Datornätverk a – lektion mks b – lektion Kapitel 3: Fysiska signaler

Datornätverk A – lektion 3 MKS B – lektion 3

• Kapitel 3: Fysiska signaler.

• Kapitel 4: Digital transmission.

• Chapter 3 – Time and Frequency Domain Concept, Transmission Impairments

Periodic vs. Non Periodic Signals

Sinusvågor

Measuring the Phase

• The phase is measured in degrees or in radians.

• One full cycle is 360o

• 360o (degrees) = 2 (radians)

• 

• Example: A sine wave is offset one-sixth of a cycle with respect to time 0. What is the phase in radians?

• Solution: (1/6) 360 = 60 degrees = 60 x 2p /360 rad = 1.046 rad

Example: Square Wave

Characteristic of the Component Signals in the Square Wave

• Infinite number of components

• Only the odd harmonic components are present

• The amplitudes of the components diminish with increasing frequency

Media Filters the Signal

Filtering the Signal

• Filtering is equivalent to cutting all the frequiencies outside the band of the filter

Förstärkning mätt i decibel (dB)

• 1 gång effektförstärkning = 0 dB.

• 2 ggr effektförstärkning = 3 dB.

• 10 ggr effektförstärkning = 10 dB.

• 100 ggr effektförstärkning = 20 dB.

• 1000 ggr effektförstärkning = 30 dB.

• Osv.

Dämpning mätt i decibel

• Dämpning 100 ggr = Dämpning 20 dB = förstärkning 0.01 ggr = förstärkning med – 20 dB.

• Dämpning 1000 ggr = 30 dB dämpning = -30dB förstärkning.

• En halvering av signalen = dämpning med 3dB = förstärkning med -3dB.

Measurement of Attenuation

• Signal attenuation is measured in units called decibels (dB).

• If over a transmission link the ratio of output power is Po/Pi, the attenuation is said to be –10log10(Po/Pi) = 10log10(Pi/Po) dB.

• In cascaded links the attenuation in dB is simply a sum of the individual attenuations in dB.

• dB is negative when the signal is attenuated and positive when the signal is amplified

What is dB?

• A decibel is 1/10th of a Bel, abbreviated dB

• Suppose a signal has a power of P1 watts, and a second signal has a power of P2 watts. Then the power amplitude difference in decibels, is:

• 10 log10 (P2 / P1)

• As a rule of thumb:

• 10dB means power ratio 10/1
• 20dB means power ratio 100/1

Noise and Interference

• Noise is present in the form of random motion of electrons in conductors, devices and electronic systems (due to thermal energy) and can be also picked up from external sources (atmospheric disturbances, ignition noise etc.)

• Interference (cross-talk) generally refers to the unwanted signals, picked up by communication link due to other transmissions taking place in adjacent frequency bands or in physically adjacent transmission lines

Signal-brus-förhållande

• Ett signal-brus-förhållande på 100 dB innebär att den starkaste signalen är 100 dB starkare än bruset.

• Ljud som är svagare än bruset hörs inte utan dränks i bruset.

• Ljudets dynamik skillnaden mellan den starkaste ljudet och det svagaste ljudet som man kan höra, och är vanligen ungefär detsamma som signal-brus-förhållandet.

Throughput (Genomströmningshastighet)

Delay (Time, Latency)

• When data are sent from one node to next node (without intermediate points), two types of delays are experienced:

• transmission time (Paketsändningstid)
• propagation delay (Utbredningsfördröjning)

• When data pass through intermediate nodes four types of delay (latency) are experienced:

• transmission time
• propagation delay
• queue time
• processing time

Transmission Time (Paketsändningstid)

• The transmission time is the time necessary to put the complete message on the link (channel).

• The transmission time depends on the length of the message and the bit rate of the link and is expressed as:

• length of packet (bits)

• bit rate (bits/sec)

Propagation Delay (Time)

• The propagation delay is the time needed for the signal to propagate (travel) from one end of a channel to the other.

• The transmition time depends on the distance between the two ends and the speed of the signal and is expressed as

• distance (m) / speed of propagation (m/s)

• Through free space signals propagate at the speed of light which is 3 * 108 m/s

• Through wires signals propagate at the speed of

• 2 * 108 m/s