Transmission of Digital Data:

1
Chapter 6
Transmission of Digital Data Interfaces and Modem
2
Figure 6-1
Data Transmission
3
Figure 6-2
Parallel Transmission
Advantage Speed Disadvantage Cost (Short
distance)
4
Figure 6-3
Serial Transmission
5
Figure 6-4
Asynchronous Transmission

• Information is received and translated by
  agreed-upon patterns.
• Asynchronuos here means asynchronous at the
  byte level.
• - The bits are are still synchronized Their
  durations are the same.

6
Figure 6-5
Synchronous Transmission

• Timing is very important.
• Advantage Speed (No extra bits and gaps)
• Byte synchronization is accomplished in the data
  link layer.

7
Figure 6-6
DTEs and DCEs

• A DTE is any device that is a source of or
  destination for binary digital data.
• A DCE is any device that transmits or receives
  data in the form of an analog or digital signal
  through a network.

• DTE Data terminal equipment
• DCE Data circuit-terminating equipment

8
Figure 6-7
DTE-DCE Interface

• EIA standards EIA-232, EIA-442, EIA-449
• ITU-T standards V X series

9
EIA-232 (RS-232)

• Defines the mechanical, electrical, and
  functional characteristics of the interface
  between a DTE and a DCE.
• Mechanical specification
• A male and a female DB-25 pin connector
• Not exceed 15 meters
• DB-9 pin connector
• Electrical specification
• Sending the data
• The definition of bit rate
• A maximum bit rate of 20Kbps (in practice open
  exceed).
• Control and timing
• Functional Specification

10
Figure 6-8
Electrical Specification for Sending Data in
EIA-232
Degraded by noise pulse precision is not
important.
11
Figure 6-9
Electrical Specification for Control Signals in
EIA-232

• 4 wires out of 25 available in an EIA-232
  interface.

12
Figure 6-10
Functions of Pins in EIA-232, DB-25
13
Figure 6-11
Functions of Pins in EIA-232, DB-9
14
Figure 6-12
Synchronous Full-Duplex Transmission
15
Figure 6-13
Using Regular Data Pin Connections with and
without DCEs
Null Modem
16
Figure 6-14
Null Modem Pin Connections
DTE/DCE ready
Request/clear to send
Received line signal detector
Receiver/transmitter signal element timing
17
Figure 6-15
EIA 449 DBs

• DB-37 pin connector
• - Category I similar to EIA-232
• - Category II new functions

• EIA-449
• - RS-423 unbalanced circuits
• - RS-422 balanced circuits

Cf.) Tables 6.1 and 6.2 DB-37 and 9 pins
18
Figure 6-16
RS-423Unbalanced Mode

• Only one line for propagating a signal
• - Use of only the first pin of pair of Category
  I pins.
• - Use of all Category II pins.

19
Figure 6-17
RS-422Balanced Mode

• Two lines for propagating each signal
• - Use of all pair of pins in Category I.
• - Not use the Category II pins.

20
Figure 6-18
Canceling of Noise in the Balanced Mode
21
EIA-530

• A version of EIA-449 that uses DB-25 pins (Legacy
  15 pins).

22
Figure 6-19
X.21

• Using data circuits for control
• Eliminate most of control circuits of the EIA
  standards.
• Need additional logic to discriminate control
  information and data upon receipt.
• Byte timing pulse
• Improve overall synchronization of transmission.
• Control and initiation
• Initial handshake

DB-15 Connector
23
Figure 6-20
Modem Concept

• Every line has an upper and a lower limits on
  the frequencies of the signals it can carry.
• The limited range is called the bandwidth.

D/A conversion (ASK, FSK, PSK, QAM)
A/D conversion
24
Figure 6-21
Telephone Line Bandwidth
25
Figure 6-22
Baud Rate in Half-Duplex ASK
26
Figure 6-23
Baud Rate in Full-Duplex ASK

• Although ASK has a good bit rate, it is not used
  today because of noise.

27
Figure 6-24
Baud Rate in Half-Duplex FSK
N baud rate
28
Figure 6-25
Baud Rate in Full-Duplex FSK
??) Table 6.4 maximum bit rate over standard
twisted-pair telephone lines.
29
Figure 6-26
Bell Modems
Direction

• Bell Telephone Company

30
Figure 6-27
ITU-T Modem Standards
??) Table 6.5 ITU/Bell compatibility
31
Figure 6-28
V.22bis 16-QAM Constellation
32
Figure 6-29
V.32 Constellation
33
Figure 6-30
V.33 Constellation
34
Figure 6-31
Traditional Modems
Maximun data rate 33.6 Kbps
35
Figure 6-32
56K Modem

• Maximun data rate
• Uploading 33.6 Kbps
• Downloading 56 Kbps

56 Kbps 7 (bits) x 8K samples/sec
36
Figure 6-33
Cable Modem
Coaxial cable up to 750 Mhz - Frequency
division multiplexing 6Mhz Downloading - 36
Mbps (64-QAM 6bits, 6Hhz) Uploading - 12 Mbps
(QPSK 2bits, 6Hhz)
37
Summary

• Digital data transmission
• Parallel/serial
• DTE-DCE interface
• EIA-232/449/530 interfaces
• X.21
• Modems
• Traditional 56k Modems
• Cable modem