PSTN trunks - Cisco Unified Communications 500 Series - Cisco Smart Business Communication Systems


Thu, 10/29/2009 - 05:46
Nov 24th, 2008
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A Public Switched Telephone Network (PSTN) trunk is typically the voice interconnect between any business and the outside world. The Cisco Unified Communications 500 (UC500) Series provides various options to terminate PSTN connections into a business. This section will deal with the traditional TDM interconnects offered while there is a separate section for IP interconnects using SIP or H.323 trunks. The TDM interconnect options include:

  • Analog:
    • Analog CO trunks, also called 1FB (using FXO ports)
    • Analog DID trunks (using DID cards in expansion VIC slot)
  • Digital:
    • ISDN BRI  (using BRI ports)
    • T1 or E1 voice circuits that use ISDN PRI or CAS (E&M wink start, E1 R2 etc) for signaling (using T1 or E1 VWIC cards)

Common PSTN features that the UC500 interfaces support include:

  • Caller ID name and number
  • Automatic Number Identification (ANI)
  • Direct inward dialing (DID)& direct outward dialing
  • Dedicated trunk mapping to phone button

A subset of the above options, that are not available in Cisco Configuration Assisant (CCA) but are supported in the UC500.


Where do I terminate different TDM Trunk connections on the UC500?

The UC500 supports a variety of interfaces to terminate TDM trunks and devices:

  1. Fax machines, analog phones & modems would normally connect to the FXS ports
  2. Paging systems can connect to either FXS or FXO ports depending on the paging system capabilities. If Paging system end is FXO, then it plugs into an FXS on the UC500 and vice versa
  3. Analog phone lines (also called 1FB) from the Telco or Provider (CO) would normally connect to the FXO ports which use Loopstart signaling. There maybe rare instances of an FXO or FXS port being required to support GroundStart signaling in which case this is supported using CLI
  4. Digital ISDN BRI lines (common in EU) would connect to the BRI ports
  5. T1 or E1 circuits terminate on the T1 / E1 port or VIC - these circuits can have 2 options for signaling:
    • ISDN PRI which is more common - has different switch types such as:
      • National ISDN or NI2 - translates to primary-ni (Most common in US)
      • NET5 - translates to primary-net5 (Most common in Europe, Asia, Australia)
      • Custom 5ESS - translates to primary-5ess
      • 4ESS - translates to primary-4ess
      • Nortel DMS-100 - translates to primary-dms100
      • NTT - translates to primary-ntt (Most common in Japan)
    • CAS or Channel Associated Signaling

Inbound calls on certain analog lines from the CO seem to ring twice on phones, then call goes away? 

This is because the tip / ring on the line is reversed. Test each analog line individually, for all lines that show the above behavior, reverse the tip & ring wires and re test.

What are the total number of PSTN interfaces that the UC500 can support?

Depending on the UC500 model or SKU - can have any of the below:

    • 4 to 12 FXO ports or 2 to 6 BRI ports (VIC slot can be used to add interfaces in certain model)
    • T1/E1 VWIC interface card for use in the 8-, 16-, and 32-user UC500 SKUs (VIC slot can be used to add this T1/E1 interface card)
    • UC500 48-user SKU is also available with integrated T1/E1 interface.

What is the predetermined outbound call routing per fxo port now and can you change this so that when dialing out a particular fxo port is used first?

Currently the FXO ports are all placed at the same preference level - so selection of the ports is random. If you do need to put the ports in a particular oder, this can be done via the CLI using the preference on each of the POTS dial peers pointing to individual FXO ports, not CCA at this time. An example can be found on the least cost routing section at First Look 2008 - Configuring Dial-peers using IOS CLI

Does UC500 support connecting to door or gate openers or intercoms for security?

