Content for TS 26.346 Word version: 19.1.0
B (Normative) FEC encoder specification p. 204
This Annex specifies the systematic Raptor Forward Error Correction code (see RFC 5053) and its application to MBMS. Raptor is a fountain code, i.e., as many encoding symbols as needed can be generated on the fly by the encoder from the source symbols of a block. The decoder is able to recover the source block from any set of encoding symbols only slightly more in number than the number of source symbols.
The code described in this document is a Systematic code, that is, the original source symbols are sent unmodified from sender to receiver, as well as a number of repair symbols.
B.1 Void
B.2 Void
B.3 File download p. 204
B.3.1 Void
B.3.2 Void
B.3.3 Void
B.3.4 Example parameters p. 204
B.3.4.1 Parameter derivation algorithm p. 204
Section 4.2 of RFC 5053 provides recommendations for the derivation of the transport parameters G, T, Z and N.
Recommended settings for the input parameters, W, Al, KMIN and GMAX are as follows:
W = 256 KB Al = 4 KMIN = 1024 GMAX = 10
B.3.4.2 Examples p. 205
The above algorithm leads to transport parameters as shown in Table B.3.4.2-1 below, assuming the recommended values for W, Al, KMIN and GMAX and P = 512:
| File size F | G | Symbol size T | G*T | Kt | Source blocks Z | Sub-blocks N | KL | KS | TL ∙A | TS ∙A |
|---|---|---|---|---|---|---|---|---|---|---|
| 100 KB | 6 | 84 | 504 | 1,220 | 1 | 1 | 1,220 | 1,220 | N/A | N/A |
| 100 KB | 8 | 64 | 512 | 1,600 | 1 | 1 | 1,600 | 1,600 | N/A | N/A |
| 300 KB | 2 | 256 | 512 | 1,200 | 1 | 2 | 1,200 | 1,200 | 128 | 128 |
| 1,000 KB | 1 | 512 | 512 | 2,000 | 1 | 5 | 2,000 | 2,000 | 104 | 100 |
| 3,000 KB | 1 | 512 | 512 | 6,000 | 1 | 12 | 6,000 | 6,000 | 44 | 40 |
| 10,000 KB | 1 | 512 | 512 | 20,000 | 3 | 14 | 6,666 | 6,667 | 40 | 36 |
B.4 Streaming p. 205
B.4.1 Void
B.4.2 Void
B.4.3 Void
B.4.4 Example parameters p. 205
B.4.4.1 Parameter derivation algorithm p. 205
This clause provides recommendations for the derivation of the transport parameter T. This recommendation is based on the following input parameters:
- B the maximum source block size, in bytes
- P the maximum repair packet payload size, in bytes, which is a multiple of Al
- Al the symbol alignment factor, in bytes
- KMAX the maximum number of source symbols per source block.
- KMIN a minimum target on the number of symbols per source block
- KMIN a maximum target number of symbols per repair packet
G = min{ceil(P·KMIN/B), P/Al, KMIN} - the approximate number of symbols per packet
T = floor(P/(Al·G))·Al
The value of T derived above should be considered as a guide to the actual value of T used. It may be advantageous to ensure that T divides into P, or it may be advantageous to set the value of T smaller to minimize wastage when full size repair symbols are used to recover partial source symbols at the end of lost source packets (as long as the maximum number of source symbols in a source block does not exceed KMAX). Furthermore, the choice of T may depend on the source packet size distribution, e.g., if all source packets are the same size then it is advantageous to choose T so that the actual payload size of a repair packet P', where P' is a multiple of T, is equal to (or as few bytes as possible larger than) the number of bytes each source packet occupies in the source block.
Recommended settings for the input parameters, Al, KMIN and KMIN are as follows:
Al = 4 KMIN = 1024 KMIN = 10
B.4.4.2 Examples p. 206
The above algorithm leads to transport parameters as shown in Table B.4.4.2-1 below, assuming the recommended values for Al, KMIN and KMIN and P = 512:
| Max source block size B | G | Symbol size T | G∙T |
|---|---|---|---|
| 40 KB | 10 | 48 | 480 |
| 160 KB | 4 | 128 | 512 |
| 640 KB | 1 | 512 | 512 |
B.5 Void
B.6 Void
B.7 Void
B.8 Void
