6677239-Circuit-Switching

8/7/2019 6677239-Circuit-Switching

http://slidepdf.com/reader/full/6677239-circuit-switching 1/51

1

Network Layer and CircuitSwitching

Networks provide for communicationbetween devices without direct connections.

Circuit Switching is the Oldest Networking

Technology



2

Kinds of Networks

� By Communication Technique� Switched Networks

� Broadcast Networks e.g. LANs

� By Geographical Coverage� Wide Area Network

� Cover large geographical areas, often crossingpublic right-of-ways

� Usually consist of several interconnectedswitching points

� Local Area Network� Small scope, usually a building or cluster

� Typically owned by the same organization thatowns the equipment



3

Switched Network Types

� Circuit Switched Networks

� Packet Switched Networks



4

Circuit-Switching

� Definition: Communication in which adedicated communications path isestablished between two devices through

one or more intermediate switching nodes� Oldest Networking Technology - more than

a hundred years old

� Dominant in both voice and datacommunications today

� e.g. PSTN is a circuit-switched network

� Relatively inefficient (100% dedication

even without 100% utilization)



5

Circuit Switching� In circuit switching network any two stations wishing

to communicate first establish a connection byrequesting to the network.

� The network responds by arranging a connection insuch a way that a dedicated/physical path isestablished between the two parties

� For the duration of the connection the path isdedicated to the connection and cannot be used forother connections

� After the connection has been established thenetwork becomes pretty much transparent to the twoparties. Data can be exchanged transparently overthe connection

� After the parties are done communicating theconnection is released by making a request to thenetwork



6

Circuit-Switching Stages

� Circuit establishment

� Transfer of information

� point-to-point from endpoints to node

� internal switching/multiplexing amongnodes

� Circuit disconnect



7

Circuit Establishment

� Station requests connection fromnode

� Node determines best route, sendsmessage to next link

� Each subsequent node continues theestablishment of a path

� Once nodes have establishedconnection, test message is sent todetermine if receiver is ready/able to

accept message



8

Information Transfer

� Point-to-point transfer from source tonode

� Internal switching and multiplexedtransfer from node to node

� Point-to-point transfer from node toreceiver

� Usually a full-duplex connectionthroughout



9

Circuit Disconnect

� When transfer is complete, onestation initiates termination

� Signals must be propagated to allnodes used in transit in order to freeup resources



10

Circuit Switching Application

� Circuit switching is well suited foranalog voice communications as inthe telephone network.

� Circuit switching turns out to berather in-efficient for data networksdue to its resource allocation nature.

� Circuit Switching is ill-suited to datacommunication because data traffic isBAD



11

Examples of Circuit

Switching

� Public Switched Telephone Network -PSTN

� Private Automatic Branch Exchange -PABX

� Integrated Services Digital Network -ISDN



12

Public Switched Telephone

Network (PSTN)

� PSTN is short for Public SwitchedTelephone Network, which refers tothe international telephone system

based on copper wires carryinganalog voice data. This is in contrastto newer telephone networks base on

digital technologies, such as ISDN andFDDI.

� Telephone service carried by the

PSTN is often called plain old



13

POTS

� POTS is short for plain old telephoneservice, which refers to the standardtelephone service that most homes use. In

contrast, telephone services based on high-speed, digital communications lines, suchas ISDN and FDDI, are not POTS.

� The main distinctions between POTS and

non-POTS services are speed andbandwidth. POTS is generally restricted toabout 33.6 kbps (33,600 bits per second)though several modem manufacturers have

developed technologies that would enable



14

Public Switched Telephone

Network (PSTN) Elements

� Subscribers

� Local loop

� Connects subscriber

to local telcoexchange

� Exchanges

� Telco switching

centers� Also known as end

office

� >19,000 in US

� Trunks

� Connectionsbetween exchanges

�

Carry multiple voicecircuits using FDMor synchronous TDM

� Managed by IXCs(inter-exchangecarriers)



15

Telephone Network Structure



16

Telephone Network



17

Typical Circuit Route for

Medium Distance Calls-



18

PSTN Connectivity



19

Is PSTN Analog or Digital?



20

Issues in Circuit Switched

Networks

� Routing

� Control Signalling



21

Routing

� Routing in the network tries to determine thepath from a given source to a givendestination

� Many connections will need paths through

more than one switch� Need to find a route

� Efficiency� Resilience

� Public telephone switches are a treestructure� Static routing uses the same approach all

the time

� Dynamic routing allows for changes in



22

Routing in Circuit-Switched

Networks

� Requires balancing, efficiency andresiliency

� Traditional circuit-switched model is

hierarchical, sometimessupplemented with peer-to-peertrunks

� Newer circuit-switched networks aredynamically routed: all nodes arepeer-to-peer, making routing more

complex - almost like packet



23

Static Hierarchical Routing

� Trace common node in hierarchy

� Resiliency provided by cross-cuttingtrunk lines

� Minimum switching costs, flexibilityonly via alternate trunks

� Not able to adapt to changingconditions

� Not able to adapt to node failureseasily





25

Alternate Routing



26

Adaptive Routing

� Traffic reporting and analysis withnew paths computed periodically,adapts to net load, events

� Need to use algorithms to determinepaths dynamically, based onload/congestion vectors



