Optical and metallic cabling for LAN and SAN networks

Note: I have attempted to briefly summarize clear information about the cabling used today. I draw from my practical experience, and I researched some theoretical areas on the internet. I hope that no information is misleading, as the field of optical cables is quite complicated. I also recently attended a seminar on this topic at Alef company, led by Jan Snížek, where there was a nice and comprehensive overview. This helped me organize some information, so I'm very grateful.

Media for Data Transmission

Data transmission in computer networks (and of course not only there) happens through waves and primarily three transmission media are used:

• metallic cables - transmission using electrical potential
• optical cables - transmission using light
• air (or even vacuum) - wireless transmission using microwaves, radio waves, or infrared light

In this article, we will focus on the first two categories and look at the currently used cables in Local Area Network (LAN) and Storage Area Network (SAN) networks. The field of computer networks has gone through a long turbulent development (and is still evolving), so we may encounter various historical cables (more options are in the field of optics), which we will not cover here.

Generally, it doesn't matter whether it's LAN (telecommunication) or SAN (data) networks and what technology is used (Ethernet or Fibre Channel, other network technologies are probably not used today). In both cases, the same type of cabling is used (other components must correspond to this). Ethernet networks can combine metallic and optical cabling. Fibre Channel networks use optical cabling; if we want to use metallic cabling, we must choose iSCSI or FCoE (and the underlying technology is then Ethernet).

Structured Cabling for Computer Networks

We will look at the two main transmission media for wired transmission in computer networks. These are metallic and optical cables. Each technology is suitable for a slightly different purpose, so let's first list the basic parameters in bullet points.

Metallic Cables (Copper Cables)

• relatively inexpensive and less susceptible to damage (bending radius 2-3cm)
• common standards are for distances up to 100m (without an active element, calculated as 90m cable distribution and two connectors and a 5m patch cable on each side)
• supports power over data cable (Power over Ethernet, IEEE 802.3af PoE 15.4W, IEEE 802.3at PoE+ 30W)
• susceptible to electromagnetic interference (EMI), although various shielding is performed
• speeds today commonly up to 10Gbps

Optical Fiber Cables

• more expensive cables (and the entire technology) and more demanding for manipulation (bending radius 8 cm, welding)
• for long distances (depends on the type, but possible even 100km, with other technologies it can be hundreds of km)
• immune to electromagnetic interference (ElectroMagnetic Interference - EMI)
• lower latency and possible higher transmission speeds (today 100Gbps on a single fiber)
• lower energy consumption

Metallic Cables - Twisted Pair

For metallic cables, we will only discuss twisted pair, as we probably won't encounter older coaxial cables today (and other options are rare).

Twisted pair, abbreviated as TP, consists of four pairs of conductors (wires) that are regularly twisted along the entire length. This improves the electromagnetic properties of the cable, minimizes crosstalk, and limits the emission of electromagnetic radiation (to and from the surroundings). Each conductor has protective insulation (often made of polyethylene) and all conductors are collectively wrapped in a jacket (often made of PVC).

Note: Even before computer networks, twisted pair was used in telephone wiring. There, a cable with three pairs of conductors was standardly used, but often only one pair was used for transmission. These distributions were also used at the beginning for building computer networks.

Unshielded and Shielded Twisted Pair Cables

Twisted pair can be of several basic types. Most often it's a classic unshielded TP cable:

• UTP - Unshielded Twisted Pair - classic and most widespread cable, where standard unshielded twisted pair is used

If we need to further limit radiation, crosstalk, and electromagnetic interference, various forms of shielding are used. I tried to find a reliable description, but it seems that in different places it's described a bit differently (also because there are multiple standards). Manufacturers label it differently and colloquially often only UTP, STP, FTP designations are used. The most widespread definitions in the ISO/IEC 11801 standard or ANSI/TIA/EIA-568-B are probably correct.

