T.38 UDPTL Redundancy (H.323)
T.38 provides facilities to eliminate the effects of packet loss through data redundancy. When a packet is sent, either zero, one, two, three, or even more of the previously sent packets are repeated. (The specification does not impose a limit.) This increases the network bandwidth required (it’s still much less than not using T.38) but it allows the receiving gateway to reconstruct the complete packet sequence, even with a fairly high level of packet loss. The T.38 redundancy number corresponds to how many previous packet payloads are repeated with each packet. The values for (I)ndication, (L)ow (S)peed and (H)igh (S)peed IFP packets are specified separately.
Indication
Specifies the number of redundant T.38 fax packets to be sent to indicate CNG, CED tones, no signal. Ranges varies from 0 (no redundancy) to 5.
LS-Redundancy
Specifies the number of redundant T.38 fax packets to be sent for the low-speed V.21-based T.30 fax machine protocol. Low speed is used during the fax negotiation phase which is done at 300 Bps and is most likely to be affected by bad QoS. Ranges varies from 0 (no redundancy) to 5.
HS-Redundancy
Specifies the number of redundant T.38 fax packets to be sent for high-speed V.17, V.27, and V.29 T.4 or T.6 fax machine image data. High speed is used during the transmission phase which is done between 2400 Kbps and 14400 Kbps. Although bad QoS will deteriorate image quality, a few missing scan lines ultimately does not effect the human's perception of the received image. Ranges varies from 0 (no redundancy) to 5.
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