Physical Media Networking

Although it is possible to use several forms of wireless networking, such as radio frequency and infrared, the majority of installed LANs today communicate via some sort of cable.There are three types of cables:

  • Coaxial
  • Twisted pair
  • Fiber optic

Coaxial Cable

Coaxial cable (or coax) contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket. A plastic such as polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP, such as DuPont’s Teflon) covers this metal shield. The Teflon-type covering is frequently referred to as a plenum-rated coating. That simply means that the coating doesn’t begin burning until a much higher temperature, doesn’t release as many toxic fumes as PVC when it does burn, and is rated for use in air plenums that carry breathable air, usually as non enclosed fresh-air return pathways that share space with cabling. This type of cable is more expensive but may be mandated by local or municipal fire code whenever cable is hidden in walls or ceilings. Plenum rating applies to all types of cabling and is an approved replacement for all other compositions of cable sheathing and insulation, such as PVC-based assemblies.

Thin Ethernet

Thin Ethernet, also referred to as Thinnet or 10Base-2, is a thin coaxial cable. It is basically the same as thick coaxial cable except that the diameter of the cable is smaller (about 1/4˝ in diameter). Thin Ethernet coaxial cable is RG-58. Figure 1.10 shows an example of Thin Ethernet.

With Thinnet cable, you use BNC connectors (see Figure 1.11) to attach stations to the network. It is beyond my province to settle the long-standing argument over the meaning of the abbreviation BNC. BNC could mean BayoNet Connector, Bayonet Nut Connector, or British Navel Connector. But it is most commonly referred to as the Bayonet Neill-Concelman connector. What is relevant is that the BNC connector locks securely with a quarter-twist motion.

A stripped back thinnet

Male and female BNC connector

The BNC connector can be attached to a cable in two ways. The first is with a crimper, which looks like funny pliers and has a die to crimp the connector. Pressing the levers crimps the connector to the cable. Choice number two is a screw-on connector, which is very unreliable. If at all possible, avoid the screw-on connector!

In order to attach the backbone cable run to each station, a passive device, known as a T-connector, is used. Picture the uncut backbone cable extending to the back of each device. In order to complete the connection, the cable needs to be cut at the point where the loop is closest to the interface. The two cut ends then need to be terminated with male BNC connectors and plugged into the two female BNC interfaces of the T-connector, with the third, male connector attaching to the female BNC interface on the device’s NIC card. It is in violation of the standard to have any sort of drop cable extending from the back of the device, unlike 10Base-5, where such an attachment was customary. This requirement introduces a minimum of two caveats. The first is that any user that gains access to the back of their computer, and that wouldn’t be very hard, could disconnect the connectorized ends of the cut backbone, thus producing two unterminated LAN segments, neither one working properly. The second is that so many interconnections introduce failure points and opportunities for noise introduction.

Coaxial cable specifications

F-Type Connectors

The F-Type connector is a threaded, screw-on connector that differs from the BNC connector of early Ethernet mainly in its method of device attachment. Additionally, as alluded to earlier, you typically find F-Type connectors with 75ohm coaxial media and BNC connectors with 50ohm applications. As with most other coax applications, the F-Type connector uses the center conductor of the coaxial cable as its center connecting point. The other conductor is the metal body of the connector itself, which connects to the shield of the cable. Again, due to the popularity of cable modems, the F-Type coaxial connector has finally made its way into mainstream data networking.

An example of an Ftype coaxial cable connector

Twisted-Pair Cable

Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. Hence, the name shielded twisted-pair (STP). (You might see this type of cabling in Token Ring installations.) More commonly, you see cable without outer shielding; it’s called unshielded twisted-pair (UTP). UTP is commonly used in twisted-pair Ethernet (10Base-T, 100Base-TX, etc.), star-wired networks.

