The Base Link Code Word
Every fast link pulse burst transmits a word of 16 bits known as a link code word. The first such word is known as a base link code word, and its bits are used as follows:
- 0–4: selector field: it indicates which standard is used between IEEE 802.3 and IEEE 802.9;
- 5–12: technology ability field: this is a sequence of bits that encode the possible modes of operations among the 100BASE-T and 10BASE-T modes;
- 13: remote fault: this is set to one when the device is detecting a link failure;
- 14: acknowledgement: the device sets this to one to indicate the correct reception of the base link code word from the other party; this is detected by the reception of at least three identical base code words;
- 15: next page: this bit is used to indicate the intention of sending other link code words after the base link code word;
The technology ability field is composed of eight bits. For IEEE 802.3, these are as follows:
- bit 0: device supports 10BASE-T
- bit 1: device supports 10BASE-T in full duplex
- bit 2: device supports 100BASE-TX
- bit 3: device supports 100BASE-TX in full duplex
- bit 4: device supports 100BASE-T4
- bit 5: pause
- bit 6: asymmetric pause for full duplex
- bit 7: reserved
The acknowledgement bit is used to signal the correct reception of the base code word. This corresponds to having received three identical copies of the base code word. Upon receiving these three identical copies, the device sends a link code word with the acknowledge bit set to one from six times to eight times.
The link code words are also called pages. The base link code word is therefore called a base page. The next page bit of the base page is 1 when the device intends to send other pages, which can be used to communicate other abilities. These additional pages are sent only if both devices have sent base pages with a next page bit set to 1. The additional pages are still encoded as link code words (using 17 clock pulses and up to 16 bit pulses).
Read more about this topic: Autonegotiation
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