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Adtran
Enterprise Network Series
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HDLC Utilization for Safe-T-Net Applications in the ATLAS 800PLUS/810PLUS
Family
Safe-T-Net(TM) is an ADTRAN technology that provides a complete disaster
recovery solution for Frame Relay networks. Safe-T-Net, when deployed through
the ADTRAN ATLAS system, allows end-users to be proactive in their disaster
recovery plans by designing a complete dial-around-the-cloud solution.
In a Safe-T-Net environment, the ATLAS 800PLUS /810PLUS will actually emulate
frame relay over the dialed backup links in the event the primary frame relay
link goes down. In addition to the HDLC resources initially required to process
the primary frame relay links, separate HDLC resources are also required to
perform the emulation of the dialed backup links. While the HDLC resources for
the primary links are dedicated to those links, the HDLCs needed for the backup
links are dynamically allocated. This allows you to overbook your HDLC resources
and support a larger number of backup links. Below is an example of HDLC
resource usage in a Safe-T-Net application:
The primary T1 frame relay circuit will be terminated on the 0.1 T1/PRI port
and will use that port's built-in HDLC resource (B1). This circuit will utilize
1.544 Mbps of the available 2.048 Mbps bandwidth. The PRI circuit for dial
backup will terminate into port 0.2 and will also utilize the port's built-in
HDLC resource (B2) to process the D channel. Although a D channel is only 64
kpbs, the remaining bandwidth capability of the HDLC resource is no longer
available because the built-in resource is dedicated to the 0.2 port. Frame
packets are being sent out the V.35 DTE port and will be processed with one of
the general resources (G1). While the maximum data rate for a V.35 port is 2.048
Mpbs, we will assign a rate of 1.544 Mbps to our V.35 port, using the maximum
data rate of our source frame relay T1 as a guide. Since the two built-in HDLC
resources (B1 & B2) and one general resource (G1) have all been used for
normal frame relay operation, we are left with only one general resource (G2) to
provide processing for all the dialed backup links. In order to accomplish
simultaneous backup capability in the event of a total failure, each of the
remote site backup links will require their own HDLC resource. Therefore, an
HDLC module will be needed to provide the additional HDLC resources required. If
you wanted to take a chance that only one remote site might go down at any given
time, you would simply utilize the remaining general resource (G2) and will not
need to add the HDLC module.
For this example, assume the customer wants to provide simultaneous dial
backup capability for each remote. The first dial backup call established will
utilize the last general resource at 128kbps bandwidth. The additional bandwidth
available on this general resource cannot be used by the other frame relay dial
backup links. Therefore, this application will require the addition of an HDLC
Module. The next two sites will use 128kbps each of the HDLC Module resources.
Note that unlike the built-in HDLC resources which can only support one frame
relay link, the HDLC Module allows multiple frame relay links to share the
available bandwidth.
The HDLC Module contains 8.192 Mbps of bandwidth for HDLC processing. This
bandwidth can be shared across multiple interfaces requiring HDLC processing.
The most granular division would be 128 different interfaces each at 56k/64k.
The parameters that determine the amount of HDLC resources used in the HDLC
Module include the number of frame relay links as well as the bandwidth required
by each of the links.[(# independent frame relay links * N<=128) where N is
the # of 56kbps/64kbps increments]. In this example, there are two independent
frame relay links each using two 64kbps increments thus 2*2=4. Thus 4 of the
available 128 resources are used.
Below is a representation of the resources utilized in this example:
This application illustrates the concept of how the built-in resources differ
from the HDLC module resources in their allocation capabilities. Any leftover
bandwidth of a built-in HDLC resource is not available to be shared by another
endpoint requiring HDLC processing. However, the bandwidth of the HDLC Module
can be divided up between multiple endpoints requiring HDLC processing. When a
port/packet endpoint is mapped that requires HDLC processing, the unit
automatically assigns the HDLC resources. The first two ports/endpoints
requiring HDLC processing that are mapped but not terminated into the base 0.1
and 0.2 ports will be assigned to the general resources. Leftover bandwidth on
the general resources is not available for use by any other port/endpoint. Thus
the ports/endpoints requiring the largest amount of bandwidth for processing
should be mapped first. This will allow for more efficient use of the HDLC
Module resources.
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