ATM networks is a relatively new development in networking. Because of its increasing importance, in fact, it will be the backbone standard for all high speed networks, especially WANs, we include a general introduction here. ATM standard doesn't fit well into the existing OSI standards. But As you will see from the discussion below, it falls somewhere below the data link layer and above the physical layer.
In addition, the development of ATM standards is motivated by several reasons. The single most important of which is the need to integrate diverse type of information traffic, with dramatically different bandwidth and traffic characteristics, into a single transmission paradigm. This need is largely due to the increase in computer processing speed and vastly improved high speed transmission technology. The development of high speed graphical based applications, multimedia, digital TV, HDTV, all demand a transmission infrastructure that can provide universal and efficient transport of high speed information over wide and local geographical areas.
What is the main difficulties in supporting the current multimedia technology?
The first problem is that shared medium LANs severely limits the use of bandwidth per user:
Basically, there are two opposing views on how to proceed to improve the network capability to handle multimedia traffic. One view is to improve the existing technology such as switching hubs, fast Ethernets, etc. There are also proposals to modify the existing TCP/IP protocol (which we will also discuss in detail later) by using the so called stream protocol (ST) to accommodate real time traffic.
The other view is to use entirely new technology, which is to use ATM. The second approach has now clearly won wide acceptance. Most people believe faster Ethernet, switching hubs, etc, are only short term fixes the potential explosive bandwidth demand of the future.
Since ATM is basically a data link layer protocol, it will be used to support higher layer protocols. In fact it is possible for ATM to support multiple coexisting virtual LANs using different higher layer protocols but all sharing the same ATM layer functionalities.
We know that typical digitize phone quality voice takes
at most 64 (can be as low as 1
),
but digital HDTV signals can take up
to 40
, supercomputing requires even higher bandwidth.
This poses tremendous challenge if
we use TDM, we have to have the minimum
channel rate at around 64, and maximum rate of over 150
or even higher. This yields a TDM frame with at least
slots per frame. Maintaining the slot allocation within
such frame at such high speed represents a major difficulty.
ATM is a cell-switching and multiplexing technology designed to combine the benefits
circuit switching (constant transmission delay, guaranteed capacity)
with those of packet switching(flexibility,
efficiency for intermittent traffic).
ATM defines the interface between the user equipment and the network
(referred to as the User Network Interface, or
UNI.) The standard is put together by a group of 300
leading network companies, called ATM forum, and released
to the public. This definition supports the use
of ATM switches (and ATM switching techniques) within
both public and private networks
Because it is an asynchronous mechanism, ATM differs from synchronous transfer mode (STM), which is a different term for TDM, where TDM techniques are employed to preassign users to time slots. ATM time slots are made available on demand, with labels identifying the source of the transmission contained in each ATM cell. TDM is inefficient relative to ATM because, if a station has nothing to transmit when its time slot comes up, that time slot is wasted. The converse situation, where one station has lots of information to transmit, is also less efficient. In this case, that station can only transmit when its turn comes up, even though all the other time slots may be empty. With ATM, station can send labeled cells whenever necessary.
Another critical ATM design characteristic is its star topology. The
ATM switch acts as a hub in the ATM network, with all
devices attached directly. This
provides all the traditional benefits of star-topology networks,
including easier troubleshooting and support
for network configuration changes and additions. Furthermore, ATM's switching fabric provides
additive bandwidth. As long as the switch can handle the
aggregate cell transfer rate, additional connections to the switch can
be made. The total bandwidth of the system increases accordingly. If a switch can pass cells among all its
interfaces at the full rate of all interfaces, it is
described as non-blocking. For example, an ATM switch with 16 ports each at
155 would require about 2.5
aggregate throughput to
be nonblocking. This can only achieved through the use
of parallel switching architectures.
Finally, ATM is flexible, in that it can carry various source material and run on various physical-layer implementations. The ATM model defines an engine that moves small, fixed-size cells through a network, but leaves issues of application and physical implementation open.
The development of ATM represents a compromise between telecommunication industry, dominated by circuit switching technology, and computer industry, dominated by packet switching technology. Because of the universal consensus on ATM technology by both industries, ATM is determined to be the standard for all future high speed communications.