Contemporary Control Systems Compact Managed Switches Software Manual for Console Access - Manual (Page 31)

TD020850-0MG
31
4.3.7.3.1 Map TOS Precedence and Priority Queue
Figure 24 — Map TOS Precedence and Priority Queue
Figure 24 displays the default TOS parameters that will apply if you have chosen TOS
as your QoS type. 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.