Alert-Info and P-Early Media Headers Interworking

Overview

The SBC supports the following AI to PEM interworking functionality using the flag aiToPemInterworking:

• Interworking between a network supporting AI header (based on the Legacy Mobile Station Domain (LMSD) format) to a network supporting PEM header. The SBC supports interworking irrespective of the existence of a provisioned tone on the SBC.
• Interworking between a network that does not support PEM header to a network that supports PEM header. For example, the ingress network supports PEM header; however, the egress network does not.
• Interworking between networks that support PEM headers.

When a tone is configured on the SBC,

• If the flag aiToPemInterworking is disabled, the SBC plays tone based on the LMSD format. For more information, refer to Tones and Announcements. 

• If the flag aiToPemInterworking is enabled, the SBC supports interworking between AI and PEM headers. The SBC plays tone when it receives AI header with sig-id=rt in the 180 provisioning response (either first 180 response or subsequent 180 response) from the Mobile Switching Center (MSC) (CDMA network). 
  

  Note

  • When all the tone playing criteria are fulfilled, the SBC inserts PEM header as SENDRECV (PEM: SENDRECV) and sends it towards the ingress network.

  • When the SBC fails to play tone, the SBC inserts PEM header as INACTIVE (PEM: INACTIVE) and sends it towards the ingress network.

When tone is not configured on the SBC; and the IPSP flag acceptAlertInfo is enabled on the egress TG, and the INVITE message is received with PEM: SUPPORTED,

• If the flag aiToPemInterworking is disabled, the SBC falls back to the existing LMSD interwoking functionality. For more information, refer to LMSD Interworking without Tones.
  

• If the flag aiToPemInterworking is enabled, the SBC supports interworking between AI header (received in the LMSD format) and PEM header.
   

  Note

  • When the User-Agent Server (UAS) plays tone, the SBC inserts PEM header as SENDRECV (PEM: SENDRECV) and sends it towards the ingress network.
  • When the User-Agent Client (UAC) plays tone, the SBC inserts PEM header as INACTIVE (PEM: INACTIVE) and sends it towards the ingress network.

P-Early Media to Alert-Info Header Interworking

The SBC supports interworking between a network supporting PEM header to a network supporting Al header. To support this functionality, the flag aiToPemInterworking is used in the IP Signaling Profile. The SBC performs PEM to AI interworking once the 180 response is received with PEM header, while forwarding 180 response. If the peer does not explicitly provide early media authorization using a PEM header in 180 response with SDP answer, the SBC monitors the RTP traffic from the egress TG and performs a cut-through if RTP is received from the egress. To support this functionality, the flag monitorRTP is added to the SIP Trunk Group.

Explicit Early-Media-Authorization or Gating

Explicit Early-Media-Authorization signifies that a given network publishes P-Early-Media header support in the INVITE request and requires "authorized" early media. These networks, unless explicit Early-Media-Authorization is received, discard early media from UAS. These networks expect the P-Early-Media header in the provisional responses from UAS to determine that early media received after this response is deemed to come from an authorized source.

• Terminating network shall insert "P-Early-Media:sendrecv/sendonly" header to authorize the UAS to send early media.
• Terminating network inserts "P-Early-Media:inactive/recvonly" header, to indicate that UAS is not authorized to send early media. 
• However, there is ambiguity when P-Early-Media is not inserted by UAS in provisional response containing SDP answer and current early media authorization state is not known. In order to handle these scenarios, Ribbon SBC shall monitor RTP traffic from UAS for specific time duration (to acknowledge a continuous stream of RTP packets) and generate P-Early-Media based on whether (sufficient) RTP packets are received or not.

The SBC uses the following "early-media-authorization" status' on a per TG level:

• When INVITE is sent to a peer that has P-Early-Media:Supported
• "no" – on no dialog associated with this request P-Early-Media header was received
• "e-no" – on dialog associated with this request P-Early-Media header was received as “recvonly”/”inactive”
• "e-yes" – on dialog associated with this request P-Early-Media header was received as “sendonly”/”sendrecv”
•  “yes”  – on no dialog associated with this request P-Early-Media header was received, however monitoring criteria was successful i.e. media is received at least once
  
  
• When Alert-Info header is supported (based on accept_alert_info)
• "no" – the most recent dialog associated with this request did not have Alert-Info header
• "e-no" – the most recent dialog associated with this request has Alert-Info header set to “rt”
• "e-yes" – the most recent dialog associated with this request has Alert-Info header set to “null”
• “yes”  – the most recent dialog associated with this request did not have Alert-Info header, however monitoring criteria was successful i.e. media is received at least once

Interworking between PEM and AI Headers

The SBC supports PEM to AI interworking when PEM header is supported on the Trunk Group towards which 180 provisional response message is received and acceptAlertInfo flag is enabled on the Trunk Group towards which 180 provisional response message is sent.

