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ATM-ASYNCHRONOUS TRANSFER MODE
Definition: ATM is a high-speed networking standard designed to support both
voice and data communications. ATM can support speeds ATM operates at the data
link layer over either fiber or twisted-pair cable.
ATM differs from more common data link technologies like Ethernet in several ways.
ATM does not involve routing for example. Hardware devices known as ATM switches
establish point-to-point connections between endpoints and data flows directly from
source to destination. Instead of using variable-length packets, ATM utilizes fixedsized cells. ATM cells are 53 bytes in length that includes 48 bytes of data and 5
bytes of header information.
The performance of ATM is often expressed in the form of OC (Optical Carrier) levels,
written as "OC-xxx." Performance levels as high as 10 Gbps (OC-192) are technically
feasible with ATM. More common performance levels for ATM are 155 Mbps (OC-3)
and 622 Mbps (OC-12).
ATM is designed to support easier bandwidth management. Without routing and with
fixed-size cells, one can much more easily monitor and control bandwidth under ATM
than under Ethernet, for example. The high cost of ATM relative to Ethernet is one
factor that has limited its adoption to "backbone" and other high-performance
applications.
ATM PROTOCOLS
The top layer of the ATM protocol stack is the ATM Adaptation Layer (AAL). Different
AALs correspond to the different data types ATM supports. Thus AAL1 permits
the ATM device to closely resemble a constant bit-rate voice circuit; AAL3/4 and
AAL5 are used for variable bit-rate data types, which are those typically found on
computer networks. The AAL is also responsible for integrating the inherently
connection-oriented ATM with connectionless data sources, enabling ATM clients
to emulate broadcasting and multicasting.
ATM:
Short for Asynchronous Transfer Mode, a network technology based on transferring data
in cells or packets of a fixed size. The cell used with ATM is relatively small compared to
units used with older technologies. The small, constant cell size allows ATM equipment
to transmit video, audio, and computer data over the same network, and assure that no
single type of data hogs the line.
Some people think that ATM holds the answer to the Internet bandwidth problem, but
others are skeptical. ATM creates a fixed channel, or route, between two points whenever
data transfer begins. This differs from TCP/IP, in which messages are divided into
packets and each packet can take a different route from source to destination. This
difference makes it easier to track and bill data usage across an ATM network, but it
makes it less adaptable to sudden surges in network traffic.
When purchasing ATM service, you generally have a choice of four different types of
service:
constant bit rate (CBR): specifies a fixed bit rate so that data is sent in a
steady stream. This is analogous to a leased line.
a. The constant bit rate (CBR) is the simplest service to define
b. It is used by applications that require a fixed data rate that is continuously
available during the communication life tile
c. CBR is commonly used for uncompressed audio and video information
d. Example of CBR are
a. Videoconferencing
b. Interactive audio
c. Audio/Video distribution
d. Audio/Video retrieval
variable bit rate (VBR): provides a specified throughput capacity but data is
not sent evenly. This is a popular choice for voice and videoconferencing data.
1. The real time variable bit rate category is indented for time
sensitive applications.
2. The real time variable bit rate category is usually bursty
3. Example of this variable bit rate is that the standard approach to
video compression results in sequence of images of varying size.
available bit rate (ABR): provides a guaranteed minimum capacity but allows
data to be bursted at higher capacities when the network is free.
unspecified bit rate (UBR): does not guarantee any throughput levels. This is
used for applications, such as file transfer, that can tolerate delays.