Baud Rate
Baud rate is the number of line signals changed per second. It is important
to note that bit rate and baud rate may or may not be the same depending upon the
modulation techniques used. In the above example, if 100 bits were transmitted
per second, the number of times that the carrier would shift its frequency will also
be 100. Thus, in this case:
Bit rate = 100 bps and
Baud rate = 100
It is possible to employ creative modulation techniques to increase the bit rate
without necessitating increased bandwidth. For example, Bell 212 modems use a group of
two bits (called Dibits) to cause change in the carrier phase as shown in the figure below.
Dibits
Degrees Shift
00
01
10
11
90 degrees
0 degrees
180 degrees
270 degrees
Figure 1-4
This method is called Phase Shift Keying (PSK) or phase modulation. In this case
for every 100 bits transmitted, the line will experience 50 phase changes, therefore the
baud rate is 50.
Multi-level Modulation
When more than one bit causes a change in the line signal, it is called
multi-level modulation. The level of modulation is 2n where n is equal to the
number of bits causing a single line signal change. In the example above, Bell 212
uses a four level modulation. In general
baud rate = bit rate / n.
Equation 1-1
Example 1.1
A modem uses 3 bits per signal change and is transmitting data at a rate of
2400 bps. Determine:
A) it’s baud rate
B) the modulation level
Solution:
A) Baud rate = bit rate / n
= 2400 / 3
= 800 baud
B) Modulation level = 2n
= 23
=8
This system is using an eight level modulation
Example 1-2
A modem which is using a 16 level modulation is transmitting at 9600
baud.
A) How many bits per signal change is it using?
B) Calculate it’s bit rate.
Solution:
A) Modulation level = 2n
16 = 2n
n=4
B) Baud rate = bit rate / n
Therefore, bit rate
= baud rate X n
= 9600 X 4
= 38400 bps
Simplex, Half Duplex and Full Duplex Transmission
A) Simplex – Transmission in one direction only. Example thermostat to furnace.
Thermostat
Figure 1-5
Furnace
B) Half Duplex (HDX) – Communication in both directions, but only one at a
time. Example Intercom system.
Transmit
Receive
Receive
Transmit
Figure 1-6
C) Full Duplex (FDX) – Communications in both directions simultaneously either
on separate pairs of wires or using some kind of modulation. Example, Bell 103
uses FSK to communicate in a FDX mode. See diagram on following page.
Transmit
Receive
Receive
Transmit
Figure 1-7
Bit Error Rate (BER)
Bit error rate is defined as the ratio of erroneous bits received to the
number of bits sent.
Bit error rate = Be / B
Equation 1-2
A typical voice grade channel may generate error rates of 10-5.
Block Error Rate (BLER)
Block error rate is the ratio of the number of blocks of data with at least
one error to the total number of blocks transmitted.
BLER = Number of blocks with errors
Total number of blocks sent
Equation 1-3
Asynchronous Systems
Data is transmitted as soon as it is available. Example – a keyboard. When a key
is pressed a character is generated and transmitted immediately. The character is enclosed
in a frame with one “low” start bit and 1, 1 ½, or 2 “hi” end bits. The receiver looks for
the low start bit for the beginning of data. Asynchronous transmission is widely used for
low data rates because it is inexpensive and easy to implement.
Synchronous Transmission
Synchronous transmission refers to sending data at a fixed periodic rate with a
synchronization character at the beginning of the message to synchronize the receiver
with the transmitter. When no data is being sent a sync character is continuously sent to
keep the receiver synchronized with the transmitter. Synchronous systems are complex
and are used for higher data rates.
Baseband Signals
An unmodulated signal is usually called a baseband signal. A baseband signal is
used to modulate a carrier. In other words, the information bearing signal in a
communications system is called a baseband signal. Keep the following distinctions in
mind when dealing with baseband and broadband signals:
A)
B)
C)
D)
Uses digital signaling
Limited Bandwidth
Usually unmodulated
Usually uses time division multiplexing for channel sharing
Broadband Systems
A)
B)
C)
D)
Uses analog waveforms
Has a large bandwidth (MHz to GHz)
Uses analog modulation
Uses frequency division multiplexing for channel sharing
Channel Capacity and Bandwidth
Shanon’s Law – Shanon’s law relates channel capacity and bandwidth
requirements in a noisy channel.
C = BW log2 (1+S/N)
C = Channel capacity in bps
BW = Bandwidth in hertz
S/N = signal to noise ratio
Also note that Log2N = (Log 10 N) / Log10N
= Log10N / 0.3
Equation 1-4
Example 1-3
A signal is to be transmitted at 19.2 Kbps through a noisy channel whose
signal to noise power ratio is 30 dB.
A) Calculate the bandwidth required.
B) Can this signal be transmitted using a standard telephone channel?
Solution:
A) C = BW Log2(1+S/N)
C = 19.2 Kbps
S/N = 30 dB
BW = ?
First convert S/N ratio into a linear ratio. Power ratio in dB is given by:
dB = 10Log(S/N)
30 = 10log(S/N)
S/N = Antilog 3
S/N = 1000
C = BW log2(1 + 1000)
19.2 X 1000 = BW log21001
19.2 X 1000 = BW ((Log101001) / (0.3))
BW = 19.2 X 1000 X 0.3
Log101001
BW = 19.2 X 1000 X 0.3
3
BW = 19.2 X 100
BW = 1.92 KHz
B) Yes, since the BW requires is 1.92 KHz and the telephone bandwidth is
3 KHz.
Example 1-4
Calculate the capacity of a 64 KHz channel whose voltage signal to noise
ratio is 25 dB.
Solution:
C = BW ((log10(1+S/N)) / 0.3)
S/N = dB = 20 Log signal voltage
noise voltage
25 = 20 LogS/N
LogS/N = 1.25
S/N = 17.782
C = 64 X 1000 (1+17.782)
0.3
C = 64 X 1000 X 18.782
3
C = 400.68 Kbps
RS – 232 Connector (Refer to attached handouts)
EIA (Electronics Industries Association) standard for data
communications designed for rates up to 19.2 Kbps. RS stands for Recommended
Standards.
TTL
RS-232
State
Logic 0
0VDC
+5 to +15 VDC
SPACE
Logic 1
+5VDC
-5 to –15 VDC
MARK
Secondary channels are used for error detection and correction.
The reader of this material is strongly suggested to review the material covered in
the Telecom course corresponding to the topics presented in this module before
attempting end of module assignment and test.