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Two-Way Frequency Use
By: Bruce Bahlmann - Contributing Author (your
feedback
is important to us!)
Created: May 6, 1998
Note: For help designing/implementing your multi-frequency network or developing tools to help you improve or implement such a program contact Birds-Eye.Net.
Overview:
This document is intended to describe how we currently provision two-way frequencies
for high-speed data and look at ways these methods could be applied towards supporting
DOCSIS (formally MCNS) equipment.
Today:
The predominant equipment used by the MSO in the two-way space is Bay Networks LANCity
rev. 3 gear. This equipment is presently used to handle two different cases:
- Serves a single frequency pair over a Down Stream Area (DSA)
- Serves multiple frequency pairs over a Merged Down Stream Area (MDSA)
Single Frequency Pair:
In the first instance, the model is quite simple. A DSA connects many Cable Modems
(CM)s to a single Cable Modem Termination System (CMTS) and several CMTS are connected
together (via hub or ethernet switch – not shown) represent a network (or VLAN). This
collection of CMTS or DSAs share some
Figure 1.0 Single frequency pair represents a singe Down Steam Area
(DSA)
commonality that is critical for provisioning (see Figure 1.0). The following represent
this commonality:
- Every CM and CMTS on a network share the same frequency pair and this information is
include in the configuration file they download as part of the booting process.
- Every CM, CMTS, and Customer Premise Equipment (CPE) share the same gateway and subnet
mask. Since secondary addressing is used, all these devices are on different networks from
one another.
- The gateway for this network is used by the provisioning system to distinguish devices
on this network from other devices on other networks. This allows the provisioning system
to deliver the correct configuration file needed by the CM and CMTS to function properly.
Note that a DSA does not always represent a single fiber node. In some deployments,
several fiber nodes are joined or combined to form a combined DSA. This is done to for a
variety of reasons. For example combining several nodes into one reduces the number of
CMTS required to support a DSA – which ends up costing less and taking up less space
in the headend. However there are problems with this approach that can negatively effect
operations and troubleshooting. For example, if the CMTS dies, the entire combined DSA is
effected. This also defeats troubleshooting methods of determining DSA specific problems
because there are more than one DSA involved. Lastly, if the combined DSA become congested
with subscribers it must be un-bundled which is labor intensive. For these reasons, a
single DSA per CMTS is the preferred model.
For the most part, the single frequency pair model works well for 99% of today’s
2-way implementations. This is because the MSO simply does not yet have enough customers
clustered on any one DSA that would warrant adding an additional frequency pair. This is
not foreseen in the near future. However, should the MSO need to, there is a plan for
implementing multiple frequencies over a single DSA using is Bay Networks LANCity rev. 3
gear.
Multiple Frequency Pairs
The plan for implementing multiple frequencies over a DSA utilizes much of the
processes for dealing with the single frequency model with a few exceptions (see Figure
1.1). The idea behind deploying multiple frequencies over the DSA (creating a Merged DSA
or MDSA) is to stack unique networks on top of one another. In this way the frequency
spectrum keeps each network (RF LAN) separate. Although all devices on a MDSA are
connected together, each RF LAN represents a unique pair of frequencies that it uses
Figure 1.1 Multiple frequency pairs represents a Merged DSA (MDSA)
to transmit and receive data. These unique RF LANs consist of a gateway, one or more
CMTS, and several CMs. Although each network’s frequency pair is combined to the
MDSA, the CM must be told via provisioning which frequency pair to use (which RF LAN to
join).
Note that similar to the single frequency pair model, CMs learn a valid frequency
pair during to booting process. However, in the multiple frequency pair model, the
frequency pair learned may or may not be what the CM is to use. Therefore during the
booting process a configuration file is downloaded to the CM which contains the actual
frequencies that the CM is to use. If the frequency pair contained in the configuration
file is different from the ones the CM learned during the boot cycle, the cycle is
repeated using the frequency pair contained in the configuration file. In this model, its
important to understand that the Operational Support Systems (i.e. database) maintains
which CM is on which RF LAN.
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