Chapter 4 Making Connections

Chapter 4

Making Connections

Data Communications andComputer Networks: A Business User’s Approach

Data Communications and Computer Networks Chapter 4

Objectives•Identify a standard modem and cite its basic operating characteristic

•Discuss advantages of newer digital modems

•List alternatives to traditional modems, including T1, cable, ISDN and DSL modems

•Recognise uses of modem pools

•List 4 components of all interface standards

•Discuss basic operation of the EIA-232E interface standard

Data Communications and Computer Networks Chapter 4

Objectives•Cite advantages of Firewire and Universal Bus interface standards

•Outline characteristics of asynchronous and synchronous data link interfaces

•Recognise the difference between half duplex, full duplex and simplex connections

•Identify the operating characteristics of terminal to mainframe connections

Data Communications and Computer Networks Chapter 4

Introduction

Connecting peripheral devices to a computer is normally not a simple task.

Let’s examine the interface between a computer and a device. This interface occurs at the physical layer.

We will start with the interface of a modem, one of the more common devices.

Data Communications and Computer Networks Chapter 4

Modems

Modern modems use combinations of amplitude, frequency, and phase modulation to achieve high data rates.

The fastest dial-up modem at the moment is 56 Kbps.

Modems can support auto answer, auto dial, auto disconnect, and auto redial.

Data Communications and Computer Networks Chapter 4

Modems

•Connection negotiation is the ability of a modem to automatically fall forward or fallback to faster or slower speeds, respectively.

•Modems can perform data compression and error correction and support the MNP 1-5 protocols e.g. MNP5 – supports data compression, MNP4 – supports error correction.

•Most modern modems can support the fax standards. E.g. V.29

Data Communications and Computer Networks Chapter 4

Modems

•Modems can support numerous security features including blacklisting callback security, and backdoor entry with password protection.

•Self-testing (loop-back) is the the ability of a modem to test itself and its connection.

•Local loop-back testing tests the local computer and modem connection while remote loop-back testing tests the connection between the local computer and the remote modem.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Modems

•Modems can be internal, in which they plug into a slot inside a computer, or external, in which they are separate from the computer and require a serial cable and their own power supply.

•Internal modems do not require a serial cable but instead require an IRQ assigned.

•Partially internal modems in laptops, PC Cards, PC Card Hosts and PC Card Software – PCMCIA (Personal Computer Memory Card International Association) standard.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Breaking Bandwidth LimitationsA 56K modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems.

A 56K modem actually only achieves approximately 53K due to noise and reduced power levels.

A 56K modem will not even achieve 53K if the connection between your modem and the remote computer does not contain an analog to digital conversion.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Alternatives to Traditional Modems•A T1 line is a service offered by the telephone companies and can transfer data as fast as 1.544 Mbps. (Available in the US)

•To support a T1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Alternatives to Traditional Modems•Cable modems allow high speed access to wide area networks such as the Internet.

•Most cable modems are external devices that connect to the personal computer through a common Ethernet card.

•Cable modems can provide data transfer speeds between 500 Kbps and 2.5 Mbps.

•A few cable modem services require a telephone line for the upstream connection.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Alternatives to Traditional Modems•ISDN modems support ISDN connections. ISDN is an all-digital service capable of supporting data and voice, with data speeds up to 128 Kbps- Basic Rate – 2B channels (64Kbps each) and 1 D channel (16Kbps)

•DSL modems support digital subscriber line service. DSL is quickly growing in popularity and provides a high-speed service between homes and Internet service providers.

Data Communications and Computer Networks Chapter 4

Modem Pools•A relatively inexpensive technique that allows multiple workstations to access a modem without placing a separate modem on each workstations.

•Modem pools can also be used to allow external users to dial into a business or corporate network via a modem in the modem pool.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Interfacing•Connecting a device such as a modem (or DCE - data circuit-terminating equipment) to a computer (or DTE - data terminal equipment).

