30 Communication Networks
Copyright © 2005 PragSoft
Figure 2.21 Time slots in concentration.
001 001
010 10101
11111 01110
001 11011
Channel data Channel address
Concentration is a popular method for connecting a set of character-based terminals to a central computer. Line capacity requirements are greatly reduced due
to the fact that terminals tend to be idle for most of their operating period.
2.4. Physical Layer Standards
The most commonly-used physical layer standards are those published by ISO, CCITT, IEEE, and EIA, many of which are inter-related. A large number of the
existing standards deal with transmission over telephone lines. The CCITT V series of standards fall into this category and are by far the most-widely adopted.
Below we will look at two very popular standards for connecting DTEs and DCEs: the analog standard RS-232 and the digital standard X.21.
2.4.1. RS-232
RS-232 has dominated the computer industry as the most-widely used standard for physically connecting devices. It is an analog standard, defining the physical layer
interface between a DTE and a DCE, and can support simplex, half-duplex, and full- duplex connections in synchronous as well as asynchronous mode. It originated in the
late 1950s, and has been revised a number of times over the years. The latest revision, EIA-232-D, is based on CCITT’s V.24 and V.28 standards and ISO’s
2110 standard.
ISO 2110 defines the mechanical appearance of the RS-232 connectors see Figure 2.22. The connector provides 25 pins for connecting the circuits derived
from the V.24 standard, as summarized in Figure 2.23. The circuits are used for data transfer, conveying of control signals, and conveying of clocking signals for
synchronization.
V.28 defines the electrical characteristics of RS-232. V.28 uses 5 to 15 Volts to represent binary value 0, and -5 to -15 Volts to represent binary value 1. It allows
for connection distances of up to 20 meters and data rates of up to 20 kbps.
www.pragsoft.com Chapter 2: The Physical Layer
31
Figure 2.22 RS-232 connector based on ISO 2110.
1 2
3 4
6 7
8 9
10 11
12 13
5 14
16 17
18 19
20 21
22 23
15 24
25
Figure 2.23 V.24 circuits. Pin
Circuit Direction
Description
1 AA
-- Protective ground shield common return
2 BA
DTE to DCE Transmitted Data
3 BB
DCE to DTE Received Data
4 CA
DTE to DCE Request to Send
5 CB
DCE to DTE Clear to Send
6 CC
DCE to DTE DCE Ready
7 AB
-- Signal Ground
8 CF
DCE to DTE Carrier Detect
9 --
-- Reserved for testing
10 --
-- Reserved for testing
11 --
-- Unassigned
12 SCF
DCE to DTE Secondary Carrier Detect
13 SCB
DCE to DTE Secondary Clear to Send
14 SBA
DTE to DCE Secondary Transmission Data
15 DB
DCE to DTE Transmitter Signal Element Timing transmitter clock
16 SBB
DCE to DTE Secondary Received Data
17 D
DCE to DTE Receiver Signal Element Timing receiver clock
18 LL
-- Local Loopback
19 SCA
DTE to DCE Secondary Request to Send
20 CD
DTE to DCE Data Terminal Ready
21 RLCG
DCE to DTE Signal Quality Detector Remote Loopback
22 CE
DTE to DCE Ring Indicator
23 CH
DTE to DCE Data Signal Rate Selector
23 CI
DCE to DTE Data Signal Rate Selector
24 DA
DTE to DCE Transmitter Signal Element Timing transmitter clock
25 TM
-- Test Mode
In most applications, only a few of the circuits specified by RS-232 are actually used. Figure 2.24 serves as an example. It illustrates how a PC may be connected to
a modem.
32 Communication Networks
Copyright © 2005 PragSoft
Figure 2.24 Typical half-duplex connection using RS-232.
PC Modem
2 3
4 5
6 7
8 20
22
2 3
4 5
6 7
8 20
22
Transmitted Data Received Data
Request to Send Clear to Send
DCE Ready Signal Ground
Carrier Detect Data Terminal Ready
Ring Indicator
Transmitted Data Received Data
Request to Send Clear to Send
DCE Ready Signal Ground
Carrier Detect Data Terminal Ready
Ring Indicator
RS-232
RS-232 has two important limitations which reduce its usefulness: it is not suitable for distances of more than about 50 meters, and it has a maximum
bandwidth of 20 kbps. Other similar standards have been devised to overcome these limitations. For example, RS-449 and EIA-530 can both support data rates of up to
2 mbps over longer distances.
2.4.2. CCITT X.21
