Telco - Return

How to Switch Over Cable Telco-Return to Two-Way RF Return Path

Document ID: 12214

Updated: May 18, 2006



This document explains how to perform a cut-over from a Telco-Return (one-way) scenario to a two-way Radio Frequency (RF) return path. In a two-way plant scenario, cable modems use an RF upstream instead of an asynchronous Point-to-Point Protocol (PPP) return path via the Public Switched Telephone Network (PSTN) (see Figure 1). This cut-over process is usually done after a cable plant has upgraded their unidirectional amplifiers to bidirectional and can therefore support a RF transmission in both the forward and return paths. The challenge for Multiple Service Operators (MSOs) is to make the cut-over with a minimal impact to paying subscribers in a live Hybrid Fiber-Coaxial (HFC) network.

Figure 1 - Telco-Return Diagram


The ability to perform such a cut-over seems complex physically, and may potentially interrupt service by losing Internet connectivity for longer than desirable periods. However, this should not be the case if executed correctly. This is because a Cisco uBR7246 Cable Modem Termination System (CMTS) can support both modes: Telco-Return and two-way return paths mode at the same time on the same headend and the same cable linecard.

Note: It is important to know that, before converting all your modems to two-way, you should carefully select the upstream frequency (the example uses 24000000 MHz). Also, make sure the return path is available and clean by using a spectrum analyzer. To learn how to measure the upstream, refer to Cable FAQ. When performing a cut-over from Telco-Return to two-way, it is recommended to first try the cut-over with a few cable modems inside the cable plant headend and a small sample out in the field. This can be done initially as a test to check if there are any return path issues before rolling out the cut-over.



There are no specific requirements for this document.

Components Used

This document uses these specific software and hardware versions:

  • uBR7223 with Cisco IOS® software 12.0(5)T

  • Cisco Network Registrar (CNR) version 3.5.3


Refer to Cisco Technical Tips Conventions for more information on document conventions.

Example of a Mixed Telco-Return and Two-Way Environment

This portion of a CMTS configuration shows the necessary commands that are relevant to Telco-Return.

interface Cable2/0 
    ip address secondary 
    ip address 
    no ip directed-broadcast 
    cable helper-address 
    no ip route-cache 
    no ip mroute-cache 
    no keepalive 
    cable insertion-interval automatic 25 500 
    cable dhcp-giaddr policy 
    cable downstream annex B 
    cable downstream modulation 64qam 
    cable downstream interleave-depth 32 
    cable downstream frequency 117000000 
    cable upstream 0 frequency 24000000 
    cable upstream 0 power-level 0 
    cable upstream 0 range-backoff 0 6 
    no cable upstream 0 shutdown 
    cable Telco-Return enable 
    cable Telco-Return spd 1 factory-default 
    cable Telco-Return spd 1 threshold 255 
    cable Telco-Return spd 1 dial-timer 1200 
    cable Telco-Return spd 1 manual-dial 
    cable Telco-Return spd 1 dhcp-authenticate 
    cable Telco-Return spd 1 dhcp-server 
    cable Telco-Return spd 1 ppp-authenticate pap 
    cable Telco-Return spd 1 phonenum 2489888 
    cable Telco-Return spd 1 username test 
    cable Telco-Return spd 1 password test 

For a complete configuration of Telco-Return and explanation of the commands used in the preceding example, refer to Telco-Return for the Cisco uBR7200 Series Universal Broadband Router.

The next example is the show cable qos profile and the show cable modem output with respect to the working configuration. The output shows an example of a mixed Telco-Return and two-way environments in the same CMTS. Notice that all the cable modems shown here are in the same cable interface (Cable2/0).