When we talk about shielded cables, two letters are used in the designation:

• S - Braid Screen (Shielded) - shielding braid
• F - Foil Shield (Foiled) - protective (metal) foil

Protection can be done by:

• pair shielding - each pair of conductors is wrapped in shielding foil, only F (foil) can be used
• cable shielding - all pairs together are wrapped in braiding (S) or foil (F)

Cables are then designated (in parentheses is the colloquial designation) as U/UTP (UTP), U/FTP (STP), S/UTP (STP), S/FTP, F/FTP, etc. Before the slash is the designation of shielding of the entire cable, after the slash is the shielding of individual pairs.

Standards for Ethernet over Twisted Pair

All common telecommunication networks today use Ethernet transmission technology, which defines (in IEEE 802.3 standards) the used cabling, network access, signal sending, speeds, etc. According to the supported transmission speed, we have various Ethernet standards that use twisted pair (other variants are defined on different transmission media).

The designation consists of a number determining the speed in Mbps (if G is added, then in Gbps). The word Base is short for baseband indicating that it's an unfiltered line without modulation. The letter T at the end means Twisted (twisted pair).

The most common variants are:

• 100Base-Tx Ethernet - IEEE 802.3u (1995), technology that transmits Ethernet frames at a speed of 100 Mbps, also referred to as FastEthernet (FE)
• 1000Base-T Ethernet - IEEE 802.3ab (1999), speed 1000 Mbps, also referred to as GigabitEthernet (GE)
• 10GBase-T Ethernet - IEEE 802.3an (2006), speed 10,000 Mbps, also referred to as 10GigabitEthernet (10GE)
• 40GBase-T Ethernet - IEEE 802.3bq (2013), speed 40,000 Mbps, also referred to as 40GigabitEthernet (40GE)

Ethernet networks initially used coaxial cables (10Base5, 10base2). Then they started using telephone wiring over twisted pair (the first 10Base-T standard was created, but before that there was StarLAN for example) and gradually evolved to today's TP cables, which are still evolving to support higher transmission speeds.

Categories of Twisted Pair Cabling

Cabling, and its categories, are defined in the standards TIA/EIA-568 (uses categories as listed below) and ISO/IEC 11801 (uses classes - Class C, D, E, etc.). Cabling categories define the minimum parameters required for cables. It determines various levels of performance in terms of signal bandwidth, attenuation, and crosstalk. Generally, the higher the category, the higher transmission speeds it supports. Various Ethernet standards require a certain category of cabling. Main cabling categories:

• category 5e (Cat 5e, Class D) - for FastEthernet (uses two pairs) and GigabitEthernet (uses all four pairs), uses shielded and unshielded cabling, bandwidth 100 MHz, RJ45 connector (8P8C)
• category 6 (Cat 6, Class E) - supports 10 Gbps Ethernet, but only up to 55 meters, uses shielded and unshielded cabling, bandwidth 250 MHz, RJ45 connector (8P8C)
• category 6a (Cat 6a - augmented, Class EA) - supports 10 Gbps Ethernet for the full distance of 100 meters, uses shielded and unshielded cabling, bandwidth 500 MHz, RJ45 connector (8P8C)
• category 7 (Cat 7, Class F) - an attempt that didn't catch on, for supporting 10Gbps Ethernet, requires shielded cabling (S/FTP) and special connectors (GG45 compatible with RJ45 or TERA), bandwidth 600 MHz, manufacturers preferred Cat 6a and RJ45 connector
• category 7a (Cat 7a, Class FA) - an attempt to extend Cat 7, could support 40 Gbps Ethernet for a distance of 50 meters, bandwidth 1000 MHz
• category 8 (Cat 8) - currently in development (ISO technical recommendation from 2013), supports 40 Gbps Ethernet, bandwidth 1600 to 2000 MHz, two variants, one uses shielded cables (U/FTP, F/UTP) and RJ45 connector (8P8C), the other cables (F/FTP, S/FTP) and TERA/GG-45/ARJ-45 connector

Metallic TP Cables and Their Designation

When designating a metallic cable using twisted pairs, an abbreviation is used indicating whether the cable is unshielded or shielded in some way, and further the category. For example:

UTP Cat6a Cable

Additional supplementary information may be provided, such as that it's 4 pairs (4x2), conductor material (most often copper - Cu), conductor thickness (23AWG is 0.57mm or 24AWG is 0.52mm), conductor insulation material (PE - polyethylene), jacket material (PVC - polyvinyl chloride).