Yes you can connect door / gate openers as intercoms to IP phones. Typically such units connect to either FXS or FXO ports on the UC500 depending on the physical characteristics  of the door opener. When doorbell is pressed, phone rings, you answer it, then press DTMF to open the door. Some vendors that offer such devices (NOTE these are based on field information, not officially tested as part of the SBCS solution):

Most configuration can be done via CCA but some additional CLI may be required such as disabling battery reversal on FXS ports as shown below:

voice-port 0/0/0

no battery-reversal

Is there a limit to the number of Direct Inward Dialing (DID) or Direct Dial-In (DDI) on the UC500?

The actual number of DIDs will depend only on what the customer's telco provisions for the PSTN line. The number of concurrent calls supported will depend on the T1/E1 channels or Analog DID ports available, as well as the number of DSP resources available to handle to handle voice calls.

What T1 parameters are supported on the UC500 for voice termination?

The below parameters are supported:

  • T1 line coding = B8ZS
  • T1 framing = ESF
  • Interface switch Type for PRI - National with FAS (User side)
  •   Hunt Type = two way, descending (channel 23 to 1)
  •   Switch Protocol = See ISDN PRI switch-types above

Will the UC500 support a fractional T1/PRI?

Yes, the UC500 will support a fractional T1/PRI. The number of channels you may have in a PRI depends on three things: 1) the model of UC500 you are using, 2) the method of conferencing you are using (software or hardware), and 3) the audio codec you are using. The list below describes the "maximum" channels you may havein "<conferencing mode> / <codec>" notation:

8/16 user models

  1. Software / g711ulaw: 22 channels
  2. Hardware / g711ulaw: 6 channels
  3. Software / g729r8: 14 channels
  4. Hardware / g729r8: None

24/32/48 user models *

  1. Software / g711ulaw:  Full T1/PRI
  2. Hardware / g711ulaw: Full T1/PRI
  3. Software / g729r8: Full T1/PRI
  4. Hardware / g729r8: 22 channels
  • * The following SKUs do not support the VWIC2-1MFT-T1/E1 card and thus cannot support a T1/PRI:
    • UC520-48U-12FXO-K9
    • UC520-48U-6BRI-K9

48-user model with built-in T1/PRI card - UC520-48U-T/E/F-K9 & UC520-48U-T/E/B-K9

  1. Software / g711ulaw:  Full T1/PRI
  2. Hardware / g711ulaw: Full T1/PRI
  3. Software / g729r8: Full T1/PRI
  4. Hardware / g729r8: Full T1/PRI

Why does hardware conferencing limit available channels on T1/PRI?

DSP resources are reserved whenever a channel of a voice T1 or PRI is brought online. In the UC500, these DSP resources are fixed and cannot be upgraded. Using hardware conferencing requires the use of extra DSP resources to support the enhanced functionality. This limits the amount of DSP resources available to bring up channels of a T1/PRI, which is why the number is limited on some models with Hardware Conferencing enabled.

For more information on Software vs. Hardware Conferencing, see the note in the System Features section of the wiki.

Can UC500 route mobile calls out a GSM Gateway?

Yes this should work but is not a validated configuration in SBCS. A GSM gateway that providers a standards based ISDN interface can be configured to talk to the UC500. Changes to the dialplan would also need to be made outside of CCA to support least cost routing or routing of specific dial patterns out the ISDN trunk to the GSM gateways. An example can be found on the least cost routing section at First Look 2008 - Configuring Dial-peers using IOS CLI

Can the T1/E1 module on UC500 terminate Voice and Data?

The VWIC-2MFT-T1/E1 card on UC500 can only be used for Voice T1 termination. It cannot be used for terminating Data T1s. On UC500, this card does not support multiplexing of Voice and Data. For Data connectivity, using an ISR based T1/E1 is recommended.

Is it possible to setup the system not to require 9 to reach PSTN?

You can configure the UC500 in keysystem mode, where the user can seize available FXO or T1 CAS to dial numbers directly to CO.

Are there any feature differences between keysystem mode & PBX mode? Are there any restrictions on platforms that support keysystem mode

UC500 plaforms that have a FXO or T1 CAS voice interface support keysystem mode. Details regarding feature differences are available in PBX versus keysystem feature matrix



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