Protocols and formats
Calls are controlled using the Session Initiation Protocol (SIP). Telephone calls are routed through SIP-to-PSTN gateways at our inbound and outbound providers. Call media is transported using the Real-time Transport Protocol (RTP). Within RTP, Aculab Cloud supports the G.711 A-law and G.711 mulaw audio codecs, and RFC2833 DTMF digits - all sampled at 8000Hz.
G.729 is available on request. Contact us if your need to use this codec with your provider.
Media file formats
REST API and UAS API applications can play media files that have been uploaded to an account's media file storage on Aculab Cloud. Similarly they can record media files to an account's media file storage.
Aculab Cloud plays and records Microsoft wav format (.wav) media files compressed using:
- G.711 A-law (default)
- G.711 mu-law
- OKI ADPCM
- IMA ADPCM
- signed 16-bit PCM
- unsigned 8-bit PCM
Only single channel or mono recordings are supported.
G.711 A-law or G.711 mu-law are preferred since they are reasonably small and provide the best quality for telephony use.
The supported sampling rates are:
- 8000Hz (default)
8000Hz is preferred since it matches the sampling rate used on calls and therefore provides the best quality.
Fax file formats
REST API and UAS API applications can send fax files that have been uploaded to an account's media file storage on Aculab Cloud. Similarly they can receive and save fax files to an account's media file storage.
Fax files to be sent or that are received are stored in the Tagged Image File Format (.tif).The compression schemes supported are:
- CCITT Group 3 1-Dimensional Modified Huffman run length encoding (RLE). Also known as MH or CCITT 1D.
- CCITT T.4 bi-level encoding as specified in section 4, Coding, of ITU-T Recommendation T.4. Also known as MR2D, CCITT Group 3 fax encoding or CCITT Group 3 2D.
- CCITT T.6 bi-level encoding as specified in section 2 of ITU-T Recommendation T.6. Also known as MMRT6 or CCITT Group 4 fax encoding.
These compression schemes do not support colour or greyscale. Images are black and white only.
The page widths supported are:
- A4 (1728 pixels, 21.95cm)
- B4 (2048 pixels, 26.01cm)
- A3 (2432 pixels, 30.89cm)
See Sending & Receiving Faxes for more details.