AN985B/BX
Functional Descriptions
Table 4
Field
Format
Description
Preamble
A 7-byte field of (10101010b)
A 1-byte field of (10101011b)
A 6-byte field
Start Frame Delimiter
Destination Address
Source Address
Length/Type
A 6-byte field
A 2-byte field indicated the frame is in IEEE802.3
format or Ethernet format.IEEE802.3 format: 0000H ~
05DCH for Length field Ethernet format: 05DD ~ FFFFH
for Type field
Data
CRC
461) ~ 1500 bytes of data information
A 32-bit cyclic redundant code for error detection
1) If padding is disabled (TDES1 bit23), the data field may be shorter than 46 bytes.
Transmit Data Encapsulation
The differences between the encapsulation and a MAC frame while operating in the 100BASE-TX mode are listed
as follow:
1. The first byte of the preamble is replaced by the JK code according to the IEE802.3u, clause 24.
2. After the CRC field of the MAC frame, the AN985B/BX inserts the TR code according to the IEE802.3u, clause
24.
Receive Data Decapsulation
When operating in 100BASE-TX mode the AN985B/BX detects a JK code for a preamble as well as a TR code
for the packet end. If a JK code is not detected, the AN985B/BX will abort this frame receiving and wait for a new
JK code detection. If a TR code is not detected, the AN985B/BX will report a CRC error.
Deferring
The Inter-Frame Gap (IFG) time is divided into two parts:
1. IFG1 time (64-bit time): If a carrier is detected on the medium during this time, the AN985B/BX will reset the
IFG1 time counter and restart to monitor the channel for an idle again.
2. IFG2 time (32-bit time): After counting the IFG2 time the AN985B/BX will access the channel even though a
carrier has been sensed on the network.
Collision Handling
The scheduling of re-transmissions is determined by a controlled randomization process called “truncated binary
exponential back-off”. At the end of enforcing a collision (jamming), the AN985B/BX delays before attempting to
re-transmit the packet. The delay is an integer multiple of slot time. The number of slot times to delay before the
nth re-transmission attempt is chosen as a uniform distributed integer r in the range:
0 ≤ r < 2k, where k = min (n, 10)
Data Sheet
25
Rev. 1.51, 2005-11-30
INFINEON [ Infineon ]
