LAN9312 datasheet(78/458 Pages) SMSC

78 / 458 page

High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface

Datasheet

Revision 1.2 (04-08-08)

SMSC LAN9312

DATASHEET

6.5.4

Transmit Priority Queue Servicing

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 Buffer Manager Configuration Register (BM_CFG) 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 Buffer Manager Configuration Register (BM_CFG) 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.).

6.5.5

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 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 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 14.5.4.14 through Section 14.5.4.19 for detailed register

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

listed in Table 6.5. When a port is transmitting at 10 Mbps, any setting above 39 has the effect of not

limiting the rate.

Note 6.3

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

Table 6.5 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 6.3)

96 Mbps (Note 6.3)99 Mbps (Note 6.3)

100 nS

66 Mbps

78 Mbps

80 Mbps

120 nS

55 Mbps

65 Mbps

67 Mbps

140 nS

48 Mbps

56 Mbps

57 Mbps

160 nS

42 Mbps

49 Mbps

50 Mbps

200 nS

34 Mbps

39 Mbps

40 Mbps

260 nS

26 Mbps

30 Mbps

31 Mbps

400 nS

17 Mbps

20 Mbps

800 nS

8.6 Mbps

10 Mbps

1580 nS

4.4 Mbps

5 Mbps

158

3180 nS

2.2 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

3973

79480 nS

87 Kbps

98 Kbps

100 Kbps

7947

158960 nS

44 Kbps

49 Kbps

50 Kbps

LAN9312 Scheda tecnica(PDF) 78 Page - SMSC Corporation

LAN9312 Scheda tecnica(HTML) 78 Page - SMSC Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Scheda tecnica Scarica

Link URL