Detecting Frame Corruption
Next, we'll remove the assumption that the channel is error free;
frames (only) may now be corrupted during transmission,
introducing the need for checksums.
We'll now introduce a FRAMETYPE
to distinguish what a frame is being used for.
Known (agreed to) by both the sender and receiver:
typedef enum { DLL_DATA, DLL_ACK, DLL_NACK } FRAMETYPE;
typedef struct {
// firstly, the frame's header
FRAMETYPE type;
int checksum; // checksum of the whole frame
int len; // length of the payload, only
// followed by the payload
char data[MAX_DATA_SIZE];
} FRAME;
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In the sender:
FRAME frame, ackframe;
int link, len, acklen;
while( true ) {
READ_NETWORK_LAYER(frame.data, &len);
frame.type = DLL_DATA;
frame.len = len;
frame.checksum = 0;
frame.checksum = checksum_crc16(&frame, FRAME_SIZE(frame));
while( true ) {
link = 1;
WRITE_PHYSICAL_LAYER(link, &frame, FRAME_SIZE(frame));
READ_PHYSICAL_LAYER(&link, &ackframe, &acklen);
if(ackframe.type == DLL_ACK)
break;
}
}
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CITS3002 Computer Networks, Lecture 3, Data Link Layer protocols, p6, 13th March 2024.
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CITS3002