29-12-2012, 05:49 PM
ATM Tutorial
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Introduction
This self-study tutorial satisfies the prerequisite for
Asynchronous Transfer Mode (ATM) that is required for
attendance at Fujitsu Network Communications Inc. (FNC)
Educational Services training classes.
Objectives
After completing this lesson, the student should be able to
describe ATM cells, discuss ATM protocol and transmission
(including ATM service classes, categories, and quality), and
define ATM terminology.
Standards
The student should complete the tutorial and take the Self
Evaluation at the end of the tutorial. If the student passes the
evaluation by missing no more than three questions, the ATM
prerequisite is satisfied. If more than three questions are
answered incorrectly, the student should review the tutorial again
and make sure the information relating to the missed questions is
understood. Each student should be familiar with concepts and
terms of the tutorial prior to attending class.
ATM
Asynchronous Transfer Mode (ATM) is a switching and
multiplexing technology that employs small, fixed-length cells to
very quickly and efficiently move all types of traffic. ATM is fast
and efficient because its cells fit into spaces too small for larger
packets or frames, traffic routes are preplanned, switching is
done without the need for time-consuming software, and payload
error checking and correction is performed only at the destination
node, not at every hop along the way.
ATM was designed to be the protocol of choice for future
Broadband-Integrated Services Digital Network (B-ISDN)
services. Because ATM is asynchronous, it provides true
bandwidth-on-demand. Additionally, its small cell size makes
ATM adaptable to any form of information—data, voice, video,
audio, e-mail, faxes—and capable of moving this information
amazingly fast across a network that can provide millions of
virtual paths and channels between end user equipment.
Characteristically, ATM has two dimensions: transport and
switching. In the transport dimension, ATM can move no faster or
slower than any other digital communication technology. It is in
the switching dimension that ATM shines. Packets and frames of
various sizes need smart switches controlled by slow-moving
software to move them through a network. Small, uniformly sized
cells on an ATM network move through switches without needing
software assistance. The cells already know the route to take
and do not need to slow down to look for road signs or stop to get
directions.
The ATM Forum, ITU and ANSI
The ATM Forum was started in 1991 by four computer and
telecom vendors. Today nearly 1000 members of the ATM Forum
work for equipment and service providers, manufacturers,
carriers, and end users interested in accelerating the definition,
development, and deployment of ATM technology. While the
forum is not a standards body, the International
Telecommunication Union (ITU), the most important
telecommunications standards body in the world, considers the
ATM Forum a very credible working group.
The ITU is rooted in the International Telegraphy Union, founded
in Paris in 1865. Its name changed in 1934, and in 1947 the ITU
became an agency of the United Nations. The ITU works with
public and private organizations to develop earth-linked and
satellite communications, while developing standards for all
types of telecommunication technology.
The ITU-Telecommunication Standardization Sector (ITU-T) is
the leader in defining Integrated Services Digital Network (ISDN),
B-ISDN, and ATM specifications. The American National
Standards Institute (ANSI) is the formal standards body guiding
the development of ATM in the US.
ATM Protocol
The ATM protocol layer model consists of four layers and three
planes (see Figure 3). The layers are closely interrelated, but
each layer addresses a specific set of functions. The physical
layer and ATM layer can be compared with the physical layer of
the OSI reference model. As with the OSI model, the various
layers function independently, but continuous interaction among
the layers is highly coordinated.
ATM Layer
The ATM layer performs many very critical functions essential to
the exchange of end-to-end communications:
• First the ATM layer takes the 48-byte payload from the ATM
adaptation layer and adds the 5-byte addressing header.
• Then it multiplexes all the cells from various connections,
prepares a single-cell stream for the physical layer, and
puts in idle cells, if needed, as fillers for synchronous
transmission systems (for example, SDH or SONET).
• Next the ATM layer provides translation (directional coding)
for every cell to get the cells switched through multiple
virtual connections. The ATM layer can do this because it
knows the capabilities of virtual connections carrying the
cells.
ATM Advantages
ATM provides several advantages (see Figure 7):
• ATM fixed-length cells require lower processing overhead
and produce higher transmission speeds than traditional
packet switching methods.
• ATM transmits asynchronous data in a synchronous
network while prioritizing time-sensitive traffic ahead of
delay-tolerance traffic to ensure that quality of service is
maintained.
• ATM delivers true bandwidth-on-demand (a big plus for
high-speed voice, data, and video service) and uses
statistical multiplexing techniques to efficiently use
resources.
• ATM is application-independent, meaning it can be used as
a common infrastructure for many network types, including
public, private, LAN, and campus backbones.
ATM Disadvantages
While ATM has several advantages, ATM disadvantages can be
seen in three areas:
• Cost
• Complexity
• Availability
Compared with voice switches, ATM switches are still much more
expensive per line. ATM was implemented before the designers
intended, and ATM standards are still trying to catch up, which
should help to bring down the cost of ATM equipment.
ATM equipment is very complex and intelligent. It takes a very
intelligent management team and system to operate ATM
successfully (that is, efficiently and cost-effectively).
ATM is not as widely available as SONET and SDH. These are
older protocols that are widely accepted and very well
standardized. Additionally, with the proliferation of Fast Ethernet
and Gigabit Ethernet, ATM may not be necessary. This is
especially true for those who transport Ethernet over already
ubiquitous SONET/SDH networks.