Telecommunication Transmission & Switching EN454
Laboratory Assignment 1
Group Members: 060371N
P.L.Porambage
060387T
R.A.S.D. Rabel
060411N
R.A.N.P Rupasinghe
060412T
M.A.D. Ruwanthilaka Ruwanthilaka
060424G
H.H.S.R. Samarasiri
060426N
B.S.S. Sameera
Date of Submission: 21-06-2010
Assignment 1 Practical 1
1) If a call is made from telephone 1 to telephone 2, what happens if telephone 2 is replaced (Switch Hook pressed), and then picked up again? Conversion is started from Telephone 1. So as long as telephone 1 is not replaced the path would be reserved. Therefore even telephone 2 is replaced and then picked up conversation can resume without any interruption. 2) With the same call what happens if the telephone 1 is replaced? Then the call will end .The reserved path would be released. Hence conversation would be terminated. Only way to start the conversation again is redialing. 3) What tones or messages do you expect for Dial Tone, Ring Tone, Busy Tone and Number Unobtainable, when using your normal telephone? Dial Tone: Continuous tone Ring Tone: On time of the tone is less than the off time (e.g. 1s on time and 3s off time tone.) Busy Tone: On time and off time is quite equal (e.g. 0.4 s on time and 0.4 s off time) Number Unobtainable Tone: Continuous tone but different frequency to Dial tone
Practical 2
1) If you have access to an internal telephone system, are the Tones used in that system different from those in the main public system? Frequencies and on and off times may be vary. But normal regulations will be the same. 2) What is the value of having similar or even identical Tones for the main public system in different countries? So if a ring tone in one country is similar to a busy tone in another country there will be a misunderstanding when a person from one country travels to the other. In order to eliminate such situations it is really advisable to use the same tones in all the countries.
3) Why does CCITT not have a standard for Ringing? Some countries are still dealing with analog systems. Since analog systems cannot provide up as much as digital systems r inging cannot be Standardized by ITUT. 4) When would it be useful to have different Ringing cadences? When we have two or more phones on a table (one from direct line and one from PABX line) it is useful to have different tones for two cases in order to inform the user which one is direct line and which is the PABX line. This also applies in the case of different operators operating in one country.
Assignment 2 Practical 1
1) What is the essential process in digital switching? Time slot mapping. 2) How many data transfers between timeslots are required for one telephone connection? Two data transfers are needed between time slots for one telephone connection. 3) How often are the data transfers made? Data transfers are made in every frame, once in every 125µ s 4) How long is each data sample available on the serial bus? It wills available about 3.9 µs. (8 bits of data) 5) How many connections can be made between telephones on one 30 channel bus? 15 connections can be made on 30 channel bus. ( 15 Inlet busses and 15 Outlet busses ) Practical 2 (Time and Space Switching)
1) Why is space switching sometimes required as well as time switching? Space switching is required to establish more connections using the same time slots 2) How many telephones can be connected through a digital switch with 8 Inlet and Outlet 30 channel serial busses? This is the format of the switch used on the Work board? 120 (15x8 = 120) telephones can be connected.
3) Consider how several similar switches could be connected to provide for 16 inlets and outlets?
8 inlets
8 Inlets and
8 Inlets and
Outlets switch
Outlets switch
8 Inlets and
8 Inlets and
Outlets switch
Outlets switch
8 outlets
8 inlets 8 outlets
Assignment 3 Practical 1
1) How many locations in the Connection Memory need to be written for one speech connection? One location in the Connection Memory need to be written for one speech connection. 2) How long does each sample of data remain in the data memory? 1.25 μs 3) What is the sequence of actions to find the correct data for the Outlet during a particular timeslot? Every location in the Data Memory keeps data of a particular time slot, which the position in the data memory indicates to which time slot it is connected. Every location in the Connection Memory is used for the same time slot as the corresponding location in the Data Memory. They contain the address of the Data Memory location which should be read and data is sent to the Outlet for each time slot. Therefore correct data for the Outlet is found for each time slot. 4) What entries are made in which locations in the Connection Memory for a connection between lines L2 and L4? 2nd location of Connection Memory contains the address of 4 th location in Data Memory. th
4 location of Connection Memory contains the address of 2 Memory.
nd
location in Data
Practical 2 (Connection of Tones)
1) What address is entered into which location of the Connection Memory to apply Dial tone to Line L3? rd
st
3 location of the Connection Memory contains the address of the 1 location in Data Memory
2) Why? st
Because 1 location of the data Memory corresponds to the Dial Tone. So the address of that location is put to the 3
rd
rd
location on Connection memory. 3 time slot
which is connected to line L3.
3) Is there any limit to the number of lines which can receive the same tone from one source at the same time? No. Each tone can be connected to as many telephones.
Assignment 04: Practical 1
1) Can you detect a difference in speed of response to the switch hook when the telephone is starting a call, or answering in the silent period (call detect), or when it is ringing (answer detect)? There is a difference in speed of response. 2) 2. Why is there a difference? SLIC (Subscriber’s Line Interface Circuit) normally uses a call detect circuit to test the switch hook when the telephone is under the normal condition. But when the telephone is receiving a ringing signal then it uses an Answer detect circuit. SLIC is very fast but Answer detected circuit takes about 200ms to respond. Therefore a considerable time delay is sensible when giving the response to the Switch hook status.
3) Consider a call which has been established from line L3 to Line L4. What action is expected by the control if
Bit 3 changes from 1 to 0 – Connection 3 is changed from Off – hook to On-Hook state.
Bit 2 changes from 1 to 0 – Connection 2 is changed from Off – hook to On-Hook state.
