SCC 130 - Part 2

The exchange of information between two devices via a transmission medium.

Systems need to maintain certian criteria for effective electronic communication

• Delivery, accuracy, timeliness, jitter

It has a sender, receiver, transmission medium (channel) and protocol

It depends on 4 characteristics:

• Delivery
• Accuracy
• Timeliness
• Jitter

Data must be delivered to the correct destination and must be received by the intended device or user and only by the device or user

Data that has been altered (or tampered with) in transmission and left uncorrected is unusable. Altered data can be as a result of distortion or error introduced in the system by noise. 

Distortion means that the signal changes in its form or shape and typically effects complex or composite signals and takes place when a composite signal carrying different frequencies suffers from the delay of some of these frequencies

Each frequency component has its own propagation speed through a medium and as a result its own delay in being received.

Noise is the main source of a signal being corrupted

There are several types of noise that can effect a signal

• Thermal noise - random motion of electrons in a wire
• Induced noise - created by sources such as motors and appliances
• Crosstalk - effect of a signal effecting another wire
• Impulse - the spike that comes from power lines, lightning and car engines

Data delivered late is rendered of no use (dropped packets or failed acknowledgements)

In the case of IP packets, if no acknowledgement is end back in time the packet will be resent by the sender

In the case of video and audio for example, timely deliverly means the delivery data in real time without a significant delay

For real-time audio-video reproduction (Rx), timeliness refers to play-back in order and with no delays.

Even though there is a 1s delay the time relationship between the packets is preserved

Refers to the variation of the packet arrival time. Uneven delay (latency) in the delivery of audio and video packets for example.

If packets are sent every 30 ms (if some packets arrive with 30 ms delay and others with 40 ms delay, an uneven quality of play-back is experienced. 

Assuming we are sending three packets (10 seconds each), the 1st packet arrives with 1s delay, the 2nd arrives with 5s delay and the 3rd arrives with 7s delay

Data can be represented as analogue or digital signals.

Whether analogue or digital, rich data occupies a range of frequencies = bandwidth.

Human speech, musical instruments are all examples of rich signals.

An analogue signal is defined as a physical time varying quantity and is usually smooth and continuous - acoustic pressure variation when speaking

Analogue signals are continuous in both time and amplitude and can be classed as simple or composite.

An example of a simple anologue signal is a sine wave which is the most fundamental form of a periodic analogue signal and can be visualised as a simple oscillating curve, its change over the course of a cycle is smooth and consistent with a continuous rolling flow

A composite signal is a signal where there is more than 1 frequency component

For example, speech annot be made up of just one sine wave but a collection of sine waves

A sine wave can be further described using the following terms:

• Peak amplitude
• Frequency
• Phase

The peak amplitude represents the absolute value of its highest intensity (volume). For electric signals, the peak amplitude is normally measured in volts.

A digital signal is made up of discrete symbols selected from a finite set, e.g. letters from the alphabet or binary data

Digital signals are continuous in time but discrete in time but discrete in amplitude

• And this is normally a representation of a "sampled" anologue signal

In digital systems the size of the discrete alaphabet is related to the number of binary digits available in hardware or is related to the properties of the channel when selecting a particular digital transmission system

Alphabet size is equal to 2ⁿ where n is the number of bits

A digital system operating with 3 bits has an alphabet of 2³ = 8

A digital signal with all its sudden changes is actually a composite signal having a large number of frequencies

In other words, a digital signal has a very large bandwidth much larger than an analogue signal

A medium or media enables the transmission of such data in a link: wired or wireless

Different media is characterised by different bandwidths

Twisted pair, coaxial and optical fibre cables occupy different bandwidths

Wireless channels are also grouped into different frequency bands

The larger the bandwidth = the more of a signa; or data you can send

In data communication the term bandwidth is used in 2 ways:

• Bandwidth in Hertz refers to a range of frequencies in a composite / complex signal or the range of frequencies that a signal occupies

Bandwidth relates to analogue signals

Remember:

• Analogue signals are "continuous" in time and level
• For example, if a composite signal contains frequencies between 0Hz and 4000Hz, its bandwidth is 4000Hz as shown below

Equally important to note that the signal below on the right occupies a bandwidth of 4000Hz even though it contains frequencies between 1000Hz and 5000Hz

The bandwidth of a speech is from 50Hz through to 10kHz and that of a music signal is from 15Hz to 20kHz

Remember:

• Digital signals are represented as "continuous" in time but "discrete" in level
• For example, the "sampled" bit stream below, a 1 can be encoded as a positive voltage and a 0 as zero voltage
• But you could use bipolar representation if you wanted
  • +1 and -1

A digital signal has 2 parameters which are used for calculatinf the equivalent of period and frequency they are:

• Bit interval (instead of period)
• Bit rate (instead of frequency)

Bit interval is the time required to send one single bit

Bit rate is the number of these intervals per second

The bit rate is therefore the number of bits send in 1 second and expressed in bps

Bandwidth in bits per second refers to the speed of rate of bit transmission that a medium, a link or a section of a communication system is sending or transmitting the data

Example:

• A digital signal has a bit rate of 10bps, what is the duration of each bit - 0.1 seconds