What Does Combo Port Mean for Your Network?

In addition to the common ports, there is a port called “combo port” on the Ethernet switches. To many newbies, the combo port always makes them feel confused as there have been many Gigabit Ethernet ports and SFP slots for used on the switches. What is combo port and what does it mean for your network? How can I used it? A lot of such questions on combo port are coming all the time. Don’t worry, if you happened to be confused with this, the following text will tell you the answer.

What Is Combo Port and What Does It Mean for Your Network?

In simple terms, a combo port is considered as a single interface with dual front ends, i.e. an RJ-45 connector and an SFP (Small Form Pluggable) module (also called Mini-GBIC) connector. In other words, it is a compound port which can support two different physical, share the same switch fabric and port number. But the two different physical ports can only be used simultaneously. That means if the SFP port is being used, then the corresponding copper port is automatically disabled and vice versa. For example, a switch has 8 x 10/100/1000TX ports and 2 Gigabit Ethernet Combo ports, therefore it could be configured to have 10 copper ports or 9 copper ports & 1 fiber port, or 8 copper ports & 2 fiber ports, etc.

A Combo port is a way to provide different types of connectivity without taking up unused switch fabric, giving users the power and flexibility to configure their switch for their unique application requirements.

Rules for Using the Combo Port Pairs

When using the combo port pairs, some rules should be followed:

• You may use either the twisted pair port or SFP slot of a combo port pair, but not both at the same time.
• If you connect both the twisted pair port and SFP slot of a combo port pair to network devices, the SFP slot takes priority and the twisted pair port is blocked.
• The SFP slot becomes active when the SFP transceiver establishes a link to a network device.
• The twisted pair port and SFP slot of a combo port pair share the same settings, such as VLAN (Virtual Local Area Network) assignments, access control lists, and spanning tree.
• Reference the switch instruction to configure your combo ports according to your requirement.

DWDM vs CWDM

Core idea of xWDM technology: frequency duplex.  A multiplexer combines all different colors (with different wavelength/frequency)  and then data will be transmitted over one fiber using different wavelengths together. On the receive end, demultiplexer will separate them into different colors. This helps to improve the capacity using single fiber.

Coarse Wavelength Division Multiplexing (CWDM): 

• Up to 16 CWDM wavelength over one pair of fiber
• CWDM channel spacing 20 nm, 1720 nm to 1610 nm
• Distances up to 120 km
• Cost-effective WDM solution, mainly used in access, metro network.

DWDM (Dense Wavelength Division Multiplexing): 

• Up to 96 DWDM wavelength over one pair of fiber
• DWDM channel spacing 0.8 nm (100 GHz grid) or 0.4 nm (50 GHz grid)
• Distances over 1,000 km can be achieved with the use of optical amplifier
• Expensive, mainly used in core network

Understanding BIDI (Bi-Directional) transceivers

BiDi transceivers is short for Bi-Directional Optical Transceiver.

The key-point of BIDI transceivers is that a single fiber.used for both receiving and transmuting data.

In order to distinguish the receiving and transiting data, they must be transmitted using different wavelength. Then Wavelength Division Multiplexing (WDM) couplers is needed to combine and separate data transmitted over a single fiber based on  the wavelengths of the light. Based on this, BIDI transceivers are also referred to as WDM transceivers.

Example:

• Transceiver A’s diplexer must have a receiving wavelength of 1490nm and a transmit wavelength of 1310nm
• Transceiver B’s diplexer must have a receiving wavelength of 1310nm and a transmit wavelength of 1490n

Advantage of BIDI transceivers: require less fiber. 

While BiDi transceivers (a.k.a. WDM transceivers) cost more to initially purchase than traditional two-fiber transceivers, they utilize half the amount of fiber per unit of distance. For many networks, the cost savings of utilizing less fiber is enough to more than offset the higher purchase price of BiDi transceivers.