Practical 2.
1) Under what condition should the microprocessor control use the DTMF code for any particular line? If a valid DTMF is detected the DTMF receiver circuit sets an output pin positive for at least 40ms 2) Why does the line scan occur every 20 ms?
To ensure the reliable response to the DTMF receiver
To ensure no valid DTMF detection inputs are missed.
3) How does the control determine whether a keypad button has been pressed more than once? DTMF detection signal is routed 50 times in a second by the microprocessor in the switching stage. To signal a valid key detection it takes nearly 40mS. So after that standard time slot, another key pressing can be detected. Even the same key being pressed more than once can be detected in the same way.
4)
What advantage is there in keeping the DTMF receivers connected continuously?
Key pressings can be detected even after a call is connected.
A pressed key can be identified even while a call is going on.
Assignment 5 Practical 1
1) Establish a call step by step between two telephones. Record the sequence of States used, including the name and code for each State. The sequences of states:
In this case the call has been established successfully to remote telephone. Therefore Busy (S5), NU (S6) won’t be encountered.
• Idle (S0)
IDLE (S0)
• Call Request (S1) • Dialling (S2)
Call Request (S1)
• Find Line (S3) • Ringing (S4)
Dialling (S2)
• Busy (S5) • Connect (S7)
Find Line (S3)
Ringing (S4)
Busy (S5)
Number Unobtainable
Connect (S7)
If other phone is engaged states S3 will be routed to Busy (S5) state and finally go to idle state. If dialled number is not in use S3 go to NU (S6)
2) What circumstances cause the system to reach states S5 or S6? If the dialled number is engaged system will reach S5 and if dialled number is wrong one state S6 would have been achieved.
3) This small system can only provide 2 connections at one time. Why are Call records provided? This small system used for the practical is having only 4 phones. Two telephones connect for a single connection; hence only 2 connections can be achievable at a given time. But it is possible for all 4 telephones to be off hock or idle. So there has to be one call record for each and every phone. Each call record tells us about the current state of particular telephone. Hence for 4 telephones, 4 call records were provided. 4) Does a large public telephone network need a Call record for every telephone connected? No. In large public telephone network most telephones are not in connection at a particular time. Therefore call record for each and every telephone is not needed. 5) What are the advantages of the Call state Diagram?
Get to know about the progress of a particular call.
Can predict the next state of a particular call.
It serves an introduction to the Specification and Description Language (SDL)
Practical 2
1) When is a new call Record started? When the telephone goes off hook (lifted). 2) When is a call Record cleared? When connection is established between two telephones 3) If a Call is made from Line L1 to Line L2 at what stage of the call is the required Line Identity entered into the call Record? Ringing (S4) 4) Consider a call at state S4 with CLI=L2 and RLI=L3 a. What is happening? Call is made from Line 2 to Line 3 b. For each case what switching is required? Switching between L3 and L2
5) What is the essential information required in the Call Record for the control of a call. Call state S and the Line Identities CLI and if appropriate the dialled number or RLI/ Telecommunication Transmission & Switching
Discussion:
Early days in telecommunication two parties were connected via a pair of copper wires. When number of customers increase it was realized that this method was inefficient. As a result telephone operators were introduced .There when a customer wanted to speak to another person he had to ask from operator to connect to that person. Modern day switches provide several functionalities; B
- Battery
O
- Over voltage protection
R
- Ringing
C
- Coder / Decoder
S
- Supervision
H
- Hybrid
T
- Test
The switch supplies line voltage (-48 V :off-hook) to the subscriber premises. Therefore even under domestic power failures, it is possible to make calls through the line. The ringing current is also supplied by the exchange. Exchanges are digital equipments, which handles the signals in digital forms. But in copper wires, voice travels in analog form, from customer premises to exchange. Therefore these signals have to be digitized and this task is performed by the exchange. Manual switching concept was not a good idea when demand increases. Then, A. B. Strowger in UK developed a method, which was having relays to connect two lines electrically. After that Cross-Bar switch was introduced in Sweden (1965) and that was controlled by a mechanical controller. Switch Mechanical
Controller (Mechanical) - Cross Bar Switch -
The first electronic switch was come in 1968, which was invented by Bell Laboratory (AT&T). Generally, a digital switching system serves several time division multiplexed (PCM) samples. These PCM samples are conveyed on PCM (Pulse Code Modulation) highways. Switching of calls, in this environment, requires placing digital samples from one time-slot of a PCM in the same or different t ime-slot of another PCM multiplex. The interconnection of time slots (i.e. switching of digital signals) can be achieved using two different modes of operation. These modes are,
Space Switching
Time Switching
Usually, a combination of both the modes is used in exchanges. TSi HWY1
TSx
Space Switch TSi
TSy Time Switch
HWY1
HWY1
HWY2 - Time & Space Switching -
In Space Switching mode, corresponding time slots of I/C (Incoming) and O/G (Outgoing) PCM
Highways are interconnected. In a given time-slot, TSi of I/C HWY (HWY1) is
switched to same time-slot, TSi of O/G HWY (HWY2). Obviously, there is no delay in switching of the sample from one highway to another highway, since the sample transfer tales place in the same time-slot of the PCM frame (32 time-slots for 1 PCM frame).
Time Switching, on the other hand, involves the interconnection of different time-slots on the incoming and outgoing highways by re-assigning the channel sequence. As shown in the figure, a time-slot TSx of an I/C highway can be connected to a different time-slot, TSy, of the O/G highway. In other words, a time switch is, basically, a time-slot changer.