• When the SBC receives 180 response with PEM header, while forwarding 180 response:  
  • if PEM is inactive and the SBC does not play tone, the SBC inserts AI header with sig-id=rt. 
  • if PEM is inactive and the SBC plays tone, the SBC inserts AI header with sig-id=null.
  • if PEM is sendrecv or sendonly independent of the SBC is configured to play tone or not, the SBC inserts AI header with sig-id=null.

• When the SBC receives 180 response with PEM header, while forwarding 180 response:

  • In case of first 180 response without SDP and PEM is inactive:
    • if the SBC does not play tone, the SBC inserts AI header with sig-id=rt.

    • If the SBC plays tone, the SBC inserts AI:rt and MSC plays the local tone

  • In case of subsequent 180 response without PEM header, the SBC goes for monitoring based on the monitoring profile and delayed RBT configuration.
    • If monitoring fails, the SBC sends additional 180 response without SDP with AI:rt towards Ingress.
    • If monitoring is successful, then the SBC sends 180 without SDP with AI:null towards Ingress.

PEM to PEM Interworking

The SBC supports PEM to PEM interworking when PEM header is supported on the Trunk Group towards which 180 provisional response message is received and PEM is supported on the Trunk Group towards which 180 provisional response message is sent.

• When the SBC is not configured to play tone:  
• if the SBC receives 180 response with PEM header, while forwarding 180 response, the SBC relays the received PEM header towards ingress.
• if the SBC does not receive PEM header, the SBC inserts PEM header based on the SDP direction in 180 provisional  response while forwarding 180 response towards ingress.
• if the SBC does not receive PEM header and the flag monitorRTP is enabled, the SBC forwards 180 response without PEM and monitors RTP.
• When the SBC is configured to play tone:
  • if the SBC receives an inactive PEM, the SBC inserts PEM sendrecv while forwarding 180 response towards ingress.
  • if the SBC does not receive PEM and the monitoring profile which is configured on EXT-PSX and attached to packet service profile is enabled, the SBC inserts PEM sendrecv provided any of the previous provisional response did not receive PEM while sending 180 response towards ingress.
  • if the SBC is configured to play tone (based on tone generation criteria) for sendrecv/sendonly, the SBC will feed tone immediately and monitor for RTP.
• The SBC will play tone based on local configuration at the ToneAndAnnouncement profile on receipt of 180 with or without SDP with P-Early-Media (inactive) or without P-Early-Media and will continue to play tone (including the new negotiated codec) and monitor while playing tone when UPDATE is received.

Tone Generation Profile

 The SBC has flexibility for tone generation criteria, such as:

• For the same provisional response message, the SBC shall trigger tone-generation in some scenarios and shall relay in other scenarios.
• For the same provisional response message, the SBC shall trigger tone-generation based on monitoring result in some scenarios and shall trigger tone-generation immediately in other scenarios and based on early media authorization state.
• The SBC shall generate tone, on receipt of 18x provisional response, based on P-Early-Media header contents. For some scenarios, this is also coupled with result of monitoring status. 

The toneGenerationCriteria profile specifies conditions under which tone is generated. The conditions are as follows:

• Whether it is first or subsequent response
  • Response codes: 180 w/o SDP, 183 w/o SDP, 180 w/ SDP, 183 w/ SDP, any18x, 18x w/ SDP, 18x w/o SDP
• whether it is received on "SDP-answered" dialog or not
• status of Early-Media-Authorization and whether it is based on Alert-Info or P-Early-Media
• Delayed RBT, or Delayed RBT only if 180 is ever received, or neither
• whether P-Early-Media: supported is received in INVITE or not

Monitoring RTP

The SBC monitors the RTP packets when all or either of the following conditions are met.