Note: The modems that have a T are for Telco-Return and those with a U0 are for two-way modems using upstream port 0. Both modes are on the same MC16C card interface Cable2/0.

ubr7223#show cable modem 
Interface   Prim Online    Timing     Rec       QoS    CPE IP address      MAC address 
            Sid   State    Offset     Power    
Cable2/0/T  94   online     0          0.00     3      2     0020.4066.b6b0 
Cable2/0/T  95   online     0          0.00     3      1      0020.4061.db5e 
Cable2/0/T  96   online     0          0.00     3      1     0020.4066.b644 
Cable2/0/U0 97   online    307         0.25     4      1     0020.4002.fc7c    
Cable2/0/T  98   online     0          0.00     3      1     0020.4003.65fe 
Cable2/0/U0 99   online    332         0.25     4      0     0020.400b.9b40    
Cable2/0/U0 100  online    277         0.25     4      1     0020.4002.ff42 
Cable2/0/T  101  online     0          0.00     3      1     0020.4066.b6c8 
Cable2/0/U0 102  online    272         0.25     4      1     0020.400b.9b84 
Cable2/0/T  103  online     0          0.00     3      1     0020.4003.6788 
Cable2/0/T  104  online     0          0.00     3      1      0020.400b.9af6 
Cable2/0/T  105  online     0          0.00     3      1     0020.4065.d75e 
Cable2/0/T  106  online     0          0.00     3      2     0020.4065.9148 
Cable2/0/T  107  online     0          0.00     3      2      0020.4066.d2b0 
Cable2/0/T  108  online     0          0.00     3      1     0020.4003.7110 
Cable2/0/T  109  online     0          0.00     3      1     0020.4003.6b22 
Cable2/0/U0 111  online    227         0.25     4      1     0020.4002.fd0e 
Cable2/0/T  112  online     0          0.00     3      0     0020.4062.1ba0 
Cable2/0/T  113  online     0          0.00     3      1     0020.400b.9a22 
Cable2/0/T  114  online     0          0.00     3      1     0020.4061.65ee 
Cable2/0/T  115  online     0          0.00     3      1     0020.4002.ffb4 
Cable2/0/T  116  online     0          0.00     3      1      0020.407e.a54c 
Cable2/0/T  117  online     0          0.00     3      1      0020.4084.1780 

An interesting point to note is that both modes (Telco-Return and two-way) are using different DOCSIS configuration files, as seen in the output show cable qos profile in comparison to the show cable modem command. All online Telco-Return modems are using QoS #3 and two-way are using QoS #4.

In this example, there are two DOCSIS configuration files. One file uses Telco-Return with QoS #3, the other file uses two-way with QoS #4.

ubr7223#show cable qos profile 
Service Prio   Max         Guarantee   Max         Max tx     TOS   TOS    Create     B 
class          upstream    upstream    downstream  burst      mask  value  by         priv 
               bandwidth   bandwidth   bandwidth                                      enab 
1       0      0           0           0           0           0x0  0x0    cmts(r)    no 
2       0      64000       0           1000000     0           0x0  0x0    cmts(r)    no 
3       1      128000      0           512000      0           0x0  0x0    cm         no 
4       7      32000       0           265000      0           0x0  0x0    cm         no 

Decision Process of the Cable Modem to Know If the System is Telco-Return or Two-Way Return Path

The General Instruments (GI) SURFboard SB2100D and 3100D modems have the ability to do both two-way and Telco-Return. The D in 2100D and 3100D stands for dual capable. When the dual modem comes fresh out of the box, and is installed in the system for the first time, it is initialized in an unknown state. In the unknown state, the cable modem listens for messages in the downstream to learn how to acquire an upstream. If the cable modem hears Upstream Channel Descriptors (UCD) it attempts to make a connection on the RF return path. If the cable modem hears Telephone Channel Descriptors (TCD) it attempts to make an upstream connection using the internal PPP modem (inside the GI modem) via the PSTN after failing to acquire the upstream RF Return path. Usually, dual GI cable modems look for UCDs first. If the cable modem is using Telco-Return, a UCD message is not received. The cable modem starts looking for TCDs.

Switching Modes in a Dual Telco-Return Modem Automatically

A Telco-Return dual D modem does not have the ability to automatically switch back and forth between modes. Once the dual cable modem acquires an upstream successfully, no matter what mode that is, the dual modem stays in that mode until modified by the system operator. The SB2100D and the SB3100D do not have the ability to automatically switch between RF and Telco-Return modes. Once the modem has registered in Telco-Return mode, it ignores UCDs.

