Simplest communication system consists of:
Transmitter that sends a signal
along a channel to a receiver.
Local Area Network (LAN)
- Computer network covering a relatively small area “company site”
- consists of: computers, printers, routers, switches
- interconnect the various parts of the network
- ensure that data is sent to the right place
public switched telephone network (PSTN)
connected to a local exchange can be connected to other subscribers on the same exchange.
The access network (the local exchange)
its connections to the subscribers it serves
- may serve millions of subscribers,
- each on a different site, but with only short distances involved
The Core Network
which is everything beyond the local exchange
- Trunk lines in the core network carry multiple calls
between two places that may be hundreds of miles apart.
Optical fiber is more effective over
long distances now used in:
Core Network
Copper still retains a key place in the :
Access Network
Whenever you send a signal along a communications channel,
two things happen to it:
- it gets smaller (it attenuates)
- gets distorted (the shape changes).
It is possible to compensate for attenuation by
Amplifying the received signal.
With digital signals, you can in principle **get rid **of the distortion entirely by the process of:
Regeneration التجديد (threshold detection).
Another reason for using digital technologies
in communications is?
Voice, music and video, can all be handled by the same techniques as computer data
if they are first converted to a digital form
To convert an Analogue signal to Digital form?
- first sampled by measuring its value at regular intervals in time.
يتم أخذ عينا ت من الإشارة عن طريق قياس قيمتها على فترات زمنية منتظمة . - Encode each of the possible quantization levels with a binary number. ترميز كل مستوى من مستويات الكم الممكنة باستخدام رقم ثنائي .
Electromagnetic radiation
1. includes radio waves
2. light
3. the radiation felt as heat
4. ultraviolet radiation from the Sun*
Light:
wave pattern carried by
**interdependent electric **and magnetic fields.
Signal:
electrical voltage:
Electromagnetic Wave:
the form in which a message is sent along a communications channel.
→ Copper cables
→ Optical Fibre and Radio.
the benefits of transmitting signals in digital form:
- Signal attenuates (gets smaller) and distorts (shape changes).
- Amplification the received signal compensates for attenuation.
- Regeneration removes distortion (if not too large).
- Voice, music, and video can be sent like computer data after converted to a digital form.
what keeps light guided along its path in an optical fibre? Why does the light not just stop when it comes to the first bend?
- The refractive index is higher in the core
than in the cladding, so light stays guided. - When light hits the boundary at a small angle,
it is reflected back, not refracted. - This process, called Total internal reflection,
keeps light travelling along the fibre even at bends.
What is Beam steering
- (beamforming) technique uses multiple transmitter antennas.
- Used for communicating with a single receiver التواصل مع جهاز استقبال واحد
- Purpose: improve reception تحسين الاستقبال
* Adjusting amplitudes and phases makes signals add constructively at the receiver تتجمع بشكل بنّاء عند جهاز الاستقبال - Increases received signal strength and resistance to fading زيادة قوة الإشارة ومقاومة التلاش
the unshielded twisted pair (UTP) and the coaxial cable, Briefly describe their construction
Unshielded twisted pair (UTP) cable: a pair of conductors that is twisted together along its length
*
Coaxial cable: the two conductors take the form of a centre conductor with a conducting shield around it
From your study of electric and magnetic fields, write down four short sentences about these fields and how they relate to electrical signals in conductors.
- An electromagnetic wave consists of electric and magnetic fields at right angles to each other.
- They are linked by mathematical relationships (Maxwell’s equations).
- Electric and magnetic fields can store and release energy.
- Conductors carrying electrical signals are surrounded by electric and magnetic fields.
A basic optical-fibre link has three main components.
Mention these components
- Suitable source of light controlled by input data in the form of an electrical signal
- The optical fibre itself, “carries the resulting pulses of light”
- Detector “converts the pattern of light and dark back to an electrical signal”
The difference in construction between step-index fibre and graded-index fibre.
- Step-index fibre: the refractive index** changes abruptly** between core and cladding.
- Graded-index fibre: the refractive index varies smoothly from a ** maximum in the centre of the core to a minimum within the cladding.**
Explain the multimode distortion
Two rays of light that set off at the same time may not reach the other end of the fibre simultaneously.
with attenuation: the effects are cumulative the longer the fibre, the worse it gets.
The signal transmitted is called a pulse, the effect is “pulse spreading”
One reason for pulse spreading in multimode fibres:
-
different path lengths result in different timings for the trip through the fibre.
Explain the advantage of a graded-index fibre over the corresponding step-index with respect to multimode distortion
In Graded-Index fibres, waves that take slightly longer paths travel slightly faster, → different waves setting off at the same time arrive nearly simultaneously at the other end of the fibre.
