Fiber optic cable considerations
The electrical signals carried over copper wire are subject to interference and attenuation. Fiber optic signaling uses pulses of infrared light, which are not susceptible to interference, cannot be intercepted, and suffer less from attenuation. Due to which the fiber optic cable supports higher bandwidth over longer distance. Fiber optic cables can be many kilometer long.
A single optical fiber is constructed from three elements:
- Core: it provides the transmission path, or waveguide, for the light signals.
- Cladding reflects signals back into the waveguide as efficiently as possible. Cladding is made up of the same material but has a different refractive index than the core. cladding facilitates the process of total internal reflection that guides the light signal through the core.
- Buffer is a protective plastic coating.
- each fiber optic strand can only transfer light in a single direction at a time. Therefore, multiple fibers are often bundled together to allow simultaneous transmission and reception of signals or to provide links for multiple applications.
- There are many different outer jacket designs and materials suited for different installations (indoor/plenum, outdoor, underground, undersea, and so on). kevlar strands and sometimes fiberglass rods are often used to protect the fibers from excessive bending or kinking when pulling the cable to install it.
Types of fiber cables:
- single mode fiber
- multimode fiber
Single Mode fiber: Single mode fiber or SMF has a small core 8 to 10 microns and uses long wavelength. It uses a laser to generate near infrared light signal(1310 nm or 1550nm). SMF supports up to 100Gbps and the cable can run many kilometers. The distance covered depends upon the quality of the cable. There are two grades of SMFs OS1 is designed to use indoors, and OS2 is designed to use outdoors.
Multi Mode Fiber: Multimode fiber or MMF has a larger core (62.5 or 50 microns) and shorter wavelength light (850 or 1300 nm) transmitted in multiple waves of varying length. MMF uses less expensive optics, which makes it less expensive compare to SMF. It also doesn’t support longer distance and high speed hence is suitable for LANS than WANs.
MMF is graded by optical multimode (OM) categories, defined in the ISO/IEC 11801 standard:
OM1/OM2 - OM1 is 62.5 micron and early OM2 is 50 microns, both cables are mainly rated for applications up to 1Gbps and use LED transmitters.
OM3/OM4 - Both 50 micron, with 850 nm vertical-cavity surface-emitting laser(VCSEL). A VCSEL is not as powerful as the laser used for SMF but it supports higher modulation than LED- based optics.
Fiber Optic Connector Types
Fiber optic connectors are available in many different form factors. Some types are popular for multimode and some are for single mode fiber.
Straight Tip (ST)
This type of connector is an early BNC connector that uses push-and-twist locking mechanism. ST was used mostly for multimode network but is not widely used any more.
Subscriber Connector (SC):
The subscriber connector (SC) is a push/pull design, allowing for simple insertion and removal. It can be used for single or multimode. It is commonly used for Gigabit Ethernet.
Local Connector (LC)
The local connector (LC) is a small-form-factor connector with a tabbed push/pull design. LC is similar to SC, but the smaller size allows for higher port density. LC is a widely adopted form factor for Gigabit Ethernet and 10/40 GbE.
Fiber Optic Cable Installation
Fiber optic can also be installed in the same topology as copper cable using distribution frames and switches.
Long-distance cables are typically laid as trunks or rings with repeaters or amplifiers between cable segments to strengthen the signal.
Fiber Optic Patch Cords:
Patch cables for fiber optic can come with the same connector on each end (LC-LC, for instance) or a mix of connector (LC-SC). Duplex patch cords must maintain the correct polarity, so that the Tx port on the transmitter is linked to the Rx port on the receiver and vice versa. Tx and Rx on the transmitter and receiver needs to be reversed, or crossover. Each element in the link must perform a crossover, and there must be an odd number of elements, such as two patch cords and a permanent link (three elements)
Finishing Type:
Ultra Physical Contact (UPC): The faces of the connector and fiber tip are polished so that they curve slightly and fit together better.
Angled Physical Contact(APC) - the faces are angled for an even tighter connection. APC cannot be mixed with PC or UPC. APC finishing is often not supported by the patch panels, transceivers, and switch ports designed for Ethernet.
Fiber Distribution Panels
- structured cabling links are installed in a manner similar to copper cabling.
Multi-fiber push on connectors:
- allows for low-footprint backbone or trunk cabling
- bundles 12 or more strands terminated to a single compact ferrule
- is used to aggregate 10Gbps lanes into a 40 gbps, 100 Gbps, or 400Gbps parallel optical link
- Each lane normally requires two fiber strands(send and receive). A 40Gbps link comprising 4 X 10 Gbps lanes therefore requires eight strands
- MPO can terminate this type of parallel optical link more efficiently than separate LC-terminated strands.
- An MPO capable of carrying 24 or 32 fibers has the same footprint as a duplex LC pair.
Wavelength Division Multiplexing
Wavelength division multiplexing is a means of using one or two strands to provision multiple channels.
A duplex fiber channel link uses one transmit lane and one receive lane and requires two fiber strands. Parallel fiber uses bundles of lane working at 10 Gbps or 25 Gbps to implement 40 Gbps or 100Gbps.
Bidirectional Wavelength Division Multiplexing:
Bidirectional transceivers (BiDi) supports transmit and receive signals over the same strand of fiber. This uses wavelength division multiplexing to tranmit the Tx and Rx signals over slightly shifted wavelength. BiDi transceivers must be installed in opposite pairs. If the upstream transceiver is using 1310nm for Tx and 1490 nm for Rx the downstream transceiver must use 1490 nm for Tx and 1310nm for Rx.
Coarse and Dense Wavelength Division Multiplexing:
Coarse Wavelength Division multiplexing (CWDM) supports up to 16 wavelengths and is typically used to deploy four or eight bidirectional channels over either a single fiber strand or unidirectional channels over dual fiber strands.
Dense wavelength division multiplexing(DWDM) provisions greater number of channels. It requires more expensive and precise lasers
Both support multi0channel 1 G 10, G and 40 G Ether net links. The transceivers must be installed in opposite pairs.
Reference: Comptia Network plus (certmaster)