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Adtran
Enterprise Network Series
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ATLAS System Configuration Guidelines
HDLC Module - When is it Needed?
The HDLC module is required for ATLAS applications requiring a large number
of HDLC controllers. Including 128 HDLC controllers, the HDLC module is a
resource module for the ATLAS system - it has no physical interfaces to the
outside world. All information is passed to and from the HDLC module via the
ATLAS internal bussing scheme.
Whether or not an HDLC module is required is determined by the application.
An HDLC Controller is needed to process:
-
An ISDN D Channel. One HDLC controller is needed for every D Channel that
ATLAS handles, regardless if it is PRI or BRI. For example a system
configured to convert a single PRI circuit into 11 BRI circuits would need
12 HDLC controllers. This can be accomplished without the HDLC module, using
the HDLC controllers that are built into the Octal BRI module and the Base
Unit (assuming the PRI circuit is connected to the interface provided on the
Base Unit).
-
An ISDN B Channel, but only if the call is being processed by ATLAS, as
in the case of remote access applications. For remote access applications,
one HDLC controller is required for every B Channel call that is being
terminated inside ATLAS. For example, an ATLAS system configured to support
a single PRI is worth of ISDN calls for remote access, that directs all the
calls to async ports, would need 24 HDLC controllers - one for the D Channel
and 23 for the B channels.
NOTE:
ISDN calls that are directed to synchronous V.35 ports at 56 kbps or 64 kbps
do not require HDLC processing of the B channels.
In situations where ATLAS is not terminating the ISDN calls, as in the
case for PRI to T1 conversion, HDLC controllers are not needed for the B
Channels, just for the D Channel.
-
A frame relay link. One HDLC controller is required for every frame relay
link in an ATLAS system. For example, a single T1 connection to the public
frame relay network would require one HDLC controller. In a private frame
relay network, each DS0 in a T1 may represent a frame relay link to a
particular remote site, in which case 24 HDLC controllers would be required.
Note that frame relay links can be found on the DTE interface as well as
the network interface. If ATLAS is passing frame relay out individual V.35
ports, each V.35 port passing frame relay is counted as a frame relay link
and requires an HDLC controller.
ATLAS Support of HDLC Controllers
Now that we know what conditions call for HDLC controllers, let's look at the
ATLAS hardware includes that HDLC controllers:
| Component |
# HDLC Controllers |
Usage |
| ATLAS 800 Base Unit |
2 |
Two HDLC controllers are tied to the T1/PRI interfaces built into the
Base Unit, and can be used to process D Channels (in case the port is
configured as a PRI). |
| ATLAS 800PLUS Base Unit |
4 |
Two HDLC controllers are tied to the T1/PRI interfaces built into the
Base Unit, and can be used to process frame relay links or D Channels
(in case the port is configured as a PRI).The other two HDLC controllers
are general purpose and can be used to process frame relay links, D
Channels, or B Channels as required. |
| Quad T1/PRI Module |
4 |
Each HDLC Controller is tied to a specific T1/PRI port on the module,
and can only be used to process the D Channel (in case the port is
configured as a PRI). If a T1 port on the Quad T1/PRI module is
configured as a frame relay link, an HDLC controller from some other
ATLAS hardware must be used. |
| Octal BRI Module |
8 |
These HDLC controllers can only be used to process the D channel of
the associated BRI interface. |
| Modem-16 Module |
16 |
These HDLC controllers can only be used to process ISDN B Channels for
remote access applications. |
| HDLC Module |
128 |
The HDLC controllers on this module are general purpose and can be
used to process frame relay links, ISDN B Channels, and ISDN D Channels
as the system requires. |
Thus, the HDLC module is used when the application requirements call for more
HDLC controllers than are provided with the particular piece of ATLAS hardware
that would otherwise be needed to address the application.
Fortunately, the ATLAS system is smart enough to know how to allocate HDLC
controllers to meet the user's application without the user being involved.
HDLC Application Examples
-
Remote Access - Mostly ISDN Calls
For remote access applications, the HDLC module should be used when the
majority of calls are expected to be ISDN. While it is possible to populate
a system with Modem-16 modules solely for the purpose of processing ISDN
calls, a more economical route would be to install a single HDLC module to
accomplish the same task.
Private Frame Relay Application
In a private frame relay application, you might see 24 individual frame
relay links inside a T1 circuit, going to 24 different destinations. This
situation would require 24 HDLC controllers, and therefore require the HDLC
module.
T3 Transporting One or More PRI Circuits
When a T3 circuit is transporting one or more PRI circuits, the HDLC module
will be required, in order for the PRIs' D Channels to be processed.
Back to the top of the page.
ATLAS Applications that Require Assessment of HDLC Resources
ATLAS 800PLUS/810PLUS
Back to the top of the page.
T3 Drop and Insert Rules
ATLAS allows some T3 bandwidth to be daisy-chained to another ATLAS system
via the T3 module's drop and insert (D&I) port. Note that this
daisy-chaining support is via dedicated mapping, where some of the T1s in the T3
circuit are statically mapped to the first chassis and the other T1 circuits are
statically mapped to the backup port. ATLAS does not allow bandwidth to be
dynamically switched to the T3 D&I port based on the dialed number or any
other criteria.
With the T3 Drop and Insert module installed, bandwidth from the T3 circuit
can be passed to another ATLAS system or to any device supporting channelized
T3, subject to the following rules:
- The source of bandwidth directed to the T3 module's secondary port can
only be that same module's primary port
Figure 1. T3 Drop and Insert Rules
- Bandwidth is directed to the secondary port in increments of two full T1s.
For example, it is not possible to terminate 27 DS1s in one chassis and pass
the 28th DS1 to a second system. You would need to terminate the first 26
DS1s in the first chassis and the last two in the second system.
Back to the top of the page.
Capacity Limits for BRI/PRI to T1 Conversion
When the ATLAS 800 is used to convert between BRI or PRI to a robbed-bit
signaling T1, the maximum number of conversions that can be supported in a
single system is the equivalent of 10 T1s. For example, you could convert 10
PRIs into 10 T1s in one system.
Also, this upper limit assumes a normal calling pattern. Applications that
stress the ATLAS system, such as initiating a large number of calls at exactly
the same time or leaving a very short time delay between calls, may result in
missed calls.
The capacity limit is not expected to change with the ATLAS 800PLUS.
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