FBFM2112F897CSLJLS View Datasheet(PDF) - Intel

Part Name

Description

Manufacturer

FBFM2112F897CSLJLS

Intel® FM2112 24-Port 10G/1G Ethernet Switch Chip

Intel 

Intel® FM2112 24-Port 10G/1G Ethernet Switch Chip Data Sheet

3.1.5.1

3.1.5.2

3.1.5.3

Theory

It is often desirable to limit the rate that a device can send data to its

link partner to a defined rate that is below the maximum rate of the link

(often referred to as rate pacing). In some situations the link partner is

not capable of consuming data at the maximum rate, sustained. By

limiting the rate (rate pacing), one can avoid overloading the receiving

device. Given that the IEEE 802.3ae specification defines a link rate of

10 Gbps, rate pacing is achieved by sending a frame at line rate, and

then stretching the inter-frame gap to some extent to achieve the

desired average data rate on the link over a specified period of time,

allowing a 10 Gigabit link to maintain an effective rate which is lower

than the clock rate.

Definition of Terms

PR

IFGS

Length

IFGC

Eligible

Pacing Rate: The target bandwidth of the link.

Inter-Frame Gap Stretch: The calculated length of byte times

that the transmitter places after a frame before the start of the

next frame in addition to the standard preamble and IFG to

achieve the pacing rate.

Length of the previous frame.

Inter-Frame Gap Constant. The traditional IFG, or the IFG when

the pacing rate = line rate. IFG = IFGS + IFGB.

The port is eligible if it has a frame in memory that the bridge

indicates is ready for transmission.

Functionality

Datapath

The transmitter calculates the IFGS for the next frame via the

equation:

IFGS[n+1] += (1/PR-1)*Length+IFGS[n]

Next Packet ≥ EOP + Preamble + IFGC (strict requirement)

Next Packet ≥ EOP + IFGS (soft requirement)

After a frame is transmitted, the transmitter does not begin

transmitting the next frame, even if the port is eligible, until it has

waited for the time it would take to transmit the IFGS worth of bytes.

Control

The pacing rate is statically controlled. (It is anticipated that the IEEE

will define a standard method for dynamically controlling this feature

by exchanging control messages with the downstream link partner.

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