DCN Transmision
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Transcript of 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