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參數(shù)資料
| 型號： | TNETA1560 |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | ATM Segmentation and Reassembly Device with SBUS Host Interface(ATM 分段和重設(shè)裝置帶SBUS主機接口) |
| 中文描述： | 自動柜員機分段和重組與SBus主機接口（自動柜員機分段和重設(shè)裝置帶SBU的主機接口設(shè)備） |
| 文件頁數(shù)： | 28/40頁 |
| 文件大小： | 804K |
| 代理商： | TNETA1560 |
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TNETA1560
ATM SEGMENTATION AND REASSEMBLY DEVICE
WITH SBUS HOST INTERFACE
SDNS010C – JANUARY 1994 – REVISED OCTOBER 1995
28
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
packet length (bits 26 – 16)
The packet-length field is expressed in units of cells in the packet. The host computes the correct number of
cells in the packet including the additional cell sometimes needed for AAL5 to accommodate the 8-byte tail. This
field represents the value used by the SBus SAR to determine the number of cells in a packet and enable EOP
processing.
The field is programmed in two’s complement. Incrementing the value by one each time a cell is sent results
in zero when the entire packet is transmitted. The maximum size of a packet is 64K bytes; therefore, 11 bits are
adequate to describe the largest packet.
Since this is a packet-level field as opposed to one that applies to individual buffers, it is only placed in the first
buffer descriptor of a packet in the transmit data-descriptor rings. The DMA channel only updates the
packet-length field on a per-packet basis. The packet-length field is general in that it is used for all three AAL
modes supported. In each case, the SAR enables EOP processing to notify the host when the EOP is detected
on transmit via the packet-length field.
buffer length (bits 15 – 0)
The buffer-length field specifies the number of bytes in the buffer represented by this descriptor-ring entry. The
maximum buffer size is 64K bytes, which is the largest packet size and allows an entire packet in one buffer.
This field is programmed in two’s complement and is equal to zero when all the bytes in a buffer are retrieved
by the SAR.
TX descriptor-ring word 1 – start-of-buffer pointer
Byte-aligned start-of-buffer pointer (bits 31 – 0)
The start-of-buffer pointer is 32 bits. Each buffer can be aligned on byte boundaries.
TX descriptor-ring word 2 – ATM header
PTI2 (bits 31 – 29)
VPI (bits 27 – 20) VCI (bits 19 – 4)
CLP2 (bit 28)
PTI1 (bits 3 – 1)
CLP1 (bit 0)
Word 2 contains the 4-byte header for every cell of the packet. The upper-order four bits of the ATM header,
representing the GFC at the user-to-network interface (UNI), are set to zero in every outgoing cell. Bits (3 – 0)
in word 2 represent the payload-type indicator (PTI) and cell-loss priority (CLP) fields used in every cell of the
packet except the last one (the cell that contains the EOP indication). Bits (31 – 28) in word 2 represent the PTI
and CLP fields used in the last cell of the packet.
The PTI field in the last cell of an AAL5 packet is set either to 001 or 011. The CLP is programmable and the
cell containing the EOP indication can have a different priority level from the other cells. This field is required
only in the first descriptor for the packet. In AAL3/4 or null-AAL packets, the PTI and CLP fields in both the upper-
and lower-order bits of word 2 are the same.
TX descriptor-ring word 3 – AAL5 control/length
AAL5 control field (bits 31 – 16)
AAL5 length field (bits 15 – 0)
The AAL5 control and length fields apply to packets, not to buffers, and this entry is required only in the first
descriptor for the packet. The AAL5 length field is not used to determine the length of the packet during transmit
processing. The software driver is responsible for entering these two fields. The control field is all zeros and the
length field is the number of bytes in the packet. Both fields are placed in the descriptor ring in an AAL5 packet
in the proper position (in the four bytes preceding the AAL5 32-bit CRC). These fields are not used if the packet
is either an AAL3/4 or a null-AAL packet.
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