 CAM can store and search binary words (made of "O"s and "1"s). Thus, binary CAMs are suitable for applications that require only exact-match searches. A more powerful and feature-rich TCAM can store and search ternary states ("1," "0," and "X"). The state "X", also called "mask" or "don't care," can be used as a wild- card entry to perform partial matching. Optical SN gain (dB) (optical noise dominant) Code A Code B Code C FIGURE 5.75 Coding gain for a few block turbo codes based on RS code components. The code characteristics are listed in the text. Log10(BER) after FEC operation Telecommunication Networks Priority encoder Stored word - N Match Stored word - 2 location Stored word - 1 Match lines Search word register Search lines Search word FIGURE 5.76 CAM functional scheme. A typical CAM cell utilizing 10 transistors all in CMOS configuration (a 10T cell) is shown in Figure 5.77. The bottom part of the figure (transistors from T1 to T6) repli- cates exactly the structure of an SRAM cell and is used to perform READ and WRITE operations. The transistors from T7 to T10 implement the XNOR logic that is needed during the SEARCH to compare the table entry with the search key. As in a SRAM, The WRITE operation is performed by placing the data on the bit lines (Bs) and enabling the word line. This turns on the access CMOS (T1-T2), and the inter- nal nodes of the cross-coupled inverters are written by the data. The READ operation is performed by pre-charging the Bs to the power supply voltage VD and enabling the word line. T7 T9 T8 T10 FIGURE 5.77 Word line Circuit representation of a CAM cell. Technology for Telecommunications: Integrated Optics and Microelectronics The conventional SEARCH operation is performed in steps: Search lines (S and S') are reset to ground. M is pre-charged to VD. The search key bit and its complementary value are placed on S and S'. If the search key bit is identical to the stored value (S=B, S = B'), = = both M-to-ground pull- down paths remain "OFF," and the M remains at VD indicating a "