There can also be two types of conductor, it's a solid wire or stranded wire. Solid wire is manufactured as a single piece of metal, it's rigid and less flexible. Stranded wire consists of a bundle of small wires, it's more flexible and more expensive. For each type of cable, there is a corresponding type of connector (stranded is pierced, solid is clamped).

In practice, we have cable either as bulk (commonly sold in 305m spools or by meter). We lay such a cable and crimp a connector onto the end or attach a wall socket or in a Patch panel (connection field). Or prepared patch cables (Patchcord) of certain length with attached connectors.

Optical Cables, Optical Fibers

An Optical Fiber Cable consists of one or more optical fibers that transmit a light beam from source to destination with minimal loss. An optical fiber contains a core, which has (in practice) a thickness of 9 or 50 or 62.5 µm, made of silicon dioxide. And a cladding of another type of glass, which has a lower refractive index, so the signal is reflected back, with a thickness of 0.125 mm. Then follows a primary protection and the thickness is 0.25 mm. We group optical fibers into optical cables and add several protective layers (often PVC), depending on the environment they are intended for.

We use two basic types of optical fibers:

Single-mode Optical Fiber (SMF)

• in Czech jednovidové optické vlákno
• the core diameter of the optical fiber is 9 µm (smaller than 10 µm)
• wavelengths of 1310 or 1550 nm are used
• due to the small diameter and high wavelength, only a single partial beam (mode) can propagate, this also leads to a large angle of reflection in the fiber and therefore minimal elongation of the beam path
• it is suitable for long distances, has a higher price
• the transmitter uses laser diodes
• chromatic and polarization dispersion occurs
• yellow color is often used to mark these cables

Multi-mode Optical Fiber (MMF)

• in Czech vícevidové optické vlákno
• the core diameter of the optical fiber is 50 µm (older fibers 62.5 µm)
• wavelengths of 850 or 1300 nm are used
• multiple modes (light modes) propagate in the fiber with different angles of reflection, it has higher luminosity, but due to modal dispersion, it limits the transmission distance
• allows the use of cheaper LED and laser diodes, overall cheaper than SMF
• has lower transmission capacity and is suitable for shorter distances (used in LAN networks)
• depending on the transition between core and cladding (we have different dispersion), two types are used
  • SI - step index profile - with a step refractive index, for shorter distances
  • GI - graded index profile - with a gradual change in refractive index
• modal, chromatic, and polarization dispersion occurs
• orange color is often used to mark these cables, alternatively blue-green (OM3, OM4) or purple (OM4)

Categories (classes) of optical fibers

Similarly to metallic twisted pair, optical fibers are defined in the standards ISO/IEC 11801 and TIA/EIA-568 (and others) and there are certain classes (fiber categories). A certain standard transmission speed is always supported for a given minimum distance (for example, MMF always supports FastEthernet for at least 2 km), but it also works at higher speeds for shorter distances.

Single-mode Optical Fiber (designation OS - Optical Single-mode)

For SMF fibers, the maximum attenuation coefficient in dB per km is specified and means the signal drop per kilometer. Yellow color is often used to mark these cables.

• OS1 - attenuation 1 dB/km
• OS2 - attenuation 0.4 dB/km

Multi-mode Optical Fiber (designation OM - Optical Multi-mode)

For MMF fibers, the minimum modal bandwidth is specified, i.e. its capacity in MHz per kilometer distance. MMF cables are often marked with orange color, alternatively blue-green (OM3, OM4), purple (OM4).

