Encryption and error correction in AES&FEC gpon

The recent testing process has encountered a case of system AES and FEC, here is a brief introduction to their principles and applications in the Gpon system.

Aes

for the gpon system, the downlink data is sent by broadcast to all ONU, involving a malicious user to eavesdrop on all broadcast frames, so the system introduces the AES encryption algorithm. It is a packet encryption algorithm, that is, the plaintext and ciphertext into a fixed-length grouping, encryption or decryption processing. Its fixed packet size is 128bits, the key length has 128,192,256 three kinds, the gpon uses the aes-128.

The method called Counter Mode (counter mode), the cipher algorithm produces a 16-byte pseudo-random code block stream, pseudo-random code block and the input of the plaintext after the XOR operation produces ciphertext output. Ciphertext and the same pseudo-random code after the XOR operation can be re-generated plaintext. For gem shards, only encryption payload, unencrypted Port-id, each 16B key with the Gem Shard payload is different or encrypted, ONU use the key and its XOR.

Procedure: Olt sends Key_request_msg,onu to generate a key in the PLOAM message and responds back, the Olt stores the key and selects a frame to start using this key to encrypt, ONU receives the frame and determines that the Olt and ONU use the same key for encryption and decryption. The downstream AES occurs before the FEC.

Fec

In order to reduce the bit error rate under the same Snr or increase the bitrate and transmission distance at the same BER, a redundant coding technique is introduced to improve the SNR of approximately 3-4db. FEC uses RS (255,239) format encoding, a block fixed 255bytes, where data 239bytes, checksum code 16bytes. Because of the introduction of the verification code, FEC reduces the data bandwidth, but improves the bandwidth utilization and reduces the retransmission rate. For a 2.488Gbps system, the frame length 2.488GBPS/8*125US=38875B,FEC produces 152 255B blocks and the last 120B block.

Reference:

http://www.fiberoptictel.com/key-technologies-of-gpon-fec/

Http://www.cs.nthu.edu.tw/~ctchiu/meeting/gpon/9843-3.pdf

Http://wenku.baidu.com/view/b29ce0d233d4b14e85246811.html

Encryption and error correction in AES&FEC gpon