What is the physical layer and its purpose
- The physical layer deals only with nodes that can be reached directly
- The purpose of the physical layer is to transport bits from one node to another.
-Bits are transmitted as signals over the transmission media
What are the types of guided media
- Twisted Pair
- Coaxial cable
- Power lines
- Fiber optics
Twisted pair
- Used in telephone lines, LANs
- Twisting reduces noise as magnetic fields
induce currents in opposite directions - Two types: Unshielded Twisted Pair (UTP)
and Shielded Twisted Pair (STP) that comes
with additional shielding
Coaxial cable
- Traditionally used for Ethernet LANs
- Used in cable television and MANs
- Good shielding reduces noise
- More bandwidth for longer distances and
higher rates than twisted pair (Capable of
1Gbps over 1km
Power lines
- A network that uses household electrical wiring
- Convenient to use, but not great for sending data
Fiber Optics
- Used in long distance ISP links, Fiber-to-the-Home
- Fiber has enormous bandwidth and tiny signal loss
- Light carried in very long, thin strand of glass
2 types of fiber optics
- Multi-mode: Light can bounce (wider core). Used with LEDs
for cheaper, shorter distance links - Single-mode: Core so narrow so light cannot bounce around. Used with lasers. Capable of carrying 100s of Gbps over 100s of kilometres
unguided media
- Radio transmission
- Microwave transmission
- Infrared transmission
Radio Transmission
Radio signals penetrate buildings well and propagate for long distances
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth (e.g. Maritime, AM radio)
(b) In the HF and VHF bands, the ground waves tend to be absorbed by the earth (FM radio). However, the HF (short wave) band can bounce off the ionosphere
Microwave transmission
- Microwaves have high bandwidth and are used indoors and outdoors
- can be used in wifi/ mobile data
- Signal is attenuated/reflected by everyday objects
- Strength varies with mobility due object in the way of the destination
Infrared transmission
- Used for short-range communication, do not pass-through solid objects
Transmission impairment
- Attenuation : Loss of signal strength over distance
- Noise: The random or unwanted signal that mixes up with the original signal
- Distortion: Alteration of signal shape or form (induced by the attenuation, noise or delay)
Transmission methods
simplex
Half Duplex
Full duplex
Simplex
One way communication. Example: keyboard to computer
Half Duplex
Two way communications link, but only one system can talk at a time.
Example: walkie-talkie
Full duplex
Transmit data in both directions simultaneously. Example: telephone. Most communications systems work full-duplex
Parallel Transmission
- Transmits a group of bits simultaneously - high bit rate
- Expensive over long distances because of need for thicker cable
- Synchronisation problem over long distances (bit skew)
Serial transmission
- Transmits bits one after another - slower than parallel transmission
- More reliable and cheaper for transmission over long distances
- Incurs additional complexity for sending and receiving devices
- (the sender must determine the order in which the bits are sent, and the receiver must determine in which order to reconstruct a byte)
Multiplexing
- Multiplexing allows the carrying of multiple signals on a single medium.
- This allows more efficient use of the transmission medium
3 types of multiplexing
- Frequency-division multiplexing (FDM)
- Time-division multiplexing (TDM)
- Code Division Multiple Access (CDMA)
Frequency-division multiplexing
- Shares the channel by placing users on different frequencies.
Time-division multiplexing
- Shares a channel over time.
- Each signal takes up the transmission medium for a little “time slice”
Code division multiple access (CDMA)
- Shares the channel by giving specific code to users.
- Widely used as part of 3G networks
