
RTL8100B(L) 
2001-11-9 
Rev.1.41 
46 
9.8 Tx Encapsulation 
While operating in 100Base-TX mode, the RTL8100B(L) 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. 
9.9 Collision 
If the RTL8100B(L) is not in full-duplex mode, a collision event occurs when the receive input is not idle while the 
RTL8100B(L) transmits. If the collision was detected during the preamble transmission, the jam pattern is transmitted after 
completing the preamble (including the JK symbol pair). 
9.10 Rx Decapsulation 
The RTL8100B(L) 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 RTL8100B(L) starts to process the preamble bytes based on the mode of 
operation. 
While operating in 100Base-Tx mode, the RTL8100B(L) expects the frame to start with the symbol pair JK in the first bye of the 
8-byte preamble.  
The RTL8100B(L) checks the CRC bytes and checks if the packet data ends with the TR symbol pair, if not, the RTL8100B(L) 
reports an CRC error RSR. 
The RTL8100B(L) 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. 
9.11 Flow Control 
The RTL8100B(L) supports IEEE802.3X flow control to improve performance in full-duplex mode. It detects PAUSE packets 
to achieve flow control tasks. 
9.11.1. Control Frame Transmission 
When the RTL8100B(L) 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 RTL8100B(L) sends the other 
PAUSE packet 
with pause_time(=0000h)
 to wake up the source station to restart transmission. 
9.11.2. Control Frame Reception 
The RTL8100B(L) 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 RTL8100B(L) is transmitting, the RTL8100B(L) starts to back off 
after current transmission completes. The RTL8100B(L) 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.