
PRS28.4G
IBM Packet Routing Switch
Functional Description
Page 30 of 131
prs28.03.fm
August 31, 2000
3.2 Header Format
3.2.1 Header Byte H0 - Packet Qualifier
3.2.1.1 Parity Bit
The parity bit is an even parity calculated on the first three bytes of the header. Even parity assures that the
resulting number of ‘1’ in the three byte header is even. This ensures that the SYNC cell has a valid header.
3.2.1.2 Reserved Bits
Reserved bits pass through the device unmodified.
Table 4: Header Byte H0 for Idle Packets
(Bits 2 and 3
≠
0)
Packet Type
Bit Position
0
1
2
3
4 and 5
6 and 7
Color bit
(1)
Parity
(1)
0
0
Color bits
(2)
Grant Priority
bits (2)
Blue Idle Packet
0
r
0
0
0 0
g g
Red Idle Packet
0
r
0
0
0 1
g g
Yellow Idle Packet
0
r
0
0
1 0
x x
Sync Packet
1
1
0
0
1 1
0 0
Reserved
1
1
1
0
r
r
r
r
0
0
0
0
0
0
0
0
0 0
0 1
1 0
1 1
x x
x x
x x
x x
Key:
x
r
gg
value = 0 or 1
even parity bit on entire header (H0, H1, H2)
value of grant sync bits (ignored for received packets - value = ‘00’
Table 5: Header Byte H0 for Data Packet
(Bits 2 and/or 3
≠
0),
and Control Packet
(Bits 2 and/or 3
≠
0; H1 and
H2
≠
0)
Packet Type
Bit Position
0
1
2
3
4
5 and 6
7 and 8
Reserved
(1)
Parity
(1)
Active Bit
(1)
Backup Bit
(1)
Drop Bit
(1)
Reserved
(1)
Priority
(2)
Red Data/Control Packet
x
r
0
1
d
x
p p
Yellow Data/Control
Packet
x
r
1
0
d
x
p p
Blue Data/Control
Packet
x
r
1
1
d
x
p p
Key:
x
r
gg
value = 0 or 1
even parity bit on entire header (H0, H1, H2)
value of grant sync bits (ignored for received packets - value = ‘00’