27

Control Signaling

� Manage the establishment,maintenance, and termination of signal paths

� Includes signaling from subscriber tonetwork, and signals within network

� In-channel Control Signalling

� In-channel signaling uses the samechannel for control signals and calls

� Common Channel Control Signalling

� Common-channel signaling uses



28

Control Signaling Functions

� Audible communication withsubscriber

� Transmission of dialed number

� Call can not be completed indication

� Call ended indication

�

Signal to ring phone� Billing info

� Equipment and trunk status info

� Diagnostic info



29

Location of Signaling

� Subscriber to network

� Depends on subscriber device and switch

� DSS1

� Within network

� Management of subscriber calls andnetwork

� More complex� SS7



30

Control Signal Sequence

� Both phones on hook� Subscriber lifts receiver (off hook)

� End office switch signaled

� Switch responds with dial tone

� Caller dials number� If target not busy, send ringer signal to target

subscriber

� Feedback to caller

� Ringing tone, engaged tone, unobtainable� Target accepts call by lifting receiver

� Switch terminates ringing signal and ringing tone

� Switch establishes connection

� Connection release when Source subscriber hangs



31

Switch to Switch Signaling

� Subscribers connected to differentswitches

� Originating switch seizes interswitch

trunk

� Send off hook signal on trunk,requesting digit register at target

switch (for address)� Terminating switch sends off hook

followed by on hook (wink) to show

register ready



32

Control Signals



33

In Channel Signaling

� Use same channel for signaling and call� Requires no additional transmission facilities

� Inband� Uses same frequencies as voice signal

� Can go anywhere a voice signal can

� Impossible to set up a call on a faulty speechpath

� Out of band� Voice signals do not use full 4kHz bandwidth

� Narrow signal band within 4kHz used for control

� Can be sent whether or not voice signals arepresent

� Need extra electronics

D b k f I Ch l



34

Drawbacks of In Channel

Signaling

� Limited transfer rate

� Delay between entering address(dialing) and connection

� Overcome by use of common channelsignaling



35

Common Channel Signaling

� Control signals carried over pathsindependent of voice channel

� One control signal channel can carrysignals for a number of subscriber channels

� Common control channel for thesesubscriber lines

� Associated Mode

� Common channel closely tracksinterswitch trunks

� Disassociated Mode

� Additional nodes (signal transfer points)

� Effectively two separate networks

C I Ch l



36

Common v. In Channel

Signaling





38

Signaling System Number 7

� SS7

� Common channel signaling scheme

� Used in ISDN and inside PSTN

� Optimized for 64k digital channel network� Call control, remote control, management

and maintenance

� Reliable means of transfer of info insequence

� Will operate over analog and below 64k

� Point to point terrestrial and satellite links



39

SS7 Benefits

� SS7 adds intelligence to a network� Basis of new end user services e.g.

� 800 and 900 services

�Mobile Telephone Service

� Mobile subscriber authentication

� Caller identification

� Charging calls to a credit card

� Charging calls to a calling card

� SS7 standards include a standardclient/server transaction protocol -

Transaction Capabilities Application Part -TCAP

SS7



40

SS7

Signaling Network Elements

� Signaling point (SP)� Any point in the network capable of

handling SS7 control message

� Signal transfer point (STP)

� A signaling point capable of routingcontrol messages

� Control plane

� Responsible for establishing andmanaging connections

� Information plane

� Once a connection is set up, info istransferred in the information plane

T f



41

Transfer

Points



42

Signaling Network Structures

� STP capacities� Number of signaling links that can be

handled

� Message transfer time� Throughput capacity

� Network performance

� Number of SPs� Signaling delays

� Availability and reliability

� Ability of network to provide services in

Circ it S itching Node



43

Circuit Switching Node:

Digital Switch

� Provides transparent signal pathbetween any pair of attached devices

� Typically full-duplex





45

Blocking or Non-blocking

� Blocking� A network is unable to connect stations

because all paths are in use

� A blocking network allows this� Used on voice systems

� Short duration calls

� Non-blocking� Permits all stations to connect (in pairs)

at once

�

Used for some data connections




46

Circuit-Switching Node:

Network Interface

� Provides hardwareand functions toconnect digital

devices to switch� Analog devices can

be connected if

interface includesCODEC functions

� Typically full-

duplex




47

Circuit-Switching Node:

Control Unit

� Control Unit� Establish

connections� Generally on demand

� Handle andacknowledgerequests

� Determine if

destination is free� construct path

� Maintain connection-while needed

� Disconnect - Breaks

Circuit Switch Design Cross



48

Circuit Switch Design - Cross

Bar Switch

� Crossbar switch� Number of

crosspoints grows assquare of number of

stations� Loss of crosspoint

prevents connection

� Inefficient use of

crosspoints� All stations

connected, only afew crosspoints inuse

� Non-blockin



49

Multistage Switch

� Reduced number of crosspoints

� More than one path through network

� Increased reliability

� More complex control

� May be blocking



50

Circuit Switch Design TDM



Circuit Switch Design - TDM

Bus Switch

� Partition low speed bitstream into pieces thatshare higher speedstream

� e.g. TDM bus switching� based on synchronous

time divisionmultiplexing

� Each station connectsthrough controlled gatesto high speed bus

� Time slot allows smallamount of data onto bus

Date post:	08-Apr-2018
Category:	Documents
Upload:	isam702
View:	218 times
Download:	0 times

6677239-Circuit-Switching

Documents