•The connections between the DTC and DCE are the interchange circuits.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Interface StandardsMany different groups contribute to interface standards:

•International Telecommunications Union (ITU)

•Electronics Industries Association (EIA)

•Institute for Electrical and Electronics Engineers (IEEE)

•International Organization for Standards (ISO)

•American National Standards Institute (ANSI)

Data Communications and Computer Networks Chapter 4

Interface StandardsAll interface standards consist of four components:

1. The electrical component

2. The mechanical component

3. The functional component

4. The procedural component

Data Communications and Computer Networks Chapter 4

Interface Standards•The electrical component deals with voltages, line capacitance, and other electrical characteristics.

•The mechanical component deals with items such as the connector or plug description. A standard connector is the ISO 2110 connector, also known as DB-25. (Dual Band)

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Interface Standards•The functional component describes the function of each pin or circuit that is used in a particular interface.

•The procedural component describes how the particular circuits are used to perform an operation.

•For example, the functional component may describe two circuits, Request to Send and Clear to Send. The procedural component describes how those two circuits are used so that the DTE can transfer data to the DCE.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

RS-232 and EIA-232E•An older interface standard designed to connect a device such as a modem to a computer or terminal.

•Originally RS-232, but has gone through many revisions.

•The electrical component is defined by V.28, the mechanical component is defined by ISO 2110, and the functional and procedural components are defined by V.24.

Data Communications and Computer Networks Chapter 4

X.21•Another interface standard that was designed to replace the aging RS-232.

•Currently popular in Europe and with ISDN connections.

•Each circuit in the X.21 standard can contain many different signals.

•Since each circuit can transmit different signals, the combination of signals on the four circuits is much larger than if each circuit performed only a single function.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Interfacing a Computer and a Peripheral•Firewire - A bus that connects peripheral devices such as wireless modems and high speed digital video cameras to microcomputers.

•Designated as IEEE 1394.

•Firewire supports asynchronous connections and isochronous connections (provides a guaranteed data transport at a pre-determined rate).

Data Communications and Computer Networks Chapter 4

Interfacing a Computer and a Peripheral•Universal Serial Bus (USB) - Modern standard for interconnecting modems and other peripheral devices to microcomputers.

•Supports plug and play.

•USB can daisychain multiple devices.

•Like Firewire, a high speed connection.

Data Communications and Computer Networks Chapter 4

Asynchronous Connections•A type of connection defined at the data link layer.

•To transmit data from sender to receiver, an asynchronous connection creates a one-character package called a frame.

•Added to the front of the frame is a Start bit, while a Stop bit is added to the end of the frame.

•An optional parity bit can be added to the frame, which can be used to detect errors.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Synchronous Connections•A second type of connection defined at the data link layer.

•A synchronous connection creates a large package (frame) that consists of header and trailer flags, control information, optional address information, error detection code (checksum), and the data.

•A synchronous connection is more elaborate, but transfers data in a more efficient manner.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Half Duplex, Full Duplex, and Simplex Connections•A simplex connection can transmit data in only one direction. E.g. Broadcast TV, radio, most pagers

•A half duplex connection transmits data in both directions but in only one direction at a time. E.g. Walkie talkies, CB radio, LANs

•A full duplex connection transmits data in both directions and at the same time. E.g. Telephone systems, cable modems, >= 100Mbps LANs

Data Communications and Computer Networks Chapter 4

Terminal-to-Mainframe Computer Connections•A point-to-point connection is a direct, unshared connection between a terminal and a mainframe computer.

•A multipoint connection is a shared connection between multiple terminals and a mainframe computer.

•The mainframe is called the primary, and the terminals are called the secondaries.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Terminal-to-Mainframe Computer Connections•To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal.

•Two basic forms of polling include roll-call polling and hub polling.

•In roll-call polling, the mainframe polls each terminal in a round-robin fashion.

•In hub polling, the mainframe polls the first terminal, and this terminal passes the poll onto the next terminal.

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Making Computer Connections In ActionThe back panel of a personal computer has many different types of connectors, or connections:

•RS-232 connectors

•USB connectors

•Parallel printer connectors

•Serial port connectors

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4

Data Communications and Computer Networks Chapter 4