Virtual interfaces (VI) and frequency stacking (FS) are two new
features which allow user-configurable MAC domains and multiple frequencies on
one physical connector. Virtual interfaces allow up to eight upstreams (USs)
per downstream (DS). It links a US port to a physical connector. The other
feature, frequency stacking, allows two frequencies to be configured on one
The MC5x20S card can initially be configured to match the DS and US
configuration of an existing card, and then the cable operator can modify their
configurations according to their needs. This supports different DSxUS port
ratios, as combining ratios evolve (1x6 � 1x4 � 1x1). The linecard can be used
as a 1x1 for a business customer and as a 1x7 for residential customers.
FS reduces cabling per channel and gives the ability to grow a service
area without modifying cable modem termination system (CMTS)
Figure 1 depicts one MAC domain from a
MC5x20S linecard, wired up in a sparse mode configuration. Sparse
mode refers to one or more fiber optic nodes feeding one US
Figure 1 – Sparse Mode Combining
In the above combining scheme, one node is wired per US port. The
MC5x20S card has four USs per MAC domain and five MAC domains per card, which
equals twenty nodes per linecard. Using QPSK at 3.2 MHz channel width will give
5.12 Mbps per node (~4.4 Mbps usable).
Figure 2 depicts one MAC domain from a
MC5x20S linecard wired up in a dense mode configuration. Dense
mode refers to one or more fiber optic nodes feeding more than one
Figure 2 – Dense Mode Combining
Figure 2 shows one node split to feed
four US ports of one MAC domain. Because the MC5x20S has five MAC domains per
card, this wiring serves five nodes per linecard. Because one physical area
(node) feeds multiple US ports, each US port must be configured for
non-overlapping frequencies. For example, U0 equals 20.0 MHz, U1 equals 23.2
MHz, U2 equals 26.4 MHz, and U3 equals 29.6 MHz with each port set at 3.2 MHz
channel width. Using QPSK at 3.2 MHz channel width for each port will yield
20.48 Mbps for one node (~17.5 Mbps usable).
Figure 3 shows one example of frequency
stacking with the MC5x20S linecard.
Figure 3 – Frequency Stacking
Frequency stacking allows two frequencies to be configured on one
connector, which allows less splitter loss and easier management.
The benefits of using virtual interfaces are numerous. It allows an
increase in US capacity on an as-needed basis in a flexible and dynamic manner.
Grouping of DS and US according to a particular class of service (CoS)—such as
a 1x1 for commercial customers and 1x7 for residential customers—might make
more sense. This feature also simplifies installation of the MC5x20S card when
replacing an existing linecard that uses 1x6 MAC domains. VIs also enhance
availability when used in combination with Load Balancing (LB), because they
can be used to create a large LB group for cable modems (CMs) to be balanced.
For Load Balancing information, refer to
Load Balancing for the Cisco CMTS.
Frequency stacking provides the benefit of eliminating two-way splitter
loss and cabling complexity. This could save approximately 4 dB of attenuation.
FS also enhances usability, when used in combination with VIs and LB, by easily
assigning another frequency to the same physical port.
These restrictions are imposed on virtual interfaces:
No DS VIs.
There can only be one DS and up to eight USs in a domain. A MAC
domain is one DS and its associated USs.
Only linecard VIs, not across linecards.
(registered customers only)
—Simple Network Management Protocol (SNMP)
support for VIs is not yet available.
You can only assign connectors that are not assigned or assign
default settings from other domains—By default, all US ports have connectors
assigned to them. When VIs are configured, the new US ports in the MAC domain
can be assigned connectors that are not assigned or can be assigned other
ports’ default connector assignments.
N+1 restrictions—All HCCP members of a group must be the same
configuration in terms of VIs (MAC domain sizes). Once a connector is assigned
to a port which is not its default assignment, the whole linecard will failover
when a fault occurs.
High Availability—Not available on the
These restrictions are imposed on frequency stacking:
Only two adjacent ports can be stacked and no more than two
frequencies—US connectors 0 and 1 share an internal chip and so
FS nullifies one physical port to provide another physical port with
High Availability, Load Balancing, VI, and FS are not available on
the MC28U with Cisco IOS® Software Release 12.2(15)BC2x and
Virtual Interface setup is relatively basic. These cable interface
commands are used:
ubr(config-if)# cable upstream max-ports ?
<1-8> Number of upstreams
ubr(config-if)# cable upstream max-ports 6
ubr(config-if)# cable upstream 4 connector ?
<0-19> Physical port number
ubr(config-if)# cable upstream 4 connector 16
ubr(config-if)# cable upstream 4 frequency 15000000
ubr(config-if)# no cable upstream 4 shut
The DS interface is assigned a total number of US ports and each US
port is assigned a connector. Each US port is assigned its frequency, other
settings, and no… shut to activate.
Frequency stacking setup is equally as easy. These cable interface
commands are used:
ubr(config-if)# cable upstream 4 connector 16 shared
ubr(config-if)# no cable upstream 5 connector 17
ubr(config-if)# cable upstream 5 connector 16 shared
A US port is assigned a connector command with the
shared keyword at the end of the command. Upstream
ports assigned the same connector and shared will be
frequency stacked. Before a port can be assigned a connector that is already
used, it must be unassigned from its own connector and the
shared keyword must be active on both ports.
