IPG signal bits in 100BASE-TX

Question

I have 2 questions related to Inter Packet Gap in 100BASE-TX:

1. During the IPG time (i.e. after Checksum is sent and Preamble of next frame is yet to be sent), what exactly is the activity on the "wire" ? Does the NIC remains idle by putting a signal representing bit "0" on the wire or is there a bit-pattern to represent this idle state in place of "0". If there is a pattern, what is it.
2. The bits which are put on the wire during IPG time is decided by MAC layer or MII sublayer of the Physical layer ? (Please correct me if none of them do it)

Answer 1

In the original 100BASE-TX, nothing is transmitted in the IPG period. Energy-efficient Ethernet adds low-power idle (LPI) mode which sends idle symbols generated by the PCS sublayer.

Answer 2

To answer your second question first: this function is in the physical coding sublayer (PCS).

Figure 24-5 specifies how the MAC frame is mapped to physical layer stream: immediately following the FCS is the end-of-stream delimiter (T/R), then idle (I).

The 4B/5B codings are given in Table 24-1

Idle (I): 1 1 1 1 1
End-of-Stream Delimiter, Part 1 of 2 (T): 0 1 1 0 1 
End-of-Stream Delimiter, Part 1 of 2 (R): 0 0 1 1 1

Energy efficient Ethernet adds Sleep (P): 0 0 0 0 0

This has to go through the PMD sub-layer before it gets onto the wire. In particular, sleep (P) symbols can be used to request the transmitter to go into low power idle state. Unlike normal idle, in this mode it isn't continuously encoding P symbols on the wire.