Transmission Media
Distinguish guided (wired) from unguided (wireless)
Wired Media
Three major guided media for data comm.
- twisted pair
- coaxial
- fiber optic
Twisted Pair
copper wires in bundle grouped in pairs
max frequency about 5 MHz.
wires in pairs twisted for uniform noise effect
(effect based on distance from noise source, so equal avg distance)
UTP (unshielded) or STP (shielded)
UTP most common, categories 1 thru 5 based on quality and max xmit rate
Cat 1: low rates,
Cat 3: up to 10 Mbps (standard telephone, 3 twists/ft),
Cat 5: up to 100 Mbps (tighter twists)
Signal attenuates (degrades) with distance
Connectors: 4-8 pin RJ (standard telephone); RJ45 handles 4 pairs
Coaxial
- a.k.a. coax
- solid copper core, braided copper surround, separated by insulator
- max frequency 500 MHz
- various RG grades: higher the number the thinner (and more flexible) the cable (lower rates, shorter distances)
- Thicknet
(.5" diameter), Thinnet(.25"), baseband (dedicated bandwidth), broadband (cable TV - FDM), 10BaseX, are terms relevant to applications of coax (mostly ethernet) See Ethernet chapter.
- barrel connectors (BNC): bayonet (1/2 turn with lock), threaded, slip-on.
- Use of T-connector or vampire tap
Fiber Optic
- inherently digital (light pulse).
- Fast but expensive; immune to electrical interference
- current bandwidth limited by signal generator/detector technology, not medium
- core fiber is very thin (typical 50-100 micrometers), fragile and relatively inflexible
- each core has very thin cladding.
- Connectors: mostly barrel. Must be very precise.
- Signal generation/detection. Light source either: LED (weak, unfocused, short distance, cheaper) or laser (the opposite). Use photodiodes for detectors.
- light transmission in fiber optic:
- light travels in straight line until density of medium changes.
- Fiber cladding is less dense than core.
- What happens when light beam in core strikes cladding (at some angle)?
- Depends on the angle. It may either be refracted into cladding (bad) or reflected back into core (good).
- Reflected beam will bounce in this fashion until destination
- Multimode fiber:
numerous light beams emitted at different angles from source. Some travel straight thru, some bounce along. Those that bounce along arrive later! Received signal is a little out of focus.
- Single mode fiber:
fiber is so thin that only beams traveling straight thru can fit. Received signal is well-focused. Only laser can do this. Very expensive.
Wireless Media
- Radio communication bandwidth is regulated 3 KHz to 300 GHz range
- Partitioned into 8 bands based on frequency subrange (e.g. VHF,UHF)
- Different bands have different propagation properties (by law or nature)
- hug earth surface
- straight line through troposphere (atmosphere+stratosphere)
- bounce (reflect) off troposphere (due to density difference and angle)
- bounce (reflect) off ionosphere (ditto)
- straight line into space
Will concentrate on technologies used primarily for data communication
Infrared or laser
- line of sight
- short distances (e.g. for LAN)
- digital
microwave
- line of sight (lengthen by using taller tower)
- signal good for 50 miles
- analog
- parabolic dish to receive (focuses incoming signal)
satellite
- line of sight
- for general use, need geosynchronous orbit (22,000 miles up)
- utilizes transceiver (aka transponder): accepts signals on one band, relays out on another
- must be spaced at least 4 degrees apart to prevent crosstalk (unless adjacent satellites use different frequency bands.
- Minimum of 3 satellites to connect any two arbitary earth points
- Frequency band 500 MHz wide
- Transmission rate fast, propagation delay long!
Related Home Pages:
notes | CSC 465 | Peter Sanderson | Computer Science | SMSU
Last reviewed: 18 February 1998
Peter Sanderson ( pete@csc.smsu.edu )