Modems & Interface Standards

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Modems and Interface Standards

Data Communications

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What we will cover

• Connections to the Computer– Modems– Other devices

• Types and characteristics• Advantages and disadvantages

• Interface Standards• Data Link Connections

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Connections in Networks

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

The interface between a computer and a device occurs at the physical layer.

The interface of a modem is one of the more common devices.

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Connection via Modems

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ModemsModem (Modulator/ Demodulator) It takes digital electrical pulses from a

computer, terminal, or microcomputer and converts them into a continuous analog signal, for transmission over an analog voice grade circuit.

It then re-converts the analog signal to its original digital format.

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Data Communications and Computer Networks Chapter 4

Modems (modulator-demodulator)

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.

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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.

(error correction and compression)

Most modern modems can support the fax standards.

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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.

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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 (interrupt request) assigned.

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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 would actually achieve 64K except:

1. the local loop is still analog, thus analog signaling

2. the analog to digital conversion at the local modem introduces noise/error

Combined, these shortcomings drop the speed to at best 56K.

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Breaking Bandwidth LimitationsA 56K modem does not achieve 56K either because the FCC will not let the modem transmit at the power level necessary to support 56K, so the best the modem can do is approximately 53K

A 56K modem will not even achieve 53K if the connection between your modem and the remote computer contains an additional analog to digital conversion, or if there is significant noise on the line.

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A/D conversion

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Alternatives to Traditional ModemsA T1 line is a digital service offered by the telephone companies and can transfer data as fast as 1.544 Mbps (both voice and computer data).

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

More will be said about T1 in Chapter 12.

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Alternatives to Traditional ModemsCable 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.

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Alternatives to Traditional ModemsISDN modems support ISDN connections. ISDN is an all-digital service capable of supporting data and voice, with data speeds up to 128 Kbps.

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.

More on IDSN and DSL in Chapter 12.

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Modem PoolsA 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.

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InterfacingConnecting a device such as a modem (or DCE - data circuit-terminating equipment or data communicating equipment) to a computer (or DTE - data terminal equipment).

The connections between the DTE and DCE are the interchange circuits.

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Interface StandardsMany different groups contribute to interface standards:

International Telecommunications Union (ITU) (formerly CCITT)

Electronics Industries Association (EIA)

Institute for Electrical and Electronics Engineers (IEEE)

International Organization for Standards (ISO)

American National Standards Institute (ANSI)

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Interface StandardsAll interface standards consist of four components:

1. The electrical component

2. The mechanical component

3. The functional component

4. The procedural component

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Interface StandardsThe 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.

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Interface StandardsThe 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.

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RS-232 and EIA-232EAn 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.

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X.21Another 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.

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Interfacing a Computer and a PeripheralFirewire - A low-cost digital bus that connects peripheral devices such as wireless modems and high speed digital video cameras to microcomputers up to 400 Mbps

Designated as IEEE 1394.

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

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

Support plug and play.

USB can daisychain multiple devices.

Like Firewire, USB is a high speed connection – 10Mbps

USB2 480 Mbps

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Asynchronous ConnectionsA type of connection defined at the data link layer.

Traditional model for modems and printers

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 (1’s) bit can be added to the frame which can be used to detect errors.

Total number of bits? ASCII + start bit + stop bit + parity bit = 11 bits

Indefinite amount of time between character transmission

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The Internet Model

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Synchronous ConnectionsA second type of connection defined at the data link layer.

Transmits sequence of many characters

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.

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Half Duplex, Full Duplex, and Simplex ConnectionsA half duplex connection transmits data in both directions but in only one direction at a time.

A full duplex connection transmits data in both directions and at the same time.

A simplex connection can transmit data in only one direction.

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Terminal-to-Mainframe Computer ConnectionsA 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.

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Terminal-to-Mainframe Computer ConnectionsTo allow a terminal to transmit data to a mainframe, the mainframe must poll (hey, are you ready?) the terminal.

Only one device is connected at a time

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.

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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

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Making Computer Connections In ActionA company wants to transfer files that are typically 700K chars in size.

If an asynchronous connection is used, each character will have a start bit, a stop bit, and a parity bit.

700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits.

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Making Computer Connections In ActionIf a synchronous connection is used, assume maximum payload size = 1500 bytes.

To transfer a 700K char file requires 467 1500-character (byte) frames.

Each frame will also contain 1-byte header, 1-byte address, 1-byte control, and 2-byte checksum, thus 5 bytes of overhead.

1500 bytes payload + 5 byte overhead = 1505 byte frames.

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Making Computer Connections In Action467 frames * 1505 bytes/frame = 716,380 bytes (5,731,040 bits).

Significantly less data than asynchronous (7,700,000 bits).

Documents

Modems & Interface Standards