LAN9353 Datasheet, PDF (223/523 Pages) Microchip Technology – Interfaces at up to 200Mbps via Turbo MII

LAN9353
10.4.5 EGRESS RATE LIMITING (LEAKY BUCKET)
For egress rate limiting, the leaky bucket algorithm is used on each output priority queue. For each output port, the band-
width that is used by each priority queue can be limited. If any egress queue receives packets faster than the specified
egress rate, packets will be accumulated in the packet memory. After the memory is used, packet dropping or flow con-
trol will be triggered.
Egress rate limiting occurs before the Transmit Priority Queue Servicing, such that a lower priority queue will be serviced
if a higher priority queue is being rate-limited.
The egress limiting is enabled per priority queue. After a packet is selected to be sent, its length is recorded. The switch
then waits a programmable amount of time, scaled by the packet length, before servicing that queue once again. The
amount of time per byte is programmed into the Buffer Manager Egress Rate registers (refer to Section 10.7.4.14
through Section 10.7.4.19 for detailed register definitions). The value programmed is in approximately 20 ns per byte
increments. Typical values are listed in Table 10-4. When a port is transmitting at 10 Mbps, any setting above 39 has
the effect of not limiting the rate.
TABLE 10-4: TYPICAL EGRESS RATE SETTINGS
Egress Rate
Setting
Time Per Byte
Bandwidth @
64 Byte Packet
Bandwidth @ 512
Byte Packet
Bandwidth @
1518 Byte Packet
0-3
80 ns
76 Mbps (Note 1)
96 Mbps (Note 1)
99 Mbps (Note 1)
4
100 ns
66 Mbps
78 Mbps
80 Mbps
5
120 ns
55 Mbps
65 Mbps
67 Mbps
6
140 ns
48 Mbps
56 Mbps
57 Mbps
7
160 ns
42 Mbps
49 Mbps
50 Mbps
9
200 ns
34 Mbps
39 Mbps
40 Mbps
12
260 ns
26 Mbps
30 Mbps
31 Mbps
19
400 ns
17 Mbps
20 Mbps
20 Mbps
39
78
158
396
794
1589
3973
7947
800 ns
1580 ns
3180 ns
7940 ns
15900 ns
31800 ns
79480 ns
158960 ns
8.6 Mbps
4.4 Mbps
2.2 Mbps
870 kbps
440 kbps
220 kbps
87 kbps
44 kbps
10 Mbps
5 Mbps
2.5 Mbps
990 kbps
490 kbps
250 kbps
98 kbps
49 kbps
10 Mbps
5 Mbps
2.5 Mbps
1 Mbps
500 kbps
250 kbps
100 kbps
50 kbps
Note 1: These are the unlimited max. bandwidths when IFG and preamble are taken into account.
10.4.6 ADDING, REMOVING AND CHANGING VLAN TAGS
Based on the port configuration and the received packet format, a VLAN tag can be added to, removed from or modified
in a packet. There are four received packet type cases: non-tagged, priority-tagged, normal-tagged and CPU special-
tagged. There are also four possible settings for an egress port: dumb, access, hybrid and CPU. In addition, each VLAN
table entry can specify the removal of the VLAN tag (the entry’s un-tag bit).
The tagging/un-tagging rules are specified as follows:
• Dumb Port - This port type generally does not change the tag.
- When a received packet is non-tagged, priority-tagged or normal-tagged the packet passes untouched.
- When a packet is received special-tagged from a CPU port, the special tag is removed.
• Access Port - This port type generally does not support tagging.
- When a received packet is non-tagged, the packet passes untouched.
- When a received packet is priority-tagged or normal-tagged, the tag is removed.
 2015 Microchip Technology Inc.
DS00001925A-page 223