Note: In this article, I attempted to briefly summarize the currently used connectors for network cables, Transceiver modules, and mention Direct Attach cables. I tried to keep the description concise, containing only important information (so it's certainly not exhaustive), and focused only on selected types. I draw from my practical experience and researched various theoretical areas on the internet. I also recently attended a seminar on this topic at Alef company, led by Jan Snížek, which provided a nice and comprehensive overview. This helped me organize some information, so I'm very grateful.
Network Connectors
As we described last time, networks with metallic cabling (twisted pair) are now based on Ethernet technology and can be designed for both telecommunication networks (like LAN) and data networks (Storage, SAN). In common practice, regardless of the operating speed of Ethernet or the cabling category, the 8P8C connector, popularly known as RJ45, is always used. I haven't yet encountered category 7 or 8 cabling, where different connectors are used.
When we use optical cabling (optical fibers), we can identically use Ethernet technology and create LAN and SAN networks. Or we can use special Fibre Channel technology, which is designed for SAN data networks. In both cases, it again doesn't matter whether the optical fibers are Single mode or Multi mode, their class, and operating speed. Today, the LC connector is most commonly used; in practice, I've also encountered the older SC connector, and that's all.
Metallic Connectors
When we look at computer networks with metallic cabling, it's currently always twisted pair and Ethernet over Twisted Pair is used. The cable used contains four pairs of wires, which are regularly twisted along the entire length. The cable can be unshielded or shielded in various ways.
8P8C Connector - RJ45
In all these cases (shielded and unshielded twisted pair), the 8P8C connector is used, commonly referred to as RJ45 (or RJ-45). 8P8C is a modular connector, the designation means 8 position 8 contact, meaning the connector has 8 positions and all are populated with a wire (there can be variants where some positions are unpopulated). The connector is described in the ANSI/TIA-1096-A and ISO-8877 standards.
The modular connector was designed back in the seventies and was used in telephony. It was used in the standardized Registration Interface System, where the term Registered Jack (RJ) is used. This specification defines the wiring of conductors, but not the geometry of the connector. In telephony, RJ11 was the most widespread, using a 6P2C connector. Because RJ45 was the first to use the 8P8C connector, this designation became common for the connector. And it remained so even when it began to be used in Ethernet networks, where it has nothing to do with the RJ45 standard. I will continue to use the RJ45 designation for the 8P8C connector.
The connector can be a plug, which terminates the cable, referred to as male, or a socket (or jack) at a fixed location on a surface (on a wall, panel, device), referred to as female.
Wire Connection in RJ45 Connector
For Ethernet, the wiring of conductors from a twisted pair cable to the modular connector is described in TIA/EIA-568 (along with the cabling definition). Cables were previously connected in two variants:
- crossover cable - has reversed wiring at the ends and was used for direct connection of computers (so that transmission would reach reception and vice versa), today (since the use of 1000Base-T) all network cards and devices support Auto MDI-X, where transmission and reception are automatically reconfigured, so a crossover cable is not needed
- straight-through cable (or patch cable) - has both ends wired identically, used to connect devices of different types (like connecting a computer to a switch)
Cable wiring is described in TIA/EIA-568 and there are two possible variants T568A and T568B. It may seem that it doesn't matter how the cable is wired, as long as it's the same on both sides. But at higher speeds, correct wiring minimizes interference.
Attaching the connector to the cable (with correct wire connection) is referred to as crimping. We use cabling of a certain category (like Cat 5e, Cat 6) and either unshielded or shielded (we also connect grounding). All passive elements (such as connectors, keystones - sockets, patch panels) should correspond to this. The classic rule applies that the weakest link in the system determines the quality of the entire system. As we described last time, cables are either solid core or stranded, and the connector must also correspond to this to be properly crimped.
Optical Connectors
Computer networks with optical cabling (where we will use optical connectors) use a cable with one optical fiber; in practice, two cables are usually used in pairs as a duplex (transmitting on one and receiving on the other). Ethernet over Optical Fiber or Fibre Channel is used.
In optical networks, the situation is more complex than in metallic networks, where the same connector is always used on twisted pair. In optics, a large number of different connectors have appeared in the past (many of them were used only rarely), but only two connectors are used most often: SC and LC. We may also encounter the ST (Straight Tip), MTRJ (Mechanical Transfer Registered Jack) or the new MPO (Multi-Fiber Push On) for Ribbon cables, but we won't describe these here.
