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參數(shù)資料
| 型號： | 21440 |
| 廠商： | Intel Corp. |
| 英文描述： | Multiport 10/100Mb/s Ethernet Controller(多端口10/100Mb/s 以太網(wǎng)控制器) |
| 中文描述： | 多端口10/100Mb/s以太網(wǎng)控制器（多端口支持10/100Mbps以太網(wǎng)控制器） |
| 文件頁數(shù)： | 62/92頁 |
| 文件大?。?/td> | 1352K |
| 代理商： | 21440 |
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21440 Multiport 10/100Mb/s Ethernet Controller
6-5
Network Interface Operation
6.4.4
Collision
A collision occurs on a half-duplex network when concurrent transmissions from two or more
nodes take place. During transmission, the 21440 monitors the line condition.
In the MII mode, the 21440 detects a collision when the collision detect signal (col{i}) asserts.
When the 21440 detects a collision while transmitting, it halts transmission of the data and transmits a
32-bit jam pattern. If the collision was detected during the preamble or the SFD transmission, the jam
pattern is transmitted after completing the SFD. This results in a minimum 96-bit fragment.
Note:
The jam pattern is a fixed pattern that is not compared to the actual frame CRC. It has a very low
probability (0.5
32
) of having a jam pattern identical to the CRC.
In standard networks, collisions will always occur before transmission of 64 bytes (including
preamble and SFD), in which case the 21440 begins the backoff wait period.
The 21440 scheduling of retransmission is determined through a controlled randomization process,
termed the truncated binary exponential backoff. The delay time is represented by an integer multiple
of slot times (1 slot is equal to a 512-bit time period). The number of the delay slot times, before the
n
th retransmission attempt, is chosen as a uniformly distributed random integer in the range:
0
≤
r
<
2
k
k = min(n, N)
The maximum backoff time is programmable by limiting the
“
N
”
number
(TX_TSHD_BOFF<BKL>). In the IEEE 802.3 Standard,
“
N
”
is equal to 10.
When 16 transmission attempts have been made, all terminated by collisions, the 21440 reports an
excessive collision event. The 21440 then stops transmission if programmed to do so
(TX_ERR_MOD<XLCS>). Otherwise, it will continue to transmit the same packet again and
again, while restarting the backoff algorithm as if it were a new packet (n = 0).
In network topology violating standard requirements, collision may occur after transmission of
64 bytes. If the 21440 was programmed to stop upon late collision (TX_ERR_MOD<LCLS>), it
will flush the transmit FIFO and stop transmitting. Otherwise, the 21440 will discard the packet
from its FIFO, and resume transmission with the next packet.
6.4.5
Terminating Transmission
A specific frame transmission is terminated under any of the following conditions:
Normal
The frame has been transmitted successfully. After the last byte is serialized, the pad and CRC
are optionally appended and transmitted, thus concluding frame transmission.
CRC error
The txerr signal was asserted during packet loading. The 21440 infects the CRC it is building and
sends a bad CRC onto the network. An MII error is generated as well (terr assertion in MII mode).
Underflow
Transmit data is not ready when needed for transmission. The packet is terminated on the
network with a bad CRC and an MII error generation (terr assertion in MII mode).
Excessive collisions
A collision occurs 16 consecutive times during transmission attempt of the same frame.
Late collision
A collision occurs after the collision window (transmitting at least 64 bytes). The transmission
is cut off.
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