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Application Note AN-2036

Frequently Asked Questions Regarding

Finisar’s 1000BASE-T SFPs

Finisar’s 1000BASE-T SFP transceivers are based on the SFP Multi Source Agreement

(MSA). They are compatible with Gigabit Ethernet and 1000BASE-T standards as

specified in IEEE Std. 802.3:2005. They can also function as 10BASE-T and 100BASE-

T transceivers, but only after programming over a 2-wire bi-directional serial interface.

This Application Note covers the most commonly asked questions about Finisar’s

1000BASE-T SFPs.

1. What versions are there of the Finisar 1000BASE-T SFP and how do they

differ?

Finisar currently offers 6 versions of 1000BASE-T SFP as listed in the following

table:

Part Number RoHS Case Temp PHY Rev RxLOS Auto-Negotation

Supported?

FCMJ-8520-3 5/6 0C–85C B0 Yes No

FCMJ-8521-3 5/6 0C–85C B0 No Yes

FCLF-8520-3 6/6 0C–85C B0 Yes No

FCLF-8521-3 6/6 0C–85C B0 No Yes

FCLF8520P2BTL 6/6 -40C–85C B2 Yes No

FCLF8521P2BTL 6/6 -40C–85C B2 No Yes

The most critical distinction is between the “8520” series products (i.e., the FCMJ-

8520-3, the FCLF-8520-3, and the FCLF8520P2BTL), and the “8521” series products

(i.e., the FCMJ-8521-3, the FCLF-8521-3, and the FCLF8521P2BTL). This

distinction is discussed in more detail in FAQ Question 11. For topics that apply

equally well to either 8520 or 8521 products, we’ll refer to “1000BASE-T SFP”

products or occasionally to “Cu SFP” products.

All 6 part number variations use a Marvell 88E1111 “PHY”, as discussed in FAQ

Question 5. The oldest “first generation” designs use the Rev. B0 of this PHY chip;

the latest “second generation” uses Rev. B2. Distinctions in performance between

first and second generation products are listed in FAQ Question 24.

2. Are there special electrical supply considerations when using the Finisar

1000BASE-T SFP?

Yes. The Finisar 1000BASE-T SFP consumes a maximum of 1.20W under worst-

case conditions (typical is ~ 1.05 W). The maximum current draw is 375 mA,

compared to 300 mA for a standard SFP. The electrical supply of the host should be

examined to ensure that it can handle these increased demands. The 1000BASE-T

SFP is generally more sensitive to supply voltage (3.13 to 3.47V) than most optical

transceivers (e.g., 3.0 to 3.6V typical).

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3. Will the extra length of the Finisar 1000BASE-T SFP cause mechanical

interference problems in my system?

While there is a chance of interference problem s, the probability is small. If you

compare the length of a Finisar 1000BASE-T SFP to a fiber SFP, with their

respective cables inserted, there is little d if ference in length. This is due to the strain

relief features on LC connectors that protrude much farther than the insulation on a

Cat 5 cable. An optical and 1000BASE-T SFP with cables inserted is shown in

Picture 1 below.

Flexible insulation Stiff insulation

Picture 1: 1000BASE-T and optical SFP transceivers with cables inserted.

4. What cable type is recommended for use with the Finisar 1000BASE-T SFP?

The Finisar 1000BASE-T SFP was designed to operate using standard Cat 5 cable

that has been configured per IEEE802.3:2005. You can safely use Cat 5e or Cat 6

cables, as these are improved versions of Cat 5. For short cable runs, with intricate

routing, stranded cable is recommended, as it is more flexible.

5. What is the PHY and how can it be accessed?

The Finisar 1000BASE-T SFP uses the Marvell 88E1111 Physical Layer IC (PHY) to

convert between the serial interface and 1000BASE-T interfaces. The first generation

designs use Rev. B0 of this PHY; the second generation uses Rev. B2. This chip has

a number of useful features available on internal registers that can be accessed via the

2-wire bi-directional serial interface at address 0 xAC. The contents at each register

address are 2 bytes wide; details for accessing the register can be found at

http://www.marvell.com

6. What is SGMII mode?

SGMII stands for “Serial Gigabit Media Independent Interface.” SGMII is a mode of

communication between the MAC and PHY to allow for 10/100/1000BASE-T

operation. In 100BASE-TX mode, the MAC still transmits to the PHY at 1.25 Gb/sec,

but each byte is repeated 10 times. The PHY then converts this repeated data to

100BASE-TX format. The process is the same in 10BASE-T mode but each byte is

repeated 100 times. SGMII is a Cisco Systems specification, document EN-46158,

and is available for download at no cost over the internet: ftp://ftp-

eng.cisco.com/smii/sgmii.pdf. At the time this FAQ was written, the latest SGMII

version is 1.8.