Let’s take a look at why the wires in this cable type are twisted. When electromagnetic signals are conducted on copper wires that are in close proximity (such as inside a cable), some electromagnetic interference occurs. In this scenario, this interference is called crosstalk. Twisting two wires together as a pair minimizes such interference and also provides some protection against interference from outside sources. This cable type is the most common today. It is popular for several reasons:

  • It’s cheaper than other types of cabling.
  • It’s easy to work with.
  • It permits transmission rates considered impossible 10 years ago.

UTP cable is rated in the following categories:

Category 1 Two twisted wire pairs (four wires). Voice grade (not rated for data communications). The oldest UTP. Frequently referred to as POTS, or plain old telephone service. Before 1983, this was the standard cable used throughout the North American telephone system. POTS cable still exists in parts of the Public Switched Telephone Network (PSTN). Supports signals limited to a frequency of 1MHz.

Category 2 Four twisted wire pairs (eight wires). Suitable for up to 4Mbps, with a frequency limitation of 10MHz.

Category 3 Four twisted wire pairs (eight wires) with three twists per foot. Acceptable for transmissions up to 16MHz. A popular cable choice since the mid-1980s, but now limited mainly to telecommunication equipment.

Category 4 Four twisted wire pairs (eight wires) and rated for 20MHz.

Category 5 Four twisted wire pairs (eight wires) and rated for 100MHz .

Category 5e Four twisted wire pairs (eight wires) and rated for 100MHz, but capable of handling the disturbance on each pair caused by transmitting on all four pairs at the same time, which is needed for Gigabit Ethernet.

Category 6 Four twisted wire pairs (eight wires) and rated for 250MHz. Became a standard in June 2002.

Connecting UTP

Clearly, a BNC connector won’t fit easily on UTP cable, so you need to use an RJ (Registered Jack) connector. You are probably familiar with RJ connectors. Most telephones connect withan RJ-11 connector. The connector used with UTP cable is called RJ-45. The RJ-11 has fourwires, or two pairs, and the network connector RJ-45 (also known as an 8P8C connector when referring to the plug instead of the jack) has four pairs, or eight wires.

Connecting UTP

In almost every case, UTP uses RJ connectors. Even the now-extinct ARCnet used RJ connectors. You use a crimper to attach an RJ connector to a cable, just as you use a crimper with the BNC connector. The only difference is that the die that holds the connector is a different shape. Higher-quality crimping tools have interchangeable dies for both types of cables.

Signaling Methods

The amount of a cable’s available bandwidth (overall capacity, such as 10Mbps) that is used by each signal depends on whether the signaling method is baseband or broadband. With baseband, the entire bandwidth of the cable is used for each signal (using one channel). It is typically used with digital signaling. With broadband, on the other hand, the available bandwidth is divided into descrete bands. Multiple signals can then be transmitted within these different bands. Some form of tuning device, or demodulator, is required to choose the specific frequency of interest, as opposed to baseband receiving circuitry, which can be hardwired to a specific frequency.

Don’t confuse this broadband with the term that is the opposite of narrowband, which is any bit rate of T1 speeds (1.544Mbps) or slower. That broadband refers to speeds in excess of T1/E1 rates, such as Broadband-ISDN (B-ISDN), which has been developed under the ATM specifications.

Ethernet Cable Descriptions

Ethernet cable types are described using a code that follows this format: N<Signaling>-X. Generally speaking, N is the signaling rate in megabits per second, and <Signaling> is the signaling type, which is either base or broad (baseband or broadband). X is a unique identifier for a specific Ethernet cabling scheme.

Let’s use a generic example: 10BaseX. The two-digit number 10 indicates that the transmission speed is 10Mb, or 10 megabits. The value X can have different meanings. For example, the 5 in 10Base5 indicates the maximum distance that the signal can travel—500 meters. The 2 in 10Base2 is used the same way, but fudges the truth. The real limitation is 185 meters. Only the IEEE committee knows for sure what this was about. We can only guess that it’s because 10Base2 seems easier to say than 10Base1.85.

