SMSC LAN9311i Manual De Usuario

Two Port 10/100 Managed Ethernet Switch with 16-Bit Non-PCI CPU Interface

Revision 1.4 (08-19-08)

78

SMSC LAN9311/LAN9311i

 

When a transmit queue is non-empty, it is serviced and the packet is read from the buffer RAM and
sent to the transmit MAC. If there are multiple queues that require servicing, one of two methods may
be used: fixed priority ordering, or weighted round-robin ordering. If the Fixed Priority Queue Servicing
bit in the 

 is set, a strict order, fixed priority is

selected. Transmit queue 3 has the highest priority, followed by 2, 1, and 0. If the Fixed Priority Queue
Servicing bit in the 

 is cleared, a weighted round-

robin order is followed. Assuming all four queues are non-empty, the service is weighted with a 9:4:2:1
ratio (queue 3,2,1,0). The servicing is blended to avoid burstiness (e.g. queue 3, then queue 2, then
queue 3, etc.).

For egress rate limiting, the leaky bucket algorithm is used on each output priority queue. For each
output port, the bandwidth 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 control will be triggered. 

Note: Egress rate limiting occurs before the 

, 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 

 through 

 for detailed register

definitions). The value programmed is in approximately 20 nS per byte increments. Typical values are
listed in 

. When a port is transmitting at 10 Mbps, any setting above 39 has the effect of not

limiting the rate.

These are the unlimited max bandwidths when IFG and preamble are taken into account.

0-3

80 nS

76 Mbps (

96 Mbps (

99  Mbps  (

)

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

800 nS

8.6 Mbps

10 Mbps

10 Mbps

78

1580 nS

4.4 Mbps

5 Mbps

5 Mbps

158

3180 nS

2.2 Mbps

2.5 Mbps

2.5 Mbps

396

7940 nS

870 Kbps

990 Kbps

1 Mbps

794

15900 nS

440 Kbps

490 Kbps

500 Kbps

1589

31800 nS

220 Kbps

250 Kbps

250 Kbps

3973

79480 nS

87 Kbps

98 Kbps

100 Kbps

7947

158960 nS

44 Kbps

49 Kbps

50 Kbps