-specific, SO they will be lost if frames are forwarded out of Ethernet networks. The good news is that because they map SO perfectly to IP prece- dence values, rules can be created to read CoS values and translate them to IP precedence values. Chapter 29: Introduction to QoS DiffServ IP precedence TOS field (3 bits) (IP preference) 101000 (6 bits) (DSCP) 10100000 (8 bits) = 160 (TOS) They are all the same binary digits, but in different field lengths, they represent different decimal values! Figure 29-6. Different decimal values depending on the number of significant bits in the TOS field In Cisco catalyst switches, there can be multiple queues for each Ethernet interface. CoS values can be mapped to these queues, allowing for prioritization. Ethernet pri- orities can be different from WAN-link priorities, though in practice, they're usually mapped one to one. Table 29-1 shows how CoS, IP precedence, and DSCP values are related. If you take a look at IP precedence 5, you'll see that it is the same as a DSCP value of between 40 and 47. The decimal value 40 is 101000 in binary. The decimal value 47 is 101111 in binary. However, 48 is 110000 in binary-the first three bits change from 101 to 110, thus changing the IP precedence significant bits in the TOS field from 5 to 6. Table 29-1. Priority levels of different QoS types IP precedence Routing (Best Effort) Priority 16-23 Immediate 24-31 Flash 32-39 Flash-Override 40-47 Critical 48-55 Internet 56-63 Network Because the IP precedence values and the DSCP values are SO closely related, they can usually be interchanged. For example, if you're running a diffserv environment that uses DSCP values, but have IP phones that will only mark packets using IP pre- cedence values, you're still OK because a diffserv value of 40 is the same bitwise as an IP precedence value of 5. QoS Mechanics Flavors of QoS These are the most commonly seen forms of QoS. If you're working with a VoIP network, you'll probably see CBWFQ on your routers, and CoS on your switches: Weighted fair queui