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Ip header ethernet- header – Siemens HIPATH 8000 User Manual

Page 209

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Alphabetical Reference

209

Assured forwarding is used for signaling messages by default (AF31) and

less stringent than EF in a multiple dropping system.

The AF values are containing two digits X and Y (AFXY), where X is describ-

ing the priority class and Y the drop level.

Four classes X are reserved for AFXY: AF1Y (High Priority), AF2Y, AF3Y and

AF4Y (Low Priority).

Three drop levels Y are reserved for AFXY: AFX1 (low drop level), AFX2 and

AFX3 (High drop level). In the case of low drop level, packets are buffered

over an extended period in the case of high drop level, packets are prompt-

ly rejected if they cannot be forwarded.

Layer 3 / IP Precedence (old recommendation, replaced by Diffserv)

IP Precedence is classifying traffic flows into 8 different precedence levels.

These 3 Bits are the same as the priority Bits from Diffserv. Value 5 (BIN:

101) is used for voice by default. And value 3 (BIN: 011) is used for signaling.

These values are already configured by configuring the DiffServ values.

If a routing partner can only work with one of the two standards (DiffServ

or IP precedence, for example an older router that only works with IP pre-

cedence), than the router can translate the ToS field accordingly. This can

be set for each PSTN peer or LAN interface.

Æ page 58 and Æ page 46

Æ

page 117

Standard IPV4: Three MSB Called IP Precedence

(DiffServ Uses Six MSB Bits)

Version

Length

ToS

1 Byte Len ID Offset TTL Proto FCS IP-SA IP-DA Data

QoS

IP header

Ethernet-

Header

DiffServ

versus

IP Precedence

IEEE802.1p

Diffserv-

Codepoint

Binary

Decimal

Hex (8Bit)

Drop level

Binary

Decimal

6-Bit

8-Bit

ToS Field

Priority

high

med

low

EF

101110

46

184

B8

very high

x

<->

101

101

5

AF 43

100110

38

152

98

high

x

<->

100

100

4

AF 42

100100

36

144

90

x

<->

100

100

4

AF 41

100010

34

136

88

x

<->

100

100

4

AF 33

011110

30

120

78

medium

x

<->

011

011

3

AF 32

011100

28

112

70

x

<->

011

011

3

AF 31

011010

26

104

68

x

<->

011

011

3

AF 23

010110

22

88

58

low

x

<->

010

010

2

AF 22

010100

20

80

50

x

<->

010

010

2

AF 21

010010

18

72

48

x

<->

010

010

2

AF 13

001110

16

64

38

very low

x

<->

001

001

1

AF 12

001100

12

48

30

x

<->

001

001

1

AF 11

001010

10

40

28

x

<->

001

001

1