Ways of Switching the Dual Cable Modem from Telco-Return to Two-Way Modes Manually

The system operator may use three different ways to change the dual modem to RF two-way mode:

Using the DOCSIS Configuration File

The DOCSIS configuration file can be modified. Change the value for the Upstream Channel ID which dictates the mode:

  • 0 = Telco-Return

  • 1 or greater = RF Return

Figure 2 - Cisco DOCSIS CPE Configurator V3.0a


This is a screen capture of the Cisco DOCSIS Configurator V3.0 used for setting the Upstream Channel ID for RF Return path.

To perform a cut-over, you can create one or two DOCSIS configuration files. This depends on how quickly you want to perform the transition. If you want to change all modems from Telco-Return to two-way in one step, you would only need to edit the current Telco-Return DOCSIS configuration file in the Upstream Channel ID field (see Figure 2) from a 0 to either a 1, or a blank. Afterwards, power cycle the modems forcing them to grab the new parameters. This enables two-way mode. If you want to perform a more gradual cut-over, where only a percentage of the Telco-Return modems are converted to two-way, you need two DOCSIS configuration files, as shown in Figure 2. It also is necessary to enable Client Class Processing on CNR. To minimalize risks, it is recommended that the cut-over is performed gradually in a production environment.

Using SNMP

The second method is to switch the mode via Simple Network Management Protocol (SNMP) using the 3Com provided public Management Information Base (MIB): TelcoReturnCABLE-DEVICE-MIB.

Using the docsTrCmMode MIB object, you can adjust the upstream selection as shown in the MIB properties value table. This object, which has the OID of, represents the cable modem operating mode. A cable modem may be operating in these modes with the MIB integer values as indicated in this table.

Mode MIB Integer Value
Other 1
Telco-Return 2
Two Way 3

Using the DOCSIS Configurator Tool (see Figure 3), you can set the docsTrCmMode MIB object from Telco-Return mode to two-way mode during a cut-over by setting the integer value equal to 3.

Figure 3 - Cisco DOCSIS CPE Configurator V3.0a


This is a screen capture of the Cisco DOCSIS Configurator V3.0 use for setting the docTrCmMode MIB.

This is a portion of the MIB definition that contains the MIB object docsTrCmMode:

Name: docsTrCmMIB  
Full path:  
Module: TelcoReturnCABLE-DEVICE-MIB  
Parent: transmission  
First child: docsTrCmMIBObjects  
Prev sibling: docsIfMib  
Last updated: July 28, 98 at 09:20 GMT (9807280920Z)  
Organization: 3Com - Cable Access  
Contact:   Jack Fijolek/Srinivyasa Murthy  Adiraju  
  Postal: 3Com  
  3800 Golf Road  
  Rolling Meadows, IL 60008  
  Tel:    +1 847 2622201  +1 847 2622205  
  Fax:    +1 847 2620258  
Description: Telco-Return MIB for Data Over Cable Access modems and termination systems 

Resetting to Factory Default

This method involves your cable modem and PC.

The third way to switch from Telco-Return to two-way mode is having you reset to factory default. This method is accomplished by using an HTML Configuration Manager User Interface web page. Using this tool, you can make the modem think that it is "right out of the box" in an unknown state. In this unknown state, the modem scans for the best return path on next boot up.

When the modem is reset to factory default, it listens for UCDs and attempts to make a connection on the RF return. If the modem cannot find an upstream path, it listens for TCDs.

To access the HTML Configuration Manager User Interface, complete these steps:

  1. Connect a straight-through Ethernet cable from your PC to the RJ-45 Ethernet port of your dual GI Telco-Return cable modem.

  2. Type in your browser.

  3. Configure the TCP/IP properties with an IP address on the same subnet as

For example, you can set the IP address with a mask of, and a default gateway of Once you set the TCP/IP properties, and can ping the IP address from your PC, you should be able to launch the diagnostic tool with a browser. A diagnostic HTML Configuration Manager page opens and allows you to reset the modem to the factory default. Figure 4 shows how to reset to factory default by clicking on the Reset to Defaults button. Notice that this picture shows only the bottom part of the web page.

Note: This method is the least desirable because you must have end users (subscribers) involved in the cut-over process, or field engineers may be sent to your location to perform the cut-over process.

Figure 4 - Reset To Factory Screen on a Surfboard SB3100D Telco-Return Modem


Related Information

Updated: May 18, 2006
Document ID: 12214