Optical connectors provide mechanical connection and alignment of the fiber core so that light can pass through. Most connectors use a spring, so the fiber surfaces are pressed tightly together to avoid an air gap, which would cause losses. Attaching the connector to the optical fiber is also a much more demanding procedure, and losses are much more pronounced in optics, and attenuation and reflection are addressed.
Note: For connecting optical fibers, fusion splicing or mechanical splicing is also used. This is a permanent connection that has better properties than using connectors (lower losses).
Parts of an Optical Connector
An optical connector generally consists of several parts:
- ferrule - the contact surface of the connector, a protective ring, made of ceramic, metal, or high-quality plastic, with a hole in the center where the optical fiber itself is inserted, the end of the fiber ends with the end of the ferrule and is polished smooth
- connector body - plastic or metal construction around the ferrule, reinforces the connection to the optical fiber
- connector cover - connected to the connector body, holds the connector in place when connected to another device (inserted into a socket), contains the coupling mechanism
The ferrule in the connector can have several shapes or grinding methods:
- Flat PC - Flat Physical Contact - flat grinding, losses 0.3 dB, reflectivity at least -30 dB
- PC - Physical Contact - slightly curved grinding, so the connectors touch only in the center, losses 0.3 dB, reflectivity at least -40 dB
- APC - Angled Physical Contact - angled grinding at 8 degrees, reflectivity at least -60 dB
Note: PC is always used in the Transceiver, so we can't directly connect an APC connector, but we must use a cable that has APC on one side and PC on the other.
Plug, Socket, Patch Cable
An optical connector for optical fiber is a plug, i.e., the termination of an optical cable. To connect two optical fibers, we can use an optical coupler (mating sleeve). A socket (or jack) is used where we connect the optical cable. It can either be wiring of structured cabling, and thus a patch panel (optical tray). Or connection to some device (such as a server network card, switch port), where in most cases an optical port is not used directly, but a slot for a certain type of Transceiver (which then has a socket of the given type).
Optical patch cables can have a different type of connector on each side. Each connector is equipped with a certain type of coupling mechanism, such as screwing, bayonet, and most commonly snap-in.
Optical connectors often use certain colors to identify the fiber and Ferrule:
- beige or grey - most common connector, multimode fiber, Ferrule PC
- blue - singlemode fiber, Ferrule PC
- green - Ferrule APC
SC Connector - Subscriber Connector
Also referred to as Standard Connector or Square Connector. The Ferrule size is 2.5 mm. It is used on GBIC, XENPAK, X2, CFP, CPAK Transceivers. A very popular connector for gigabit and 10 gigabit speeds years ago.
LC Connector - Lucent Connector
Currently probably the most used type of connector, also referred to as Little Connector. It's a miniaturized version of the SC connector, the Ferrule has a diameter of 1.25 mm. In a duplex cable, the connector cover is connected for both fibers, so it's plugged in at once. It is used on SFP, SFP+, XFP, QSFP+ Transceivers, so for all current Ethernet and Fibre Channel speeds.
Transceivers (converters)
The term Transceiver was created by combining the words TRANSmitter and reCEIVER. Today, these are compact modules that serve to convert signals (between the motherboard of a network device and the network cable). They are powered from the device, can be connected and disconnected while running, and are used for both telecommunication and data networks. There are several manufacturer-specified (Multi-Source Agreement - MSA) types of Transceivers, with SFP, SFP+, and QSFP+ being predominantly used today.
The principle and advantage of Transceivers is that the device (switch, router, network card, etc.) contains only a slot (socket) for a certain type of Transceiver. These are manufactured with different connectors (for example, metallic RJ45 or optical LC), different standards determining speed (such as 1000Base-T Ethernet, 10GBase-SR Ethernet, 8G Fibre Channel) and different principles of transmission and reception (power, metallic, Single mode or Multi mode optical fibers, different wavelengths, Short Range or Long Range, FC Longwave or Shortwave, etc.). Thanks to this, we can, for example, connect different types of media to individual ports on a switch. The price of the basic system is reduced (but the total price increases because most types of Transceivers are not cheap, but we can use Twinax cables) and flexibility is gained.
Note: In practice, there's often a problem that manufacturers officially support only their own Transceivers, even though others often work, problems can occur.
GBIC (GigaBit Interface Converter)
The first type of Transceiver that I encountered in practice already 15 years ago. And although it was quite quickly replaced by its successor SFP, the term GBIC (pronounced "jeebik") was colloquially used for Transceivers in general for a long time.
