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Asynchronous Transfer Mode (ATM)
ATM (Asynchronous Transfer Mode)
is an even higher speed technology that, like X.25 and frame relay,
is used to connect LANs to create a WAN, as well as being used
within individual LANs. Although the increasing speeds of Ethernet
in LAN environments are calling into question ATMs future in the
local office, it is alive and well in WAN environments.
In ATM, data is transferred in cells
or packets of relatively small size. The small, predictable
size of ATM packets makes for efficient, reliable transmission
of newer types of time-sensitive Internet data such as audio and
video, while at the same time making sure that transmission
of these larger types of data doesnt hog the
network and make it difficult for those just requesting small
file transfers or print jobs to get their work done in a timely
fashion. This is made possible through ATMs implementation of
QoS (Quality of service) functions, much like those in TCP/IP.
ATM transfer rates range from
25Mbps to 622Mbps. These speeds are higher than most office LANs
run today, allowing for efficient interconnection of multiple (3, 4
or even more) sites around the country (or world!) into a WAN. ATM
can run as a layer on top of SONET, and can also run over other
The different types of ATM
You can purchase ATM connectivity
based on several different throughput/pricing models. The different
models are each appropriate for different uses of your network and corporate
requirements for consistency of transmission speeds. The options are:
- Constant Bit Rate (CBR), which provides
your ATM link with a guaranteed reliable, constant transmission speed
at all times, much like a private leased line. If you have critical
systems such as electronic manufacturing monitoring systems running
over your WAN, this may be the most appropriate choice.
- Variable Bit Rate (VBR), a step down from
CBR, which provides a specified average throughput over time, but which
does not send data as evenly as CBR. This option costs less than CBR,
because it allows the ATM network capacity to be more efficiently shared
among multiple customers by providing the ATM carrier with a small amount
of flexibility regarding when individual data packets are sent. This
model is often used for audio and video transmission.
- Unspecified Bit Rate (UBR) that provides
no guaranteed throughput levels. This is a cost-effective ATM throughput
model for applications, which can tolerate unpredictable throughput
rates and packet delays, such as file transfer or email.
- Available Bit Rate (ABR), which guarantees
a minimum level of throughput, but which can optionally burst data
through at higher rates when excess network capacity is available.
This model is analogous to frame relays CIR.
- Guaranteed Frame Rate (GFR), similar to
ABR, which can access additional bandwidth that might become available
on the network after the connection has been established, by dynamically
adjusting its frame rate.
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