Contemporary Control Systems Compact Managed Switches Software Manual for Web Browser - Manual (Page 37)

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Middle Table — Map TOS Precedence and Priority Queue
Although TOS priority is supported by few TCP/IP implementations, it is provided as a
QoS option in managed switches from Contemporary Controls. One of the earliest
methods of QoS for Internet Protocol, TOS uses the second octet (the TOS field) of the
IP frame header — as described in RFC791 and RFC1349. The first three bits of this
octet set the priority (Precedence). The next four bits (known as the TOS bits) define
the tradeoffs among these four service objectives:
• minimize monetary cost (M-Type Priority) *
• maximize reliability (R-Type Priority)
• maximize throughput (T-Type Priority)
• minimize delay (D-Type Priority)
* This is sometimes referred to as “C-Type” priority or type of service.
The final bit of the TOS field was unused until DiffServ was defined.
Figure 32 — Map TOS Precedence and Priority Queue