Detailed description of Separation of Bearer and Signaling

Last Updated : Oct 08, 2020 |
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SBS provides a low-cost, virtual private network (VPN) over IP trunks with the high voice quality that is expected of the PSTN. By using SBS, you can save on the costs of private leased lines.
The Separation of Bearer and Signaling (SBS) feature provides a Distributed Communications Service Plus (DCS+)-like feature set using the Public Switched Telephone Network (PSTN) for the bearer (voice) element, and QSIG private network signaling over a low-cost IP Network for the signaling element of calls.With SBS Distributed Communication System Plus (DCS+), you can replace an expensive VPN service if you need Dial Plan Expansion (DPE) functionality. Therefore, you can get enhanced signaling without private leased lines. DCS does not work with 6-digit or 7-digit dial plans. Although QSIG works with 6-digit or 7-digit dial plans, QSIG does not work over VPNs. These VPNs include SDN from AT&T or V-Net from MCI.
SBS also provides a transport mechanism for application data, and other enhanced functionality in locations where ISDN trunking is unavailable or expensive.
The SBS feature supports:
  • QSIG private network signaling over a low-cost, IP-based network
  • Voice calls, or bearer traffic, over the PSTN
  • Correct association between QSIG feature signaling information and each voice call.
You must always use Automatic Alternate Routing (AAR), Automatic Route Selection (ARS), or Uniform Dial Plan (UDP) to originate an SBS call. You cannot use a Trunk Access Code (TAC) or a Dial Access Code to originate an SBS call.
Note:
This SBS feature is strictly a proprietary Communication Manager implementation and does not operate with non-Communication Manager systems. There is no known industry standard that supports Separated Bearer and Signaling calls.
When a user dials a call that is routed to an SBS trunk, two different and separate calls originate. One call carries the signaling portion of the call and the other carries the bearer portion of the call. The following is a high level description of a point to point SBS call.

Call originated

  • A signaling call is setup between the SBS Originating Node and the SBS Terminating Node (SETUP and Call PROCEEDING messages).
  • An initial SETUP message is sent which has the final destination extension number as the called party.

Complete Number constructed

  • The SBS Terminating Node selects a new extension type, SBS Extension, to use for the duration of the call setup. This new extension type indicates that the call was received on an SBS trunk group.
  • The SBS Terminating Node, using the public unknown-numbering table, constructs a Complete Number (area/city/extension) for the SBS extension.
  • The SBS Terminating Node then adds its country code to the Complete Number and sends this entire number (country code/area/city/extension) to the SBS Originating Node (INFO message).
  • With this new number, the SBS Originating Node can establish the bearer call to the SBS Terminating Node.

Complete Number Initiated to SBS Terminating Node

  • The SBS Originating Node receives the Complete Number from the SBS Terminating Node and compares the country code in the Complete Number to its own country code.
  • If the country codes match, the SBS Originating Node discards the country code.
    or
    If the country codes do not match, the SBS Originating Node adds its International Access Code to the Complete Number.
  • The SBS Originating Node uses Automatic Route Selection (ARS) to initiate a call to the SBS Terminating Node using the Complete Number. ARS is always used to initiate the bearer call. ARS uses normal call routing (ARS analysis to pattern/preference) to complete the call. Through the use of digit conversion, for example, the call can be routed using AAR and a private network.

Call completed

  • The SBS Terminating Node receives the bearer call to the SBS Extension.
  • The SBS Terminating Node determines which SBS signaling call is currently using this particular SBS Extension. The SBS Terminating Node provides a unique ID over the signaling call (INFO message) to the SBS Originating Node.
  • The SBS Originating Node passes the unique ID back to the SBS Terminating Node, through DTMF digits over the bearer call.
  • The SBS Terminating Node uses the DTMF unique ID to verify that the bearer call is the correct call to associate with the signaling call.
  • Once the SBS Terminating Node has associated the signaling and bearer call it completes the call to the called party and returns an ALERTING message to the SBS Originating Node on the SBS signaling call.
Signaling for the SBS signaling call is the same as existing H.323 trunks using QSIG signaling with two exceptions. First, the SETUP and CALL PROCEEDING messages will contain Null Caps. Second, at least two INFO messages will be sent from the SBS Terminating Node to the SBS Originating Node. The INFO messages indicate the number to route the SBS bearer call to, and then the unique ID to signal on the SBS bearer call. These INFO messages are sent after the CALL PROCEEDING, and before ALERTING. They are encoded in a standard fashion, containing a Called Party Number - Information Element (IE).