• when the SBC sends an INVITE with PEM=supported and an early media SDP answer (in any 18x response) is received without PEM header.
• when the flag defaultGatingMethod is set as none. For more information on the flag defaultGatingMethod, refer to SIP Trunk Group - Media - CLI. 

Monitoring Profile

Monitoring functionality is enhanced to cover additional interworking scenarios and allows the flexibility to monitor more than one RTP packet. The basic criteria for monitoring to begin is an SDP answer received from peer.

The following configuration parameters are provided:

• packets_for_authorization: specifies how many RTP packets must be received in the monitoring period for monitoring to succeed.
• monitoring_period: this defines a monitoring period on the Egress RTP stream (UAS). RTP packets received are relayed during the monitoring period if the SBC media pin hole for the call is open. When the monitoring period expires, the count of the packets received is compared with the packets_for_authorization
• silent_period: the SBC uses silent period based on the call state. It is used when the call has already been answered. It means an silent window is started to discount RTP packets of previous early media dialog that may arrive due to a prior 18x having SDP answer (a 180 w/o SDP received and a previous SDP was received in a 183), before triggering monitoring.
• no_of_iterations: defines how many windows the monitoring period spans.

If the packets_received count in the monitoring period is:

• >= packets_for_authorization, the monitoring criteria is considered successful

Playing Tone Locally

The SBC is enhanced to play LRBT locally without considering PEM header. To achieve this functionality, the flag withOrWithOutSdp is added to the toneAndAnnouncementProfile.

The SBC plays tone locally when it receives:

• first 180 response without SDP and PEM header or with PEM=inactive.
• first 180 response with SDP and PEM=inactive.
• first 180 response with SDP is received without PEM header.
• subsequent 180 responses without SDP and PEM header and the previous provisional response does not contain PEM header (even during an RTP monitoring).
• subsequent 180 responses without SDP with PEM=inactive.
• subsequent 180 responses with SDP and without PEM header or PEM=inactive and the previous provisional response does not contain PEM header (even during an RTP monitoring).

SBC does not Stop Playing Tone when UPDATE is Received from the UAS

The SBC is enhanced to continue playing the ringback tone after receiving an UPDATE message from the User Agent Server (UAS), rather than stopping the tone. The SBC then monitors the egress leg, stops the tone, if it receives an RTP packet or 200 OK message and opening the audio path in both directions. The UAS sends an UPDATE message, a codec upgrade, or a media hold. If the UPDATE message is due to a codec upgrade, the SBC continues playing the ringback tone using the new codec. If the SBC is not configured to continue playing the ringback tone after UPDATE, the caller may hear a very short ringback tone followed by a long period of silence until the final response is received. To achieve this functionality, the flag monitorRtpOnEgressUpdate is added to the egressIpAttributes of the IP Signaling Profile.

The SBC supports early media authorization in UPDATE, 200 OK to UPDATE, and PRACK messages towards the Trunk Group that supports PEM. 

• The SBC processes:

• • if PEM is received, the data path mode is set based on SDP direction attribute.
  • if PEM is not received in the egress UPDATE, the SBC relays the UPDATE towards the ingress without PEM.
  • if PEM is not received in 200 OK (UPDATE), the SBC does not add PEM header. It only adds PEM header in the 200 OK if UPDATE is received from the ingress with PEM: inactive or without PEM and the SBC is configured to play the tone.
• The SBC receives and processes:

• • UPDATE without PEM and forwards UPDATE without PEM header, when SBC is not playing tone. 

  • UPDATE without PEM and forwards UPDATE with PEM=sendrecv header, when SBC is playing tone (when RTP monitoring is configured). 

  • if UPDATE is received on a leg, on which tone is being played, UPDATE is locally handled and tone is played with the new codec.
• The SBC receives and processes:
  • 200 OK to UPDATE with PEM and relays 200 OK to UPDATE with PEM header.
• The SBC processes the PEM header received in:
  • PRACK message and  relays PRACK with PEM header.
  • PRACK message and if PRACK message is received without PEM header, the SBC relays PRACK without PEM header.
• The SBC is configured to play tone:
  • if UPDATE is received without PEM or PEM:inactive from egress, the SBC inserts UPDATE with PEM:sendrecv towards ingress.
  • If UPDATE is received with sendrecv/sendonly, the SBC switches tone play to new codec and monitors for RTP. If RTP is received, the SBC stops the tone and does a media cut through.

