SILK Codec Transcoding Support
SILK is an audio codec developed by Skype Limited that supports bit rates from 6 kbit/s to 40 kbit/s and sampling rates of 8, 12, 16, or 24 kHz. It can also use a low algorithmic delay of 25 ms (20 ms frame size + 5 ms look-ahead). This feature adds the SILK codec as well as support for transrating, transcoding, and pooled transcoding.
SILK Supported Options
Required SDP Parameters:- rate — Specifies the sampling frequency. SILK supports four different audio bandwidths – narrowband at 8 kHz, mediumband at 12 kHz, wideband at 16 kHz, and super wideband at 24 kHz. This parameter is mapped to the RTP clock rate in “a=rtpmap”. The DSP only supports audio sampling rates of 8 kHz and 16 kHz for transcoding; the 12 kHz and 24 kHz bandwidths are not transcodable.
Optional SDP Parameters:
- ptime — Specifies the packetization interval in milliseconds. The DSP supports packetization intervals of 20, 40, 60, 80, and 100 ms. This parameter is mapped to “a=ptime” in the SDP. The default is 20 ms.
- maxptime — Specifies the maximum packetization interval in milliseconds. The default is 100 ms.
- minptime — Specifies the minimum packetization interval in milliseconds. The default is 20 ms.
- maxaveragebitrate — Specifies the maximum average rate of bits received for a session in bits per second. Bit rates of 5000 to 30000 bps are transcodable by the DSP.
- usedtx — Specifies whether the SILK decoder utilizes Discontinuous Transmission (DTX). The possible values are 0 (no) and 1 (yes). The default is 0.
The payload type is dynamic for this codec.
Sample media-profile configuration for adding SILK
| Parameter | Value |
|---|---|
| name | SILK |
| subname | wideband |
| media-type | audio |
| payload-type | 103 |
| transport | RTP/AVP |
| clock-rate | 16000 |
| req-bandwidth | 0 |
| frames-per-packet | 0 |
| parameters | N/A |
| average-rate-limit | 5000 |
| peak-rate-limit | 0 |
| max-burst-size | 0 |
| sdp-rate-limit-headroom | 0 |
| sdp-bandwidth | enabled |
| police-rate | 0 |
| standard-pkt-rate | 0 |
Monitoring and Debugging
CLI commands:
- The show sipd codecs command is modified to add SILK Count.
SNMP:
- New SNMP OID apSysXCodeSILKWBCapacity is added to transcoding utilization statistics as reported in the apSysMgmtGroupTrap. When utilization falls below 80%, the apSysMgmtGroupClearTrap is sent.
- SILK realm statistic
apCodecRealmCountSILK
is added to the
apCodecRealmStatsEntry
table located in the
ap-tc.mib.
Alarms:
- SILK Transcoding Capacity Threshold Alarm — A warning level alarm that doesn't affect health is triggered when the SILK transcoding utilization exceeds 95% of capacity. The alarm is cleared when the SILK transcoding utilization falls below 80% of capacity.