Physical Media

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.In following section, we'll look at three types of cables:

• Coaxial
• Twisted Pair
• Fiber Optic

Coaxial Cable:
Coaxial cable 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) 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 plenums that carry breathable air, usually as nonenclosed fresh-air return pathways that share space with cabling.

types of coaxial cables are: (RG= radio guide)

RG8 :

• segment length is 500 m.
• Interference is good compared to any copper cable.
• resistance 50 ohm.
• BNC-T or Vampire Tap connector is used to connect this cable.
• Used in 10base5 network

RG6:

• broadband quad-shield cable that offers an impedance of 75 ohm.
• lower attenuation
• Used in cable TV, CCTV, satellite dish antenna.

RG59:

• Better breakdown characteristics allows for high voltage use in scientific experiments.
• Characteristic impedance 75 ohm.
• Used in security camera, cable TV, home theater.

using Thin Ethernet: Thin Ethernet, also referred to as Thinet or 10base-2, is a thin coaxial cable.It is basically the sane 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. With Thinnet cable, we use BNC ( BayoNet Connector or British Navel Connector) to attach stations to the network.

 

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 shield twisted-pair (STP). 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, 10Base-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, some electromagnetic interference occurs. 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.

 

UTP cables:

• Category 1: two twisted wire pairs. Voice grade (not rated for data communications).Oldest UTP, used in PSTN (Public Switched Telephone Network) .
• Category 2: Four twisted wire pairs. Suitable for up to 4 Mbps, with a frequency limitation of 10 MHz
• Category 3: Four twisted wire pairs with three twists per foot.Acceptable for transmissions up to 16 MHz.
• Category 4: Four twisted wire pairs and rated for 20MHz.
• Category 5: Four Twisted wire pairs and rated for 100MHz
• Category 5e: Four Twisted wire pairs and rated for 100MHz, but capable of handling the disturbance on each pair caused by transmitting on all four pairs at the same time.
• Category 6: Four twisted wire pairs and rated for 250MHz.

Fiber-Optic Cable :

A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.Electrical signals are converted into light signals. TIR (Total Internal Reflection) phenomena is responsible for transmission of data. 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).

 

          The cable itself comes in two different styles: single-mode fiber (SMF) and multi-mode fiber (MMF). The difference between single-mode fibers and multi-mode fiber is in the number of light rays ( and thus the number of signals) they can carry. Multi-mode fiber is used for short distance application and single-mode fibers for longer distance.

Fiber-optic connectors: 

 

• Straight tip (ST) : joins individual fibers to optical device.
• Subscriber Connector (SC) : attaches two fibers to send and receive signal.
• Medium interface connector (MIC) : joins fibers to FDDI
• Sub Miniature Type A (SMA) : uses individual connectors for each fiber stand.

 

NOTE:      mBase-n

•  m indicates transmission speed in megabits
• n can have different meaning. for example n=5, means 500m segment length but for n=2, segment length is 185m. Only the IEEE committee knows for sure what this was about.

for examples:

• 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters)
• 10BASE-F (optical fiber cable)
• 10BASE-T (ordinary telephone twisted pair wire)
• 10BASE-36 (broadband multi-channel coaxial cable with a maximum segment length of 3,600 meters)