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参数资料
| 型号： | ZL50418/GKC |
| 厂商： | ZARLINK SEMICONDUCTOR INC |
| 元件分类： | 网络接口 |
| 英文描述： | DATACOM, LAN SWITCHING CIRCUIT, PBGA553 |
| 封装： | 37.50 X 37.50 MM, 2.33 MM HEIGHT, MS-034, HSBGA-553 |
| 文件页数： | 98/155页 |
| 文件大小： | 1928K |
| 代理商： | ZL50418/GKC |
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ZL50418
Data Sheet
47
Zarlink Semiconductor Inc.
4.0
Data Forwarding Protocol
4.1
Unicast Data Frame Forwarding
When a frame arrives, it is assigned a handle in memory by the Frame Control Buffer Manager (FCB Manager). An
FCB handle will always be available because of advance buffer reservations.
The memory (SRAM) interface consists of two 64-bit buses, connected to two SRAM banks, A and B. The Receive
DMA (RxDMA) is responsible for multiplexing the data and the address. On a port’s “turn,” the RxDMA will move 8
bytes (or up to the end-of-frame) from the port’s associated RxFIFO into memory (Frame Data Buffer, or FDB).
Once an entire frame has been moved to the FDB, and a good end-of-frame (EOF) has been received, the Rx
interface makes a switch request. The RxDMA arbitrates among multiple switch requests.
The switch request consists of the first 64 bytes of a frame, containing among other things, the source and
destination MAC addresses of the frame. The search engine places a switch response in the switch response
queue of the frame engine when done. Among other information, the search engine will have resolved the
destination port of the frame and will have determined that the frame is unicast.
After processing the switch response, the Transmission Queue Manager (TxQ manager) of the frame engine is
responsible for notifying the destination port that it has a frame to forward to it. But first, the TxQ manager has to
decide whether or not to drop the frame, based on global FDB reservations and usage, as well as TxQ occupancy
at the destination. If the frame is not dropped, then the TxQ manager links the frame’s FCB to the correct per-port-
per-class TxQ. Unicast TxQ’s are linked lists of transmission jobs, represented by their associated frames’ FCB’s.
There is one linked list for each transmission class for each port. There are 4 transmission classes for each of the
16 10/100 M ports and 8 classes for each of the two Gigabit ports – a total of 80 unicast queues.
The TxQ manager is responsible for scheduling transmission among the queues representing different classes for a
port. When the port control module determines that there is room in the MAC Transmission FIFO (TxFIFO) for
another frame, it requests the handle of a new frame from the TxQ manager. The TxQ manager chooses among
the head-of-line (HOL) frames from the per-class queues for that port using a Zarlink Semiconductor scheduling
algorithm.
The Transmission DMA (TxDMA) is responsible for multiplexing the data and the address. On a port’s turn, the
TxDMA will move 8 bytes (or up to the EOF) from memory into the port’s associated TxFIFO. After reading the EOF,
the port control requests a FCB release for that frame. The TxDMA arbitrates among multiple buffer release
requests.
The frame is transmitted from the TxFIFO to the line.
4.2
Multicast Data Frame Forwarding
After receiving the switch response, the TxQ manager has to make the dropping decision. A global decision to drop
can be made, based on global FDB utilization and reservations. If so, then the FCB is released and the frame is
dropped. In addition, a selective decision to drop can be made, based on the TxQ occupancy at some subset of
the multicast packet’s destinations. If so, then the frame is dropped at some destinations but not others and the
FCB is not released.
If the frame is not dropped at a particular destination port, then the TxQ manager formats an entry in the multicast
queue for that port and class. Multicast queues are physical queues (unlike the linked lists for unicast frames).
There are 2 multicast queues for each of the 16 10/100 M ports. The queue with higher priority has room for 32
entries and the queue with lower priority has room for 64 entries. There are 4 multicast queues for each of the two
Gigabit ports. The size of the queues are: 32 entries (higher priority queue), 32 entries, 32 entries and 64 entries
(lower priority queue). There is one multicast queue for every two priority classes. For the 10/100 M ports to map
the 8 transmit priorities into 2 multicast queues, the 2 LSB are discarded. For the Gigabit ports to map the 8
transmit priorities into 4 multicast queues, the LSB are discarded.
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