Page 1186 - 5G Basics - Core Network Aspects

2 Transport aspects

Figure 17-17  GFP frame format for FC-1200

GFP framing is used to facilitate delineation of the superblock structure by the receiver. The leading flag
bits from each of the eight 513B blocks are relocated into a single octet at the end of the 513-octet
superblock data field (labelled "Superblock flags").
To minimize the risk of incorrect decoding due to errors in the 1 to 65 octets of "control" information
(Flags, FC, POS, CB_Type), a CRC-24 is calculated over the 65 octets within each superblock that may
contain such "control" information and appended to form a 516 octet superblock. The 65 octets in the 516-
octet superblock over which the CRC-24 is calculated are the octets (1+8n) with n=0..64 (i.e., octets 1, 9, 17,
17
11
15
24
21
20
6
5
8
9
.., 513). The generator polynomial for the CRC-24 is G(x) = x + x + x + x + x + x + x + x + x + x + x + 1
24
0
with an all-ones initialization value, where x corresponds to the MSB and x to the LSB. This superblock
CRC is generated by the source adaptation process using the following steps:
1) The 65 octets of "control" information (Flags, POS, CB_Type) are taken in network octet order (see
Figure 17-17), most significant bit first, to form a 520-bit pattern representing the coefficients of a
polynomial M(x) of degree 519.
24
2) M(x) is multiplied by x and divided (modulo 2) by G(x), producing a remainder R(x) of degree 23
or less.
23
3) The coefficients of R(x) are considered to be a 24-bit sequence, where x is the most significant
bit.
4) After inversion, this 24-bit sequence is the CRC-24.
Exactly 17 of these 516-octet superblocks are prefixed with the standard GFP core and type headers and 16
octets of "reserved" (padding). Because the number of 516-octet superblocks per GFP frame is known a
priori, it is possible for this mapping scheme to operate in a cut-through (as opposed to store and forward)
fashion, thus minimizing the mapping latency.

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