Another 10Base standard is 10Base-T. The T is short for twisted-pair. This is the standard for running 10-Megabit Ethernet over two pairs (four wires) of Category 4, 5e, or 6 UTP. The fourth, and currently final, 10Base is 10Base-FL. The F is short for fiber, while the L stands for link. 10Base-FL is the standard for running 10-Megabit Ethernet over fiber-optic cable to the desktop. Table, shown a bit later, summarizes this data.

Similarly, there are also standards for 100Base, 1000Base, and 10GBase cabling. Let’s take a closer look at these standards:

100Base-TX As network applications increased in complexity, so did their bandwidth requirements. Ten-megabit technologies were too slow. Businesses were clamoring for a higher speed standard so that their data could be transmitted at an acceptable rate of speed. A 100- megabit standard was needed. Thus the 100Base-TX standard was developed.

The 100Base-TX standard is a standard for Ethernet transmission at a data rate of 100Mbps. This Ethernet standard is also known as Fast Ethernet. It uses two UTP pairs (four wires) in a minimum of Category 5 UTP cable.

1000Base-TX 1000Base-TX, most commonly known as Gigabit Ethernet, allows 1000Mbps throughput on standard twisted-pair, copper cable (rated at Category 5e or higher).

1000Base-SX The implementation of Gigabit Ethernet running over multimode fiber-optic cable (instead of copper, twisted-pair cable) and using short wavelength laser.

1000Base-LX The implementation of Gigabit Ethernet over single-mode and multimode fiber using long wavelength laser.

1000Base-CX An implementation of Gigabit Ethernet over balanced, 150ohm copper cabling and uses a special 9-pin connector known as the High Speed Serial Data Connector (HSSDC).

10GBase-SR An implementation of 10 Gigabit Ethernet that uses short wavelength lasers at 850 nanometers(nm) over multimode fiber. It has a maximum transmission distance of between 2 and 300 meters, depending on the size and quality of the fiber.

10GBase-LR An implementation of 10 Gigabit Ethernet that uses long wavelength lasers at 1310 nm over single-mode fiber. It also has a maximum transmission distance between 2 meters and 10 kilometers, depending on the size and quality of the fiber.

10GBase-ER An implementation of 10 Gigabit Ethernet running over single-mode fiber. It uses extra long wavelength lasers at 1550 nm. It has the longest transmission distances possible of the 10-Gigabit technologies: anywhere from 2 meters up to 40 kilometers, depending on the size and quality of the fiber used.

IEEE Standard 1394 (FireWire)

One unique cabling type that is used in a limited sense is IEEE standard 1394, more commonly known as FireWire (or as Sony calls it, i.Link). Developed by Apple Computer, FireWire runs at 100, 200, 400Mbps (800Mbps in the 1394b standard), but in its standard mode it has a cable length limitation of 15 feet (4.5 meters), which limits it to specialized applications like data transfer between two computers located in close proximity or data transfer between a computer and another device (like an MP3 player).

FireWire uses two types of connectors: the 6 pin and the 4 pin. The 6-pin connector is for devices that need to be powered from the computer. FireWire cables with the 6-pin connector contain two pairs (four conductors) of copper wire for carrying data and one pair for powering devices, all within a common, braided metal shield. Cables using the 4-pin connector are for data transfer only, and they contain only the four conductors for data, none for power.

Universal Serial Bus (USB)

Over the past few years, computer peripherals have been moving away from parallel or serial connection and to a new type of bus. That bus is the Universal Serial Bus (USB). The built-in serial bus of most motherboards generally offers a maximum of 2 external interfaces for connectivity to a PC, although add-on adapters can take that count up to as many as 16 serial interfaces. USB, on the other hand, can connect a maximum of 127 external devices. Also, USB is a much more flexible peripheral bus than either serial or parallel. USB supports connections to printers, scanners, and many other input devices (such as keyboards, joysticks, and mice).

When connecting USB peripherals, you must connect them either directly to one of the USB ports (as shown in Figure) on the PC or to a USB hub that is connected to one of those USB ports. Hubs can be chained together to provide multiple USB connections. Although you can connect up to 127 devices it is impractical in reality. Most computers with USB interfaces will support around 12 USB devices.

