Most of us usually think of ATM as the automated teller
machine that bank customers use to make transactions without a human teller. However, in
the context of telecommunications, ATM stands for asynchronous transfer mode.
Asynchronous transfer mode is a dedicated connection switching service that organizes
digital data into packets (53-byte cells), then transmits them over a medium using digital
signal technology (DSL). A cell is processed asynchronously relative to other related
cells and is queued before being multiplexed over the line.
ATM switches are used mainly in Internet networks, carrier networks, and frame relay
networks. The fixed cell size, synchronous switching, and switching of cells in hardware
in a connection-oriented manner enable the switches to carry voice, data, video, and
multimedia images at very high speeds. This greater speed however is reflected in the
higher cost service, as compared to frame-relay, for end-users to implement.
The fixed-size cells (53-bytes) means less processing as compared to processing
variable-sized cells. This is because the ATM switch does not have to look for bits
telling it when the cell is over, since the cells are a standard 53-bytes long. As such,
the switch knows when each cell ends. The first five of the 53 bytes are header
information, which includes bits that identify the type of information contained in the
cell like voice, data or video, so that the cell can be prioritized. The remaining 48
bytes contain user information such as voice, video or text. Voice and video need higher
priorities than LAN data since they need constant transmission to ensure no interruption
in the voice and picture. Other header information is used for routing, sequencing the
cells, and checking errors.
The switching is done in the hardware, which is a significant reason why ATM is fast,
since now, the ATM switch need not look up each cells address in the software, which
routers must do. The ATM switch sets up a route through the network when the first cell of
the transmission is seen. This information is then put into the hardware so that when the
next switch comes across another cell with the same header routing information, the cell
is sent down the previously established virtual path.
Another reason why ATM is so fast is due to the asynchronous switching, which improves
network utilization. Asynchronous switching is different from synchronous multiplexing
technologies like T-1 and T-3 in which every one of the 672 (in T-3) input transmissions
are assigned a time slot. If a terminal does not have anything to send, the time slot is
sent through the network empty. With asynchronous switching, every bit of network capacity
is available for every single cell, and there are no synchronous requirements. ATM
statistically multiplexes cells onto the network path based on the header
informations quality of service. For instance, voice and video need better service,
fewer delays, and greater speeds than text messages like e-mail, and this is accomplished
by ATM without wastage of network capacity.
ATM is also a very flexible switch. It can carry traffic from speeds as low as 56
kilobits, to high-speed video and multimedia applications. As such, inputs from telephone
systems or video devices can be accepted and transmitted across paths or virtual circuits
previously established by the ATM switch.
ATM at Work
ATM is currently installed mainly in carrier networks. However, taking into consideration
the maturing rate of technology, in the near future, ATM will be used by an increasing
number of large corporations to carry both voice and data traffic over wide area networks.
Such as T-1 lines running from the private branch exchange to an ATM switch.
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A way for organizations to upgrade
to ATM switches is by upgrading their routers which route calls between wide area and
local area networks. They can be upgraded to ATM by adding network interface cards. The
prevalent problem is local area network (LAN) congestion. The usage of Windows OS on PCs
is one factor in causing traffic jams on corporate networks since Windows is a bandwidth
hog. Other high-bandwidth consumers are other applications such as attachments on e-mail
messages, desktop videos and sales proposals with lots of graphics. Due to the high costs
of ATM, only very large customers are using ATM currently.
ATM switches are common in frame relay carrier networks for switching multiple
customers traffic at 622 megabits, which is faster than the T-3 multiplexers
speed of 45 million bps. Frame relay traffic is passed from the port to ATM switches where
it is the carried through the frame relay devices to connect directly to their customers.
Frame relay access to ATM switched located in the frame relay carriers network is an
interim technology for customers who want higher speed wide area network (WAN)
connections. The next step would be to allow customers to access carriers directly from an
ATM switch to an ATM port on the service providers network, enabling customers to
access virtual private network services at faster speeds.Currently, the most prevalent ATM
speed is 155 megabits. Although now, the top speed is only 622 megabits, ATM switches that
will run at gigabit speeds are being developed.