It was used for Gigabit Ethernet and Fibre Channel and provided a standard (Small Form Factor Committee - SFF-8053 Specification for GBIC, first version 1995) for an interchangeable electrical interface, where one gigabit port could support various physical media, from metallic (1000BaseT) to various types of optics (1000BaseSX, 1000BaseLX/LH) with an SC connector. In Fibre Channel, it was used for the first generation 1GFC.
SFP (Small Form-factor Pluggable)
SFP appeared as a significantly smaller successor to GBIC, so it was sometimes referred to as Mini-GBIC. It was described in 2001 by the SFP committee (Small Form Factor committee - INF-8074i Specification for SFP (Small Formfactor Pluggable) Transceiver).
SFP modules are still commonly used and are available for speeds of 100 Mbps (Fast Ethernet on fiber) and primarily 1000 Mbps (metallic and fiber). On fiber, an LC connector is used. In Fibre Channel, it can also be used for the first generation 1GFC, as well as the second 2GFC and third 4GFC (speed 4.25 Gbps). Depending on the distance, we have FC variants Shortwave SFP and Longwave SFP (and Extended Reach SFP+).
For an overview, we can look at the offer of Cisco SFP Modules for Gigabit Ethernet Applications Data Sheet.
SFP+ (Enhanced Small Form-factor Pluggable)
An extension of the popular SFP to higher speeds around 10 Gbps (in FC it recently moved up to 32 Gbps) while maintaining the same module dimensions. The specification was published in 2006. For 10 Gigabit Ethernet, other Transceivers (XENPAK, X2, XFP) were used until then, but have now been replaced by the SFP+ module.
SFP+ modules are used as a standard for 10 Gigabit Ethernet (fiber and, perhaps rarely, metallic). In Fibre Channel, it can be used for the fourth generation 8GFC, fifth 16GFC, and also for the newest sixth 32GFC (speed 28.05 Gbps). On fiber, an LC connector is used.
For an overview, we can look at the Ethernet offer Cisco 10GBASE SFP+ Modules Data Sheet or Fibre Channel Cisco MDS 9000 Family Pluggable Transceivers Data Sheet.
XENPAK, X2, XFP
MSA defined the XENPAK module as early as 2001 as the first Transceiver for 10 Gigabit Ethernet. It was significantly larger than the SFP module. In 2002, a similar module XPAK was created, and a year later X2, which Cisco used in its devices. Then an even smaller module XFP appeared, which was already approaching the size of SFP. But everything has now been replaced by SFP+ modules. On fiber, an SC connector is used.
Note: Cisco also manufactures various converters, for example, a module for the X2 slot that contains two 1 Gbps SFPs, or one SFP+.
QSFP and QSFP+ (Quad Small Form-factor Pluggable)
Ethernet network speeds are constantly moving forward. Today, it's not so unusual to use 40 Gigabit Ethernet, and 100 Gigabit Ethernet is slowly starting to be used as well. There was a need for a new type of Transceiver, and in 2013, the QSFP module was specified, which is slightly larger than SFP, and especially several extended variants generally referred to as QSFP+ (QSFP10, QSFP14, QSFP28).
The word Quad in the designation means that four channels of a certain speed are supported, for example, QSFP+ 10 Gbps, and we can combine them into one link at a speed of 40 Gbps. Special breakout direct-attach cables are also manufactured, which split one port (40 Gigabit Ethernet or 100 Gigabit Ethernet) into four separate ports (10 Gigabit Ethernet or 25 Gigabit Ethernet). On fiber, an LC connector or MPO connector is used.
For an overview, we can look at the offer of Cisco 40GBASE QSFP Modules Data Sheet.
Direct Attach or Twinax cables
Along with SFP+ Transceivers, direct connection cables Direct Attach Cable, also referred to as Twinax Cable, appeared, which connect two SFP+ slots over a shorter distance. The cables have permanently attached connectors that plug into the SFP+ slot. The advantage is a significantly lower price.
Direct Attach cables are manufactured in several types and corresponding lengths. Metallic passive cables (Passive Copper Twinax) up to 10 meters, metallic active cables (Active Copper Twinax) up to 20 meters, and optical active cables (Active Optical Twinax) even for tens of meters.
Direct Attach cables are also manufactured for QSFP+ slots. Here, a special variant of splitting cables Breakout Direct-attach is also used, where there's a QSFP+ on one side and four SFP+ on the other.
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