Verifying virtual interfaces and frequency stacking can be done by
issuing the show controller and show
show controller cable_interface upstream mapping
Cable6/0/0 Upstream 4 is up
Frequency 15.008 MHz, Channel Width 1.600 MHz, QPSK Symbol Rate 1.280 Msps
This US is mapped to physical port 16
Spectrum Group is overridden
SNR - Unknown - no modems online.
Nominal Input Power Level 0 dBmV, Tx Timing Offset 0
!--- Output suppressed.
no ip address
cable bundle 1
cable downstream annex B
cable downstream modulation 64qam
cable downstream interleave-depth 32
cable downstream frequency 453000000
cable downstream channel-id 0
no cable downstream rf-shutdown
cable upstream max-ports 6
cable upstream 0 connector 0
cable upstream 0 frequency 16000000
cable upstream 0 power-level 0
cable upstream 0 channel-width 1600000
cable upstream 0 minislot-size 4
cable upstream 0 modulation-profile 21
cable upstream 0 s160-atp-workaround
no cable upstream 0 shutdown
!--- Output suppressed.
cable upstream 4 connector 16 shared
cable upstream 4 frequency 15008000
cable upstream 4 power-level 0
cable upstream 4 channel-width 3200000
cable upstream 4 minislot-size 4
cable upstream 4 modulation-profile 21
cable upstream 4 s160-atp-workaround
no cable upstream 4 shutdown
cable upstream 5 connector 16 shared
cable upstream 5 frequency 18208000
cable upstream 5 power-level 0
cable upstream 5 channel-width 3200000
cable upstream 5 minislot-size 4
cable upstream 5 modulation-profile 21
cable upstream 5 s160-atp-workaround
no cable upstream 5 shutdown
There are some things to keep in mind when configuring virtual
Caution: When doing N+1 redundancy, the Protect linecard has no connector set
by default. Cisco IOS Software Release 12.2(15)BC2 synchronizes all interface
configurations from the Working to the Protect. If the user happens to
downgrade the uBR from BC2 to an earlier Cisco IOS Software release, the
Protect linecard will have to be pre-configured for connector commands, because
earlier releases of Cisco IOS Software do not synchronize those commands from
Working to Protect.
These are the different possible mapping configurations:
Map the last two MAC domains into the first three to make three 1x6
MAC domains. This yields:
DS0 with connectors 0 to 3
cable upstream 4 connector 14
cable upstream 5 connector 15
DS1 with connectors 4 to 7
cable upstream 4 connector 16
cable upstream 5 connector 17
DS2 with connectors 8 to 11
cable upstream 4 connector 18
cable upstream 5 connector 19
DSs 3 and 4 could be assigned connectors 12 or 13 or
If you use sequential numbering, this yields:
DS0 with connectors 0 to 5
DS1 with connectors 6 to 11
DS2 with connectors 12 to 17
DSs 3 and 4 could be assigned connectors 18 or 19 or
Assign connectors based on the dense connector bundles and the
“cleaner” failover bundles for N+1. Since DS2 normally has its four USs across
two dense connectors, use it as the spare MAC domain. Assuming 1x6 and 1x4 MAC
domains, this yields:
DS0 with connectors 0 to 3
cable upstream 4 connector 8
cable upstream 5 connector 9
DS1 with connectors 4 to 7
DS3 with connectors 12 to 15
cable upstream 4 connector 10
cable upstream 5 connector 11
DS4 with connectors 16 to 19
DS2 could be activated later with its connectors reassigned from
DSs 0 and 3.
Caution: Another key point of which to be aware is the potential for user
error if copying interface configurations from one interface to another. Many
users get careless and copy an interface configuration verbatim to another
interface. Connector commands can not be arbitrarily copied from one interface
to another. Use extreme caution.
Note: Also be aware that using a default connector assignment from another
domain will automatically delete it from that domain. If you unconfigure it, it
does not automatically go back to the original domain.
These are some frequency stacking concerns:
You must use physical connector bundles (0 1), (2 3), (4 5), (6 7),
and so forth—The MC5x20S linecard has twenty US connectors, but only ten US
If configured incorrectly, you see this message:
%Invalid config. Please check existing config on physical connector 19 and/or 18
Only one front-end for both frequencies, pre-amp, and so forth—If two
frequencies are selected that are spread far apart, each frequency may require
different pre-amp or equalization settings.
Modems must re-acquire after changes are made to either VIs or
FS—Configuration changes made for virtual interfaces or frequency stacking
requires cable modems to re-register with the
Virtual interfaces and frequency stacking are complimentary with each
other and with load balancing, not to mention advanced time division multiplex
access (ATDMA). This is another way that Cisco sets itself apart from
competitors, with features that can be used to expand existing architectures
The cost of physical segmentation of a fiber node can be on the order
of ten times more than the cost of the simple addition of another US frequency.
Having the flexibility to add more US ports to the same MAC domain—or to
eliminate some US path attenuation—makes this chore less daunting, when
customers start demanding more throughput.
N+1 linecard failovers and intelligent upstream spectrum management
increases availability even more, with the features mentioned in this