ACD81024 데이터 시트보기 (PDF) - Unspecified
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ACD81024 Datasheet PDF : 17 Pages
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• Preamble is repetitive pattern of 1010…. of any
length longer than 30 bits.
• SFD (Start Frame Delimiter) is defined as an
octet pattern of 10101011.
A port’s MAC address register is cleared on power-
up, by hardware reset, or when the MAC logic cir-
cuitry enters its Link Fail state due to loss of link pulse
reported by the PHY module.
• DA is a 48-bit field which specifies the MAC
Unicast Frame Forwarding
address of the DTE to which the frame is head-
ing. If the first bit of DA is 1, ACD81024 will treat If the first bit of the destination address is 0, the frame
the frame as a broadcast/multicast frame and will is a unicast frame. The destination address is passed
forward the frame to all ports except the source to the Lookup Engine which returns a destination
port itself.
port(s) number(s) to identify which port(s) should the
frame be forwarded. If the Lookup Engine cannot find
• SA is a 48-bit field which contains the MAC
any match for the destination address, the frame will
address of the DTE which is transmitting the
be forwarded to all the ports which has not learned its
frame to ACD81024.
source address, plus the expansion port. The re-
turned destination port number is passed to the
• Type/Len field is a 2-byte field which specifies the Fabric Control logic which checks if the destination
type (DIX Ethernet frame) or length (IEEE 802.3 port(s) is ready to receive the frame. If it is, the Switch
frame) of the frame. ACD81024 will not try to
Control Logic constructs the communication
interpret this field.
channel(s) between the source port and the destina-
tion port(s). The source port MAC logic forwards the
• Data could be any information which is totally
incoming frame to the destination port(s) MAC logic
transparent to ACD81024.
through the communication channel(s). While the
data is forwarded, the MAC logic of the destination
• FCS (Frame Check Sequence) is a 32-bit field of port keeps on monitoring if collision occurs on the
CRC (Cyclic Redundancy Check) value based on destination port. If so, the source port MAC logic will
the destination address, the source address, the send a jam pattern to the source DTE. If the any one
type/length and the data field. ACD81024 will
of the destination port(s) is not ready to receive the
verify the FCS field for each frame. The proce-
frame, the source port MAC logic will start a conges-
dure of computing FCS is described in section of tion control process. The details of congestion control
“FCS Calculation”.
is explained in the section of “Congestion Control.”
Destination Address Processing
Broadcast/Multicast Traffic Handling
As a frame comes into a port of ACD81024, the
destination address field embedded inside the frame
is retrieved and passed to the Lookup Engine to
match it with the MAC addresses stored in source
MAC address registers of all ports. The destination
port is found if a match is found.
Source Address Learning
As a frame comes into a port of ACD81024, the value
of the source address embedded inside the frame is
automatically read into a data register. At the end of
the frame, if there is no FCS error and, if there is no
value stored in the MAC address register of the
source port, or if the value of this new source address
is different than the port’s current MAC address, the
recorded value is used to update the MAC address
register of the source port inside the Lookup Engine
module. The value will be used by the Lookup Engine
to match with the destination address embedded in a
frame to identify the destination port.
If the first bit of the destination address is a 1, the
frame is a multicast or broadcast frame. ACD81024
treats multicast frame the same as broadcast frame.
For a broadcast frame, the destination port contains
all the ports of the switch except the source port itself.
The Fabric Control logic checks if all the destination
ports are ready for receiving this broadcast frame. If
all of the destination ports are ready to receive the
frame, the source port will forward the frame data to
all destination ports in cut-through mode. If any one of
the destination ports is not ready, the source port
MAC logic will send a Jam pattern to the source DTE,
followed by Back Pressure signal. When all ports are
ready to transmit the broadcast frame, the Back
Pressure signal applied to the source DTE will be
removed. Upon the time the source DTE retransmits
the broadcast frame to the switch, the frame data is
forwarded to all destination ports.
FCS Calculation
Each port of ACD81024 has a CRC checking logic to
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