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Chapter 1: Introduction

Data Communications

The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data.

Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.

The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter.

  1. Delivery: The system must deliver data to the correct destination.
  2. Accuracy: The system must deliver the data accurately.
  3. Timeliness: The system must deliver data in a timely manner.
  4. Jitter: Jitter refers to the variation in the packet arrival time.

Components

  1. Message: The message is the information (data) to be communicated.
  2. Sender: The sender is the device that sends the data message.
  3. Receiver: The receiver is the device that receives the message.
  4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver.
  5. Protocol: A protocol is a set of rules that govern data communications.

Data Flow

  • simplex mode: the communication is unidirectional, only one of the two devices on a link can transmit; the other can only receive.
  • half-duplex mode: each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa.
  • full-duplex mode: both stations can transmit and receive simultaneously.

Networks

A network is the interconnection of a set of devices capable of communication. A device can be a host such a computer or connecting device such as a router.

Network Criteria

A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security.

Physical Structures

Type of Connection

There are two possible types of connections: point-to-point and multipoint:

  • A point-to-point connection provides a dedicated link between two devices.
  • A multipoint connection is one in which more than two specific devices share a single link.

Physical Topology

There are four basic topologies possible: mesh, star, bus, and ring.

In a mesh topology, every device has a dedicated point-to-point link to every other device.

In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub.

A bus topology, is multipoint. One long cable acts as a backbone to link all the devices in a network.

In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it.

Network Types

Local Area Network

A local area network (LAN) is usually privately owned and connects some hosts in a single office, building, or campus.

Each host in a LAN has an identifier, an address, that uniquely defines the host in the LAN. A packet sent by a host to another host carries both the source host's and the destination host's addresses.

Wide Area Network

A wide area network (WAN) is normally created and run by communication companies and has a wider geographical span which connects connecting devices.

Point-to-Point WAN

Switched WAN

Internetwork

When two or more networks are connected, they make an internetwork, or internet.

Switching

An internet is a switched network in which a switch connects at least two links together. A switch needs to forward data from a network to another network when required. The two most common types of switched networks are circuit-switched and packet-switched networks.

Circuit-Switched Network

In a circuit-switched network, a dedicated connection, called a circuit, is always available between the two end systems; the switch can only make it active or inactive.

Packet-Switched Network

In a computer network, the communication between the two ends is done in blocks of data called packets.

A router in a packet-switched network has a queue that can store and forward the packet.

The Internet

Summary

  • Data communications are the transfer of data from one device to another via some form of transmission medium.
  • A data communications system must transmit data to the correct destination in an accurate and timely manner.
  • The five components that make up a data communications system are the message, sender, receiver, medium, and protocol.
  • Text, numbers, images, audio, and video are different forms of information.
  • Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex.
  • A network is a set of communication devices connected by media links.
  • In a point-to-point connection, two and only two devices are connected by a dedicated link.
  • In a multipoint connection, three or more devices share a link.
  • Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology.
  • A network can be categorized as a local area network or a wide area network.
  • A LAN is a data communication system within a building, plant, or campus, or between nearby buildings.
  • A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks.
  • The Internet is a collection of many separate networks.