Introduction

1.1 Physical Structures

Computer Network

Two or more computers that communicate with each other.

Internetwork (internet)

Two or more networks that are connected together (e.g., the Internet).

Communication scheme

Example: A workstation communicating with a server through modems and a public telephone network.

• Simplex -- Data may flow only in one direction

• Half-duplex -- Data may flow in either direction, but only in one direction at a given time

• Full-duplex -- Data may flow in both directions simultaneously.

Transmition signals

Handled by the transmitter and receiver (e.g., digital and analog signals).

Transmission technologies

None All the computer have direct connections. (No sharing of lines of communication.)

Circuit switching Lines are shared. Temporarily dedicated links are created by switch boxes when needed, but devices have to wait for their turn.

Packet Switching Data is is submitted in pieces, called packet. The sender creates the packets. Different packets may follow different paths. The receiver assembles the packets. Designed for data rate of 64 kbps.

Frame relays An improvement on packet switching which reduces the overhead needed to handle transmition errors. Relies on modern communication systems that provide high data rates and low error rates, and on higher layers of logic that catch and correct errors. Designed for data rates of 2 Mbps.

Asychronous transfer mode (ATM) / Cell relay Improves on the frame relay, by using fixed-sized packets instead of the variable-sized packets of the frame relays. Hence, farther reducing the processing overhead. Designed for 10-100 Mbps. Can also provide multiple virtual circuit switching channels with data rates dynamically set by demand.

Extent of networks

Networks are classified by the criteria of how far information has to travel: local area networks (LANs), metropolitan area networks (MANs), and Wide area networks (WANs).

LANs are typically faster than WANs, making use of broadcasting technologies and so eliminating the need for intermediate switching nodes.

Integrated services digital network (ISDN)

A worldwide telecommunication network under construction, with standarized user interfaces supporting a broad range of traffic types. An older version, called narrowband ISDN, supported frame relays. A newer version, called broadband ISDN supports ATM.

Network Topologies

Star Connections are made through a centeral hub.

Ring Data streams in one direction. A break anywhere brings down the entire network.

Bus The computers are attached to a bus. Data can flow in either direction.

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