
and intricacy. As networks become more complex, the information that must be
carried in the header becomes more extensive. The result is larger and larger
headers relative to the size of the data unit. In response, some protocols have
enlarged the size of the data unit to make header use more efficient.
Unfortunately, large data fields create waste. If there is not much information
to transmit, much of the field goes unused. To improve utilization, some
protocols provide variable frame sizes to users.
3. Mixed Network Traffic
variety of frame sizes makes traffic unpredictable. Switches, multiplexers, and
routers must incorporate elaborate software systems to manage the various sizes
of frames. A great deal of header information must be read, and each bit
counted and evaluated to ensure the integrity of every frame. Internetworking
among the different frame networks is slow and expensive at best, and impossible
4. Cell Networks
the problems associated with frame internetworking are solved by adopting a
concept called cell networking. A cell is a small data unit of fixed size. In a
cell network, which uses the cell as the basic unit of data
exchange, all data are loaded into identical cells that can be transmitted with
complete predictability and uniformity. As frames of different sizes and
formats reach the cell network from a tributary network, they are split into
multiple small data units of equal length and are loaded into cells. The cells
are then multiplexed with other cells and routed through the cell network.
5. Asynchronous TDM
asynchronous time-division multiplexing-that is why it is called Asynchronous
Transfer Mode-to multiplex cells corning from different channels. It uses
fixed-size slots (size of a cell). ATM multiplexers fill a slot with a cell
from any input channel that has a cell; the slot is empty if none of the
channels has a cell to send
ATM is a
cell-switched network. The user access devices, called the endpoints, are
connected through a user-to-network interface (UNI) to the switches inside the
network. The switches are connected through network-to-network interfaces (NNIs).
between two endpoints is accomplished through transmission paths (TPs), virtual
paths (YPs), and virtual circuits (YCs). A transmission path (TP) is the
physical connection (wire, cable, satellite, and so on) between an endpoint and
a switch or between two switches. Think of two switches as two cities. A
transmission path is the set of all highways that directly connect the two
transmission path is divided into several virtual paths. A virtual path (VP)
provides a connection or a set of connections between two switches. Think of a
virtual path as a highway that connects two cities. Each highway is a virtual
path; the set of all highways is the transmission path.
networks are based on virtual circuits (VCs). All cells belonging to a single
message follow the same virtual circuit and remain in their original order
until they reach their destination. Think of a virtual circuit as the lanes of
a highway (virtual path).
virtual circuit network, to route data from one endpoint to another, the
virtual connections need to be identified. For this purpose, the designers of
ATM created a hierarchical identifier with two levels: a virtual path
identifier (VPI) and a virtual-circuit identifier (VCI). The VPI defines the
specific VP, and the Vel defines a particular VC inside the VP. The VPI is the
same for all virtual connections that are bundled (logically) into one VP.
7. ATM Layers
standard defines three layers. They are, from top to bottom, the application
adaptation layer, the ATM layer, and the physical layer. The endpoints use all
three layers while the switches use only the two bottom layers.
Ethernet and wireless LANs, ATM cells can be carried by any physical layer
layer provides routing, traffic management, switching, and multiplexing
services. It processes outgoing traffic by accepting 48-byte segments from the
AAL sublayers and transforming them into 53-byte cells by the addition of a
two formats for this header, one for user-to-network interface (UNI) cells and
another for network-to-network interface (NNI) cells.