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Figure 2.26 X.24 circuits. Circuit
Direction Description
G --
Protective ground shield Common Return Ga
DTE to DCE DTE Common Return
Gb DCE to DTE
DCE Common Return T
DTE to DCE Transmit
R DCE to DTE
Receive C
DTE to DCE Control
I DCE to DTE
Indication S
DCE to DTE Signal Element Timing
B DCE to DTE
Byte Timing F
DCE to DTE Frame Start Identification
X DTE to DCE
DTE Signal Element Timing
Figure 2.27 Typical full-duplex connection using X.21.
DTE DCE
Byte Timing Ground
DTE Common Return
Signal Element Timing Indication
Receive Control
Transmit
X.21
X.21 bis is a variation of the X.21 standard with similarities to RS-232: it uses the V.24 circuits and is usually used with the 25-pin connector of ISO 2110.
2.5. Further Reading
Black 1988, Blahut 1990, Bic et al 1991, and Gitlin et al 1992 provide detailed descriptions of physical layer topics such as interfaces, coding, modulation,
transmission, synchronization, error-handling, and standards. McClimans 1992 describes different transmission media and their properties. Stone 1982 describes
detailed examples of mainly RS-232 physical layer interfaces for microcomputers.
2.6. Summary
•
Network equipment are classified into DTE user equipment, DCE connect DTE to network, and DSE perform switching between DCEs.
•
A connection may be of type simplex, half-duplex, or full-duplex.
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•
A signal may be analog continuous variation of some property or digital sequence of binary values 0 and 1.
•
Digital data is transmitted over analog lines using modulation and converted back to digital format using demodulation. These two functions are performed
by a modem. Modulation method are classified into AM, FM, and PM.
•
Converting an analog signal into digital is called digitization and is performed by a codec. PCM is a popular digitization method for voice signals.
•
Transmission methods are classified into synchronous clock-based and asynchronous
.
•
Popular transmission media include: copper wire, coaxial cable, optical fiber, radio
, and infra-red.
•
Multiplexing methods are divided into SDM multiple wires in a common enclosure, FDM subdivision of the frequency bandwidth into logical channels,
and TDM allocation of time slots to each logical channel.
•
Concentration
is a variation of TDM where time slots are allocated on demand.
•
RS-232 is a popular analog standard for the physical interface between a DTE
and a DCE.
•
X.21
is a popular digital standard for the physical interface between a DTE and a DCE.
2.7. Exercises
2.9 Describe the role and functions of DTEs, DCEs, DSEs, and name an example of
each device type.
2.10 State the differences between an analog signal and a digital signal. Provide an
example of either signal type.
2.11 Describe the differences between modulation and digitization. Name the devices that
perform these functions.
2.12 Using a sample bit stream and a diagram, illustrate the difference between
synchronous and asynchronous transmission.
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35
2.13 Consider the problem of providing a 2 mbps physical connection between two LAN
sites which are 10 kms apart and are located in the same city. Discuss the merits of using different types of transmission media for this purpose.
2.14 What is the purpose of multiplexing? Compare the strengths and weaknesses of
FDM and TDM.
2.15 Describe how a 100 MHz line with a data rate of 200 mbps can be divided into 20
channels using FDM and TDM.
2.16 Describe the differences between RS-232 and X.21 standards. Provide an
application example of either standard.
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3. The Data Link Layer
This chapter looks at the data link layer of the OSI model. The data link layer transforms the logical communication channel provided by the physical layer into a
reliable channel by splitting the data into frames which are subjected to error control and flow control procedures.
We will first look at various link protocol types, and then describe the constituent functions of link protocols, such as acknowledgment of frames, error checking, and
flow control. These functions are embodied by a general technique called the sliding window protocol, which will be described next. Finally, we will discuss two popular
data link standards: BSC and HDLC.
After completing this chapter you should be able to:
•
Distinguish between different data link protocol types and know the characteristics of each type.
•
Have a general understanding of the various data link protocol functions.
•
Explain how the CRC error checking method works and how a CRC code is calculated.
•
Understand the sliding window protocol and explain how it can be used for flow control.
•
Describe the BSC character-oriented data link protocol, including its block format and functions.
•
Describe the HDLC bit-oriented protocol, including its modes, frame format, different frame types, and subsets.