Configuring AI to PEM Headers Interworking

• Use case 1: Network that Supports PEM
• Use case 2: Network that does not Support PEM and AI
• Use case 3: Network that does not Support PEM but Supports AI

Use case 1: Network that Supports PEM

Configuring PEM

% set addressContext default zone defaultSigZone sipTrunkGroup TG1 media earlyMedia method pEarlyMedia defaultGatingMethod none monitorRtp enabled egressSupport enabled
% commit

Configuring the flag announcementBasedTones

% set profiles media toneAndAnnouncementProfile EXAMPLE localRingBackTone signalingTonePackageState enable flags announcementBasedTones enable

Configuring the flag convertAlertToProgress

% set addressContext default zone defaultSigZone sipTrunkGroup TG1 signaling convertAlertToProgress enabled
% commit

Configuring the flag monitorRtpOnEgressUpdate

% set profiles signaling ipSignalingProfile DEFAULT_SIP egressIpAttributes flags monitorRtpOnEgressUpdate enable
% commit

Configuring the parameter localRingBackTone

% set profiles media toneAndAnnouncementProfile T1 localRingBackTone signalingTonePackageState enable flags useThisLrbtForIngress enable earlyMediaMethod pEarlyMedia withOrWithOutSDP enable
% commit   

Configuring Common IP Attributes of the IP Signaling Profile

% set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes flags disableMediaLockDown enable minimizeRelayingOfMediaChangesFromOtherCallLegAll enable relayDataPathModeChangeFromOtherCallLeg enable sendOnlyPreferredCodec enable
% commit 

Use case 2: Network that does not Support PEM and AI

Configuring Session Answer

% set addressContext default zone defaultSigZone sipTrunkGroup TG1 media earlyMedia method sessionAnswer
% commit

Configuring the flag convertAlertToProgress

% set addressContext default zone defaultSigZone sipTrunkGroup TG1 signaling convertAlertToProgress enabled
% commit

Configuring the flag monitorRtpOnEgressUpdate

% set profiles signaling ipSignalingProfile DEFAULT_SIP egressIpAttributes flags monitorRtpOnEgressUpdate enable
% commit

Configuring the parameter localRingBackTone

% set profiles media toneAndAnnouncementProfile T1 localRingBackTone signalingTonePackageState enable
% commit

Configuring the parameter localRingBackTone

% set profiles media toneAndAnnouncementProfile T1 localRingBackTone signalingTonePackageState enable flags useThisLrbtForIngress enable earlyMediaMethod pEarlyMedia withOrWithOutSDP enable
% commit   

Configuring Common IP Attributes of the IP Signaling Profile

% set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes flags disableMediaLockDown enable minimizeRelayingOfMediaChangesFromOtherCallLegAll enable relayDataPathModeChangeFromOtherCallLeg enable sendOnlyPreferredCodec enable
% commit 

Use case 3: Network that does not Support PEM but Supports AI

Configuring Session Answer

% set addressContext default zone defaultSigZone sipTrunkGroup TG1 media earlyMedia method sessionAnswer
% commit

Configuring the flag announcementBasedTones

% set profiles media toneAndAnnouncementProfile EXAMPLE localRingBackTone signalingTonePackageState enable flags announcementBasedTones enable

Configuring AI

% set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes flags acceptAlertInfo enable

Configuring the flag monitorRtpOnEgressUpdate

% set profiles signaling ipSignalingProfile DEFAULT_SIP egressIpAttributes flags monitorRtpOnEgressUpdate enable
% commit

Configuring the flag aiToPemInterworking

% set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes flags aiToPemInterworking enable

Configuring the parameter localRingBackTone

% set profiles media toneAndAnnouncementProfile T1 localRingBackTone signalingTonePackageState enable flags useThisLrbtForIngress enable earlyMediaMethod pEarlyMedia withOrWithOutSDP enable
% commit   

Configuring Common IP Attributes of the IP Signaling Profile

% set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes flags disableMediaLockDown enable minimizeRelayingOfMediaChangesFromOtherCallLegAll enable relayDataPathModeChangeFromOtherCallLeg enable sendOnlyPreferredCodec enable
% commit