Universal Serial Bus (USB)

Fiber-Optic Cable

Because fiber-optic cable transmits digital signals using light impulses rather than electricity, it is immune to Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).

Fiber-Optic Cable

Anyone who has seen UTP cable for a network run down an elevator shaft would, without doubt, appreciate this feature of fiber. Light is carried on either a glass or a plastic core. Glass can carry the signal a greater distance, but plastic costs less. Regardless of which core is used, the core is surrounded by a glass or plastic cladding, which is more glass or plastic with a different index of refraction that refracts the light back into the core. Around this is a layer of flexible plastic buffer. This can be then wrapped in an armor coating (where necessary), typically Kevlar, and then sheathed in PVC or plenum

The cable itself comes in two different styles: single-mode fiber (SMF) and multimode fiber (MMF). The difference between single-mode fibers and multimode fibers is in the number of light rays (and thus the number of signals) they can carry. Generally speaking, multimode fiber is used for shorter-distance applications and single-mode fiber for longer distances.

If you happen to come across a strand of fiber in the field and want to know if it’s single mode or multimode, here are some general guidelines. First of all, if it’s got a yellow jacket, it’s probably single mode. If it’s got an orange jacket, it’s most likely multimode. Also, check the writing on the cable itself. You’ll find a number like 62.5/125. These are the outside diameters of the core and the cladding (respectively). If the first number is a 8, 9, or 10, it is most likely a single mode. On the other hand, if the numbers read as before (62.5/125), it’s most likely a multimode strand of fiber. Use these two tips to help you identify that errant strand of fiber.

Although fiber-optic cable may sound like the solution to many problems, it has pros and cons just as the other cable types. Here are the pros:

  • Is completely immune to EMI or RFI
  • Can transmit up to 40 kilometers (about 25 miles)

Here are the cons of fiber-optic cable:

  • Is difficult to install
  • Requires a bigger investment in installation and materials

Fiber-Optic Connectors

Fiber-optic cables can use a myriad different connectors, but the two most popular and recognizable are the straight tip (ST) and subscriber (or square) connector (SC) connectors. The ST fiber-optic connector, developed by AT&T, was one of the most widely used fiber-optic connectors. It uses a BNC attachment mechanism similar to the Thinnet connection mechanism, which makes connections and disconnections relatively easy. Its ease of use is one of the attributes that makes this connector so popular.

The SC connector (sometimes known also as a square connector) is another type of fiber-optic connector. SC connectors are latched connectors. This latching mechanism holds the connector in securely while in use and prevents it from just falling out. SC connectors work with either single-mode or multimode optical fibers, and they will last for around 1000 matings. They are seeing increased use but aren’t as popular as ST connectors for LAN connections.

Fiber-Optic Connectors

A sample SC connector

A sample Sc connector

Small Form Factor Fiber-Optic Connectors

One of the more popular styles of fiber-optic connectors is the small form factor (SFF) style of connector. SFF connectors allow more fiber-optic terminations in the same amount of space over their standard-sized counterparts. The two most popular are the mechanical transfer registered jack (MT-RJ or MTRJ), designed by AMP, and the Local Connector (LC), designed by Lucent.

MT-RJ

The MT-RJ fiber-optic connector was the first small form factor fiber-optic connector to see widespread use. It is one-third the size of the SC and ST connectors it most often replaces. It had the following benefits:

  • Small size TX and RX strands in one connector
  • Keyed for single polarity
  • Pre-terminated ends that require no polishing or epoxy
  • Easy to use

Small Form Factor Fiber-Optic Connectors

LC

Local Connector is a newer style of SFF fiber-optic connector that is overtaking MT-RJ as a fiber-optic connector. It is especially popular for use with Fibre Channel adapters and Gigabit Ethernet adapters. It has similar advantages to MT-RJ and other SFF-type connectors but is easier to terminate. It uses a ceramic insert as standard-sized fiber-optic connectors do.

A sample LC fiber optic connector

Cable Type Summary

Cable Type Summary
Cable Type Summary
Cable Type Summary


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