• OM1 - core 62.5 µm, bandwidth 200 MHz-km at 850 nm (higher for 1300 nm), commonly used for FastEthernet at a distance of 2 km
• OM2 - core 50 µm, bandwidth 500 MHz-km at 850 nm, commonly used for GigabitEthernet at a distance of 550 m
• OM3 - core 50 µm, bandwidth 2000 MHz-km at 850 nm, commonly used for 10GigabitEthernet at a distance of 300 m, transition from LED to VCSEL lasers
• OM4 - core 50 µm, bandwidth 3500 MHz-km at 850 nm, the latest standard supports 40 and 100GigabitEthernet at a distance of 150 m, optimized for laser

Standards for Ethernet over Optical Fiber

Ethernet of various speeds is also defined (IEEE 802.3) when using optical fibers. Compared to twisted pair, there are more variants and more complications ;-). The beginning of the designation is the same, consisting of a number determining the speed in Mbps (if G is added, then in Gbps). The word Base is short for baseband indicating that it's an unfiltered line without modulation. The letters at the end are used in pairs, the first can be F for Fiber (optical cable), S means Short-range MMF, L means Long-range SMF or MMF. The second letter X determines block coding 4B/5B for FastEthernet or 8B/10B for GigabitEthernet, R is block coding 64B/66B.

Selection of some used standards:

• 100Base-FX Ethernet - IEEE 802.3u (1995), speed 100 Mbps (FastEthernet - FE), uses wavelength 1300 nm and MMF optical fibers, one for receiving (RX) and another for transmitting (TX), supports distance of 2 km for full-duplex
• 1000Base-SX Ethernet - IEEE 802.3z (1998), speed 1000 Mbps (GigabitEthernet - GE), uses wavelength 850 nm and MMF optical fibers, distance 275 m for OM1 and 550 m for OM2
• 1000Base-LX Ethernet - IEEE 802.3z (1998), speed 1000 Mbps (GigabitEthernet - GE), uses wavelength 1310 nm and SMF optical fibers for distance of 5 km or wavelength 1300 nm and MMF optical fibers for distance of 550 m
• 10GBase-SR Ethernet - IEEE 802.3ae (2002), speed 10,000 Mbps (10GigabitEthernet - 10GE), uses wavelength 850 nm and MMF optical fibers, distance 300 m for OM3 and 400 m for OM4
• 10GBase-LR Ethernet - IEEE 802.3ae (2002), speed 10,000 Mbps (10GigabitEthernet - 10GE), uses wavelength 1310 nm and SMF optical fibers, distance 10 km
• 40GBase-SR4 Ethernet - IEEE 802.3ba (2010), speed 40,000 Mbps (40GigabitEthernet - 40GE), uses wavelength 850 nm and MMF optical fibers, distance 100 m for OM3 and 150 m for OM4, digit 4 means that 4 parallel lanes are used for data each at speed of 10.3125 Gbps
• 100GBase-SR10 Ethernet - IEEE 802.3ba (2010), speed 100,000 Mbps (100GigabitEthernet - 100GE), uses wavelength 850 nm and MMF optical fibers, distance 100 m for OM3 and 150 m for OM4, digit 10 means that 10 parallel lanes are used for data each at speed of 10.3125 Gbps

Standards for Fibre Channel

We described the use of optical cables for Ethernet (and thus primarily for telecommunication networks), so we must also mention Fibre Channel, the standard network technology for SAN networks. For Fibre Channel networks, we use the same optical fibers as described above. Both Multi-mode Optical Fiber and Single-mode Optical Fiber are used, classically mentioned OM classes (uses designation M6 for OM1, M5 for OM2, M5E for OM3, M5F for OM4) and OS (uses designation SM for OS1 and OS2).

