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An introduction to networks, discussing their goals, typical types, basic technology, and required components. It covers wan, lan, man, wireless lan, san, and active networks, as well as dial-up services, channel capacity, and modems. It also touches upon baseband and broadband systems, nyquist theorem, and data and signals.
Typology: Study notes
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Hosts: Router, Imp, switch: Initially ARPANET ►Internet
Dial your ISP number Your modem gets then connected to the server via PSN Server connects you to the host that you want Your computer, your ISP system are both hosts PSN
● packets are all heard by all machines ● packets have an address field, machines sporting different address discard them ● sending a message to all machines is easier. ● topologies: Bus (Ethernet) Ring (Token ring) Radio ♪… ♫ …. ♫
Satellite :
Bandwidth of a link = measure of what can pass through = max allowed freq – min allowed freq. Bandwidth is measured in bps (bits per sec) Or in Herz. Low bandwidth signals are less susceptible to noise interference. But higher bandwidth is desirable. Bandwidth == throughput (?) How fast a file can be transmitted? Bandwidth test. Idea is to find an approx measure of channel capacity. Depends on: Internet traffic (speed generally decreases as volume increases) Freq response Frequency Bandwidth
Variable propagation delays (can artificially inflate or degrade the result) Noise on data lines (has a real detrimental effect) The size(s) of file(s) used for the test The number of files used for the test The demand load on the test server at time of test Geomagnetic and/or thunderstorm activity Naturally, statistically sampled result would be used. Two types of systems (not concerned with narrow band) would be used: Baseband systems: allowing a single nonmultiplexed digital signal for transmission, reception and pass through. Broadband systems: allowing a wide band of frequencies available for transmission. Information can be multiplexed, and sent on many different frequencies and channels allowing more throughput than baseband. Baseband ↔ digital broadband ↔ analog Narrow band ↔ 50 Hz – 64 Kbps (for paging, mobile ..) Nyquist Theorem (a limit on available bandwidth) A sampling theorem for digitization of analog signals.
) N S Cmax B log 2 ( 1 bps Note that signal to noise ratio is usually expressed in decibels: i.e. Instead of N S we quote 10 N S log 10. e.g. Signal to noise ratio is 30 dB. Bandwidth is 3000 Hz. C (^) max 3000 log 2 ( 1 1000 ) = 30,000 bps How long does it take to transmit a signal? Transmission time sec B M M : Message size in bits B : Bandwidth in bits/sec Transmitter ■ ■ ■ ■ ■ ■ ■ ■■ ■ ■
Once the bits are on the cable, they travel at a speed of light (= 2c =^2 10 meter/sec ^8 ).
F C C C C C F C C C C F … → Computer-to-computer High-throughput. High-line efficiency. Error-dectection code might be necessary. Self-clocking signaling technique required. Expensive. Asynchronous Transmission: Each character is framed before transmitted. Though inter-character time is nonuniform, timing of each bit within a character is specified. F C F C F C F C … → Terminal-to-computer Low cost. For low speed lines (< 16kbps). Easy to implement. Clock synchronizes with each character. Asynchronous character format Sender and receiver must agree on Number of bits per character Length of a bit time Length of the START and the STOP character Signal Levels START BIT BIT BIT BIT STOP LEAST SIGNIFICANT BIT
Order of bit transmission Character code Therefore, we can have Synchronous Message Format Both the sender and the receiver agrees on Transmission format Duration of a bit Signal levels SYNC characters Size and content of header and trailer
Voltage amplitude Bit representation 0 Volts (min) 0 0 2 Volts 0 1 4 Volts 1 0 6 Volts (max) 1 1 This would give us 10 bps rate.
These can handle several TV channels, CD audio channels, digital streams. Broadband needs directional amplifiers. These transmit signals in one direction. Head-end catches it, retransmits at a different frequency on another cable. Another trick is to fold it as below. H E A D E N D
1 f f . Frequency hoping to avoid transmitter/receptor detection.
