| 摘要 |
<p>For a packet identification (PID) contained at a variable possible location which comprises part only of a 32-bit packet header, a corresponding DES key value is located. A table (10) stored in memory contains for each DES key (12) (i) a packet header (14) comprising 32 bits with a possible PID of e.g. 12, 9 or 8 bits contained at a defined location and with zero values elsewhere, and (ii) a mask value (16) also comprising 32 bits with ones contained at the said defined location of the PID and zeros elsewhere. The table is divided into regions (18) for respective packet format types. An incoming packet header at an input (22) is combined (26) with a first one of the mask values from the table so as to provide a combined value which consists of the value held in the input packet header at the defined location and zeros elsewhere. This combined value is compared (36) with the corresponding packet header stored in the table. If they are not equal, the combining and comparison is repeated for the next row of the table. If they are equal, the corresponding DES key value is read (32) from the table and provided as an output (38). The system can cope with variable PID formats within the packet header without alteration to the hardware but merely with re-programming of the table contents. <IMAGE></p> |