Fibre Channel is standardized by the ANSI committee INCITS (InterNational Committee for Information Technology Standards). The Physical layer (FC-0), which defines cables, connectors, etc., is described in Fibre Channel Physical Interfaces (FC-PI-5 and FC-PI-6). Similar to Ethernet, there are different versions of Fibre Channel that support different speeds. They are designated by their speed in Gbps and the abbreviation FC for Fibre Channel, also referred to as a certain generation of Fibre Channel. The main versions are (a specific version like 10GFC is not listed here):

• 1GFC - Gen1 (1997), 8b/10b coding, line rate 1.0625 GBaud, throughput 200 MB/s (in both directions, i.e., 100 MB/s in one direction)
• 2GFC - Gen2 (2001), 8b/10b coding, line rate 2.125 Gbps, throughput 400 MB/s
• 4GFC - Gen3 (2006), 8b/10b coding, line rate 4.25 Gbps, throughput 800 MB/s
• 8GFC - Gen4 (2008), 8b/10b coding, line rate 8.5 Gbps, throughput 1600 MB/s
• 16GFC - Gen5 (2011), 64b/66b coding (due to this, a lower line rate is sufficient for the required transmission speed), line rate 14.025 Gbps, throughput 3200 MB/s
• 32GFC - Gen6 (2016), 64b/66b coding, line rate 28.05 Gbps, throughput 6400 MB/s

Wavelength Division Multiplexing - WDM

There are methods for achieving higher transmission speeds on a single optical fiber. Each fiber can carry many independent channels with different wavelengths. Wavelength division multiplexing - WDM is used, where multiple optical signals are combined (multiplexed) into one fiber using different wavelengths (colors) of LEDs or lasers. WDM (Wavelength Division Multiplexing) technology uses a multiplexer in the transmitter to combine signals and a demultiplexer in the receiver to separate them.

In practice, two types of WDM are used:

• CWDM - Coarse WDM - cheaper, uses a channel width of 20 nm, primarily uses the band 1470 nm to 1610 nm (i.e., 8 wavelengths)
• DWDM - Dense WDM - dense wavelength division multiplexing, uses a channel width of 0.8 nm, uses the band 1530.33 nm to 1560.61 nm

With this technology, we can achieve that one cable carries, for example, 32 independent services/customers. Although they share one fiber, the communication is completely separate (more securely than using VLANs).

Optical Cables and Their Designation

For optical cables, the situation is more complicated than for metallic ones. Patch cables (Patchcord) are of two general types:

• Simplex - optical cable composed of only one fiber, for situations where we only need reception or transmission, or when multiplexing is used (so both directions go on one fiber)
• Duplex - an optical cable consisting of two optical fibers, typically used for duplex communication, where one fiber is for receiving (RX) and the other for transmitting (TX)

Commonly used are duplex patch cables, where two optical fibers are utilized, but they are actually separate (including full insulation) and only used in pairs.

When we lay an optical cable somewhere, whether outside or inside, it is usually worth choosing a cable that contains multiple optical fibers (e.g., 4, 8, 12). These optical cables in meters are referred to as DROP cables, intended for connection to distribution networks and building wiring. Backbone networks of ISPs use much more fiber cables (microducts and fiber blowing are also used).

With cables, we may encounter the designation that it is intended for FTTx. This is a series of abbreviations that indicate where the optical cables are led from the central office (further distribution continues with metallic cables). Common options are:

• FTTH - Fiber To The Home - the optical fiber is led to the boundary of the apartment
• FTTB - Fiber To The Building - the optical fiber is led to the boundary of the building
• FTTC - Fiber To The Cabinet - the optical fiber is terminated in a distribution cabinet, which is at a certain distance (up to 300 meters) from the customer
• FTTN - Fiber To The Node - the optical fiber is terminated in a distribution cabinet, which is at a greater distance from the customer

In the designation of optical cables, the core diameter and cladding are indicated, as well as the fiber class, whether it is a simplex or duplex patch cable, the number of fibers in DROP cables, and various other details. Example:

9/125, OS2 Singlemode, duplex, 2m, LC-LC
50/125, OM3 Multimode, 8 fibers, 100m spool