Chapter 3

Types of Transmission Media

(i) Twisted Pair Cable – It consists of 2 separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective sheath. They are the most widely used Transmission Media. Twisted Pair is of two types:

1. Unshielded Twisted Pair (UTP): This type of cable has the ability to block interference and does not depend on a physical shield for this purpose. It is used for telephonic applications.

   Advantages:

   • Least expensive

   • Easy to install

   • High-speed capacity

   • Susceptible to external interference

   • Lower capacity and performance in comparison to STP

   • Short distance transmission due to attenuation

2. Shielded Twisted Pair (STP): This type of cable consists of a special jacket to block external interference. It is used in fast-data-rate Ethernet and in voice and data channels of telephone lines.

   Advantages:

   • Better performance at a higher data rate in comparison to UTP

   • Eliminates crosstalk

   • Comparatively faster

   • Comparatively difficult to install and manufacture

   • More expensive

   • Bulky

Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras (northern/southern lights). EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.

EMI can be used intentionally for radio jamming, as in electronic warfare.

(ii) Coaxial Cable – It has an outer plastic covering containing 2 parallel conductors each having a separate insulated protection cover. The coaxial cable transmits information in two modes: Baseband mode(dedicated cable bandwidth) and Broadband mode(cable bandwidth is split into separate ranges). Cable TVs and analog television networks widely use Coaxial cables.

Advantages:

• High Bandwidth

• Better noise Immunity

• Easy to install and expand

• Inexpensive

Disadvantages:

• Single cable failure can disrupt the entire network

Coaxial Cable usually used in Bus Topology

In fiber-optic communication, a single-mode optical fiber (SMF) is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions. These modes define the way the wave travels through space

For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface so that the cladding is of a lower refractive index. The step-index profile corresponds to a power-law index profile with the profile parameter approaching infinity. The step-index profile is used in most single-mode fibers and some multimode fibers

In fiber optics, a graded index is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber.

Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The most common refractive index profile for a graded-index fiber is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.

Multi-mode optical fiber can be built with either graded index or step index. The advantage of the multi-mode graded index compared to the multi-mode step index is the considerable decrease in modal dispersion. Modal dispersion can be further decreased by selecting a smaller core size (less than 5-10μm) and forming a single mode step index fiber.

Fibers Connectors

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1. SC Connector

The SC was developed by the laboratories at Nippon Telegraph and Telephone (NTT) in the mid-eighties, and was one of the first connectors to hit the market following the advent of ceramic ferrules. Sometimes referred to as the ‘square connector’ the SC has a push-pull coupling end face with a spring loaded ceramic ferrule. Initially intended for Gigabit Ethernet networking, it was standardized into the telecommunications specification TIA-568-A in 1991 and slowly grew in popularity as manufacturing costs came down. Due to its excellent performance it dominated fiber optics for over a decade with only the ST rivalling it. Thirty years on, it remains the second most common connector for polarization maintaining applications. The SC is ideally suited for datacoms and telecoms applications including point to point and passive optical networking.

2. LC Connector

Considered by some to be the modern replacement of the SC connector; its introduction was less successful, in part due to initially high license fees from inventor Lucent Corporation. Also a push-pull connector, the LC utilizes a latch as opposed to the SC locking tab and with a smaller ferrule it is known as a small form factor connector. Having half the footprint of the SC connector gives it huge popularity in datacoms and other high-density patch applications, as its combination of small size and latch feature make it ideal for densely populated racks/panels. With the introduction of LC compatible transceivers and active networking components, its steady growth in the FTTH arena is likely to continue.

3. FC Connector

The FC was the first optical fiber connector to use a ceramic ferrule, but unlike the plastic bodied SC and LC, it utilizes a round screw-type fitment made from nickel-plated or stainless steel. The connector end face relies on an alignment key for correct insertion and is then tightened into the adaptor/jack using a threaded collet. Despite the additional complexity both in manufacturing and installation, it’s still the connector of choice for precise measuring equipment such as OTDRs.

Initially intended for datacoms and telecoms applications, its use has reduced since the introduction of the SC and LC. These deliver similar performance to the FC but both have less expensive components and are quicker to connect. However, the screw-on collet of the FC does make it particularly effective in high vibration environments, ensuring that the spring-loaded ferrule is firmly mated.

4. ST Connector

The ST connect was developed by AT&T shortly after the arrival of the FC. At a glance they can be mistaken for one another but the ST uses a bayonet fitment rather than a screw thread. Usage has declined in recent decades, for the same reasons as the FC. Additionally it cannot be terminated with an angled polish, which limits use in single mode fiber and FTTH applications.

Deployed predominately in multi-mode datacoms it is most common in network environments such as campuses, corporate networks and in military applications where the quick connecting bayonet had its advantages at the time. It is typically installed into infrastructures that were built at the turn of the century; when retro-fitting, STs are typically swapped out for more cost effective SC and LC connectors.

5. MTP/MPO connector

The MT ferrule connector is another of NTT’s inventions and has been around since the 1980s, although the technology has only recently become popular under branded versions of the Multiple Fiber Push-On/Pull-Off connector, such as MTP and MPO. It is larger than the other connectors but for good reason - it can support up to 24 fibers in a single ferrule.

Multi-fiber connectors are not currently designed for field-fit applications so must be lab terminated. In high density patch environments such as datacenters they are used extensively, both at single mode and multi-mode wavelengths. On a ‘per-fiber’ basis the costs are relatively inexpensive. However as might be expected, the attenuation loss can be higher than a single ceramic ferrule connector. That being said, it is possible to order ‘low loss’ MTP/MPO connectors which have comparable insertion loss performances. These are more costly however.

Network planners should also consider that whilst still using a uniter/adaptor much like other connectors, the MTP/MPO must also be mated to an opposing male or female connector. This may require more than one connector specification or type within inventory, adding to cost and complexity.

Because the sequence of the fibers cannot physically be changed after termination, the connector is often supplied with a fan-out assembly at the opposing end (such as LC, SC FC etc.). This allows the operator to change channels simply by re-patching the fanned-out side of the cable. The consequence of this is that the small form high density design of the MTP/MPO will only benefit one side of the assembly.

More common in datacoms, these connectors are starting to appear in FTTH applications. They should therefore be considered if drivers include quick deployment of aggregate fibers, high density patching or where smaller ODFs and nodes might be crucial.

The differences between types of connectors can easily overlooked in the complex planning around fiber deployments. However taking the time to select the right one for the job can deliver big benefits when it comes to speed and cost, so take the time to investigate your options before making your connector choice.

OSI Second layer.

Repeater Layer 1 : Physical Layer device , it doesn't care about the content it just care about the signal and change it to the original signal.

Hub Layer 1 : Multi port repeater

Bridge, Switch Layer 2

Router Layer 3

Gateway Layer 7

Router : to find way from server to destination

Layer 3 Switch is cheaper because it contains only routing.