RTL8101L
2003-05-28
Rev.1.3
56
10.8 Tx Encapsulation
While operating in 100Base-TX mode, the RTL8101L encapsulates the frames that it transmits according to the 4B/5B
code-groups table. The changes of the original packet data are listed as follows :
1. The first byte of the preamble in the MAC frame is replaced with the JK symbol pair.
2. After the CRC, the TR symbol pair is inserted.
10.9 Collision
If the RTL8101L is not in full-duplex mode, a collision event occurs when the receive input is not idle while the RTL8101L
transmits. If the collision was detected during the preamble transmission, the jam pattern is transmitted after completing the
preamble (including the JK symbol pair).
10.10 Rx Decapsulation
The RTL8101L continuously monitors the network when reception is enabled. When activity is recognized it starts to
process the incoming data.
After detecting receive activity on the line, the RTL8101L starts to process the preamble bytes based on the mode of
operation.
While operating in 100Base-Tx mode, the RTL8101L expects the frame to start with the symbol pair JK in the first byte of
the 8-byte preamble.
The RTL8101L checks the CRC bytes and checks if the packet data ends with the TR symbol pair, if not, the RTL8101L
reports an CRC error RSR.
The RTL8101L reports a RSR<CRC> error in any of the following cases :
1. In the 100Base-Tx mode, one of the following occur.
a. An invalid symbol (4B/5B Table) is received in the middle of the frame.
RSR<ISE> bit also sets.
b. The frame does not end with the TR symbol pair.
10.11 Flow Control
The RTL8101L supports IEEE802.3X flow control to improve performance in full-duplex mode. It detects PAUSE
packets to achieve flow control task.
10.11.1 Control Frame Transmission
When the RTL8101L detects that its free receive buffer is less than 3K bytes, it sends a PAUSE packet with
pause_time(=FFFFh) to inform the source station to stop transmission for the specified period of time. After
the driver has processed the packets in the receive buffer and updated the boundary pointer, the RTL8101L
sends the other PAUSE packet with pause_time(=0000h) to wake up the source station to restart transmission.
10.11.2 Control Frame Reception
The RTL8101L enters a back off state for a specified period of time when it receives a valid PAUSE packet
with pause_time(=n). If the PAUSE packet is received while the RTL8101L is transmitting, the RTL8101L
starts to back off after current transmission completes. The RTL8101L is free to transmit the next packets when
it receives a valid PAUSE packet with pause_time(=0000h) or the backoff timer(=n*512 bit time) elapses.
Note: The PAUSE operation cannot be used to inhibit transmission of MAC Control frames (e.g. PAUSE packet). The
N-way flow control capability can be disabled, please refer to Section 6, EEPROM (93C46) Contents for a detailed
description.
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