3-mux & switching

7/31/2019 3-Mux & Switching

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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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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.



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More precisely, the transmitted frame contains a collectionof data groups.



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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!



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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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3-mux & switching

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