Data Communication and Networking332 Hardware Components of Data Communications

Lecture Overview

• DTE• DCE• Modem Standards

Data Terminal Equipment• DTE can be virtually any binary digital

device that generates, transmits, receivers, or interprets data messages.

• In essence, a DTE is where information originates or terminates.

• DTEs contain the hardware and software necessary to establish and control communications between endpoints in a data communications system.

• DTE includes the concept of terminals, clients, hosts and servers.

• E.g. video display terminals, printers and personal computer

Data Circuit-Terminating Equipment• Also known as Data Communication Equipment

and Data Carrier Equipment• DCE is a general term used to describe

equipment that interfaces data terminal equipment to transmission channel, such as an analog telephone circuit.

• The output of a DTE can be digital or analog, depending on the application.

• In essence, a DCE is a signal conversion device, as it converts signals from DTE to a form more suitable to be transported over a transmission channel.

• A DCE also converts those signals back to their original form at the receive end of a circuit.

Data Circuit-Terminating Equipment• Several types of DCEs depending on the

type of transmission channel used:– Channel Service Unit (CSU) and Digital

Service Unit (DSU)• Used to interface DTEs to digital transmission

channels.– Data Modems

• Used to interface DTEs to analog telephone networks

Modems Standards• Modems operations for the Telephone line

interfacing is standardized by the International Telecommunications Union – part of ITU.

• Modem standards are set as ITU-T “V-series” Specifications

• When 2 modems attempt to establish a connection a process called “handshaking” takes place.

• Three most important things negotiated are the speed, compression technique and error control.

Modems Standards (Speed)• Speed is the data transmission rate

between DCE to DCE. • Speed depends on line quality

• Examples of speeds used today are:o 14400 bps (V.32 bis)o 28800 bps (V.34bis)o 33600 bps (V34 bis -with software

enhancement)o 57000 bps (V.90)

Modems Standards (Compression)• Compression the process of reducing the

repetitive bits pattern and redundant bits within a data set.

• Eg. a text message contains the string XXXXXX, a compression technique code the string into 2 bytes i.e. :i. one byte to identify the repetitive character andii. one byte to specify the number of times it is

repeated. • Compressed file can be sent at a higher speed than

uncompressed file. • The two primary standards used for modems are

V.42 and the Micro Networking Protocol (MNP) level 5.

Modems Standards (Compression)• V.42 generate 4 to 1 data compression ratio

depending on the type of file being transmitted.

• MNP5 compress data by a factor of 2 to 1.

• To benefit from compression the PC must be able to transmit with its connected modem at a rate that is equal to the speed achieved by the compression ratio.

Modems Standards (Error Control)• Error Control refers to error detection and

correction.

• The standards used are V.42 and MNP levels 1 through 4.

• V.42 uses cyclic redundancy check (CRC) for error detection and automatic repeat request (ARQ). ARQ prevents the modem from accepting any more data until the defective frame has been retransmitted successfully.

Types of Modem Standards• Most recent series based on the V-series

standards published by the ITU-TThree types of modem standards:– V.32 and V.32 bis– V. 34 bis– V 90– V 92

Modem – V.32• The V.32 modem uses a combined modulation and

encoding technique called trelliscoded modulation.

• Trellis is essentially QAM plus a redundant bit. • The data stream is divided into 4-bit sections.

– Instead of a quadbit (4-bit pattern), however, a pentabit (5-bit pattern) is transmitted.

– The value of the extra bit is calculated from the values of the data bits.

– The extra bit is used for error detection.• The Y.32 calls for 32-QAM with a baud rate of 2400.

Because only 4 bits of each pentabit represent data, the resulting data rate is 4 x 2400 = 9600 bps

Modem – V.32 bis• The V.32bis modem was the first of the ITU-T

standards to support 14,400-bps transmission.

• The Y.32bis uses 128-QAM transmission (7 bits/baud with I bit for error control) at a rate of 2400 baud (2400 x 6 = 14,400 bps).

• An additional enhancement provided by Y.32bis is the inclusion of an automatic fall-back and fall-forward feature that enables the modem to adjust its speed upward or downward depending on the quality of the line or signal

Modem – V.32 and V.32 bis

Modem – V.34bis• The V.34bis modem provides a full-duplex

bit rate of 28,800 with a 960-point constellation and a bit rate of 33,600 bps with a 1664-point constellation.

Modem – V.90• V.90 modems with a bit rate of 56,000 bps are

available; these are called 56K modems• These modems may be used only if one party

is using digital signaling (such as through an Internet provider).

• They are asymmetric in that the downloading rate (flow of data from the Internet service provider to the PC) is a maximum of 56 kbps, while the uploading rate (flow of data from the PC to the Internet provider) can be a maximum of 33.6 kbps– How we arrive at the 56-kbps figure? – The telephone companies sample 8000 times

per second with 8 bits per sample. One of the bits in each sample is used for control purposes, which means each sample is 7 bits. The rate is therefore 8000 x 7, or 56,000 bps or 56 kbps.

Uploading and downloading in 56K modems

Modem – V.92• The standard above V.90 is called V.92.

These modems can adjust their speed, and if the noise allows, they can upload data at the rate of 48 kbps. The downloading rate is still 56 kbps.

• The modem has additional features:– For example, the modem can interrupt

the Internet connection when there is an incoming call if the line has call-waiting service.

QUESTIONS?