Tie Line Management Overview
Technical Note
Date: 2/25/98
Products: SC-3 Controller, Utah-300
SC-3 Tie Line Management Overview
Tie lines are a very useful feature expanding the
overall functionality and capability of routers. Tie
lines provide a simple way of sharing sources between
two routers increasing the number of sources that
may be switched to any of the destinations of either
router. The SC-3 controller accommodates tie lines
between all Utah routers connected to a common SC-3
controller. By simply inserting a signal format converter
into the tie line path, switching between two routers
of different signal formats can also be accomplished.
This allows, for instance, a digital video router
to switch a source from an analog video router to
the digital video router's outputs.
Once tie lines are implemented correctly the tie
line and any converters in the tie line path are transparent
to the user and the switch from the input of one router
to the output of the other router happens seamlessly.
Configuring and managing tie lines is where Utah
Scientific's SC-3 System Controller excels with its
tie line management capabilities.
What is a Tie Line
A tie line is one or more physical connections from
the output(s) of one router to the input(s) of another
with the two routers requiring the sharing of sources.
These physical connections may have converters in
the path to convert from one format to another, matching
the formats handled by the routers. Tie lines can
be used for all signal types as long as the signal
types of the two routers being used are the same i.e.
video to video, audio to audio, etc. or the appropriate
converters are in place. Once the physical connections
are made, the tie line must be configured in the controller
to allow the necessary switching to occur.
SC-3 and RMS Software
The SC-3 controller provides the intelligence for
configuring and managing tie lines and is interfaced
using Utah Scientific's Router Management System (RMS),
a PC based software program. RMS runs on a Windows
NT platform and provides the user with a Graphical
User Interface to configure and manage tie lines.
RMS accesses the SC-3 over Ethernet allowing either
stand-alone configuration or any RMS computer on the
customer's network to access the SC-3 (Figure 1).
The SC-3 and RMS software assist the user in resolving
one of the more difficult issues surrounding tie lines
which is how many tie lines are required for a specific
application. RMS and the SC-3 provide visibility to
the user at any time as to how tie lines are configured,
what tie lines are currently in use, and usage stats
over a given time frame.
Figure 1 - Ethernet
Configurations
Tie Line Applications
Tie lines may be implemented for a number of various
reasons the most common of which is to provide a source
from one router to another when both routers are of
different format standards. The diagram (Figure 2)
below depicts this application in simple form.
Figure 2 - Tie Line
use for Format Conversion
The above application of tie lines allows for seamless
switching of any source located on either of the two
routers to any destination on either of the two routers
even though the two routers are of different video
formats.
Another common use for tie lines is to expand the
size of a router beyond the size allowed by the original
chassis. For instance a 128x128 chassis could be expanded
to include a 64x64 chassis. This allows each router
to switch sources from the other router to its own
destinations but it does not create, in this case,
a fully functional 192x192 router. Limitations include
the number of tie lines between the two routers and
the overall number of cross points available at any
given time compared to a true 192x192 router. Once
all of the tie lines are in use, no more switches
are possible between the two routers until a tie line
becomes free. This is, however, a very effective way
to share sources across two routers. The implementation
for this would be the same as figure - 2 above minus
the A/D and D/A converters.
A third application for tie lines would be "distributed"
routing where a number of sources connected to a main
router are made available to several routers. Tie
lines in this scenario are typically only one direction
going from the main router to the remote routers as
shown below in figure 3.
Figure 3 - Distributed
Routers
Tie Line Configuration and Usage
Tie line configuration is somewhat complex in general
to set up and can become very complex with large implementations.
The SC-3 and RMS software provide the ability to configure
tie lines and effectively manage them. The screen
below shows tie line group configurations for all
tie lines controlled by this SC-3 and where a Utah-300
is the primary router. If an AVS router controlled
by this SC-3 were setup up as a primary router for
tie lines, a similar screen under the AVS tab would
be used to configure those tie line groups. A tie
line mapping table must also be configured to provide
"offset" information to differentiate the
inputs and outputs of the two routers. For example,
when input 2 is switched to output 147, the mapping
provides the information required to determine which
router input 2 and output 147 resides on and whether
a tie line is required to accomplish the switch. This
offset may be done physically using dip switch settings
on the routers so that their actual inputs and outputs
are physically different, or it may be done logically
in the mapping tables in RMS in which case the inputs
and outputs could be identical on the two routers.
Panel source encoding must also be completed to provide
SCP and/or CSP panels information about the tie lines
and to fully implement the tie line configurations.
Once this configuration is complete the switch is
as simple as a "take" within a single router.
Figure 4 - RMS
Tie line Configuration
The SC-3 and RMS software manage tie lines in groups.
A tie line group specifies a group of connections,
which may include converters, and manages them as
a resource group. Statistics are kept for each tie
line group to help in managing the tie line usage
and ensure that the correct number of tie lines are
in place. The number of requests are accumulated to
show how many total tie line paths are asked for since
the stats were last reset. This provides an overall
feel for tie line usage. RMS also displays the total
number of requests that could not be accommodated
due to all of the tie lines being in use at the time
a request was made. Maximum tie lines in use at any
given time are also displayed for the period since
the last reset of the statistics. These tools are
very valuable in determining whether the current number
of tie lines is adequate or if additional tie lines
need to be added or subtracted.
Once an input from router 1 is switched to router
2 that same input may be switched to any number of
outputs using the same tie line. The number of outputs
an input is switched to is reflected in the "Use
Cnt" column in the configurations table to show
the user at all times how the tie lines are being
utilized. The input switched onto a tie line is also
reflected in the "Tie Src" column of the
configuration table. To free up a tie line, all of
the outputs on the tie line must be switched to a
source that does not require a tie line.
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