This is a frequently discussed Topic, In which particular case the DSP SPA could be required for supporting DTMF Interworking in ASR1K CUBE(SP) ?
Answer : The DSP SPA is required to allow detection of DTMF in the incoming circuit (Inband Signaling with no RTP rfc2833 headers) and Interworking with RFC2833/OOB (SIP INFO/NOTIFY). DSP SPA is needed only for the case of transcoding inband DTMF to out of band SIP signaling.
To Summarize, the purpose of the DSP SPA would be to provide "Media:DTMF Interworking:InBand and RFC2833/OOB (SIP INFO/NOTIFY)".
About SPA DSP Services in CUBE (SP Edition)
What is SPA DSP ?
The shared port adapter (SPA) digital signal processor (DSP) is a single-width, half-height, high-power, SPA module that can be used across multiple Cisco platforms. The SPA DSP is designed for DSP-based voice and video solutions in the SPAs on the Cisco mid-range and high-end routers.
In Cisco IOS XE Release 3.2S, the following SPA DSP features have been deployed on the Cisco ASR 1000 Series Router for the session border controller (SBC):
Cisco Unified Border Element (SP Edition) was earlier known as Integrated Session Border Controller, and is referred to as SBC in this document.
•Associating SBC configuration with a DSP farm profile.
•Voice transcoding and transrating support using onboard DSP services.
•Dual tone multifrequency (DTMF) interworking using onboard DSP services.
•VoIPv4 and VoIPv6 transcoding and transrating support.
•Transcoding, transrating, and DTMF interworking call control and signaling control.
Cisco IOS XE Release 3.2S The SPA DSP onboard services were introduced on the Cisco ASR 1000 Series Routers.
Note: This IOS XE Release 3.2S is End of Sale. Those who are using this IOS and Feature, can refer this document.
The Cisco® Unified Border Element (SP Edition) is a high-scale, carrier-grade session border controller (SBC) that is integrated into Cisco routing platforms; it can be integrated with many router functions to provide a very feature-rich and intelligent SBC application. Formerly known only as Session Border Controller, Cisco Unified Border Element (SP Edition) provides a network-to-network demarcation interface for signaling interworking, media interworking, address and port translations, billing, security, quality of service (QoS), Call Admission Control (CAC), and bandwidth management.
Information About the SPA DSP Services:
A SPA DSP contains digital signal processors and related hardware to provide voice transcoding capability for the SBC. In addition, Cisco Unified Border Element, Enterprise can use a SPA DSP for simple voice transcoding services.
The SPA DSP allows the translation of one type of media stream or codec to another type of media stream that uses different media encoding and decoding technologies. Other translation activities include:
•Translation between different codecs
•Translation between different packetization settings (transrating)
In-Band DTMF Interworking
The Cisco ASR 1000 Series Aggregation Services Routers support DTMF interworking between Real-Time Transport Protocol (RTP) in-band waveform, RTP telephone-event codec (RFC2833), and SIP DTMF indication types.
A DTMF tone can be generated using the following methods:
•SIP digit detection and generation package—A SIP message is sent from an endpoint to a SIP proxy, indicating that there has been a DTMF event, along with information about the type and duration of the event.
•RTP payload for DTMF (telephone-event codec)—The RTP packets contain information in their headers, indicating that a DTMF is being generated. The endpoints interpret these messages and play the DTMF locally.
•RTP in-band waveform—The DTMF is sent as part of the voice waveform.