Table of Contents
Advertisement
Quality of Service
QoS (Quality of Service) is an industry-wide initiative to provide preferential
treatment to certain subsets of data, enabling that data to traverse the
Internet or intranet with higher quality transmission service.
There have been two generations of quality of service architectures in the
Internet. The interpretation of the Type of Service Octet in the Internet
Protocol header varies between these two generations.
The First generation: Precedence and type of service bits The refined
definition of the initial Type of Service Octet looks like this：
2^7
Precedence
The Second generation: Differentiated services code point
The Differentiated Service Code Point is a selector for router's per-hop
behaviors (PHB). As a selector, there is no implication that a numerically
greater DSCP implies a better network service. RFC2474 redefined the Type
of Service Octet to be：
2^7
Differentiated Services Code Point
The fields ECT and CE are nothing to do with quality of service. They are
spare bits in the IP header used by Explicit Congestion Notification. As can
be seen, the DSCP totally overlaps the old Precedence field. So if values of
DSCP are carefully chosen then backward compatibility can be achieved.
This leads to the notions of "class", each class being the group of DSCP with
the same Precedence value. Values within a class would offer similar
network services but with slight differences. Classes were initially defined
as：
2^6
2^5
2^4
2^6
2^5
2^4
DSCP
Precedence
0
0
8
1
16
2
24
3
32
4
40
5
48
6
56
7
Wireless IAD User Manual
2^3
2^2
Type of Service Field
2^3
2^2
Purpose
Best effort
Class 1
Class 2
Class 3
Class 4
Express forwarding
Control
Control
2^1
2^0
2^1
2^0
ECT
CE
83
Advertisement
Table of Contents
