TRANSMISSION MEDIA
Transmission media is data that transformed from one place to another is called transmission or communication media. it's physical path between transmitter and receiver, may be guided (wired) or unguided (wireless), communication achieved by using em waves.
The characteristic of trasmission media :
Characteristics and quality of data transmission
- Dependent on characteristics of medium and signal
- Guided medium
- Medium is more important in setting transmission parameters
- Bandwidth of the signal produced by transmitting antenna is important in setting transmission parameters
- Signal directionality
- Lower frequency signals are omnidirectional
- Higher frequency signals can be focused in a directional beam
- Concerned with data rate and distance
- Bandwidth
- Higher bandwidth implies higher data rate
- Attenuation
- Twisted pair has more attenuation than coaxial cable which in turn is not as good as optical ber
- Can be minimized by proper shielding in guided media
- In a shared link, each attachment introduces attenuation and distortion on the line
1. BOUNDED MEDIA:
Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are
- Coaxial Cable
- Twisted Pairs Cable
- Fiber Optics Cable
Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.
Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.
Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.
Outside the copper mesh is the final protective cover. (as shown in Fig)
The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.
Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.
Here the most common coaxial standards.
- 50-Ohm RG-7 or RG-11 : used with thick Ethernet.
- 50-Ohm RG-58 : used with thin Ethernet
- 75-Ohm RG-59 : used with cable television
- 93-Ohm RG-62 : used with ARCNET.
- Single cable failure can take down an entire network
- Transmission capacity (bandwidth and data rate) depends on distance and type of network (point-to-point or multipoint)
- Twisted pair
{ Physical description
- Two insulated copper wires arranged in regular spiral pattern
- Number of pairs are bundled together in a cable
- Twisting decreases the crosstalk interference between adjacent pairs in the cable, by using di ferent twist length for neighboring pairs
- Most common transmission media for both digital and analog signals
- Less expensive compared to coaxial cable or optical ber
- Limited in terms of data rate and distance
- Telephone network
- Individual units (residence lines) to local exchange (end o ce)
- Subscriber loops
- Supports voice tra c using analog signaling
- May handle digital data at modest rates using modems
- Communications within buildings
- Connection to digital data switch or digital pbx within a building
- Allows data rate of 64 kbps
- Requires ampli ers every 5-6 km for analog signals
- Requires repeaters every 2-3 km for digital signals
- Attenuation is a strong function of frequency
- Higher frequency implies higher attenuation
- Susceptible to interference and noise
- Improvement possibilities
- Shielding with metallic braids or sheathing reduces interference
- Twisting reduces low frequency interference
- Di erent twist length in adjacent pairs reduces crosstalk
- Unshielded twisted pair (utp)
- Ordinary telephone wire
- Subject to external electromagnetic interference
- Shielded twisted pair (stp)
- Shielded with a metallic braid or sheath
- Reduces interference
- Better performance at higher data rates
- More expensive and di cult to work compared to utp
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