Post on 05-Apr-2018
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Data Communication and Networks
(Multiplexing and Switching)
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MULTIPLEXING
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Figure 1 Dividing a link into channels
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Figure 2 Categories of multiplexing
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Figure 3 FDM
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FDM is an analog multiplexing
technique that combines signals.
Note:
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Figure 4 FDM process
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Figure 5 FDM demultiplexing example
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Frequency Division Multiplexing
FDMUseful bandwidth of medium exceeds required
bandwidth of channel
Each signal is modulated to a different carrierfrequency
Carrier frequencies separated so signals do notoverlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
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Frequency Division Multiplexing
Diagram
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Time Division Multiplexing
Sharing of the signal is accomplished by dividing available
transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time division
multiplexing.
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Figure 8 TDM
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TDM is a digital multiplexingtechnique to combine data.
Note:
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Figure 9 TDM frames
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In a TDM, the data rate of the link is n
times faster, and the unit duration is n
times shorter.
Note:
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Figure 10 Interleaving
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Figure 11 Multiplexing and inverse multiplexing
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Time Division Multiplexing
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Time division multiplexing
Time division multiplexing comes in two basicforms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time divisionmultiplexing.
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Synchronous Time Division
Multiplexing
Data rate of medium exceeds data rate of digitalsignal to be transmitted
Multiple digital signals interleaved in time
May be at bit level of blocks
Time slots pre-assigned to sources and fixed
Time slots allocated even if no data
Time slots do not have to be evenly distributedamongst sources
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Synchronous Time Division Multiplexing
If one device generates data at a faster rate than other
devices, then the multiplexer must either sample the
incoming data stream from that device more often
than it samples the other devices, or buffer the fasterincoming stream.
If a device has nothing to transmit, the multiplexer
must still insert a piece of data from that device intothe multiplexed stream.
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Synchronous time division multiplexing
So that the receiver may stay synchronized with the incomingdata stream, the transmitting multiplexer can insert alternating
1s and 0s into the data stream.
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Statistical TDM
In Synchronous TDM many slots are wastedStatistical TDM allocates time slots dynamically
based on demand
Multiplexer scans input lines and collects datauntil frame full
Data rate on line lower than aggregate rates ofinput lines
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Statistical Time Division Multiplexing
A statistical multiplexer transmits only the data
from active workstations (or why work when
you dont have to).
If a workstation is not active, no space is wasted
on the multiplexed stream.
A statistical multiplexer accepts the incomingdata streams and creates a frame containing
only the data to be transmitted.
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Statistical Time Division Multiplexing
To identify each piece of data, an address is included.
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If the data is of variable size, a length is also included.
Statistical Time Division Multiplexing
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More precisely, the transmitted frame contains a collectionof data groups.
Statistical Time Division Multiplexing
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A statistical multiplexer does not require a
line over as high a speed line as synchronous
time division multiplexing since STDM doesnot assume all sources will transmit all of the
time!
Good for low bandwidth lines (used forLANs)
Much more efficient use of bandwidth!
Statistical Time Division Multiplexing
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Switching
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Simple Switched Network
Th C t f P th i
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The Concept of Path in
Communication
A path is a route for data to travel between twonodes in a network
Dynamic pathIn this case, the path often changes from packet to packet
Example: Packet Switching
Static pathPath is initially established and it remains unchanged for the
duration of the communication session
Example: Circuit Switching
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Switching Networks
Long distance transmission is typically doneover a network of switched nodes
Nodes not concerned with content of data
End devices are stationsComputer, terminal, phone, etc.
A collection of nodes and connections is acommunications network
Data routed by being switched from node tonode
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Nodes
Nodes may connect to other nodes only, or tostations and other nodes
Node to node links usually multiplexed
Network is usually partially connectedSome redundant connections are desirable for
reliability
Two different switching technologies
Circuit switchingPacket switching
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Circuit Switching
Dedicated communication path between twostations
Three phases
Establish
Transfer
Disconnect
Must have switching capacity and channel
capacity to establish connectionMust have intelligence to work out routing
P bli Ci it S it h d
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Public Circuit Switched
Network
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Circuit Establishment
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Circuit Switching - Issues
Circuit switching is inefficient (designed forvoice)
Resources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed
Both ends must operate at the same rate
Set up (connection) takes time
Once connected, transfer is transparent
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Packet Switching
Circuit switching designed for voiceResources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed
Both ends must operate at the same rate
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Basic Operation
Data transmitted in small packetsTypically 1000 octets
Longer messages split into series of packets
Each packet contains a portion of user data plus
some control info
Control info
Routing (addressing) info
Packets are received, stored briefly (buffered)and passed on to the next node
Store and forward
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Use of Packets
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Switching Technique
Station breaks long message into packetsPackets sent one at a time to the network
Packets handled in two ways
DatagramVirtual circuit
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Datagram
Each packet treated independentlyPackets can take any practical route
Packets may arrive out of order
Packets may go missingUp to receiver to re-order packets and recover
from missing packets
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Datagram Diagram
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Virtual Circuit
Preplanned route established before anypackets sent
Call request and call accept packets establishconnection (handshake)
Each packet contains a virtual circuit identifierinstead of destination address
No routing decisions required for each packet
Clear request to drop circuitNot a dedicated path
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Virtual Circuit Diagram
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Virtual Circuits v Datagram
Virtual circuitsNetwork can provide sequencing and error control
Packets are forwarded more quicklyNo routing decisions to make
Less reliableLoss of a node loses all circuits through that node
DatagramNo call setup phaseBetter if few packets
More flexibleRouting can be used to avoid congested parts of the
network
Transmission of Audio and
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Transmission of Audio and
Video in Packet Switching
Transmission of audio and video are time sensitiveTimely arrival of packet is important
The process that would ensure the orderly and timelyarrival of data packets is known as streaming
Streaming also compresses the data to minimize
bandwidth requirements thus ensuring that the packetshave adequate bandwidth to travel to the destination inthe order in which they are supposed to arrive at thedestination
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Buffering in Streaming
Streamed audio or video is often buffered toensure that the audio or video is playedcontinuously
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Packet Size
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Circuit v Packet Switching
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Circuit Switching
Figure 2-38. (a) Circuit switching. (b) Packet switching.
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Message Switching
A store-and-forward network where the block oftransfer is a complete message.
Since messages can be quite large, this cancause:
buffering problems
high mean delay
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Cell Switching
A network where the unit of transfer is a small,
fixed size block of date (i.e., one cell).ATM (Asynchronous Transfer Mode) networks
use 53 byte cells.
53 bytes
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THANKS?