TRANSMISSION MEDIA

Logical Links(Virtual LAN)• A virtual  local area network (virtual LAN) is the logical grouping of network nodes.  A virtual LAN allows geographically dispersed network nodes to communicate as if they were physically on the same network. 

• A switch that supports VLANs allows the administrator to select which ports will participate in the VLAN. These ports are then grouped to become one VLAN, and any broadcasts or information passed among these ports will not be seen by the remaining ports on the switch.

Transmission Media• There are several types of physical channels through which data can be sent from one point to another in a LAN.

• Bounded/Guided Media (Wired)• Unbounded/Unguided Media (Wireless)

• Media differ in the capability for the following reasons:1. Noise absorption: Susceptibility to external noise2. Radiation: Leakage of signal3. Attenuation: decline of magnitude of signal with

distance4. Bandwidth: transmission speed

Guided Media• Wired media

1. Twisted-pair wire : LAN

2. Coaxial cabling : Cable TV

3. Fiber-optic cabling : Broadband

Twisted-pair wire• two insulated copper wires (1 mm thick)

• The purpose of twisting the wires is to ensure that each wire is equally exposed to any external electrical magnetic field that could induce unwanted noise.

• commonly used in local telephone communication and for digital data transmission over short distances up to 1 km. (9600 bps)

Twisted-pair wire

Insulator (Teflon in Cat 5)

Advantages• being the oldest method, trained manpower is easily available

• In a telephone system, signals can travel several kilometers without amplification

• can be used for both analog and digital data transmission

• least expensive

• if a portion of a twisted-pair cable is damaged, the entire network is not shutdown

Disadvantages• easily picks up noise signals

• thin (break easily)

Types of TP Cable• Category 3 : 4 pairs

• Category 5 : 4 pairs but more twisted with Teflon insulation

• Category 6 : Gigabyte Ethernet

Coaxial Cabling• stiff (rigid) copper wire surrounded by an insulating material• The insulator is encased within a cylindrical conductor (braided mesh)• The signal is transmitted by the inner copper wire

Types of Coaxial Cabling• Two types:

1. 50-ohm : digital transmission2. 75-ohm : analog transmission (Cable TV)

Coaxial Cable Implementation

Terms Implementation

10Base2 Thin Ethernet (RG-58), 10MBPS, 185m cable segment

10Base5 Thick Ethernet, 10MBPS, 500m cable segment(bus)

10BaseF

Fiber-optic cable

10BaseT

UTP, uses RJ-45 connectors, Star Topology, 100m length

Thick Ethernet

0.4 inch thick, 1006m

Thin Ethernet

0.2 inch thick, 165m, normally used for office installation

Advantages• better shielding against EMI(Electromagnetic interference) than twisted-pair cable

• can be used for both analog and digital

• higher bandwidth (noise immunity)

• more expensive than TP but easy to handle

• capable of carrying over 50 std 6MHz color TV channels

Security• Cables that employ copper conductors can easily be breached by listening equipment.

• If the main consideration is security, then fiber cable is the only choice to avoid espionage.

Optical Fiber• newest form of bounded media• superior in data handling and security• transmits light signals rather than electric signals• each fiber has an inner core of glass/plastic that conducts light

Optical Fiber• total internal reflection

Optical Fiber• Two light sources:1. LEDs (Light Emitting Diodes)2. LASERs(Light Amplification by Stimulated Emission

Radiation)

Photoelectric diodes

LED/Laser

Types of Optical Fiber• Two types:1. Single-mode fiber (8-10µ (microns) core)2. Multimode fiber (50µ microns core) 1µ = 10-6 m

Laser

LED

Characteristics• Cost: more expensive than copper cable (associated equipments)• Installation: difficult to install• Bandwidth capacity: provides data rates from 100MBPS to 2GBPS• Node capacity: 75 nodes• Attenuation: much lower attenuation but has a different problem, namely, chromatic dispersion• EMI: immune to eavesdropping • Mode of transmission: half duplex

Chromatic Dispersion

Advantages• handle much higher bandwidth with low attenuation

• not affected by power surges, EMI or power failures

• lighter than copper cable

• do not leak light and are quite difficult to tap (excellent security)

Disadvantages• unfamiliar technology requiring skills

• Being unidirectional, two-way communication requires either two fiber cables or two frequency bands on one fiber

