dcn transmision

8/2/2019 DCN Transmision

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ASYNCHRONUS AND SYNCHRONUSTRANSMISSION

serial->bit by bit transmission fromsender to receiverParallel->simultaneous transfer


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serial-> conversion devices are required at theinterface between the device and the line oftransmission

Asynchronous transmission->

timing is irrelevant, bits are send as group of 8

1start bit at beginning and 1 stop bit at end

There may be gap between each byte

At byte level sxr and rxr are not synchronized

Synchronous transmission

Sends bit 1 after another without start or stop bits or gaps

Bit stream is converted to longer frames(multiple bytes)


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Multiplexing

Frequency Spectrum - the combination of all signals that make up the

signal. It is a collection of all component frequencies of the signal

Bandwidth - refers to the range of frequencies of frequency spectrum or

simply the width of the frequency spectrum

Bandwidth = highest frequency minus lowest frequency

Bw = fh fl

Multiplexing-> allows simultaneous transmission of multiple signals


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Frequency division multiplexing

Bandwidth of link is greater than combined bandwidth of all

the signals to be transmitted Signals generated by each device is modulated at different

carrier frequencies and is combined to form a composite signal.

Guard bands prevent signals from overlapping

Synchronous Time-Division Multiplexing

Time-division multiplexing (TDM) is a digital process thatallows several connections to share the high bandwidth of alink

Shares time instead of portion of bandwidth

Statistical Time-Division Multiplexing

In statistical time-division multiplexing, slots are dynamically

allocated to improve bandwidth efficiency.


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MO-DEM

ADSL-asymmetric dsl

Higher downstream speed than upstream

For residential users

Uses existing local loops

Adaptive technology that uses data rate based on condition oflocal loop line

Modulation technique used is DMT

Channel 0 voice;1-5 idle,6-30 upstream;31- 255 downstreamdata transfer and control


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Tecnology Downstreamrate

Upstreamrate

ADSL LITE 1.5mbps 500 kbps

HDSL 1.5-2.0 mbs 1.5-2.0mbps

SDSL 768 kbps 768 kbps

VDSL 25-55mbps 3.2mbps


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Spread Spectrum

Spread spectrum is designed to be used in wirelessapplications

Spread spectrum achieves its goals through two

principles:1. The bandwidth allocated to each station needs to

be, by far, larger than what is needed. This allowsredundancy.

2. The expanding of the original bandwidthB to thebandwidth Bss must be done by a process that isindependent of the original signal. In other words, thespreading process occurs after the signal is created bythe source.


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Frequency Hopping Spread Spectrum (FHSS)

The frequency hopping spread spectrum (FHSS)technique usesM different carrier frequencies thatare modulated by the source signal.

At one moment, the signal modulates one carrierfrequency; at the next moment, the signal modulatesanother carrier frequency


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Direct Sequence Spread Spectrum

The direct sequence spread spectrum (dSSS) technique alsoexpands the bandwidth of the original signal, but the process isdifferent. In DSSS, we replace each data bit with n bits using a

spreading code.

Eg: barker sequence where n is 11

Privacy

Immunity against interference


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Code-Division Multiple Access (CDMA)

In CDMA, one channel carries all transmissionssimultaneously

CDMA differs from FDMA because only one channel

occupies the entire bandwidth of the link.It differs from TDMA because all stations can send

data simultaneously; there is no timesharing.

Communication with different codes

Multiplying each code by another we get 0

Multiplying code by itself we get the no of stations


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Circuit switching

Set of switches connected by physical links

Connection is dedicated path made of 2 or more links;each connection uses only one dedicated channel on

each link.Takes place at physical layer

Data transferred are not packetized

No addressing involved3 phases

Set up:- a dedicated link is established

Data transfer phase:-2 parties transmit data


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Space division switching

Path in the circuit are separated from each otherspatially

Cross bar switch

Connects n i/ps to m o/ps in a grid

Uses electronic micro switches (transistors) at cross point

Limitations

n x m cross pointsMore idle cross points


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Multistage switches

Crossbar switches in several stages

Middle level has less switches than first & finalstages

Outputs of switches are given as inputs of otherswitches

Multiple paths are given

Reduced cross point numbers

First stage must have output to each nextintermediate stage

Last stage must have input from each previous


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DATAGRAM

In a datagram network, each packet is treatedindependently of all others. Even if a packet is part of amultipacket transmission, the network treats it as thoughit existed alone. Packets in this approach are referred to

as datagrams.Datagram switching is normally done at the network

layer

Packets may get lost or arrive at out of sequenceConnectionless networks:- switch does not keep

information about the connection state


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Virtual-circuit networks

A virtual-circuit network is a cross between a circuit-switchednetwork and a datagram network. It has some characteristics ofboth.

There is tear down and set up phase

Resources are allocated during set up

Data are packetized and each packet carries an address in header-global address and local address

All packets follow same established path

Implemented at data link layer


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Effect Of Packet Size

Smaller packets and much larger packets increases delay

Connection oriented:- connection is established before

transmission Connection less:- connection is not established between the

device before transmission


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TH

dcn transmision

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