To provide some background, there are three things that control our ability to reliably send data over longer distances than 300 meters.
- The intensity (brightness) of the transmitter (for long distances this is normally a laser as opposed to an LED). The transmit value is measured in dBm. There is a concept called a link budget that is arrived at by adding the number of joins and the length of the fibre to determine if the Tx (transmit) value will fall below the minimum Rx (receive) value of the SFP at the receiving side. If it does, we will have an optical quality issue. If its too bright on the other hand, we will need a device called an optical attenuator to dim the light. There is a good Wikipedia article here: Link Budget
- The wavelength of the laser. Traditional longwave is 1310nm (that value effectively being the 'colour' of the light). For real longhaul (like WDM and CWDM SFPs) we use SFPs in the 1500-1550nm range. The main point is that the SFPs at each end of a link need to use the same wavelength, or they won't be able to communicate with each other. Wikipedia again: Optical variants
- The number of BB (buffer to buffer) credits available to the link, especially if this is a cross site Inter Switch Link (ISL). This is not a big issue for director class switches, but can be a major gotcha if your using small/midrange switches. If you have a Brocade switches and dont have the Extended Fabrics license, you could be in trouble if your link is more than 10km long. The bad news is that this license is not free. The good news is that Brocade can provide an evaluation license so you can test to see if purchasing the license will really help. Cisco doesn't need an equivalent license, but the 4 Gbps capable MDS9124 and MDS9134 are limited in the maximum number of buffers that can be dedicated to one port (there are only 64 buffers per group of 4 ports). At 2 Gbps you need to have at least 1 buffer per kilometer of fibre. At 4 Gbps you need 2 buffers per km and at 8 Gbps you need 4 buffers per km. The 8 GBps capable Cisco MDS9148 has 128 buffers per port group meaning one port at 8 Gbps could utilize 32 km of fibre. This great article is about FICON but its an excellent read: BB Credits and FICON
Standard Longwave versus Extended Longwave
When you see a 10km SFP versus a 25km longwave SFP , the main difference between them is that one uses a more intense (brighter) transmit signal than the other.
Without intending to indicate a vendor preference, I will use the specs of Brocades SFPs to make a point. You can visit the specifications page here:
Compare this 8 Gbps capable 10km range SFP:
With this 8 Gbps capable 25km range SFP:
The 10km SFPs transmits between -8.4 and 0.5 dBm and needs to receive a signal of at least -15.4dBm.
The 25km SFPs transmits between 0 to 5.0 dBm and needs to receive a signal of at least -13.8dBm. The higher transmit value gives you a bigger link budget, which is why the SFP can go 15 extra km.
Remember you can increase these distance with WDM or CWDM technology, but you need to ensure the available BB credits on the ISL will be able to fully utilize it.
For Cisco switches check out the specs here:
Displaying actual transmit and receive values
The great thing is that both Cisco and Brocade switches use SFPs that report diagnostic information that can be displayed using the relevant management GUI.
In the screen capture below you can see the output from a shortwave length (850 nm) 4 Gbps capable SFP installed in an IBM R18 (Brocade Silkworm 7500) switch. Because this is a very short connection, the Rx level is very close to the Tx level.
In the screen capture below you can see the output from a CWDM long haul (1530 nm) 4 Gbps capable SFP installed in a Cisco MDS9509. This screen capture is more interesting because it shows that the Rx level is too low (which is why I got the screen capture in the first place - the link wasn't working!) So while its a good example of failed link, it doesn't give an example of seeing how close to reality your link budget calculations were.