• higher cost

Comparison

Factor UTP STP Coaxial Fiber-optic

Cost Lowest Moderate Moderate Highest

Installation Easy Fairly easy Fairly easy Difficult

Bandwidth 1-155 Mbps 1-155 Mbps 100 Mbps 2 Gbps

Attenuation

High (few hundred meter)

High (few hundred meter)

Lower (few kilometer)

Lowest (tens of kilometer)

EMI Most vulnerable to EMI and eavesdropping

Less vulnerable than UTP

Less vulnerable than UTP

Not affected by EMI or eavesdropping

Unguided Media• signals are not guided through a solid medium (Air)

• Radio wave (including VHF and microwave links)• Satellite links• VSATs(Very Small Aperture Terminals)• Infrared and millimeter waves

Radio Waves• frequencies between 10 KHz and 1 GHz• broadcast omnidirectionally or unidirectionally• The power of the Radio Frequency (RF) signal is determined by the antenna and trans-receiver.

Characteristics• easy to generate• travel long distances• penetrate buildings easily• omnidirectional• At low frequency, radio waves pass through obstacles easily, but the power falls off sharply with distance from source.• At high frequency, radio waves tend to travel in straight lines and bounce off obstacles.• absorbed by rain

Microwave Transmission• Above 100 MHz, the waves travel in straight lines and can, therefore, be narrowly focused.• Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other.• uses repeaters at intervals of about 25-30 km to amplify weak signal

Microwave Transmission• do not pass through buildings well• Multipath Fading (delayed waves may arrive out of phase)• The higher the towers are, the further apart they can be.

Characteristics• transmission depends on weather and frequency. (10GHz)• widely used for long-distance telephone communication, cellular telephones, television distribution• inexpensive as compared to fiber-optic system (congested area)• 16 Gbps

Types• Two Types:

1. Terrestrial2. Satellite

Terrestrial Microwave Systems• use directional parabolic antennas to send and receive signals• Relay towers and repeaters are used to extend signals• used whenever cabling is cost-prohibitive such as in hilly areas or crossing rivers etc.

Characteristics• Frequency range : 4-6 GHz and 21-23 GHz• Cost : inexpensive for short distance but expensive for long distance• Installation: difficult due to Line-of-sight requirements• Bandwidth capacity: 1-10 Mbps• Attenuation: affected by atmospheric conditions (rain and fog)• EMI: vulnerable to EMI, jamming and eavesdropping

Satellite Microwave Systems• one antenna is on the satellite in geo-synchronous orbit about 36,000 km above the equator (can reach the most remote places)• the transmitted frequency is different from the receiving frequency to avoid interference

Characteristics• Frequency range: 4-6 GHz and 11-14 GHz• Cost: The cost of building and launching a satellite is extremely high• Installation: extremely technical and difficult • Bandwidth capacity: 1-10 Mbps• Attenuation: depends on frequency, power, antenna size and atmospheric conditions. Higher frequency microwaves are more affected by rain and fog

Advantages and Limitation• visible from any point• transmission and reception costs are independent of the distance• a transmission station can receive its own transmission (testing)

• cost of placing the satellite• a signal sent to a satellite is broadcast to all receivers within the range (security measures need to be taken)

VSATs• Very Small Aperture Terminals (1m wide antennas)• the micro-stations do not have enough power to communicate directly with one another, hence • a special ground station, the hub, with a large high-gain antenna is needed to relay traffic between VSATs

Light Wave Transmission• connect two LANs in two buildings via lasers mounted on their roof-tops• Coherent optical signaling using lasers is inherently unidirectional. So, each buildings needs its own Laser and photo detector

Advantages and Disadvantages• Advantages:1. bandwidth is very high at very low cost2. easy to install3. does not require any license

• Disadvantages:1. cannot penetrate rain or thick fog2. heat from sun causes convection currents

Infrared and Millimeter Waves• used for short-range communication (remote control in TV)• directional, cheap and easy to build but do not pass through solid objects• Security if Infrared systems against eavesdropping is better than that of radio systems• No government license is needed (radio systems must be licensed)

Fiber-optics communication

Factor LED Laser

Data rate Low High

Mode Multimode Multimode or single-mode

Operating distance 3 Km 30 Km

Lifetime Long life Short life

Temperature sensitivity

Minor Substantial

Cost Low Substantial

• Light Source: LED/Laser• Transmission medium: Fiber-optic cable• Detector: converter

Intranet• a network that connects the computer and networks within an organization by using the hardware and software that is used on the Internet (TCP/IP protocol)

• secure & can have thousands of users

Intranet(HCL)

Intranet(Wipro)Interne

t

Extranet