Computer Networks A 1

DT024G, Datornätverk A, 7,5 hp

DT075G, Multimedie- och kommunikationssystem, 6 hp

Kapitel 1 - Introduktion

Föreläsningsmaterialet är författat av Magnus Eriksson. Material är även hämtat från Iskra Popova samt läroboksförfattaren Behrouz

Fourozan.

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Tänk på ett tal mellan 0 och 31

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Amount of information

- Think about a number between 0 and 15. I am now going to guess it using as few yes and no questions as possible. I start by asking: Is the number larger than or equal to 8?

- Yes- Is it larger than or equal to 12? (The interval is successively divided

by 2.)- No.- Is the number larger than or equal to 10?- Yes- Is the number larger than or equal to 9?- Yes.- The amount of information you give me when you tell me that the

number is 9 is 4 bits, because the amount of information in bits is the minimum number of yes and no questions that are required. We had 16 options, which is = 24 = 2·2·2·2, corresponding to 4 bits. If the number of options was 32 = 25 , it would require 5 bits.

- If yes is represented by the binary digit ”1”, and no by ”0”, the value in the above example can be represented by 1 0 1 1.

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Bits and Bytes

N bit can represent M=2N different values. M values can be represented by

N = 2log M =log M / log 2 values Example: The N=7 bit ASCII character code

consists of M=128 codes. 8 bits = 1 byte (a unit for measuring amount

of data) 1 kbit = 1000 bit (previously 1024 bit). 1 Mbit = 1000 kbit (previously 1024 kbit). 1 Gbit = 1000 Mbit. 1 Tbit = 1000 Gbit.

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Data Representation

Text – using different codes Each character is represented by certain

number of bits The number of bits in the code determins the

number of different characters ASCII (7 bits), Extended ASCII (8 bits), Unicode

(16 bits), ISO (32 bits) Numbers – Binary number system Images – A matrix of pixels represented by

bit patterns Video – A combination of images Audio – Digitized voice and music

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Bandbredd kan betyda många saker

Analog bandbredd i Hertz. Övre gränsfrekvens minus undre gränsfrekvens.

Net bit rate = Digital bandbreddskapacitet i Bit/s. = Datatakt exklusive felupptäckande koder. Net bit rate ≤ Gross bit rate

Maximal genomströmninshastighet = Maximal throughput

Genomströmningshastighet = Throughput = Digital bandbreddskonsumtion

Goodput = Filöverföringshastighet

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Chapter 1Introduction

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Figure 1.1 Five components of data communication

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

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Figure 1.2 Simplex communication

Example: Video monitor. TV and radio broadcasting.

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Figure 1.3 Half-duplex communication

One at a time.

Example: Communication radio. 2-wire Ethernet.

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Figure 1.4 Full-duplex communication

Example: Telephony. 4-wire Ethernet.

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1-2 NETWORKS1-2 NETWORKS

A A networknetwork is a set of devices (often referred to as is a set of devices (often referred to as nodesnodes) ) connected by communication connected by communication linkslinks. A node can be a . A node can be a computer, printer, hub, switch, router, modem, or any computer, printer, hub, switch, router, modem, or any other device capable of sending and/or receiving data other device capable of sending and/or receiving data generated by other nodes on the network.generated by other nodes on the network.

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Different Line Configurations

Point-to-point Two devices on a

single channel (dedicated channel)

Multipoint Many devices on a

single channel (shared channel)

Example:Bus Networks. Ring networks.Wireless Networks

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Topology of Networks

Topology

BusStarFull Mesh RingPartial Mesh

Topology defines the arrangement of links in a network

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Figure 1.5 Mesh topology (Maskformigt nätverk)

Example: 5 stations. 4 ports per station. 4+3+2+1=10 links.

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Figure 1.7 Bus topology (Bussnätverk)

Examples: -”Old” ethernet with coaxial cable- Cable TV networks with cable modems

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Figure 1.6 Star topology (Stjärnnätverk)

Hub (Nätnav) or switch (växel)

Examples: -Ethernet with twisted pair cable-Telephone networks

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Figure 1.8 Ring topology (Ringnätverk)

Example: Token ring and FDDI networks

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Figure 1.9 A hybrid topology: a star backbone with three bus networks

Hub (Nätnav) or switch (växel)

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LAN – Local Area Network (Lokalt nätverk)

Single building LAN

Backbone(ryggradsdatanät)

Multiple building LAN

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MAN - Metropolitan Area Network (Stadsnät)

Public city network

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WAN – Wide Area Network (nationellt eller globalt nätverk)

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Figure 1.12 A heterogeneous network made of four WANs and two LANs

LAN = Local Area Network

WAN = Wide Area Network

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Nättopologier för WAN

Hopkopplade LAN och WAN = internetwork T.ex. Internet.

Växlat WAN (stjärnnät)T.ex. X.25 eller ATM

Växel. Kopplar ihop punkt-till-punkt-länkar.

Datorer, terminaler, skrivare, etc.Router=vägväljare, kopplar ihop nätverk med olika teknologier.

Nätnoder:

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The Internet

Internet routers are network nodes that interconnect different networks.Network Access Points (NAP) are routers that interconnect national ISP:s. ISP = Internet Service Provider (internetleverantör)

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The internet versus the Internet

Internetwork or internet (small ”i”) – generic term to mean an interconnection of networks

Internet (Uppercase I) – the specific worldwide network that uses the IP protocol (Internet protocol)

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